Tuesday, August 26, 2014

How Not to Wire a Rotator

My tri-band yagi has arrived and is in temporary storage until I'm ready for it. That primarily entails installing the mast bearing, rotator and mast. Of these the only significant item is the rotator. For that I have unpacked the rotator that I have had in storage for over 20 years: a Ham-M series 5.

The rotator dates from the 1970s and I bought it used in 1985. Although it has the original small brown control box I upgraded both the control unit and motor unit back then to be equivalent to the Ham-III. So it's sufficient to the task. That is if it still works.

I cleaned up both units and then unburied the long roll of 8-conductor cable from a back corner of the basement. Deciding to get right to it I connected everything together and powered up. Nothing blew up or apart. The brake solenoid had a loud and healthy sound to it. That, too, was a good sign. My luck ended there.

The motor wouldn't turn and the direction indicator didn't indicate. The first problem was expected since the rotator relies on an non-polarized electrolytic (start/run) capacitor to shift the relative phase between motor windings. The capacitor is an original so it is well past its best-before date.

I cut the leads and visited a electric motor repair shop. I've done this in years since it's typically faster and cheaper than ordering an exact replacement part from the US.

It was an interesting experience. The folks at the shop I randomly selected were eager to help me but somewhat puzzled by my quest. First, they don't sell motor run capacitors. They stock them only for repairs. Second, the fellow helping me was very particular about the numbers on the part being replaced. I couldn't satisfy him by being helpfully flexible. I knew for certain that the voltage rating is pretty much irrelevant (these shops rarely carry anything rated less than 115 VAC) and the capacitance tolerance is wide. His insistence on getting an exact voltage match turned out to be his concern that the replacement part wouldn't fit inside my motor.

I assured him that size wasn't an issue and 115 is very favourable in comparison to 30. He remained skeptical. The closest part he had in stock was 145-175 μF. The rotator requires a 108-155 μF capacitor. I shrugged and told him I'd take it. The poor guy looked flabbergasted that I'd be so cavalier about the business. I assured him that the part would work out until I could order an exact replacement from the US. Giving up on me he invented a price for the capacitor, which I paid, and left him standing there still looking confused.

The larger capacitor does not fit inside the cramped Ham-M control box. I tried various methods to stuff it in there. I gave up and wired it to the outside of the unit. Some hams wire the capacitor to the motor unit which not only works but increases torque and requires two less wires in the cable. I did it the easy but ugly way by placing the capacitor on the shelf behind the control unit. I'll buy a proper replacement later or a used, later-generation control unit. In the picture you can see it clip-leaded to the control unit, with the old one alongside for comparison.

The motor still wouldn't turn. It seems that more can go wrong after long-term storage. So I started troubleshooting the control unit and motor unit. Both checked out okay. Even the direction indicator circuits looked fine.

There is a quick-release connector installed at the motor end of the cable. I opened these connector terminations. One side was in pristine condition, while the other was heavily corroded and had broken wires and pins. Those still intact could not withstand the wiggle test.

The corrosion must have been present when my antenna system was dismantled in 1992. Perhaps water got in and pooled on only the one termination. I did have protection on the connectors, though it must not have been very effective. It takes only a small flaw in the seal, and time, to destroy cables and connectors.

Luckily I have spares of the same type in my junk box so I warmed up my pencil iron and replaced the faulty connector. I also resolved to pay closer attention to sealing the connectors this time.

Twisting the connectors together showed that I had resolved the motor issue: it now turned nicely in both directions. Yet one more problem remained: the direction indicator still would not work.

The meter needle quivered a bit as the rotator turned but otherwise stayed fixed in the southeast direction. There was continuity through the connector and the control unit seemed fine so I opened the motor. I expected to find a light coat of oxidation on the direction potentiometer wiper or windings.

I cleaned the pot and checked it over its range. It was fine. The direction indicator still would not work. Now, finally, I had the idea of testing the long length of 8-conductor cable. That's where I found the problem. So after an inspection of the now-open motor unit (mechanically sound and grease in good condition) I closed it and dealt with the cable.

What I seem to have forgotten is that my rotator cable is not one continuous run. It is in 3 sections that are spliced together. My memory of this is vague at best. I do hope I paid a good price for it back in 1985! I could tell as I uncovered the splices that this was definitely my work, so there was nowhere else to place the blame. Expedient solutions that seem like such great ideas at the time often come back to haunt us. I was now being haunted by my former incompetence.

There was light corrosion on the wires at two of the splices and heavy corrosion at the third. It was there that I found a failed splice, and several that were on the verge. The proximate cause of the indicator failure was in the white wire (at bottom of the picture, bridged with green clip leads. I repaired all 8 wire joints in that splice. The rotator then worked as it should.

From the dirt on the outer jacket adjacent to the splice this section would have been buried. In my former station all cables were buried under the 10 meters of lawn from the tower base to a row of shrubs. The coax, at least, was burial grade. That splice in the rotator cable, even encased in silicone, did not keep out the water. Consider that a lesson. Over the years just a small trickle of moisture will do serious damage.

This is a problem that not only happens to amateurs. Earlier this month my landline phone and DSL were out of service for several days. The telephone repair tech discovered that, over 20 years of service, the entrance box and cables were soaked and badly corroded. She had to replace the lot.

Another thing I discovered was that none of my three multimeters is in good condition. Oxidation of internal switches is usually the culprit in the case of inconsistent or failed ohmmeter function. If you can believe it the old Radio Shack device pictured above performed best. Modern auto-ranging devices are so cheap today that I plan to buy new rather than bother with repairs.

At last I mounted the rotator on the tower and ran the cable back into the shack. It works great. Except, of course, that it is turning nothing. That item is now at the top of my priority list.

Monday, August 18, 2014

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.


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.


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.

Sunday, August 17, 2014

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.


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.

Saturday, August 9, 2014

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.

Monday, August 4, 2014

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.