Multi-band Inverted Vee - Version 2.0

The multi-band inverted vee I had on the previous house-bracketed mast worked well and served my purposes at the time. Since then my interests and objectives have changed. The inverted vee still serves an important purpose in my station but one that requires changes.

The new and improved antenna mast was the first step. The time had come for a more permanent and robust structure. The antenna came next, with changes both to its operating characteristics and its mechanical design. Both were small changes and were quickly put into effect once the weather improved. It has been very wet here recently. The antenna is up in the air for testing and tuning, almost but not quite ready for the fall season.

Objectives

In the past year my operating objectives have changed. My original plan of low-impact QRP operation put the inverted vee in the spotlight as key element of my antenna plan. It included elements for 30, 20, 17 and 15 meters, and also worked well on 10 meters. Its apex at 14.3 meters was higher than the multi-band dipole for 20, 17, 15 and 10 meters, so it not only did better on some longer path DX it filled the nulls in the dipole's pattern. It was also my only antenna for 30 meters.

DXing and QRP remain as objectives for the immediate future. At least that hasn't changed. But now that I've returned to contest operating and put up the new tower the purpose of the inverted vee has changed.

• Short path 40 meters: The delta loop was a great antenna for DXing but not so good for working short paths. Contesting requires an antenna with more high-angle radiation for this band.
• Diversity: The planned tri-band yagi on the 15 meters high tower will be a boon for QRP DX and contests. That benefit comes with disadvantages. In particular the F/B, F/S and the time to swing the yagi around to work perhaps just one or a few stations. The inverted vee will allow the ability to hear, then quickly call and hopefully work a rare DX station or contest multiplier.
• Reliability: The inverted vee has to survive normal wear-and-tear and weather events. The old antenna was prone to problems that required maintenance and repair.

New electrical design

The 15 meters elements was extended to make it a 40 meters element. That's it; no other changes were made. Even so there are implications.

The 40 meters element length was modelled in EZNEC beforehand since the wire is insulated and the interior angle and height impact on the resonant frequency. This calculated to 10.2 meters per side, and that is how I cut it. From previous modelling and experimentation I found that the longest element in a fan dipole of this type (multi-wire cage) is least affected by the shorter elements. This didn't quite work out since the resonant frequency is close to 6.9 MHz, or nearly 3% below 7.1 MHz. I'll have to trim it to improve the SWR in the SSB segment.

Problems were expected for the other elements since they all must now contend with an additional adjacent wire at their ends, which is the situation that most impacts tuning. This is just what I found on 20 and 17 meters, with both antennas having their resonant frequencies reduced by about 1.5%. Interestingly this did not happen on 30 meters (formerly the longest element), where the SWR stayed put at almost exactly 1.

I have not yet trimmed the antenna yet since I want to "test drive" it for a few days to determine its performance. An SWR of 1.5 on the CW segment of 20 and 2.5 on 17 is good enough to allow testing. Somewhat troubling is that even at resonance (17.9 MHz) the SWR of 1.9 is higher than I'd like. However since the rig isn't complaining I may just trim the antenna and then leave it be.

The situation on 15 meters is more interesting. I expected a problem since a 40 meters dipole typically resonates quite a bit higher than simply 3x the fundamental frequency. The standalone EZNEC model (mentioned above) showed resonance around 21.4 MHz for a fundamental at 7.05 MHz. With the resonant frequency lower than planned the 15 meters resonant frequency is 21.1 MHz (SWR 1.7). I measured the antenna below the band edge to get an idea how the SWR would behave after I trim the 40 meters element. It will be acceptable but not ideal for the CW segment.

To better understand this antenna I may take the trouble to modelling it in EZNEC. It may be difficult because of the closely-spaced wire. In any case I don't have the time for that now.

New mechanical design

The bulk of the improvement to the antenna is its mechanical. This may not be obvious in the picture above so I'll step through what I did.

• Insulators: Tying the ends of each element with nylon rope was expedient at the time but not a great idea. Precipitation would wet the rope and cause the resonant frequency on all bands to drop quite dramatically. Since I still need the antenna to be light I made a pile of insulators from the same PVC pipe I used to make the X-spreaders for the cage. (That may be the best $3 I have ever spent.) As you can see in the picture they are simple and ugly, but they work. When it rains I'll know for sure.
• Wire ties: The wires and ropes were held in notches at the ends of the X-spreaders by friction alone. They sometimes popped out in the wind or when the antenna was lifted and lowered. I also had the problem of the spreaders leaning over in the places where the friction was insufficient to securely hold all the wires and ropes. I solved this by drilling small holes just below every notch in every spreader, sliding an AWG 22 wire through the hole and wrapping it around the wire element (or rope) on both sides of the spreader (I have lots of surplus telephone quad cable). The picture shows how this looks (along with an insulator) at the north end of the 30 meters element.
• Rope routing: The 30 meters element and the ropes holding the other element ends in the first version all tied back to a steel ring (flat washer). The ropes had a habit of twisting and tangling. I simplified the structure and made it stronger. The 20 meters element ties to the rope holding the 17 meters element so that it doesn't have to be any longer. That longer rope passes through the centre hole in the I-spreader for the 30 and 40 meters element (one half of an X-spreader, as pictured immediately above) to a short spreader near the end of the 40 meters element. Result: no tangling and all inter-element distances are held consistent. Ropes for the shorter wires can be correctly tensioned provided that the 40 meters element is held taut.
• Spreader spacing: I had been careless with placement of the spreaders. This causes additional sag in some places. In this version the distance between X-spreaders is held consistent at 2.3 meters (93"). The position of the new I-spreaders was placed at the end of the 30 meters element to best manage consistent element spacing.

The result was impressive. When I finished construction I was able to drag the antenna across the yard, lift it by attaching the centre insulator to my climbing harness, and then winch it to the top of the mast without a single wire or spreader moving out of place through all of this abuse. The few tangles were unravelled by simply pulling on the ropes holding the ends of the element halves. In other words, I seem to have gotten it right this time.

Performance

After finishing the antenna raising just before sunset on Sunday evening I ran the coax into the shack and confirmed that the antenna worked on all bands. It did, with the caveats about tuning mentioned earlier. I then proceeded to put it on the air and make some CW DX contacts.

I hope it is an omen that my very first QSO was with JT1AA/5 on 17 meters for my QRP DXCC country #202. He was weak here and I expect my 10 watt signal was much weaker there. He needed some time to copy my call correctly. I then moved to other bands, working DX on 20, 30 and 40 meters without any real difficulty. I didn't work anything on 15 but I did hear a weak A35 working a stateside pile-up.

With that out of the way I sat down with the EZNEC model of the 40 meters element and had a closer look. Notice that the radiation is mainly horizontal broadside and vertical off the sides. This is expected from an inverted vee.

The pattern is not much worse than a vertically-polarized delta loop with regard to omnidirectionality. Its elevation pattern is, not surprisingly, worse. However the difference is no worse than -2 db at 10° elevation. The good news is that the gain is higher where short-path communication should peak.

It will be interesting to do a side-by-side comparison to a vertically-polarized 40 meters antenna this fall. I may put up the delta loop again if the interactions with the tower and yagi are managable.

The only other band I will discuss for now is 15 meters. That's a special case since its length is 3λ/2. The other bands are mostly uninteresting.

Not unexpectedly the pattern on 15 is a mess. A long antenna like this has multiple lobes and nulls. You can see the complexity in the 3D plot. If you look at the azimuth pattern at 20° elevation (where maximum gain is located) and tilt your head left it looks a bit like a gingerbread man. While this is not what a reasonable person would call omnidirectional it is close enough to serve that purpose for my stated objective of directional diversity.


The elevation pattern shows more gain at high angles than I'd like since it's wasted. So, not great but it'll do for now.

Since the antenna is tied on the south end to the new tower the 40 meters element crosses the upper guy wire. The distance between them is less than 2 meters. That 26' (7.9 m) guy section is resonant on 17 meters but not at all on 40. Since they cross at nearly a right angle interaction ought to be small. Initial operation of the antenna on 40 tells me that there is no significant impact. Interaction modelling could prove useful in this instance.

That and interactions with tower-mounted antennas is a task yet to be done. That and more is planned for my interaction model. It should be interesting.

Addendum (August 20): The antenna has now been tuned. Fan dipoles, even in cage form, are strange beasts when it comes to element interactions and tuning.

First I trimmed one side of the 40 meters element by 1% (20 cm), adjusted the ropes and raised it back in the air to see what would change. Resonance moved from 6.9 MHz to 7.05, MHz, which is a 2% change. This is close enough to what I wanted that I decided to leave it be. None of the other bands were affected by this trimming. Okay, 15 meters did change since it utilizes the 3rd harmonic of this element. Resonance there increased to 21.25 MHz, and the SWR is a little above 2 at the bottom of the band.

Next I trimmed the 20 and 17 meters elements (30 meters was already perfectly tuned) from the same side. This is the side where I had added lengths of wire while tuning the original version of the antenna last year. I cut 12 cm off the 20 meters half-element and 10 cm off the 17 meters half-element. This is less than I calculated to raise resonance from 13.9 to 14.1 MHz and from 17.9 to 18.1 MHz. I was being cautious. Again I adjusted the ropes and raised the antenna for testing.

The 20 meters resonant frequency moved up to 14.075 MHz and on 17 meters to 18.15 MHz. Both bands showed a higher Q (smaller SWR bandwidth) than an individual dipole for either band would have. Also interesting is that the SWR at resonance on 17 meters improved from 1.9 to 1.5. While the SWR at 14.3 MHz and 18.068 MHz is higher than I'd ideally like the antenna works well and the rig doesn't complain.

At this point I decided to step away from further tinkering. The SWR curves are good but not perfect on a few bands. In consideration of the extensive interactions among elements I suspect that further tuning could prove frustrating.

Managing Interactions: Creating an Interaction Model

Modelling interactions among antennas and metal supports is not easy. It is so 'not easy' that it is rarely done by amateur radio operators. Instead most hams proceed by rumour, lore and what they read in articles and antenna books. Nowadays we can add in the advice offered via the internet. Some is true, some is false, and much of it goes unverified for years, even in respected and widely-distributed books.

That is what I used to do for many years. Basically I did what seemed best, getting advice from other hams who really had no better understanding of the subject or I would just shrug and hope for the best. Since any antenna will work, even reasonably well under the burden of serious interactions, there is some truth to a certain silly saying: what you don't know can't hurt you. Although antennas in this situation can be successful they can be made to perform better if attention is paid to interactions.

Modern NEC-based modelling software is a tremendous help. However it remains a challenge to build a model with all those antennas, masts and tower and get useful results out of it. It's an individual choice. Since I am writing these words you can guess which way I lean. Even so I am not a glutton for punishment; I want to get the best results for the least amount of work. That is, I want a model that is simple enough to retain the key elements needed to identify and mitigate unwanted interactions but without worrying about high precision. I focus more on precision for models of individual antennas.

My immediate motivation is the tower I just put up. In addition to a tri-band yagi I intend to put up wire antennas for at least 40 and 80 meters. There will also be antennas nearby on the smaller tower plus mast bracketed to the house. For the present I will ignore these latter items so that I can start with a basic interaction model. Other antennas will be added over time. I will start with the larger tower and the antennas it will support.

If all goes well I will have received the tri-band yagi to be mounted atop the tower in the coming days. It is not a TH3 but is close enough that I used it in my model. To be more precise, I am using the model I developed during my series of articles on high-band yagis. At right is an EZNEC view of this antenna on the tower, along with a prototype loaded half-sloper for 80 meters. This is the first iteration of my comprehensive model.

Since it is difficult to see in the plot I will point out a few noteworthy items in the model. I will elaborate on these further in this article.

• The yagi has a boom, but the original tri-bander model does not. I omitted this earlier since while the boom does alter antenna tuning by a small amount it is not pertinent to measuring performance in isolation. In a comprehensive model with diverse interactions that is is no longer sufficient.
• The rotator and mast bearing are not modelled. It is assumed that there is sufficient conductivity between the mast and the tower to exclude those items. There is some risk here since the rotating surfaces are coated with grease and might not reliably yield metal-to-metal contact. Some hams attach a flexible strap between the mast and tower to guarantee continuity for antennas that include the tower, mast and yagi(s).
• The tower and mast for the yagi are modelled as simple wires. This is not accurate but with some care can be made to be sufficient.
• Guy wires are not modelled. There are two guy stations and a total of 6 guys. They were designed and built to be largely non-resonant for all antennas supported by the tower. Each guy has 3 sections, separated by insulators: first section is very short (~45 cm), to isolate the tower; second section is 8 meters long (26'), which is near resonant on 17 meters but no other band; the final section is of whatever length is needed to reach either an anchor or a terminal insulator.
• Cables are not modelled. These will be added later. However I will only add cables in select instances and then only until reaching a common-mode choke, which is assumed to act as an infinite impedance.
• Every antenna has a source, only one of which will be powered on each run of the model. The other sources represent unused transmission lines that terminate in a short or a finite-impedance transceiver (receiver or transmitter) in the shack. But those will in most instances be excluded from the model.
• The tower is not connected to the ground. NEC2 does not support wire connections to the ground or wires in the ground. This does not permit modelling of directly-connected ground rods. It is true that at present in its unadorned state the tower is not grounded, but this will change.

Now let's look at a few subjects in more detail.

Yagi model

I have several key objectives in modelling the yagi in the interactions model:

• Permit a reasonably-accurate top hat model for low-band antennas that include the tower and all its attachments.
• Retain its performance metrics of gain and directivity (F/B, F/S). I am less interested in retaining accurate SWR calculation since this is less pertinent to interactions; accurate SWR is already designed into the standalone yagi model.
• Accurately model coupling induced by other antennas.
• The design should allow the yagi to be rotated to test interactions for when wire antenna elements are parallel or orthogonal to the yagi elements. Since this is a software model we can instead choose to rotate the other antennas if that's easier. Modelling software typically allows selective rotation, but it is up to us to set up the model to make it convenient.

The yagi boom is modelled by 2 wires of the requisite diameter. These attach to the wire representing the mast. If the mast continues higher than the yagi the mast must be made with 2 or more wires. The parasitic element centre sections are split into 2 wires each so that they can be attached to the ends of the boom (you can only connect wires at their ends with NEC).

The driven element does not touch the boom. This represents any yagi with a dipole feed, such as Moseley, Hy-Gain and others. I had to "bend" them so that they pass each other without touching. A short centre wire connects the dipole halves. This is where the source and beta match are attached. The other end of the beta match is attached to the boom as on a typical Hy-Gain yagi.

This last connection is preferred to modelling a virtual short-circuited transmission line since the connection ensures that the driven element is included in the "top hat". It is assumed that a common-mode choke or current balun is at the yagi feed point. (Please note that the Hy-Gain BN-86 is not suitable for this application since it is not a current balun.)

One issue is the attachment of the short-circuited end of the beta match to the boom. Doing so introduces a modest mismatch, raising the SWR to 2 or even 3. You can see that there is excess capacitive reactance in the chart above. However there is a negligible impact on the yagi's performance metrics of gain and pattern. This is acceptable in an interaction model. Just be sure to do it differently in the yagi model itself, or find a way to mimic the impedance behaviour in the model. Although the latter may be possible I am not motivated to spend time on it since it is tangential to my objectives.

Tower model

You can forget about modelling a lattice tower with any hope of accuracy. Even if you took the long, painful step of creating wire for each cross-brace and leg sections between braces of a continuous-taper tower it would still not work. That degree of accuracy in such a complex arrangement cannot be expected using NEC2 or even NEC4. I briefly touched on this subject in earlier articles on 40 meters loop antennas.

A better approach is to model a single wire with a diameter equal to the average width of one face of the (triangular) tower, and as much as (or even more than) 10% shorter than the tower, as measured by W8WWV. Since you can't really shorten the tower at the top you should do it at the bottom by lifting the wire end off the ground. This of course does not permit accurate representation of radials or ground connection for lightning protection.

I suggest ignoring the lightning protection ground protection (impossible in any case with NEC2) as the lesser evil. W8JI provides some graphic examples of how to provide lightning protection for ground-isolated towers if that is of interest to you. Radials can still be connected to the tower bottom when it is above ground level in the model. With EZNEC at least it is easy to extend the tower downward while also keeping radial connections intact for test cases where the radial interactions are more important than accurately modelling tower resonance.

It is not practical to give better guidance since towers have diverse designs, behave differently on different bands and have various cables running down their length (inside and outside the tower), and are often buried where they exit the tower. There may be better ways to attack this problem.

Wire composition

In a comprehensive model of this type there are conductors mode from different metals. These typically include steel (tower, mast and guy wires), aluminum (yagis) and copper (wire antennas). EZNEC, like similar modelling software, often only allows one in a model. We need to choose which one. To do that there are a couple of considerations:

• Loss: Steel is far lossier than copper, with aluminum intermediate. However zinc-coated steel (galvanized towers and cables) is almost as good a conductor as aluminum at RF. It is also true that in most installations that the steel and aluminum conductors are of large diameter, and therefore better RF conductors.
• Resonance: As we saw with the tower model it is possible for large conductors and tapered elements to have different effective (RF) lengths from their physical lengths. We usually don't need to worry about this since these conductors are not parts of antennas. In an interaction model these differences can impact results.

For my model I am ignoring the loss implications since these do not directly impact interactions. The resonance and coupling factors are more important than individual antenna performance. This is unlike modelling of antenna performance where the wire composition can be significant. I specify either aluminum or copper in these complex interaction models. It is also acceptable to specify perfect (no loss) wires.

Segmentation

EZNEC tells you, sometimes quite forcefully, when you push the NEC2 engine beyond its capabilities. These warnings tell you when your model is liable to produce unreliable results. Pay attention to these warnings.

In the interaction model you can quite easily elicit warnings because of the wide range of test frequencies, perhaps as low as 1.8 MHz and as high as VHF. Here are the warnings I commonly run across in the interaction model:

• Segments are too small (or large) for the test frequency. For example, the segment length that is optimum for 20, 15 and 10 meters on the yagi is often too small for 80 meters. The reverse can occur in the opposite case.
• The tower can violate the maximum diameter-to-length ratio for a wire, especially if you break the tower into multiple wires to simulate the taper of a self-supporting tower. As indicated earlier it is better to model the tower as one wire. Tapering wires (in towers or yagi elements) is in any case a problem with NEC2 so you gain nothing with a manual taper.
• Wires with short segments that attach to the tower could have one or two segments entirely inside the tower diameter. This is common with wires that form an acute angle with the tower. You can increase segment length (decrease number of segments) of the offending wire or add a short horizontal wire between the tower and the offending wire.
• Wires that meet at acute angles or are near parallel should have equal segment length. This is especially vital if the wires are part of the same antenna. If one of the wires is only interacting with the other one -- not part of the same antenna -- it may be safe to proceed even if the segment lengths are unequal. Test with different segment length ratios to be sure. But if you can you should make them equal.

The first point requires some further consideration in the model since there is typically no way to avoid segment length violation at all test frequencies without compromising the model integrity of one or more antennas. I suggest paying attention to where the violation occurs and only taking action if there is a high probability of a problem.

For example, with a test frequency of 3.5 MHz on a half-sloper the segments of the yagi elements in my model were too short. After running the test anyway I found that the current on the yagi elements was small. This tells me that any error resulting from the yagi segmentation will also be small. So I ignored those warnings.

Coaxial cables

All transmission lines to my planned antennas are coax. The only time I use open wire lines is to build wire antenna arrays, none of which are in the current plan. The interaction model therefore only needs to model the transmission lines for common mode (direct conduction or coupling) and termination.

For interaction purposes I model coax as wires between the antenna feed point and the first common-mode choke. I am assuming that the chokes are perfect (infinite impedance components), whether coax coils, baluns or ferrite beads/toroids. Even with this restriction there can be a problem for coax running down the tower since if there is substantial current on the tower (e.g. half sloper antenna) it will couple to the coax runs. Also, whether buried or run between tower and shack in the air there can be coupling to horizontal antennas. If that becomes a problem it is perhaps best to add another common-mode choke so that there are no resonant sections of coax (outer conductor) that can couple to nearby antennas. Yes, it's messy but that is the actual situation whether we like it or not.

As already noted parallel conductors must be modelled with equal-length segments. For the purpose of modelling interactions this is typically more important than optimizing the segment length for any particular frequency.

If the coax is not choked at the feed point there should be a directly-connected wire to the feed point representing the outer conductor of the coax. If there is a choke at the feed line the coax can be modelled as a wire with no connection at the feed point. If a choke is placed where the coax leaves the tower (towards the shack) in most cases the coax run from feed point to the coax does not need to be in the model.

With the coax so close to the tower there are modelling issues with NEC2. Expect inaccuracies.

Use your judgment whether the section from the choke towards the shack is close enough to any horizontally-polarized wire antennas that it can have induced current on it. Don't worry about yagis on top of the tower since they will have little radiation in the direction of that section of coax.

Antennas that are not fed in each test (all but the one with an active source) will conduct any induced current towards the shack via their connected transmission lines. The impedance presented will depend on the length of coax and whether the shack end is shorted, open or connected to a rig's receiver or transmitter. My choice is to avoid this complex issue by not modelling the transmission line other than as (above) for common-mode currents. My reason is that if there is significant and unwanted current of that antenna the problem is the induced current and not its precise behaviour due to the transmission line and shack termination. I instead try to reduce or eliminate that current by moving or otherwise modifying one or more antennas.

Concluding notes

Even with its many flaws a comprehensive NEC2 interaction model of a complete antenna installation can prove very useful. It is certainly easier than building antennas and live testing their interactions in various configurations. Without extensive testing it is often impossible to tell if a problem is in an antenna's design or its interactions with other objects. It is therefore beneficial to test interactions before investing time and money, and to select antennas and their placement to optimum effect at the outset.

Be prepared for investing a bit of time and effort to make the model, and then don't be surprised that running the model (SWR or far-field patterns) is slow, even on a fast computer. It's a good thing I have EZNEC+ since with 2 or 3 wire antennas added to the model I have more than the 500 segments permitted by the basic version of EZNEC.

I have used the model I developed to begin testing a variety of low-band antennas. It has been very interesting and enlightening. This is something I'll return to in some future articles so that you can see how these modelling investigations are guiding my thinking about low-band antennas that I'll soon be building and erecting for the fall operating season. I started with an 80 meters half-sloper but have since gone on to inverted vees and loops.

Here are a few concluding notes on where I find that the interaction model is most useful:

• Wire antennas that utilize the tower and and yagis as active elements.
• Directional antennas, including yagis, in the vicinity of wire antennas below or beside them.
• Interaction of cables and transmission lines with adjacent wire antennas.

Site-B Antenna Mast 3.0

After the mast supporting my multi-band inverted vee collapsed in early winter I put up a newer, stronger mast that replaced the fibreglass parts with an 18 gauge steel fence rail. I called this antenna mast version 2.0. Both sat on a 19' long length of Schedule 40 steel pipe bracketed to the house. On my site plan this location was designated 'B'. Both the mast and the pipe were taken down in early June in preparation for a complete rebuild of my antennas and supports.

The Golden Nugget tower that had been at Site C (supporting a multi-band dipole and delta loop) has since taken the place of the steel pipe. That is the bottom support for what I am calling antenna mast version 3.0. It is stronger and simpler that its predecessors. And this time it's all steel construction.

At right is the newly completed antenna mast, complete with steel back stay and the pulley to pull up the antenna. The steel stay (or guy) and pulley are the sole survivors from the version 2.0 mast.

The new mast consists of two 10' (3 meters) sections of steel pipe. The bottom pipe is the 18 gauge mast that comes with the tower. It is swaged at one end to allow stacking.

Rather than purchase an overpriced mast from the tower manufacturer I found a suitable alternative at my local big box home improvement store. This is an EMT 1.25" (nominal) galvanized steel conduit. Its outer diameter is 1.51" and the steel gauge according to the EMT industry specification (and my measurement) is 16. It weighs 10 lb (1 lb per foot) and fits perfectly onto the swaged end of the lower pipe.

I drilled a hole just below the top for a bolt from which to hang the pulley and secure the steel cable. With all attachments in place it was quite easy to stand at the top of the tower in a stiff breeze and plant the pipe onto the lower mast.

The top of the completed mast is about 14.3 meters above grade: the tower is 8.8 meters high and the mast (minus overlaps) is 5.5 meters long. Wind load on the mast is 2.4 ft², or 22.5 kg (50 lb) at 135 kph (85 mph). If the load were close to the top of the tower this would be within the bracketed tower's capacity, but not when extended 18' above it. Hence the need for guying. The back stay is one guy, with the other two to be provided by the inverted vee legs.

The horizontal bar is the same rigid plastic pipe that was the back stay supporting the mast of the 40 meters delta loop. Its purpose is to allow the stay to be anchored to the tower rather than on the ground. As in the discussion of vector forces acting on a gin pole it is best that the pipe be midway between top and bottom cable anchors. Otherwise the wind would bend and possibly break the plastic pipe. The wind load carried by the steel stay is designed to result in a net horizontal load from the tower to the house bracket.

My plan for the next week is to modify the old multi-band inverted vee (30, 20, 17, 15 and 10) and mount it on the mast. I'll have more to say later about those changes and my motivation for making them. I am also making repairs to my old Ham-M/II rotator. It's amazing how things can deteriorate by simply being stored in the basement for 22 years.

For the present I am, again, QRT since I had to remove the dipole from the small tower to build the new mast.

Nested Delta Loops for 30 and 40 Meters

It's a curiosity to me that one of the most popular articles I've written is on nested delta loops (vertically polarized) for 20, 15 and 10 meters. I say this for a couple of reasons: they're fed with 2 feed lines, not 1; then in a later article I demonstrated by model and experiment that this antenna is a poor choice for DX. Even at modest heights a horizontal antenna does better for DX than one that is vertically polarized on 20 meters and above.

Below 20 meters is where vertically-polarized antennas start to shine since they almost always outperform a horizontal antenna at modest heights. Certainly my 40 meters delta loop has done well for me most recently, netting me well over 100 countries on that band in 9 months. But that antenna came down when I took my station apart in preparation for putting up my new tower.

With the tower up and a brief delay in purchasing and installing a tri-band yagi my thoughts returned to the low bands. I do plan a 40 meters dipole, mainly for working VE/W in contests, but I still need a DX antenna. Since I also enjoy working 30 meters this became an opportunity to reconsider nested delta loops, although this time with a single feed line.

Before going further I will point out that I did come up with a suitable design quite quickly, yet I still had to reject the antenna. You'll see why I came to this conclusion towards the end of this article.

The concept is simple enough: model the full-wave (1λ) delta loops for each band, nest them and then come up with a scheme whereby one coax feed line can be used. There are several challenges in getting this antenna to work:

• The λ/4 coax transformers to match the high feed point impedance of the loops is frequency dependent. That is, the transformers for the bands are of different lengths yet there is only one feed line. However I do know that the transformer length isn't critical, with non-exact lengths adding reactance that can usually be compensated by adjusting the antenna length.
• The antenna will interact by being fed from a common source and by mutual coupling. Ideally we want each antenna to exhibit a high SWR (severe mismatch) on the non-resonant band so that the source mostly terminates on the desired antenna element. Mutual coupling mostly is of the non-resonant variety, but this can still have a significant impact on tuning and even far-field pattern.

Rather than struggle to solve these problems analytically I went directly to EZNEC and see what would happen. For the λ/4 transformer I cut it for the 30 meters band and terminated it to the 30 meters loop. I then ran a short section of transmission line from there to the 40 meters loop. Both loops are fed λ/4 down from the apex, which is the best approach to get an omnidirectional vertically-polarized pattern.

To my surprise the model was not too far off the mark. I was able to optimize the antenna by simple trial-and-error changes to loop diameters and transformer lengths and impedance. I'll cut to the chase and give you the EZNEC data for wires and transmission lines. The antenna is modelled over medium, real ground, and the wires are insulated 12 AWG. The 50 Ω source (coax from the shack) is connected to virtual wire "V1".

The loop lengths are slightly off their expected values due to the peculiar transmission line lengths. The loop for 30 and 40 meters are 31.3 and 42.1 meters in diameter, respectively. Although I kept the transmission line impedances to standard values it may be that not many hams have RG-62 or similar 90 Ω coax in their junk boxes. You could instead make one of the required short length from a pair of parallel (rigid) wires.

It was not possible to get ideal SWR for both bands using only interconnecting coaxial cable. If the 40 meters SWR is not to your preference you will have to devise a more elaborate matching system. My intent was to keep it simple, and with SWR below 2:1 that works well for my purposes. On 30 meters (not shown) the SWR is 1.5 across the band.

I used EZNEC to plot the antenna current to see just how active the non-resonant element would be. The adjacent plot is for 40 meters. It is similar but opposite for 30 meters. The current on the non-resonant element is low enough to have only a modest effect on the far-field patterns.

The elevation plots show that the antennas perform as desired. They are vertical radiators with good low-angle radiation (DX) and rejection of high-angles (short path). The azimuth plots (not shown) are similarly ideal with a front-to-side ratios of -3 db, the same as for a single band delta loop fed in the described manner. Ground loss for both bands is also as expected for these heights, in the range of -5.5 to -5.8 db.

On 40 meters the gain at 10° elevation is -1.24 dbi, which is a fraction of a decibel worse than a single band delta loop with a 14 meters apex. The additional loss of -0.2 to -0.3 db is negligible. Nothing has been given up by the addition of 30 meters to the basic 40 meters delta loop.

I made the antenna apex only about 14 meters since my intention was to have this antenna hang off my new tower with the maximum separation between it and the tri-band yagi at 15 meters height. Even at the small separation of 1 meter the bottom of the antenna would not clear the head of a very tall person. That isn't ideal but nothing more can be done without distorting the shape of the elements, which would impact both pattern and match.

The delta loops could be replaced with a different loop shape, such as the higher-gain narrow diamond loop, but not without increasing interaction with the yagi. In my preliminary modeling the sharp downward angle of the delta loop wires results in minimal coupling with a tri-band yagi only a short distance away. However this is not conclusive and would require further modelling and testing.

All for nought...

Now we move on the crushingly bad news: the antenna cannot work as intended in my station. When I reprise the model with the tower present the patterns and match are totally disrupted. It is really awful. The tower strongly couples to the antenna on both 30 and 40 meters.

Accounting for the coupling can, in principle, be accomplished but not without a lot of modelling and adjustments once the antenna is in the air. Modelling by itself is insufficient since there is great sensitivity to the specifics of the tower dimensions, grounding, and so forth. Removing resonance with tower traps is certainly possible, but complicated by the need to trap two bands rather than one.

For those of you who can mount this antenna on a non-resonant structure, such as hanging from a tall tree, this antenna may be worth consideration. I don't have this option so I will likely put up a single-band delta loop for 40 meters and adjust it to fit the environment I must deal with. It will still couple to the tower but this is a tractable problem when restricted to one band. I've done it before.

Tower Up, Lessons Learned

My objective was to get the DMX-52 tower up by the end of July and I just managed to accomplish that feat. It is standing, guyed and ready to be adorned with antennas. The adjacent picture shows the tower near completion. In it you can see the as-yet untensioned upper guys and the temporary rope guys to steady the tower as it was topped off.

This is of course nothing remarkable. I expect that most of you reading this have a tower already (as I have had in the past), so it's nothing of surpassing interest. What makes this particular project interesting are the following:

• All steps of tower erection were done by myself, with no helpers.
• There is no concrete involved, neither for the base nor for the guys.
• Specialized tools were improvised, mostly with parts I had on hand.

The experiment was interesting and educational despite the fact that as an experienced (amateur) tower worker there were no surprises and no big doubts to overcome. It was just that when I was younger it was so easy to solicit helpers since we were all energetic and eager to help each other out with building our stations. After being QRT for 20 years I found that many hams have resorted to hiring professional help due their advancing age and the general aging of the entire ham population.

To conclude this project I will list some of the lessons learned since they may be of use to others out there.

Gin pole performance

My homemade gin pole performed quite well at its assigned task, provided I paid close attention to its limitations. As should perhaps be expected of its construction from readily-available parts (such as those found in my garage) and put together with fasteners (no welding) it had some shortcomings.

• The brackets that grab the X-braces were a bit too long on the upper side. I intended those to grab the inside of the adjacent tower leg for redundancy and, possibly, added strength. In practice it never quite touched the tower leg. Instead it complicated the attachment and removal of the gin pole due to that extra length getting caught in the X-braces.
• Although the angle aluminum attached to the top of the steel pipe was plenty strong the clamps that hold it to the pipe were a problem. First, since the back of the angle aluminum isn't round the muffler clamp could rotate a bit under load. You may be able to see the abrasion marks the saddles made as they shifted from side to side. This is a concerning even though the coupling didn't slip. More aggravating was the projecting U-bolt that eagerly hooked on to tower sections as they slid past. Next time I'll purchase a pipe of the requisite length.

I don't know if this gin pole will get used again, but I will keep it intact for as long as I don't need the parts for another project. It'll have lots of company hanging on the garage wall until it gets called back into use or disassembled.

Through a thicket of trees

Since I have no concrete base to make this a free-standing tower the tower has to be guyed. Although this adds complexity it is cheaper, easier to erase when/if I sell the property, and increases the load capacity of this light-duty tower.

Two of my guy anchors are mature trees that just happen to be perfectly positioned on my property (the third anchor is the house frame). A consequence of this opportunistic guying is that the guy wires have to pass through the tree. That makes for an interesting challenge.

The adjacent picture was taken from beside the southwest tree anchor. In it (if you look carefully) you can see the two steel guys with the still-attached temporary rope guy. The bottom guy wire is ⅛" aircraft cable and the upper guy wire is 3/16" aircraft cable.

The lower wire is unimpeded by the tree branches. This is not the case with the upper guy. I faced a choice between cutting off all the branches in the line-of-site to the tower guy station or route the guy wire through the tangle of branches. While it may be difficult to pick out in the picture the rope guy deflects around a branch. That will not do for the permanent guy.

The way I went about it (after failing at the "brute force" technique) was to tape the guy to a heavy socket wrench and drop it through a 20' long aluminum yagi boom. You can see it propped up by the other tree in the first picture of this article. This is the same boom that formerly supported my multi-band inverted vee. I then threaded that combination through the branches until the end of the boom was in the target "window" among the branches. I then pulled the boom back, exposing the guy wire, and proceeded to attach it to the anchor. After pulling it taut I removed one small branch that strayed too close to the guy.

The other tree, a spruce, was not so difficult but it did require some acrobatics to slip the guy over a large branch that could not be readily manipulated from the ground with that 20' boom.

As I mentioned, the upper guy wires are 3/16" cable. I chose it even though ⅛" cable is sufficiently strong (2,000 lb breaking strength) so as to minimize the risk of wear on the guy by the branches under the influence of wind and ice.

Upper guy station

As earlier discussed the DMX guy stations attach at the joins between sections. It is also possible in most cases to place the guy station at the top of the top section by drilling out the rivets holding the top plate (for the mast bearing) and using those bolt holes. That does not work for the DMX-1T top section since neither stock guy station fits that position.

I had originally planned to place the GS123 guy station at the join between the top two sections, which is 8' below the top. With mast installed this would work out to 11' below the main antenna load. Although this is the simplest approach I was not comfortable with that large distance between guy station and antenna load for a light-duty top section.

The manufacturer suggests setting the GS123 adjustment bolts to the smallest size that will fit over the top of the DMX-1T, finding the position where it naturally comes to rest on the tapered section and mounting it there by drilling ⅜" bolt holes in the tower legs.

At first I laughed at that idea. As I said earlier, it is wise to reject a used tower that has been modified. Drilling large holes in tower legs can qualify. However, in this case these holes are no different than the holes that are used to bolt together tower sections. The critical need is that the hole be filled with a properly torqued bolt to compensate for the weakness created by the hole. Left open a hole this size will weaken the tower.

I eventually decided this really was the best thing to do. But it's trickier than it sounds.

Getting the position right is the first problem. Even a slight deviation from the precise position where the GS123 fits snugly will stress the tower section when the bolts are torqued. A bit too high and the legs will be pulled outward; too low and the legs will be squeezed together. I spent some time experimenting with the parts and came up with what worked out to be a near ideal position: bolt hole centres 47.5" from the bottom of the legs.

Now the top guy station is 4' below the top plate, 2' below the rotator plate and 7' below where the antenna will be mounted. The height of the mast is the highest I can go (15 meters above grade) and stay exempt under city and federal regulations.

All dressed up and no place to go

The tower is up but I still do not have a suitable tri-band yagi. Since doing an analysis of the various possibilities earlier this summer I have not come across a reasonably nearby used yagi that suits my needs for performance and wind load.

I have a mast bearing and I have a rotator so those can be mounted on the tower. I will also further adjust the guy tension to find the best balance between rigidity and tree movement. Then it's back to antenna shopping.

Notes on Gin Pole Mechanics

If you've ever erected a tower you are likely familiar with gin poles. The alternative, raising tower sections is to do this by brute force. I've done this many times, but only with a reliable partner. This is not recommended practice even if it is common among amateurs. Other alternatives are typically expensive and inconvenient: cranes and helicopters (sky hooks are yet to be invented).

Rather than borrow or manufacture a tower-specific gin pole to raise my DMX tower I chose to fabricate a simple gin pole that, while a bit unconventional, is up to the task. My reasons were to avoid the need for a welder (and the associated expense of parts and labour) and simply for the joy of it. Now that I have one it is available for other projects.

DMX gin pole recommended
by the manufacturer

Before we look at the gin pole itself it would be helpful to understand what we are asking it to accomplish. It is vitally important that it is up to the demanding task of safely and effectively lifting heavy and bulky tower sections. Failure can result in serious injury or death, or at least damage to property.

Forces on the gin pole

If the tower section weighs 25 kg what is the force on the gin pole when it is lifted off the ground? The correct answer is not 25 kg.

When the tower section is in the air and not moving, and both sides of the rope are vertical, there is an equal downward force on the side of the rope being pulled. This doubles the force on the gin pole to 50 kg.

An additional downward force is required to move the section upward. The greater the force the faster the section rises. This force must also be supported by the gin pole. Another consideration is acceleration: the force needed to change the upward velocity of the tower section: F = ma, according to Newton.

If you pull on the rope hard enough (large accelerating force) you can lift yourself off the ground. Less amusing is that the acceleration can break even a strong gin pole. Never jerk hard on the rope; use gradual, fluid movements when lifting tower sections.

Vectors, not scalars

As a physicist would tell you: force is a vector, not a scalar. What this means is that calculation of the net force requires taking into account the directions of the contributing forces. You can't simply add together the magnitudes of X and Y. When you have that section hanging in the air the scalar net force is 25 + 25 = 50 kg, but that is only a valid sum if both forces are in the same direction.

Since you cannot easily stand directly underneath the gin pole when pulling on the rope the net force will be in a direction other than straight down. This not only stresses the gin pole it requires more scalar force (how hard you pull on the rope).

When you pull on the rope the gin pole will bend towards you. This creates bending stress along the length of the gin pole and lateral stress on the bottom attachments between the tower and gin pole. Since the tower section is hanging vertically the additional force causing the stress must be coming from you pulling on the rope. That is, in addition to the force that is equal to the weight of the tower section you must pull harder (additional force) so that the force causing the aforementioned stress, when added to the weight of the tower section, equals the pulling force. Not only are you working harder that extra effort is creating a serious safety risk.

If at any point on the gin pole the stress exceeds its yield strength it will fold or split. This is something we do not want to occur.

Let's assume that you are standing where the angle between the pulley and the rope in your hand is 30°. Just as in the guy station/wire calculation there is a lateral force component additional to the 25 kg vertical (downward) force. The scalar value is tan 30° x 25 = 14.4 kg. The tension of the rope in your hand is therefore SQRT(25² + 14.4²) = 28.9 kg. When you pull on the rope to raise the section (as discussed above) this value is even higher, as is the lateral force.

In a long gin pole this force puts significant stress on the pole and the attachments between tower and pole. Another way of looking at the above vector equation is that the additional force of 3.9 kg (28.9 - 25) multiplies 3.7x to a lateral force at the top of the gin pole of 14.4 kg. That is, if you stand a little way from the gin pole and you pull hard you can easy gain enough mechanical leverage to destroy the gin pole, and all the mayhem that entails.

Another area of concern is where the bottom of the gin pole attaches to the tower (typically at two points). The top attachment acts as a fulcrum for the lateral force of 14.4 kg at the pulley. If the two attachment points are on the tower are 1 meter apart and the gin pole is 4 meters long the lateral force on the bottom attachment point is 4 x 14.4 = 58kg. This is in addition to the vertical force of 25 + 25 + lifting force + acceleration force.

Bottom line: stand as close to the tower as possible when manually lifting tower sections with a gin pole, and wear a hard hat. Gin poles can and do break. Don't turn yourself or your friends into statistics.

Material

The major components to be selected in any gin pole include:

• Pole: More than anything this determines the strength and weight of the gin pole. A steel cylinder (tube or pipe) provides the optimum balance between strength and weight. Wall thickness and heat treatment, if any, determines yield strength. The longer the pole the greater the stress on it, so the stronger it must be. It must be long enough to securely attach to the tower and project high enough above it to, at a minimum, place the pulley above the lifted section's centre of gravity.
• Pulley: The pulley should be labelled with its working load rating. A pulley without a rating should be avoided. For light duty tower work the rating should be a minimum of 100 kg but even so avoid a pulley that is rating less than 150 to 200 kg. I recommend avoiding one with a centre axle that is secured to the housing in rivet style. The axle should have wide flanges and/or clips outside the housing body that cannot easy break or wear through. Get a decent match with the rope diameter you'll be using.
• Tower attachment brackets: As with the other components the brackets must support all the loads on the gin pole. Since the brackets are at stress points the pole can break at the brackets if they are improperly joined, allowing the pole to fracture or bend. The brackets must also ensure that the gin pole cannot slip off the tower structural members on which it rests, that the tower at the attachment can deal with the additional load, and do not require more than two hands (!) to position and secure the gin pole.
• Rope: Match the rope diameter and the pulley, but ensure that the rope's working load strength is at least twice the forces involved (4x the weight of the heaviest tower section). If the rope is too small for the pulley there is a chance that the rope can get trapped between the wheel and housing. You don't ever want that to happen. I can assure you from experience it is difficult and dangerous to free a trapped rope from a pulley that is holding a tower section in mid-air and is out of hand's reach above you.

In summary, the objectives in selection of gin pole components include:

• Light enough to be safe and easy enough to be maneouvered  by one person on the tower, with optional assistance by ground crew. I generally do this sort of tower work with at least one other person, but I decided to design the equipment and process in this instance to permit me to raise the tower on my own as a challenge to myself.
• Withstand all calculated forces with a large safety margin.
• Easy to construct, or at least fall within a comfortable budget.

With all the forgoing in mind I fabricated a gin pole for the DMX tower. It is pictured at right.

• I used material on hand to keep the cost low since I do not know if it'll get used again, other than to eventually take this tower down. The 7.5' 18 gauge fence rail used to be the replacement bottom section of my version 2.0 antenna mast for the inverted vee. A 4' length of heavy gauge angle aluminum is attached at the top with muffler clamps. The pulley is fastened at the top, on the side facing the tower. Total length is 10.5' (the photograph perspective altered the apparent length ratio between the pole and angle stock).
• A close-up of the pulley fastened to the angle stock is shown above. It is rated at 200 kg (440 lb) and was bought at Canadian Tire (of all places) for $7. It comfortably accommodates ⅜" and ½" rope without binding. The pulley mounting has just enough free pivoting to respond to the load. It should turn so that the rope enters and exits in the plane of the wheel as the load shifts. Its motion does not allow the rope to be trapped between housing and pole or to spin the pulley and twist together the two sides of the rope.
• 5/16" Grade 5 hardware is used to fasten the pulley and the two tower brackets. I used ⅜" nuts as spacers for the pulley mount, and allow room for the brackets to comfortably grab and rest on the tower's X-braces. The rectangular pieces are galvanized steel, thick gauge construction brackets that hold the X-braces between the pole and brackets.
• I did not weigh the gin pole but it is very light. I can manipulate it on the tower with just one arm.
• There is an important difference between this gin pole and the design Wade (Delhi) recommends (see earlier picture). The "official" gin pole uses two vertical rods at each bracket which surround the tower leg and rest on the top of an X-brace on both sides of the leg. Mine uses one vertical rod at each bracket and these fit the bottom of X-braces on the left side of a tower face. This is important since the distance between X-braces on a DMX tower is not constant, varying between 36.5" and 37.75" on those I measured. As a result typically only the top bracket on both gin pole designs rests on an X-brace (the brackets are 36.5" inches apart), and therefore the bottom brackets can move sideways. On the official design the tower leg prevents this motion. On mine the free bracket must be tied to the tower so that it doesn't pivot. I use a $2 heavy-duty rubber tie-down which is easy to attach and remove with one hand.
• Another difference is the total height of the gin pole. The official design is 15' tall, for the purpose of picking up a tower section from the top. This is not strictly necessary since it is only mandatory to pick up a tower section from above its centre of gravity. This allows for a shorter and lighter device, and one that is stronger for the same steel gauge. That is why I can get away with using 18 gauge steel rather than the recommended 16 gauge. However it causes the section to stray further from the vertical, requiring the section to be manually rotated when dropped into the lower section.

Safety

Tower erection is dangerous. You can be killed or seriously injured by nothing more than a moment's inattention. Erecting a tower on your own without requisite experience is foolish. Don't do it. Get experience working with others on tower projects before going solo.

That said, here are a few points on safety relevant to this project.

• Have a plan for emergencies. If something breaks and stuff falls are you standing in a safe place? If a snag occurs up in the air do you have a way to secure everything while you effect a solution? Is a partner nearby to watch over you, and help if necessary? Are you carrying a phone in case you can't move and need to call for help? These are uncomfortable thoughts but you must think about them before you begin. This goes for any tower work, no matter how many people are involved.
• If you have someone helping you do you trust them? If you tell them "do this" or "don't do that" will they listen and obey? Of equal importance, will you take direction when someone points out a flaw in your plan or execution? Learn to listen to what others are telling you. Stubbornness and pride can kill.
• Wear a certified hard hat and sturdy shoes when you are underneath any heavy equipment, tools, dangling tower sections, etc. These many not save you from injury but that injury will be reduced, and you get to live a little longer.
• Test your equipment before starting. Look over those forces described earlier and then, with the gin pole near the ground, lift some test weights a short distance, and try it at various rope angles. Do this even with borrowed or bought equipment, not just for (as in my case) homemade equipment. For one test, since I weigh 55 kg, I could subject the gin pole to 2x+ the weight of a tower section by grabbing both ropes and lifting myself off the ground.
• If you must climb the tower while a tower section is in the air keep your body and hands away from the gin pole and climb on a side of the tower that is not under the tower section.
• Plan, don't improvise. Know exactly what will happen, what to do when the wrong thing happens, and lay out all the needed tools and parts where you'll need them. Clean up afterwards.
• Use a rope cleat on or near the tower to easily secure a load with one hand and no knots. I typically bolt a cleat at shoulder height on my own towers.

The anti-climax: lots of planning and preparation makes for short work

Be safe out there

To close off this article I'll post one more picture. Above is the third section (DMX-4) dangling in the air, ready for me to climb up and bolt it into place. The lift rope is secured to a cleat and then tied off to the tower for redundancy. I did have to climb the tower once during lifting when the section got caught on a temporary rope guy. My homemade equipment performed well.

That tower section is now attached, the bolts torqued to spec, and the first set of guys attached to the "ugly" guy station. The guys are not yet at final tension but they have been adjusted to bring the tower into good vertical alignment. There are now 3 more sections (half the tower) to go, plus one more guy station.

Lifting and securing the tower section took only 30 minutes, including the time taken to unsnag it from the guy rope. It took longer, 90 minutes, to attach and tension the guys and plumb the tower.

Overhauling a BBMB Mast Bearing

The majority of rotatable yagis mounted on a tower require two plates (platforms) at or near the top of the tower to mount the rotator and a mast bearing. The bearing fits over the mast and transfers (lateral) wind forces on the mast and antenna to the tower while allowing the mast to freely rotate and greatly reducing bending moment at the rotator.

Since the bearing acts as a fulcrum to the mast's lever it is important that it be correctly placed when the mast extends far above the top of the tower to support stacked yagis. This is accomplished by moving the rotator plate lower in the tower. Either way the bearing is subject to extensive abuse from wind loads and precipitation. Eventually every mast bearing (sometimes called a thrust bearing around here) will require maintenance or replacement.

Many tower manufacturers sell mast bearings that mate to pre-drilled holes on the tower plate. There is a multitude of industrial bearings and matching flange housings that are cheaper and capable of being employed as thrust bearings but for the following shortcomings:

• Weather - The bearings, while nominally sealed, are open to the elements.
• Mast adjustment - There is no provision for a mast size other than the one that exactly fits the bearing's inner diameter.
• Vertical force - Although the mast bearing is not designed for vertical forces it can deal with a modest amount of it. A typical industrial bearing has less ability in this respect since that is not its intended use.

Although the purpose-built thrust bearing tend to be expensive -- this is a low-volume specialty market -- they are almost always the superior choice. That doesn't mean we must always buy new, if you are willing to put in some effort.

The BBMB mast (or thrust) bearing made by Wade Communications (Delhi) for their DMX towers is typically sold by Canadian retailers for $85 to $90, plus tax and shipping. It is made from 2 pieces of cast aluminum that are held together by the ball bearings in the mated half-races, as shown in the diagram at right (taken from the spec sheet linked to above). There is a hole in the outer surface through which the bearings are inserted at the factory. A steel slug is sealed into place by pressing the aluminum of the cast body over the edge of the slug. This product is designed to be replaced not repaired.

Replace is just what many hams with DMX towers will do when the bearing succumbs to the effects of age. Yet it is possible to repair these sealed bearings if the internal wear is modest and it has not been abused by excessive loads or poor installation. Often it only requires cleaning and greasing the bearings and races. That is, if we can get inside to do it.

I know many hams who have repaired these bearings and I have done a few myself. A web search failed to turn up any instructions from hams who have elected to tell others how it's done. Since I was going to repair an old BBMB anyway it seemed to be an ideal opportunity to fill this gap. Although this procedure is specific to the BBMB it may also prove helpful to owners of other types of towers and bearings. It only takes an hour, or perhaps two hours if you're doing it for the first time.

Opening the bearing

The only difficult task in the overhaul process is the removal of the slug that seals the bearings inside the race. Until this comes out the bearings cannot be removed and the halves of the bearing cannot be separated.

To remove the slug it is necessary to grind or file away the aluminum from the casting that is holding the slug onto the housing. From what I've seen of these bearings there are 3 equally-spaces arcs of aluminum pressed in over the edge of the slug. Unfortunately the area is quite small and I couldn't take a picture (with my camera phone) that gets in close enough to show this. Look for it on your own BBMB and you'll see it.

A Dremel tool can be used or a small cylindrical grinder that fits an electric drill. Alternatively, if you have a small, curved file you can manually remove the aluminum. This is not precision work so don't worry if you remove more of the housing material than necessary. Just try not to enlarge the circular slot in which the slug rests.

If there is still resistance to the slug coming out you can drill a small hole in the steel plug and use a sharp, strong tool to lever it out. In this instance I did it with a steel awl with a hardened tip.

Don't lose the slug! You'll need it later.

Disassembly

You will need a small container for this next step. While holding the outer half of the bearing over the container slowly rotate the square flange. The bearings will, one by one, roll over the now open hole and drop into the container. You may need to shake or tap the housing if the balls stick to the grease residue.

This time around I counted the ball bearings: there were 29. There is room for more in the race so don't quote me on this figure.

You'll know when the last ball bearing is out when the halves of the bearing slip apart in your hands.

Cleaning

Pour a grease solvent into the container holding the ball bearings until they're submerged. I used the cheapest available: paint thinner. For this application there is no value in spending money on higher-quality solvent. Drop the slug into the container so that it also gets cleaned.

Dip a paper towel or disposable rag in the solvent and use it to clean the dirt and grease from both halves of the bearing race and other surfaces that are in close contact to the other half of the bearing. Some vigourous rubbing may be required.

When the parts are clean to your satisfaction wipe and dry the surfaces and ball bearings with a clean paper tower or rag. Letting them air dry may leave some residue -- this is a disadvantage of cheap solvents.

Inspection

When you look at the cleaned races you'll almost certainly notice that the races are indented, one indentation per ball bearing. This is to be expected with steel balls and an aluminum race. The same thing occurs in Hy-Gain rotators, though to a lesser degree.

If the indentations are shallow and smooth you can get more service life out of the bearing. If the aluminum is so badly galled or abraded that there are sharp edges or deep pits in the race it may be time to purchase a new thrust bearing.

Pitted balls can be replaced. Take one to a motor or bearing shop and buy replacements of the same size (I didn't measure them). It is (far) more likely that some balls bounce away during disassembly and you can't find them. It is also not unknown that the slug falls out of the housing on its own and steel balls rain down from the top of the tower from time to time. Yes, that really does happen with this bearing. At least that saves you the trouble of removing the slug!

Assembly

Assuming that the bearing passes your inspection you will need a few items to put it back together.

• Grease - I use the same white lithium grease that I also use for rotators and protecting hardware. Use a liberal amount on the races and on the top and bottom rims of the lower housing (the one with the flange).
• Slug - If the slug is damaged during disassembly you may need to replace it. If it's just bent you can hammer it flat again. If you do need a replacement keep in mind that it is smaller that any coin I have. The smallest in my collection is a pre-Euro 1 peseta coin (Spain) and it is too large. The slug should be steel but I think that any reasonably thick metal will suffice. Just be sure whatever you use to replace the slug does not penetrate into the hole since that will interfere with the balls' movement.
• Hose clamp - A 3½" hose clamp (stainless preferred) to secure the slug after reassembly.

After reassembly test that the bearing freely turns, only showing resistance due to the grease. As the excess grease is pushed aside by the balls the resistance will decrease. Wipe off any grease that is pushed out at the seams between the bearing halves.

Also test that the bearing turns reasonably well with a modest amount of vertical force. Do this by placing the bearing on a flat workbench and turning it while pressing down with your hand. If there is metal-to-metal contact you'll feel it. A small amount of contact is not a problem.

If the resistance is significant you should disassemble the bearing and sand the surfaces of the bearing housing that show signs of contact. You can see in the cutaway view at the top of this post where this contact is likely to occur. Do not sand or file the races! Be sure that all metal filings are removed before greasing and assembly.

You can see that the bearing is not designed to turn under high vertical force by the fact that the the housing only behaves as a bushing to this force, with only grease to prevent binding. The balls offer little support against vertical forces.

To finalize reassembly insert the slug into the bearing hole and tighten the hose clamp around the housing. You may need a shim to hold the slug securely since it sits slightly below the housing surface. The final result should look like the one in the adjacent picture, which is the one I just finished overhauling.

Installation

You will likely need new hardware to bolt the flange to the tower plate and screws and nuts for the mast adjustment. The stock hardware supplied with the bearing is not stainless and readily rusts. Use a bolt cutter if necessary to remove old hardware; the old hardware isn't worth saving.

To mount the flange to the tower plate use 5/16" bolts that are at least ¾" long. You'll need a lock washer as well. The mast adjusting bolts are ¼" and should be at least 1" long, or longer if the mast diameter is small.  If you want to avoid excess length of adjustment screws protruding from the housing you can choose screws that are just long enough to secure the mast you're using. Two nuts are needed on each bolt: one on the inside to set the distance to the mast and one on the outside to lock the screw. High strength hardware isn't required, so use coated (and greased) or stainless hardware to inhibit rust.

Impatience Breeds Action

With the solar flux pushing 200 and my tower projects taking longer than anticipated I grew impatient. Since I now have a short, perfectly-usable tower bracketed to the house it was time to get some use out of it.

It only took 90 minutes to fit together my 4-band trap dipole (what I call a TH1vn) with a steel mast, lift it into place and hook up the already-prepared run of coax and coax choke to it. QSOs rapidly followed on 20 through 10 meters.

While I can't work anything on 6 meters with this antenna I was able to copy EA8 and W6 among a host of others. I'm tempted to piece together a small yagi from the antenna parts in the garage and point it east.

I have another plan for this tower but for now this will keep me occasionally active until the larger tower is complete.

Guy Stations - the Good, the Bad and the Ugly

After a brief hiatus I have returned to the building of towers. This brings me to guying since the DMX-52 tower I am putting up will be guyed rather than free-standing, which would typical for this class of tower. In particular I want to discuss guy stations (or guy brackets): what they are, what they do and why they are needed.

In brief, a guy station is the structure that attaches to, or is integrated with, the tower to terminate the upper end of a set of guys. Guyed towers have one or more of these at regular intervals specified by the tower manufacturer to support the tower under a set of static and dynamic loads.

In this article I will not delve into the mechanics of guyed towers and guys, which is a large topic of its own. I will assume that the tower guying is being done according to engineering requirements for the structure.

Why Guy Stations?

Why not just attach the guy wires to the tower directly and save the cost and trouble of using guy stations? This is a good question. Certainly that is what I did with my Golden Nugget tower.

Should I have done so when I knew full well that it was a bad idea? I judged that in that particular case it would be acceptable. There is no bright line between good and bad, just a continuous band of gray shading. It is unlikely that I'd advise anyone else to do what I did since I have no control over others' practices and situations. Prudence dictates using the proper guying hardware when you cannot easily or reliably do the stress calculations.

Let's take a short, mainly qualitative look at the forces on a tower at the point where the guy wires are attached. Keep in mind this is an example directed to a conventional guyed tower, not the tower I am currently erecting.

In the standard configuration the tension in each guy wire (pre-load) is 10% of the guy wire's breaking strength. In the case of ¼" EHS guy wire this is 600 lb. Since the tower is laterally stabilized by 3 or more guys with equal tension there is a vector transformation of that tension into a vertical (downward) compression of the tower. With 30° guying (angle between the tower and guy wire) the compressive force on the tower is ~500 lb (cos 30° x 600), multiplied by the number of guy wires at the guy station, or ~1,500 lb for the typical 3 guy wires, at each guy station.

There is also a normal stress on the tower at the guy station (horizontally outward in the direction of the guy wire) of sin 30° x 600, or ~300 lb per guy wire. Since each of the 3 guy wires points in a different direction there is a large force that is essentially trying to pull the tower apart!

Dynamic loads (ice, wind, vibration, etc.) can increase the various forces and stresses. For example ice on the guys wires increases the normal stress and tower compression. Wind has a more complex affect since the tension increases in the windward guy(s) and decreases in the leeward guy(s), and not necessarily in exact proportion.

Towers that are specifically engineered for guying often integrate a high-strength set of cross braces, typically located at the centre of each section. It is designed to withstand the normal stress of static and dynamic guying loads, thus ensuring the tower is not torn apart. Guying hardware attaches directly to these braces. On other towers, especially those typically those that are tapered for free-standing applications, a separate guy station (bracket) must be installed to strengthen the tower to withstand the guying stress.

That is all that a guy station really is: an integrated or supplemental structural component to increase lateral strength of a tower where guy wires are attached. Now we can look at some examples, including those that I will be using on the DMX-52.

GS123 at the top of the DMX-2 tower section

The Good

A good guy station (or bracket) is one that is built by the tower manufacturer for that application. This is almost always the most expensive option but also the best. Provided that the tower is built and loaded within the rated height and capacity, and the manufacturer's guy stations and guys are similarly installed per specification, you can be assured of a safe and reliable tower.

Add-on guy stations are most often of two types: attached at the joint between sections, or; attached mid-section, usually resting on a horizontal cross brace. Wade (Delhi) guy stations are of the former type. Rohn guy stations are of the latter type.

The Wade guy station (in my case, the GS123) attaches to the lower of the set of bolts that bind the sections. As should be obvious (and can be seen in the picture) there can be no cross braces where the sections join so the guy station must supply the lateral strength on its own.

Rohn guy bracket

Rohn and Trylon guy stations grip the tower from the outside. In both cases the lateral members are heavy-gauge steel to withstand the guying stresses and ensure that when there is lateral force or motion that the tower moved as a single unit, not allowing the legs to move relative to one another. Unlike the Wade guy station the Rohn guy bracket complements existing tower cross braces, and in fact rests upon them.

Whether by gripping the legs, abutting the cross braces or bolted to the legs the guy station effectively transfers compressive force (parallel shear) to the tower legs without slipping on the legs or "hinging" the legs.

The hardware that attaches the guy wire to the bracket must be high strength and able to accommodate a range of guy angles. The better designs allow resistance-free pivoting (see the Rohn bracket above). Many commercial towers utilize a shackle that freely hinges in the vertical plane. The Wade guy station utilizes flanges with a fixed angle, and achieves freedom of movement from the pivoting of the thimble (supplied with the guy station). This imparts a shear differential between the bottom and top of the flange. While not ideal I have yet to hear of this being a problem in the field.

I purchased a new GS123 for this tower. I wanted the best option for the guy station positioned near the top of the tower, close to the antenna load (between the DMX-1T top section and DMX-2). This will suit for the loads I am planning. The GS123 cannot be placed higher without modifying the tower.

The Bad

The worst option is to wrap the guy wires around the tower legs. The reasons are simple enough:

• The guy wire must rest on a cross brace which may be inadequate to handle the stress of static and dynamic loads. The cross brace may also be inadequate to resist the normal stress caused by the tower legs being pulled outward by the loads.
• Abrasive active of the guy wire on the tower leg and cross braces will first remove the protective coating on all three, then by abrasion and rust will cut through each strand of the guy wire. Failure will occur more quickly for towers formed from sheet metal than from rods and tubes.

Despite these serious problems this is how I had guyed the Golden Nugget tower. But I knew it would not be for long -- just one year -- and the static and dynamic loads would be far less than in a conventional guyed tower.

I also used polyester rope at the lower Golden Nugget guy station. Rope is a poor choice since it has a very high modulus of elasticity (stretches easily in response to increased tension) and is degraded by ultraviolet radiation from the sun. Polyester is rated "UV resistant", but don't let this fool you. After one year there is evidence of UV damage on this rope. Dacron is a superior choice if you must do this. I recall a VK who, years ago, used heavy hemp rope (around 1" or 2" diameter) to support a couple high towers. It seemed to work for him, so who knows.

The picture above shows that I am using this style of "bad" guy station to temporarily support the bottom two DMX sections. Once the third section is lifted into place it will use a more appropriate guy station at the top. When that is done the ropes will be removed.

The Ugly

I did not want to purchase two new guy stations for this tower. Rather than use a GS456 guy station at the top of the DMX-4 section I improvised a guy station. Yes, it is ugly. When I say "ugly" I mean a guy station that is up to the assigned task but looks as if it is not.

The guy station I designed and built is shown in the adjacent picture. The total price is $6, with all parts purchased new:

• 5' of coated twisted-link, 5/32" chain, cut into 3 equal lengths
• Four ¼ x 1¼" Grade 5 bolts and nuts, and 8 ¼" flat washers

The ends of each chain are bolted together, but not tightened. Each chain is looped around a tower leg and the other end is loosely bolted to a link on the same chain. Flat washers sandwich the chain links, acting as pressure plates.

The objective is to keep the common tie point centred within the tower and the tension in the chains high enough that some effort is required to slide the final link over its connecting bolt. As the bolts are then tightened the twisted links at each connection will shift to accommodate the pressure. When this happens the chains becomes taut. The improvised guy station is now rigid, a key requirement of a "good" guy station.

The guy station is completed by inserting guy wire thimbles into links on the outside of each tower leg. These are not yet there in the picture since I had run out of thimbles of the correct size.

Final tensioning is to be done after the tower section above this one is bolted in place. Otherwise the upper section might not easily fit into the lower one. Also, the grip of the chain is strong enough that it cannot be slid down into its resting position on the cross braces when it is tensioned.

Since the ugly guy station rests on cross braces to support the vertical load it is not the same quality as a good guy station. For this reason I am using it for the bottom guy station where the loads will be lower than at the top guy station.

The Plan

I have now been off the air since dismantling the antennas and towers in early June. Unfortunately this means I am now missing some good conditions due to the high solar flux. But all I've had time for since returning home is construction of the guy stations. I may put up a temporary antenna on the house-bracketed tower just so that I don't completely miss out.

Next up is the construction of a gin pole and lifting the section with the ugly guy station into place. It will be guyed at low tension until after the next section (the fourth) is in place. Once these guys are at working tension and the tower is plumbed the rest of the tower can be raised.

If all goes well the tower should be up later this month (July).