Friday, August 30, 2019

Tower Alignment With a Transit

For structural integrity a guyed tower must be vertical and straight. Although perfection isn't required it is highly recommended to get as close as practical. Doing this job without proper tools is prone to error; you cannot rely on your eyeballs alone! Take the extra effort to do it right.

When I put up my first guyed tower I used what I called my poor man's transit. I later improved the usability of this technique by mounting the straightest pipe I could find in my bicycle repair stand. When carefully made vertical with a long level it held its alignment quite well. Or it did if the wind wasn't too strong! In this way I roughly aligned my 150' tower and my new tower up to the 120' level (it will be 140' when complete).

Standing (or, better, sitting) back from the pipe and sighting along the length of the tower is not as easy as it sounds. Freezing your body in one position is more difficult than you might guess. Go ahead and try it by keeping a close and a distant object in optical alignment. It's made harder by the distance difference for eyes that need corrective lenses and cannot focus on both objects at the same time.

I knew that the alignment of the towers was less than what it could or should be. One tell was that the towers were not parallel to each other. The error was no more 2" or 3" (5 or 8 cm) from top to bottom, which while not large and not a significant safety risk does offend my sensibility. A few of my ham friends call me a perfectionist, and that is not necessarily intended as a compliment.

A local tower pro after asking me how I aligned the towers offered to loan me his transit. It was an offer I could not refuse. When I was ready to do the job he handed me the instrument and suggested I search on YouTube for videos on how to use the thing.

A transit is a type of theodolite used in the building trades. Other than YouTube there are many resources on the internet that explain a transit's function, features and use. Here is one that is straight-forward and easy to follow.

I took the transit home, fiddled with it a bit and read the dreadfully poor manual. But that was enough to put it to use. Most of the features are not needed to align a tower, but can be useful to survey a plot of land for a new tower. It wasn't even necessary to turn on the power except to play with the advanced features in the comfort of my shack. Setting up the transit for tower alignment involves the following steps:
  1. Plunk the transit down in the field far from the tower.
  2. Level the transit.
  3. Point the telescope at the tower base centre and lock its horizontal motion.
  4. Rotate the telescope up and down to track deviation from a vertical line.
Sound simple? Well, not quite. Let's look at the process in more detail.


This is the most critical step in setting up transit for use. It absolutely must be level. The method I was taught and that works well for me is as follows:

  • Extend the tripod to working height, but not so high that you can't easily see the levels. Point one leg towards the tower so that you have unencumbered access to the telescope.
  • Roughly level the tripod top by eye then firmly push the leg spikes into the ground (assuming you're not on bare rock).
  • Mount the transit to the tripod. Adjust tripod legs until the bubble level (bottom) is approximately centred.
  • Rotate the transit so that two adjustment knurls are at either side. Adjust the knurls to level the transit using the the linear level (top). Do it for each of the 3 positions, then repeat until the transit is level in all directions. You can usually get away with only being precise about the level for the one position where the telescope points at the tower if the only purpose is tower alignment. But don't get sloppy: get it level in all other directions as best you can.
You should periodically check the level as you work. Unfortunately you must repeat the process every time you move the transit, and that must be done many times during alignment. It goes faster as you become accustomed to the process.

Pick a spot

The best spot is directly opposite a guy anchor, and farther from the tower than its height. The first is to ease sighting of the tower and for left-right symmetry. The second is to keep the maximum telescope angle below 45° elevation for ease of use. Farther is better than nearer, although it requires more walking to and from the turnbuckles after each adjustment.

The best spot is not always available because of obstacles such as buildings and bush. Or the sun is behind the tower from that vantage point which makes sighting the tower difficult and dangerous for your eyes. An alternative is the place the transit behind a guy anchor, slightly offset so that the guys don't block sighting the near tower leg. This is shown in the adjacent picture.

When you do offset in this fashion be aware that your view of the tower is slightly rotated. The difference may be less than 1" but it will loom large in the telescope. Also, the pier pin will not be vertically aligned with the nearest tower leg. Keep that in mind as you read the section below on the alignment process.

Preparing the tower

The guys are under a lot of tension. When you adjust one turnbuckle the tension of other guys at that level and especially those higher can distort the tower in unexpected ways. It is advisable to loosen all the guys equally before starting. Obviously this should not be done when the tower is heavily loaded and a storm is imminent.

The guys should not be so loose as to be slack. In my case with 5/16" EHS guys the pre-load tension is a little over 1000 lb I reduce the tension to no more than 500 lb. Do it for all guys above the lowest guy set. Leave the full pre-load tension on the lowest guys. As alignment proceeds we want the guys above the set being adjusted slack enough that they cannot distort the tower due to excess downward force. Similarly we want the guys below the set being adjusted to be at full pre-load tension.

All that said, if alignment is for periodic maintenance and not the initial alignment of the tower you likely don't need to loosen the guys. This is because the expected lateral shift required ought to be small. For new towers I find that the initial eyeballing rough alignment can be out by several inches. With experience more than one guy level can be adjusted at a time and tension becomes less of a concern. Don't overestimate your ability and you won't make any mistakes.

Aligning the tower

Guyed towers are aligned from bottom to top. You start by aligning the lowest guy set and move upward until the top. Starting at the top or middle will only make the job more difficult so avoid the temptation. This point was impressed on me by several experts. Loosening guy tension before you begin, as recommended above, greatly aids this process.

With the transit in position and levelled aim and focus the telescope at the tower's pier pin or suitable centre mark at the base. The telescope includes a reticle to do this accurately. On my transit the reticle looks somewhat like the image at left. I found the double vertical lines useful for bracketing bolt holes and leg ridges.

There is a knob to lock the horizontal motion, so lock it when your aim is close. There will be another knob for fine adjustment (concentric with the lock control on mine). Swing the telescope up and check for deviation from vertical at the lowest guy station.

Estimate the lateral correction required. Loosen the turnbuckle on the side the tower leans towards, then tighten the turnbuckle on the other side the same amount. You can make a good guess at how many turns of the turnbuckle it'll take by noting the tpi (turns per inch) and doubling it (since there are two screws) and reducing the travel a bit due to the angle of the guys -- lateral motion per turn is less the higher up the tower the guy attaches; this is basic trigonometry.

For large offsets do it in steps rather than all at once. This is also good advice if you've never done an alignment before. A bit of lubrication on the turnbuckle screws helps. Turning a balky turnbuckle carrying 1000 lb of tension generates a lot of heat (and loud screeching) that can damage the threads.

When you're satisfied with the alignment do the same for the other two positions around the tower. You'll likely need to repeat the circuit once and possibly more for towers badly out of alignment. After aligning the guys at each level tighten the three turnbuckles an equal number of turns until you reach the required tension (approximately 10% of breaking strength). Use a suitable gauge that is calibrated for the guy cable you are using. If all is in order the tension should be approximately equal on all 3 guys.

Repeat the process for each higher guy set until done. You should check the alignment at lower guy stations as you go along since as each higher guy set is tightened it will place additional load on the tower that can accentuate any imperfections. Be on the watch for significant deviations since this can indicate a weak tower section or guy component.

Common difficulties

The alignment process is straight-forward but tedious. Having friends to help makes the job go much faster. I find I spend a lot of time walking between guy anchors and transit, time which can be eliminated by asking your friends to tighten/loosen turnbuckles while you operate the transit.

It is good practice to have the turnbuckle screws approximately half inside the turnbuckle to allow adequate room for future adjustment in either direction. I found that with each new tower my initial rough alignment when corrected resulted in turnbuckles at one anchor with too much screw and with too little at another anchor. Don't shirk correcting these problems.

If there is too little screw thread inside the turnbuckle or the turnbuckle bottoms out during alignment you'll need to detach the guy to reposition the screws. Use a winch or cable puller of sufficient capacity to relax tension on the turnbuckle, unwrap the guy grip, adjust the screws and reattach the guy grip. Tighten the turnbuckle until the tension on the winch relaxes and remove the winch.

Use a tensiometer to return the tension to what it was before. Then align the tower only adjusting that one turnbuckle. This is a good time to note that when you cut the guy cable be sure to leave a longer tail in case you need it after alignment and repositioning a turnbuckle. You can cut it closer after the alignment is complete.

If you are uncertain whether the grip is safe to reuse replace it; they aren't expensive. I will reuse them once, but not twice, and only when the strands are not distorted and there is lots of grit remaining. From my own experience and those I've spoken to in the business no one can recall a grip failing on its second use. Guy grips are tougher than they look. However none of this guarantees that a reuse failure cannot happen.

Between guy stations the tower sections may not track in a straight line. Assuming that this is not due to damage (corrosion, metal fatigue, failed weld, etc.) and the deviation is large enough it is worthwhile to correct it. The problem is typically due to a section splice in which the bolt holes on one or more legs were not properly aligned when bolted.

Correction involves slightly loosening the bolts so that the vertical force seats all the splice bolts in the same position. Unfortunately the opposite can occur in some cases. I've done it though I'll admit it makes me nervous. Just be very certain the deviation is not due to damage. Better, call in an expert to do or supervise the work. In a guyed tower damage or deviations are more dangerous the lower they are on the tower, which is opposite to the expectations of many hams.

Finishing the guying

Guy components will relax when first put under full tension. For example, grips conforming to the shape of insulators and thimbles. The tower will drift out of alignment within days. For this reason professionals return to a tower a few days or weeks later to redo the alignment. We should do the same. It is worth the trouble even if money must be spent to rent a transit.

When you are satisfied that the tower is properly aligned and the steel has settled into its final shape you should add safety cables to the turnbuckles at each anchor to prevent them from turning under load and keeping the tower upright if a turnbuckle breaks. Clean and paint them to keep them in good working order.

As part of station general maintenance check alignment at least once each year. Do not rely on adjusting guy tension alone since, while indicative of potential trouble, tension changes don't tell you what has specifically happened.

Friday, August 23, 2019

Stacking Scenarios for the New Tower: 15 and 20 M

As I grind my way towards completion of my stacks of 5 element yagis for 15 and 20 meters it is time to decide what goes where. The objective is to maximize performance, when used as a stack and when used individually. The major decisions on placement: height and separation.

There is also the matter of interactions with yagis for the other bands and with the guy wires. For the present exercise I will ignore those interactions since the way I broke up the guys to be non-resonant and with sufficient yagi separation the pattern impact will be small.

In addition to electrical considerations there are physical constraints:
  • The yagis atop the tower cannot be separated by more than 3 meters (10') to keep mast stress within reason. Similarly the smaller yagi for 15 meters will go at the top of the mast and the 20 meter yagi at the bottom of the mast.
  • Lower yagis must be above a guy set, but not too far above to avoid tangling with the guys further up. The constraint is tighter if those yagis are rotatable rather than fixed. At least t his year they will be fixed towards Europe.
When completed the tower for these antennas will stand approximately 133' tall (40.5 meters) and the mast will rise another 10' (43.5 meters). Guy stations are nominally at 30', 65', 100' and 135', however due to section overlap the heights are more like 28', 61', 93' and 128'.

To recap the yagi dimensions:
  • 15 meters: 5 elements on a 32' boom (10 meters). This antenna has low SWR across the full band and has been optimized for best gain while having a good match and F/B. Design details can be found in an earlier article.
  • 20 meters: 5 elements on a 40' boom (12 meters). This antenna is a minor optimization of the yagi having the same dimension in recent versions of the ARRL Antenna Book.
Modeling is done with EZNEC. I use dual sources for in-phase stacking rather than phasing harnesses to make the modelling exercise easier. This does not materially affect the pattern but does affect the impedance, and the latter is not the focus of this article. SDC (stepped diameter correction) is used for all elements per my tapering schedule. Construction detail will appear in a future article.

If you're new to stacking you may want to review my article on the basics of stacking. There are of course many other resources on the topic of stacking around the internet and in the amateur literature.
20 Meters

I'll look at 20 meters first since the stack is more constrained; that is, there are fewer viable options for placement of the yagis. The upper yagi is at the bottom of the mast, which I waffled on with regard to the precise height, as you can see in the plot traces. The 2' (0.6 m) different is negligible. The lower yagi is in the vicinity of guy stations, ideally slightly above as discussed earlier.

Height of the upper yagi is ~2λ. Height of the lower yagi is at either 1λ or 1.35λ. The modest difference in height of the lower yagi has a significant impact on the stack behaviour.

The respective separations are 41' (12.5 m or 0.6λ) and 67' (20.5 m or 1λ). The first is equal to the boom length, usually considered the minimum separation for good performance; this is discussed in more detail below. The second is well separated at the cost of placing the lower yagi relatively close to ground (1λ).

The lowest lobe of the upper yagi is 7°, and 11° or 15° for the lower yagi. Lowest lobe of the stack is 8° and 9°, respectively.

Gain of the stack and individual yagis varies by less than 0.4 db for the two scenarios. This is indicative of low interaction, and that is confirmed in the EZNEC model. Although not confirmed in this exercise the low interaction (mutual impedance) between yagis promises low pattern distortion when they are pointed in different directions.

Stacking gain over the individual yagis varies from 1.5 to 2.5 db, which is quite good for 20 meters at these heights. The nominal 3 db stacking gain is just that: nominal. There are many variables that determine this performance metric.

Notice that the patterns of the lower yagis differs quite a bit due to their different height, and this affects the pattern of their respective stacks.
  • Vertical radiation is higher for the lower yagi at 92' since that is the only position that is not a multiple of λ/2.
  • Inter-lobe nulls coincide more closely when the stack separation is smaller, as expected. Better elevation angle diversity is achieved with greater separation. That is desirable for paths with an elevation angle close to a null so that we can avoid dead spots. 
  • The major lobe of the lower yagi is significantly higher at the lower height. This, too, is desirable for path diversity. When fixed to Europe with its wide range of probable elevation angles the lower yagi becomes more useful.
  • High angle minor lobes are attenuated with the lower yagi at 1λ height.
The choice for 20 meters is quite easy. The lower yagi will go directly above the second guy station. When (if) the lower yagi becomes rotatable it will also perform well on paths to Africa, Caribbean and North America.

15 Meters

With the upper yagi at the top of the mast and the shorter wavelength it is easier to achieve low interaction between yagis on 15 meters in comparison to 20 meters. The shorter boom length is also helpful in this respect.

The maximum height possible for the lower yagi is below 120', otherwise the elements get dangerously close to the top guys. I therefore chose this height for the first test case since while not physically practical it highlights a too small stack height of 20' (6 m or 0.42λ), which is less than the boom length.

In the second case the lower yagi is at the more practical height of 100', a little above the second highest guy station. This maximizes the distance from the outermost elements to the upper guys. The stack height is 40' (12m or 0.85λ). This is much better spacing.

When modelled the difference in performance is readily apparent. As with the 20 meter stack the greater stack height there is better distribution of the elevation nulls, reduced high angle radiation and greater stack gain.

If I had a tower dedicated to each band I could exploit the height to add a third yagi to the stack. This is not practical in my case where 15 and 20 meter yagis present since optimum and equal spacing would have yagis tangling with the guys or parked too close to a yagi for the adjacent band. A friend of mine who does manage to have a 15 meter 3 stack shared with 20 meters uses a taller tower than mine.

My next test case was to move the lower yagi even lower, though not too close to the lower 20 meter yagi at approximately 68'. The chosen height of 80' is, like 120', risky since it places the outermost elements close to the next highest set of guys.

The result is a mixed bag. Rearward lobes are small but there is an increase in radiation at high angles. Differences in gain of the stack and individual yagis compared to 100' are negligible at less than 0.3 db.

My tentative choice is to place the lower yagi at 100' or a little lower to increase distance to the upper guys while not getting too cozy with the set of guys immediately below.

Other antennas on or near the tower

Interactions limit further use of the tower. I plan to use my TH6 and TH7, possibly stacked, at lower heights for short paths such as the US and Caribbean. There is no good place for these tri-band yagis on the tower that would not impact performance due to interactions with at least the lower 20 meter yagi in the stacks. The tri-band yagis will therefore go on the other big tower, well below the 40 meter and 10 meter yagis.

I would like to leave open the possibility of a wire 40 meter yagi pointed at Europe, strung on a catenary between the big towers. With 5 elements the wire yagi would be 30 meters longs, which is about half the distance between the towers. Interaction with 20 meter yagis should be managable. Impacts on the performance of the 15 meter yagis could be more severe. The same is true of the tri-band yagis which are, of course, resonant on 15 meters.

I am looking at alternative placements of the wire yagi to best advantage. There is modelling work to be done.

Vertical antennas for 160 meters are less concerning due to the polarization difference. However the T-top of my current 160 meter antenna will require a close look to confirm that it will not causing any problems with the yagis on both towers.

Yagi separation vs. boom length

There exists a rule of thumb that two yagis in a stack should not be closer together than the yagis' boom length (for identical yagis). This is good guidance even if it obscures the reason for why it is a useful rule. Without delving into excruciating detail we can dig a little deeper for some insight.

But first let's review how a stack develops gain. For two identical yagis fed in phase, in free space and far enough apart that the mutual impedance is negligible, the stacking gain is exactly 3 db. This fact alone is a curiosity since we are dividing the power so that each yagi get half of it and the yagis' far field patterns sum in the far field. Fortunately I wrote an article on how this is possible if it seems paradoxical.

In the real world the yagis interact with real ground and there will always be some mutual impedance between the yagis. The closer together the greater the mutual impedance since the various elements will have near field coupling to those in the other yagi.

It is possible to have stacking gain greater than 3 db with carefully engineered close spacing. However this is only advisable if the yagis always point in the same direction, whether fixed or rotatable. Otherwise when one of them is turned the high mutual impedance will distort the patterns of both yagis. In general it is a poor idea.

Ground reflections for the two yagis place their lobes and nulls at different elevation angles which affects stacking gain. By closely spacing the yagis the lobes, especially the lowest main lobes, will reinforce each other and give close to the theoretical but nominal 3 db gain. This is at the price of a reduced ability to sidestep elevation angle nulls since these, too, will coincide, as we saw in the discussion earlier in the article.

With the preliminaries out of the way let's return to yagi separation and coupling. Yagi gain increases with boom length. Gain comes from a narrowing of the main lobe. As the lobe narrows the mutual impedance to a vertically separated identical yagi decreases. This is due to field cancellation away from the main lobe, directions which would intersect elements of the other yagi. Note that for this purpose we are primarily concerned with the free space elevation pattern (shown on the right).

However the lobe narrowing happens in concert with a longer boom length. Therefore the angle of, say, the -6 db point is lower but will still interact with the outermost elements of the other yagi because they are further out. The relationship is not exactly proportional but close enough that the rule of thumb is justified: increase the separation as the boom length increases to achieve approximately the same amount of element coupling (interaction).

Keep in mind this is a minimum distance not an optimum distance. When you allow the yagis to point in different directions more separation reduces pattern distortion. This is usually a better choice than trying for maximum stack gain.


I have ample time before winter to refine the stacking arrangements. This can be done in parallel with the construction and tuning of the four yagis. Although I had hoped to be further along by now I am getting closer. It's been a learning experience, one with unexpected stumbling blocks.

You can see from the picture of aluminum shavings (the dark patches are steel) filling most of a 5 gallon pail that I have not been idle! Construction of the yagis is a story unto itself and will be covered in future articles.

Sunday, August 11, 2019

6 Meter Season Wrap-up for 2019

We are now into 6 meter withdrawal season. This is the time of year when Es (sporadic E) openings rapidly decline in number and distance yet aficionados cannot quite let go. All I hear now on FT8 is endless CQing from those within direct or tropospheric propagation (within ~500 km). Although there is still some DX to be found if you are patient the season is effectively at an end.

This is a good time to reflect on my accomplishments this season and what I've learned. I did a similar season end report last year.


Purple is my favourite colour
I ended last year with 56 DXCC countries on 6 meters using FT8. I tally countries separately from other modes to track my progress. Official counting methods mean little to me since I do not chase DXCC awards. As I write this I have worked 15 more countries this season for a total of 71 DXCC countries on 6 meter FT8. I don't believe there are any countries I've worked the last few years on CW and SSB but not on FT8.

I heard many more countries than I worked. That's the nature of 6 meter propagation. Of note I worked Japan for the very first time on 6 meters, using any mode. These QSOs were my first with Asia. With respects to continents only Oceania still eludes me. I heard several Hawaii call signs this year but all were in the continental US. American call sign practices can be frustrating to the DXer. I know the path exists since several hams in this corner of the continent worked Hawaii this year.

In my season's end article last year I noted that I worked 40 European stations on August 4 during an amazing multi-hour opening. Perhaps more amazing is that I broke that mark this year, working 43 Europeans on July 20. My European distance record now extends to Ukraine. Even so I have heard but failed to work many European countries such as OH, LX, 9H, LZ, YO among others.

I was unlucky with African openings apart from adding CN and EA9. I heard 5T5PA a few times, though very weakly and never workable. Better was 6W1TA who had a fine strong signal for almost 2 hours one day. In this instance I caught the opening near its end and failed to make it through the large North American pile up.

Looking south I worked HC5VF for a new one. CP1GJ was heard but not worked. Many stations were worked in northern South American and Caribbean. One that was active but not snagged was P41E.

My final country for the season was CY9C. In this case I got lucky since many others in this region had difficulty despite the relatively easy single hop path. You win some and you lose some. It would be no fun if every station heard is worked.

50.323 MHz

What seemed to be a great idea to move intercontinental DX activity to its own FT8 window is not working out. There was little to be found there this year. I did QSY to the DX window when domestic contest activity crowded me away from 50.313 MHz but with no success. The DXers, here and abroad, wouldn't move even when the QRM became overwhelming.

CW and SSB

The digi-averse die hards are still on conventional modes. The only significant operating I did on CW and SSB was to use those modes exclusively during the ARRL June VHF contest. Unlike my experience in 2018 this year I had far better success. With only a little effort I managed well over 100 contacts, reaching as far as the west coast. There are still signs of life at the low end of the band!

Despite my almost 100% focus on FT8 for DXing I have not used it in a contest. Perhaps I never will, but you never know. It's too slow and mechanical to hold my interest.


I have yet to use FT4, restricting myself to occasional monitoring. The 6 meter window is 50.318 MHz. It is certainly fast enough to interest me but I don't know if it'll catch on for DX work. Certainly it has promising attributes for 6 meter DXing. Maybe I'll try it next year.

Decoding sensitivity

There is an ongoing debate regarding FT8 decoding success rate between WSJT-X and alternative software branches, in particular JTDX. This may very well be true. What gives me pause is a few of the methods JTDX uses: correlation to known or expected messages and correlation to a data base of known active calls.

I am still thinking about whether this is a good idea. There is a fuzzy line between viable decoding and guessing, and I am ethically opposed to the latter. Even when I use Super Check Partial (SCP) during contests I always request a repeat or confirmation of the call.

Until now I have been happy to do no more than tweak upward the aggressiveness of WSJT-X decoding, yet even it does a bit of guessing. Where should the line be drawn? I don't know. As time permits I will download and experiment with JTDX on HF, listening only, to learn what it can do and how I feel about it. In the 2020 sporadic E season I will likely run higher power and I can expect more weak callers and therefore have greater need for decoding sensitivity.

Improvement is possible beyond the software. I have altered how I set up my rig and computer to better match the dynamic range of the receiver and sound card ADC. While preliminary results are ambiguous it looks promising. I may have more to say about this if I learn something of substance.

It is unfortunate that WSJT-X calculation of signal reports is not usable for determining signal levels, SNR or receive performance since it is relative to other signals and noise within the pass band. I only learned this recently. It explains some bizarre disparities between received and sent signal reports that I've witnessed.

Transmit quality

There is more activity on 50.313 MHz than ever before. Getting on is easy since every modern transceiver has 6 meters and many HF amplifiers include 6 meters. If you use RTTY or other data modes (or you do SSB contesting, like I do) you already have the required audio connections to your computer.

The trend may continue for the next two years until the solar flux edges upward and thoughts return to HF. I may be responsible in my own small way by encouraging local activity (FN25 and adjacent grids) during several talks I gave to local clubs last winter.

Unfortunately it is all too easy to transmit a dirty FT8 signal and its incidence is increasing in proportion to rising activity. Monitoring your own signal quality isn't easy and few bother to make the effort to do so. Most rely on recommended practices but without feedback don't realize when they've done it wrong.

I got more serious about it this year. With the help of a buddy we transmitted back and forth while tweaking our rigs to see what works best. I am now reasonably confident that my signal is clean. Over the winter I will build a test system so that I can monitor and make adjustments on my own. Perhaps I'll write it up if it looks to be useful to others.

The adjacent screen capture shows an FT8 signal with adjacent images that are often decoded. These are not harmonics. The DSP and equalization circuits in modern transmitters create novel difficulties. These images are present on far more FT8 signals than is good for peaceful coexistence. I get them on my own signal under some conditions and I want to make further improvements.

W9MDB has written a document on setting up your transmitter that looks quite good to me and explains a few peculiarities I've run into. Some surely need the lesson since there are too many signals far worse than is seen in the spectrogram above.

Improving QSO throughput

One of my objectives this year was to improve the number of QSOs I can squeeze into a shorter time. The key to this is understanding the software in greater depth and making liberal use of auto sequence features. The difficulty is understanding what auto sequence does in the critical crossover between one QSO and the next.

I have had good success once I let the software free to do what it thinks is best. It does always work out, and in those instances you simply need to be paying close attention and quickly alter the message or the station you are responding to. In short DX openings, with all their QSB, it is possible to interleave QSOs when one is interrupted by propagation or QRM yet be able to complete it a minute or two later.

There is much more to be said on the subject so I won't dwell on the topic now. I believe it is worthy of its own article. There is always more to learn about the subject and it is possible that many readers will benefit from knowing more, even veterans of digital modes.

Looking forward

As I've said before it is very likely that I'll have a 6 meter capable kilowatt amplifier next year. That is perhaps the biggest change I am planning for the near future. Once my HF antenna farming matures I will pay more attention to VHF. This includes an improved antenna of some kind and antennas for 2 meters and 70 cm. That's the upper limit of my interest.

With regard to other digital modes I have ventured back into MSK144 for the Perseid meteors. I haven't worked much, just playing with it to get more experience. Meteor scatter is something that interests me more for 2 meter use; on 6 meters the distance is easily spanned using more common ionospheric propagation modes.

Of more immediate interest is HF. Now that the peak sporadic E season is over there are antennas and towers to build and get operational for the fall and winter season. It also severs an inconvenient and invisible tether to the shack that keeps me from antenna work.

Tuesday, August 6, 2019

Draining Tower Static

We are all familiar with lightning, in which charge moves between ground and the atmosphere. There is less familiarity with the role that static plays on our towers and antennas. Charge doesn't move until the potential is large enough to bridge the distance from a location with a different charge. This static discharge can play havoc with reception. It is better to continuously bleed the charge so that its potential remains low. The best place to move that charge is to ground.

Let's begin with a story. Many years ago I visited a friend who was doing work on his modest suburban tower and antenna system. As is quite common during autumn in this climate there was light snow falling. As we were chatting we could hear a periodic snap sound coming from the doorway to the basement shack. We walked over to see what it might be.

Lying on the desk was the disconnected coax for his VHF yagi located about 15 meters up the tower. As we were looking around for the source of the sound a spark jumped across the coax connector. The snow was depositing a charge on the antenna which increased until the potential was enough to jump the gap and the charge would flow to ground via the coax shield. We could reproduce the spark at will with a screwdriver to reduce the gap with a wait of perhaps 15 seconds. At the time I was surprised this could occur with such a small antenna at a modest height.

Ordinarily the charge would be constantly bled through the receiver front end or a grounding style coax switch. When drained in this fashion the potential cannot rise high enough to cause any damage. But it would be unwise to plug the coax into the rig when there is a static charge present. Better to wait for the weather to settle.

A tower has a static charge on it due to its height despite being grounded. The ability to bleed the charge to ground can be difficult due to the potential gradient and the potential of the ground surrounding the ground rod. Although you cannot directly experience it there is a charge gradient along the tower, and indeed right up through the atmosphere. Just as there is no absolute protection from lightning the same is true of static charge, but the risk can be reduced. Our main concern is with reception. In extreme environments (high and very dry) transmission can be a problem but rarely at amateur power levels.

The Cushcraft XM240 40 meter yagi has a couple of inherent challenges with respects to static charge:
  • Elements insulated from the boom
  • Small diameter rods in the capacity hats
The former impedes the flow of charge to the boom and from there to the tower and ground. The latter increases the risk of corona and therefore more energetic discharges. Many hams connect the reflector element to the boom to permit charge flow. This has negligible effect on the resonance of the element. Although the driven element cannot be connected to the boom there is at least a long path to ground via the coax and into the shack.

I am motivated to do what I can since the static discharge noise on 40 meters can be dreadful when it is raining and sometimes when it is snowing. It is bad enough that the static discharges can be heard on nearby antennas for other bands. The XM240 is at particular risk of static charge since it is at the top of the mast. If it were completely bonded to the mast and tower to bleed the charge the QRN ought to be greatly reduced, protecting reception with it and antenna further down the tower.

Which brings us to the next difficulty: continuity between the mast and tower. If you've never thought of it before this may seem a strange question. Most would assume there is an electrical path between the mast and tower due to all those set screws and clamps holding everything together at the bearings and rotator. The problem is that electricity is being asked to conduct through bearings, which due to surface grease on the rolling components may not have reliable metal-to-metal contact.

The routine solution is a flexible wire between the tower and mast. Think of it as an insurance policy against poor continuity through the bearing surfaces. It might not help but it might help a great deal.

During a maintenance climb up the 150' tower several days ago one of my tasks was to install just such a wire. As you can see it's very simple. I cut a 2' length of heavy gauge, multi-strand speaker wire that was handy (though unlikely to be UV safe) and fitted ⅜" tinned lugs at both ends. One end connects to the tower via a top plate bolt and the other end conveniently fits onto a galvanized muffler clamp already on the mast. I was going to use the stainless hose clamp (visible in the picture) but I saw an opportunity and took it.

Will it help? I'll find out soon enough. I will need to see how it performs during rainfalls in the coming months. Unfortunately the insulated XM240 elements could muddy my observations. In any case the XM240 will come down soon and this time around I'll remember to ground the reflector to the boom. The "to be determined" antenna that will replace it at the top of the mast (46.5 meters high) will be bonded to the mast.

If the strap works I will do the same to my other towers. The fix is simple and justified even if the data of its performance is inconclusive.