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.

Summer (blog) Vacation

I'm still here. New blog articles are slow to appear because I am very busy with antenna and tower projects, 6 meter DXing and a large number of non-ham summer fun stuff and chores. I thought I would fill the blogging gap with a progress report of sorts so that regular readers don't think I've vanished. I haven't. Regular blogging will resume, eventually.

To give you an idea of what I am up to I've listed the most important projects below. From this you will get an idea how busy I am. All will be written up in future articles as each project is completed and I have the time. But right now the weather is too good to waste. I have not provided links to articles which discuss these projects so if interested try the search box.

• The 80 meter vertical yagi is near completion. The parasitic elements are tuned and the switching system is operational, as is the control head in the shack. What's missing is the control box containing the switching matrix, cable interconnections and switchable L-networks. It is partially built and I have all the parts.
• Options and specs for the mast and rotation system for the new 140' tower have been discussed with my local machine shop and I should have the fabrication complete in the first half of August. I will be using my spare prop pitch motor as the rotator.
• One boom left to be built for the 2 × 5-element stacks of 20 and 15 meter yagis. That one requires machining of a 20' long pipe the machinist can't handle in his shop. But it'll get done one way or another.
• I have fabricated 2 sets of 5 element-to-boom clamps for those yagis. The rest are awaiting an order of fasteners to make drilling templates.
• All the aluminum for the 20 yagi elements (4 × 5) is on hand and the bulk of the machining is complete. Once that is done the sections will be joined and yagi construction and testing should begin around mid-August. Elements have been scaled in software for the taper schedules that I settled on.
• Design of the elements for a forthcoming full size 40 meter yagi is proceeding. I hope to at least get one complete element in the air as a dipole for testing over the winter to see how it performs electrically and mechanically.
• Tower maintenance is ongoing. This includes patching surface rust, vertical alignment and checking of all hardware. Repair of malfunctioning antenna requires getting them down using the same tram line used to put them up them.

Reading the list is fatiguing enough, yet these are things I have to get done in the next few months.

Haying on my land should be complete this week and I will have more freedom to work on towers and antennas. This includes malfunctioning yagis that need to come down for repair or to be moved to a different tower.

I hope the weather is as good where you are as it is here and that you, too, are making progress on your station projects. Soon enough the weather will turn cool and the race will be on to complete work before winter settles in. Then I will have the pleasure of cold weather projects such as low-noise receive antennas and shack equipment. It'll be a while yet before I can truly relax.

Working Japan on 6 Meters

From my start on 6 meters in the mid-1970s, to the tremendous solar maximum of 1989-1990 and into the modern era of FT8 it is interesting that I had not achieved WAC (Worked All Continents) on the magic band. The one continent that confounded me was Asia. I came very close in 1989, eking out several partial CW QSOs with Japan. That was my sole shot since almost all of west Asia, like most of Europe, did not have a 6 meter band at the time.

With FT8 prospects improved, despite the lack of sunspots. Sporadic E openings to Japan and the Far East from eastern North America have been known about for a long time but due to their fleeting nature have only rewarded the most dedicated 6 meter aficionados both here and there. These openings are easier to catch with FT8 since all activity is captured by the software. This is what I've coined the discovery problem which FT8 solves so very well.

Historically it is known that the probability of an opening between Japan and northeastern North America peaks in the last week in June between 2130Z and 2300Z, our late afternoon and early morning in Japan. In fact the times between this daily window and their sunrise and our sunset are nearly equal. Although that may be nothing more than a coincidence it is nonetheless interesting.

To my chagrin I missed a fantastic opening in late June last year, the first year that I and most of the 6 meter community were on FT8. It was dinnertime and by the time I discovered what had been occurring the opening was ending. Sitting in front of the rig every day during the propagation window is not realistic, nor very enticing.

This year I was not so complacent. I really wanted a QSO with Asia, and openings to the Middle East have so far yielded no QSO. So I monitored, watched and waited for another opportunity to work Japan.

Statistically speaking I would likely have one or two chances, each lasting no more than 15 minutes. This would be terribly daunting were it not that it is early evening when I am more likely to be in the house. But this time I would have to pay close attention to activity.

During the second half of June there was no workable opening to Japan from this QTH. Others to the south, west and east had better luck. Other than a couple of decodes of very weak signals nothing was workable here in FN24. That changed in July.

The first opening arrived on July 2. Unfortunately it was only JA7QVI who was sporadically copied over a period of 10 minutes. Several other likely but very weak signals that appeared on the spectrogram did not decode. I called him without any luck. This isn't surprising since he is a big gun who typically runs QRO and his weak received signal boded ill for my 150 watts.

Success had to wait two more days. On July 4 signals were being reported by others in this part of the continent as early as 2200Z. I did not hear any QSOs being completed but clearly something was brewing. A strong opening to OX was on the wane at that time and the Nunavut beacon VY0SNO/B was widely heard. It was a challenge to monitor the band while cooking the evening meal! Happily the real fireworks waited until meal preparation was completed.

When signals began decoding I noticed that more were stations calling KL7HBK. Alaska's bearing is only 10° west of the Japan path but I never did copy him.

JE1BMJ was calling CQ NA but I couldn't get through to him as he faded in and out for several minutes. None of the other stations heard were decoded more than twice.

They say that patience is a virtue, so I made a sacrifice by eating my dinner in the shack. I watched and hoped for signals to grow stronger. Of course I was not quite that patient so I filled the silence with my own CQ JA.

As you can see in the screen shot that eventually signals did rise. A few decibels made all the difference. First in the log was JH1IFS. Then for a few short minutes I was the object of a small pile-up. I worked just 4 stations before the opening faded, but even one was enough to leave me delighted. During the fade I made a couple of partial QSOs as signals briefly rose out of the noise. Within 10 minutes they were gone for good.

It is interesting that I never heard the KL7 the JAs were calling and big guns only a few hundred kilometers from me appeared to work nothing in this opening. This is a great example of the spotlight nature of 6 meter openings. The spotlight narrows as the path length increases. All those points of intense E-layer ionization must line up in space as well as time to make the QSO possible. That's part of the attraction, working with what nature tosses our way to put the magic in the magic band.

Notice that every Japanese station calling me skipped message #1 in the standard FT8 sequence and used RR73 rather than separate RRR and 73 messages. That is an excellent practice for fleeting openings. They can be so short that the 30 seconds saved is critical to success. It also allows for more QSOs to be made by everyone during the opening.

I suspect the Japanese operators are acutely aware of this since almost all their DX QSOs on 6 meters are on paths of this type. Once you miss out on some juicy DX using the standard (and longest possible) message sequence you quickly learn. Exchange of grid squares is nice but is not necessary. The minimum required information for a valid QSO is call sign, report and confirmation of both.

Afterward I sat back and contemplated this achievement. It was one more of persistence than any particular skill. Sometimes that's all it takes. Just as in contests the one under-appreciated difference between the winners and losers is BIC: butt-in-chair. That was perhaps the most important factor that put these rare QSO into my log.

Technically I now have WAC worked on 6 meters. However I restarted my DXCC count when I returned to the air in 2013, on which basis I still need Oceania. My best shot for that is a Sporadic E openings to KH6. From here these are rare but certainly workable, perhaps no worse than Japan but with fewer active 6 meter operators. I've had a few false alarms since with US call sign portability the only KH6/AH6/WH6 stations I've heard were within the continental US. However I know that some in this region have worked Hawaii this season.

Returning to the topic of spotlight openings, these openings to Japan, Hawaii and other distant locales cannot be so unique. There must be many more similar openings to the unpopulated or lightly populated expanses of the Pacific Ocean and northwest Asia. Without active stations those openings remain undiscovered. Certainly there are other islands and UA0 is hardly empty, but the population density is low and the ham density is lower still. We need a confluence of rare conditions and a dedicated group of hams attuned to the nature of 6 meter propagation.

Calling CQ into a silent band is not a bad strategy even though there is rarely a reply. Somebody has to CQ, so why not you? It may be a rare occurrence but every so often a strong DX signal will answer. Sporadic E DX propagation has existed for many millions of years and will continue long after amateur radio is just a memory. It's up to us to appreciate this gift and put it to good use.

Resolving Amplifier Arcing

A vacuum tube high power RF amplifier is full of high voltages. The large physical size of the tubes requires high voltage between cathode and anode to function. The combination of high output impedance and high power results in a high RF voltage (by Ohm's Law). Prevention of high voltage arcs requires careful design, component selection and operation. Those arcs are potentially destructive: to the tube, power supply and your ease of mind.

Arcing is more common in older amplifiers due to chemical degradation of insulators, insulator cracks and component warping from thermal and electronic stress, and the oxidation and burning of the contacts in physical switches. Although arcing is rare in solid state amplifiers since they operate at relatively low voltages that is no panacea since they suffer from other ills. But this article is about tube amplifiers.

Not long after putting my recently acquired vintage Drake L7 to work it suffered from intermittent arcing at the high end of its power range. Because the high DC plate voltage is independent of power level the arcing was almost certainly at RF. Although the arcing has been eliminated I expect it to return because the faulty part requires replacement. It's on my very long to-do list and it'll take a while until I get around to a implementing a long term solution.

If you've never worked on a high power tube amplifier my relating of how I investigated and resolved the arcing may be of interest. Despite the large size and lethal operating voltages and currents tube amplifiers are actually quite uncomplicated. With attention to safety and the peculiarities of dealing with high voltage, current and power they are not too difficult to work on.

Locating the arcs

The case of the L7 has no opening except on the bottom and rear for forced air cooling. These are useless as windows into the amplifier's interior. The case must be removed. This is the first problem in any properly designed amplifier. There are interlocks to prevent accidental electrocution and burns to budding technicians who are out of their depth.

The Drake L7 has two interlocks: one shorts the 3000 VDC plate supply and other (well hidden) prevents the amplifier from being turned on. Both interlocks must be disabled to operate the amplifier without the case. Before you try this, or even consider doing so, you must educate yourself about what you're attempting to do. Better still, have a knowledgable friend help you out. If you get yourself killed don't say I didn't warn you.

Yes, that's really a brick! I needed something flat, heavy and non-conducting to safely disable the high voltage interlock. The mains interlock (not visible) on the bottom is disabled with a chunk of plastic wedged under it. In the picture the amp is on and idling with a plastic (non-conducting) LED desk lamp for added illumination. Arcs are so bright that no ordinary lighting will wash them out.

I mentioned my suspicion about the loading capacitor in an earlier article. I dutifully straightened the multitude of plates until they maintained a decent gap for their full rotation. Unfortunately that repair resulted in no improvement. Hence the deep dive into the amp's innards.

As I increased drive the amp arced as expected but not where I expected. The loading capacitor sat quietly when the fireworks began.

It was the plate capacitor that was arcing. Although the spacing was generally good (and much wider than the loading capacitor) a number of rotor plates didn't track well. I straightened them without needing to remove the capacitor from the amp. There is an unrelated problem that will require its eventual removal for repair but that can wait.

Straightening the plates did not fix the problem. Worse, the arc location was seemingly random. Each one occurred in a different location. The arcs were evidently due to a fault elsewhere that caused an excess voltage condition beyond the rating of the capacitor. The capacitor is working just fine.

This is interesting so let's take a detour to review the design of a tube amplifier's output network.

Amplifier pi-network

The simplified schematic below is that of a typical pi-network found in many tube amplifiers. It transforms the high impedance of the tube output to the low impedance of the antenna system. The operator adjusts the plate capacitor to resonate the plate circuit and the loading capacitor for a high efficiency match to the impedance presented by the antenna system.

The resonance condition is a typical feature of an impedance transformation network, as previously covered in this blog. The circuit also attenuates harmonics since it is a low pass filter.

The blocking capacitor keeps DC out of the antenna circuit and the choke keeps the RF out of the power supply. Band switching (not shown) alters the range of the variable capacitors and the inductor value. The T/R switch (input side not show) bypasses the amplifier during receive.

RF voltage is determined by the power and impedance in accord with Ohm's Law: E = SQRT(PZ). From the data sheet for a pair of 3-500Z tubes a little arithmetic suggests there is approximately 3000 volts across the plate capacitor at 1000 watts RF output. The recommended capacitor rating is 4.5 kV because the impedance, and therefore voltage, can be higher depending on operating parameters.

On the high bands the L7 places a fixed capacitor in series with the variable capacitor to reduce the capacitance. This also lowers the voltage across each capacitor since capacitors in series act as a voltage divider. Because of this arcing incidence is greater on the low bands. However it is not eliminated. Something more dire is going on to cause arcing when the RF voltage is low.

The voltage across the loading capacitor is easier to calculate since the antenna system impedance is nominally 50 Ω. At 1000 watts the potential is a less than 300 volts. The voltage will often be higher when the SWR is greater than 1, which is very common for most hams. If the SWR is too high the loading capacitor with its smaller plate spacing can arc.

Sequencing

Since the capacitors appear to be in good shape and the output impedance is well within the acceptable range the problem must originate elsewhere. Most likely is a fault that affects the impedance at the output port. The antennas, external switches and transmission lines were ruled out by additional testing. High power can aggravate weak components and loose connections in an antenna system to create intermittent and permanent impedance changes.

By this process of elimination I focussed my attention on the T/R relay. It is common for slow or faulty amplifier relays to cause plate capacitor arcing when transmitter power appears at the amplifier input before the output relay contacts have settled. Unsettled contacts cause a momentary high impedance (open condition) at the output port. Once an arc starts it can continue after the relay stabilizes since the path to ground is always lower impedance than the antenna system.

The solution is sequencing to ensure the amplifier relays settle before power is applied to the input port. This can be coordinated with the transmitter, signal source (e.g. PTT in advance of transmit) and even with the amplifier itself by having the output port relay close faster than the input port relay.

The open frame relay in the L7 is typical of many vintage amplifiers. It iss slow at best, and with age the contacts are suspect. Replacements can be found but better solutions are available.

While arcing at turn on was occurring its incidence was less than that of arcing during a transmission. That is sufficient evidence to rule out sequencing as the cause of my problem. It does not mean sequencing isn't a concern, just that it isn't responsible for the observed behaviour.

Relay woes

I had good reason to suspect the relay. I earlier had to clean the contacts on the input and output of the bypass side due to intermittent signal attenuation on receive. With an ohmmeter connected to the centre pins of the input and output SO239 jacks you clean the contacts until you reliably read 0 Ω. Since the relay arms can shift laterally it is important to test for this by manipulating the relay arms.

The bad contact can be isolated by connecting one ohmmeter probe to the bypass bridge seen on the right. The output port is at the bottom of the picture and the input port is at the top. The centre arm applies tube cutoff bias during receive. Just my luck that both sides of the relay were corroded.

I had to resort to aggressive cleaning with an abrasive when a deoxidizing contact cleaner and non-abrasive buffing were insufficient. Do this only when absolutely necessary since abrasives can easily damage the thin contact coating, assuming there is any left (usually silver) after several decades of use. This is discussed in more detail by W8JI. I used a thin strip of 3000x sandpaper to be as gentle as possible. It worked.

Checking the through amplifier relay contacts is more difficult since an ohmmeter cannot be easily employed to measure resistance. Deoxidizing cleaner and buffing didn't resolve the arcing problem, but it did seem to reduce its frequency. Having gone that far I resorted once more to the sandpaper. After thoroughly clearing the contacts of debris I did another test. This time the arcing vanished entirely.

Permanent solution

I don't know how long the repair will last. The relay needs to be replaced. In any case it is slow and loud. Not only is that very annoying it is cause for ongoing worry. I don't want to take the risk of it failing during a contest.

Designs and even kits to replace T/R relays in vintage amplifiers are available. Some are fast enough to enable QSK operation. I don't need QSK just a solution fast, reliable and quiet.

Another problem with the existing T/R switching is the long lead lengths along the bypass path. On 6 meters it is enough (almost 0.03λ) to significantly raise the SWR. With 6 meter season in full swing I manually bypass the amplifier when I am not using it. Any T/R relay replacement will need to address this issue.

Vintage amplifiers are cost effective assuming you have the time and motivation to repair aging equipment and add modern features. My next amplifier will likely be a new purchase, one with a stiff power supply, silent operation and 6 meters. That way I can tolerate the quirks and faults of its older cousin at the secondary operating position.

80 Meter Stinger Version 2.0 (and Pipe Fitting)

You might think that because I write a blog about antennas and station building that I always do things right. I only wish! Perhaps I make fewer mistakes than some but I have my share of them. The original stinger for the driven element of my 80 meter array is one.

I was economical with the aluminum tubes and pipes I had on hand, saving the longer pieces for other projects. The stinger was 21' long, with 19' (6 m) projecting above the tower top. That was topped by over 3' (1 m) of 1" PVC pipe to get more height for the parasitic wire element support ropes. The total amount of aluminum and PVC above the tower was 7 meters long. That's a lot.

Although the stinger is guyed by the catenaries for the wire elements the tension produces a downward force (compression). It was obvious during setup that a butt joint between 1-½" x 0.095" aluminum tubes was not up to the stress. My hope was that it would last long enough that I could focus on other projects until I would have to replace it. All seemed well for a year as it survived one wind storm after another. Then the joint suffered a fatigue failure and the top half of the stinger fell down.

Stinger version 2.0

The stinger needs to be robust but not necessarily lightweight. The stinger must resist modest horizontal tension of the parasitic element catenaries. It must also be up to the compression force due to those same catenaries, especially with regard to high stress points where the yield point could be exceeded in a high wind and icing, or from tension imbalance among the 4 catenaries. It is laterally stabilized by those same catenaries which act as guys.

Being lightweight is beneficial when installing the stinger since it is long and must be lifted overhead to be dropped into the tower top. It can be assembled in pieces and raised from below at the price of more time and effort. As you will see I did a bit of both.

Rather than a butt joint between two lengths of 1-½" aluminum tubes the main improvement is a butt joint between two lengths of 1-½" aluminum pipe (1.9" OD). This allows me to use the existing 2" saddle clamps that secure the stinger to the tower plates. I also have a supply of these surplus pipes on hand and I know where I can get a few more at a good price.

Pipe fitting

My introduction to fitting pipes together for antenna construction was simply out of expediency: I had sources of cheap surplus aluminum pipe and tubes are expensive. I later came across the same idea in W6NL's book Physical Antenna Design (now out of print), which increased my confidence.

Although aluminum pipe is almost always 6061-T6 -- excellent tensile strength -- they have a seam, hard as it can be to find one on these aluminum pipes. They are theoretically weaker than seamless tubes although I have yet to see a seam failure. Indeed, some of my stock comes from commercial antennas that have survived harsh Canadian winters at great heights.

Aluminum pipe follows the same size schedules as steel pipe in the US and Canada. That's very convenient. Plastic pipe that is now in common use for water pipe and conduit -- ABS and PVC -- are similarly sized. I'll provide several examples of how pipes and tubes can be mated with respect to my 80 meter stinger, and additional ideas covered in earlier articles. There are other combinations of pipes and tubes that can work well together.

On the left are two schedule 40 pipes: a 1-½" pipe inside a 2" pipe. The inner pipe OD is 1.9" and the outer pipe ID is 2.067". The gap is 0.167", or 0.083" all around. Depending on the application the pipe can be simply bolted together. For improved rigidity a shim made of aluminum flashing can be used. An alternative is to increase the amount of overlap to reduce wobble, at the expense of greater weight.

The middle example is almost the same except that the outer pipe is schedule 80 with an ID of 1.939". That is a much better fit. Both are options to butt join two lengths of 1-½" pipe. I considered using the schedule 80 pipe until I found what is, to me, a more favourable solution. I prefer to save the schedule 80 pipes for my various yagi projects.

On the right is the 1" schedule 40 PVC pipe that is fit to a 1-½" aluminum tube at the top of the stinger as a non-conductive extension for the catenary attachments. The OD is 1.315" and the ID is 1.049". The 1" pine dowel purchased at a hardware store provides structural strength and fits well enough for the intended use.

This size PVC pipe will also fit well over 1" tubes. I may use PVC pipe as an insulator on the driven element 1" centre segments if I decide to use a beta match. Insulation is not needed for a gamma or T match.

Here are a few more pipe fitting ideas. When I used a 2-½" aluminum pipe to mate with 3" tubes for my first set of long yagi booms I had a machine shop turn down the pipe a few hundredths of an inch. The mast on my 150' tower is a 2-½" (2.875" OD, 0.25" wall) steel pipe slipped inside a 3" (3.068" ID, schedule 40) steel pipe used as the drive shaft for chain driven prop pitch motor. There the fit is so poor that I needed shims to prevent slippage caused by the large mechanical load. For one long yagi boom I fit 2" OD heavy wall tubes into both ends of a 2" schedule 40 pipe, which is a good fit. I bought several lengths of heavy wall 2" tubes to mate with these pipes to make booms for the 20 meter and 15 meter long boom yagis I am building.

There are other applications of pipes that I will discuss in future articles about those projects. Consult charts of pipe sizes and trawl through surplus yards for cost effective solutions in your antenna farm.

Stinger butt splice

In my junk box are Hy-Gain yagi parts that have been collected over the years. Hy-Gain booms are mostly 2" OD and spliced at the centre for the longer yagis. I have two of these surplus brackets. I tested a bracket on 1.9" OD pipes and was successful in achieving a secure fit despite the smaller size.

I butt spliced 10' and 7' pipes in the bracket. Holes were drilled through the pipes to make use of the bracket holes for that purpose in lieu of using the inner perimeter holes intended for a mast clamp.

I briefly experimented with plastic pipe as insulators to electrically isolate the pipes. I would need do this to attach a switchable coil to add 160 meters to the 80 meter array. I slit a scrap length of 2" white PVC pipe to slide over the pipe. It had to be slit since the ID is slightly less than 1.9".

At the end of the dressed pipe is a round insulator made of pressure treated lumber, cut with a hole drill, to provide mechanically robust isolation between the upper and lower pipes. The lower pipe does not need an insulating sleeve except perhaps to achieve a consistent diameter within the bracket.

I put the idea aside as not quite ready for implementation. It can be retrofit later. I first need to ensure high voltages between the pipes when the coil is active (not shorted) cannot jump the gap through the slit while maintaining high mechanical strength. It may be as simple as a wrap of thick polyethylene sheet and a seal to keep water out.

Upper stinger

Spliced to the upper pipe is a 7' (2 m) length of 1-½" x 0.095" aluminum tube. This is the only piece of the original stinger used in version 2. The 0.11" gap (the pipe ID is 1.61") is filled with a wrap of aluminum flashing coated in conductive grease. Stainless screws with nylocs hold it together. The 1" PVC pipe (1.315" OD) with its inner wood dowel are attached to the 1-½" tube (1.41" ID) in a similar fashion.

The original holes for attaching the rope catenaries are reused, and holes drilled through the wood dowel inside. The raw pine is protected by a cap of pressure treated wood, the top of which is sealed with caulk. The hose clamp adds tensile strength to the PVC and wood to better withstand the tension on the catenaries. The tension isn't high but I want to ensure years of trouble-free service.

The final stinger is ~2' (60 cm) longer than the original. This is intentional. I found that with my parasitic T-element design there is some slack on the vertical wire. The longer stinger removes the slack, and is easier than rebuilding the wire elements.

Installation

The new stinger was raised in two steps. The lower 10' pipe with Hy-Gain bracket went up first and dropped into the tower clamps until the bracket rested on the top clamp. The nice thing about aluminum pipe is high strength-to-weight ratio. The 10' of pipe with bracket attached is only ~9 lb (4 kg). It is easy to hold it vertical over my head as I fuss with inserting it through the tower clamps.

The catenary ropes are tied to the top of the stinger and detached from the wire elements in preparation for the next step. I briefly considered leaving the elements attached until I realized that the lateral tugs of those small weights would prevent safe lifting of the 17' long upper stinger.

With everything in place I lifted the upper stinger and dropped it into the Hy-Gain bracket. Once that was secured the elements were reattached. The complete stinger was then pushed up through the tower top and clamped in place.

Back on the ground I tensioned all the catenaries. The test for tension equalization was to have the stinger straight and in line with the tower. That was after the adjacent picture was taken.

Although a simple procedure it is tedious. I took an overnight break at one stage when I felt that I was too tired to do the lift with complete safety. A brief delay is preferable to an unnecessary risk.

Matching network changes

The new stinger's mechanical length is 2' longer and its electrical length is 3' longer. The reason is explained below. But having done so the new electrical properties of the driven element must be dealt with. For a λ/4 monopole on 80 meters the approximate rate-of-change (dF/dL) is10 kHz/6 cm; that's ~150 kHz lower due to the 3' extension.

There is no reason to make the driven element resonant at any particular frequency. The low impedance still requires a matching network, both as an omni-directional vertical and in yagi mode. I am using a switchable L-network.

My next task is to remeasure the antenna's impedance across the band, in both omni-directional and yagi modes. I will then use TLW to determine what changes are required. They should be small. I'll describe the details in my final article about this antenna, which will be written once it is complete and fully operational.

Catenary rope length changes

Changing the stinger height presents an interesting geometry problem: to lengthen the catenary ropes in a manner that keeps the parasitic elements vertical and preserves yagi performance. My first inclination was to ignore the problem since the change is quite minor. However it isn't difficult to check this on paper so I took that precaution.

Since the two sides of the right angle triangle are approximately equal each increment in height lengthens the full catenary (including the T-top of the wire element) by 0.7 increments; that is, 1.4' for a 2' height increase. However we only need to lengthen A to B, the distance from the top to the vertical wire, since we can freely add rope at the bottom. With A and B only 40% of the total length -- 10.5/25.5 -- we need only 40% of 1.4' or 6" (15 cm).

By not lengthening the top rope the wire element will lean towards the driven element ~4" (10 cm). As confirmed by modelling (and as you'd likely guess) this is negligible. Since I had no slack to lengthen the upper rope section I built and installed long insulators between the rope and upper end of the T shaped wire element. These are made from PVC pipe. The height of the vertical component of each wire element was increased by 0.8' (25 cm) -- 40% of 2' -- which took up all of the slack. Mission accomplished.

Returning to work

With the stinger rebuilt, better and stronger than before, work can resume on the yagi. All the parasitic switch boxes are installed and working. Tuning of the elements is partially done. Then comes the final step: the main switching system at the base of the driven element (tower).

Progress on the antenna has slowed due to more urgent projects, especially the 20 and 15 meter stacked yagis. Not to mention 6 meter DXing and otherwise simply enjoying the warm weather. With my 80 meter interest being DXing and contesting the 80 meter yagi is not urgently needed. It can wait until late summer, but may be completed earlier depending on circumstances.

In consideration of the weather and my busy schedule don't be surprised by a slowed rate of articles through the summer.

Potential for FT4 on 6 Meters

The E_s (sporadic E) season is well underway. It peaks at the solstice, which is less than 3 weeks from now. Although the season is reasonably long it is less so for DXing, which requires multiple clouds and long path lengths. When it does occur the openings are usually fleeting.

Openings for any one signal are often too short for most stations to complete an FT8 QSO. Bigger stations -- power and antennas -- or those in excellent locations do better. There are in principle two factors to be overcome in achieving DX success on 6 meters E_s:

• Weak signals: The combination of multiple bounces and forward scatter keep DX signals in almost all cases quite weak, even for those with big antennas.
• Brief opportunity: For the same reasons each signal can be in and out in less than one minute, the minimum duration for an FT8 QSO.

In the first case the challenge can be overcome with more power and more antenna. Unfortunately that is not practical for most hams, especially when it comes to towers and and antennas. But if you can do it you will see a tremendous improvement in your results.

In the second case you can increase the window of opportunity with more power, a bigger antenna or both. However, for the majority the greater opportunity is with modes that allow for quicker QSOs. That way you can exploit the propagation peaks and not just their long heads and tails.

The diagram is copied from the article linked above since I believe it makes clear the QSO duration challenge. See the article for a description of the diagram.

Traditional modes are fast, but...

Although CW and SSB are faster than FT8 (and even the twice-as-fast FT4) you are rarely in the right place at the right time. I discussed this "discovery" problem in an article last summer.

It is a major reason why FT8 has been so successful on 6 meters that it reduced CW and SSB activity by at least 80% in just one E_s season. You can't argue with success. Well you can but arguing won't put DX QSOs in the log. It's the only reason I made the move to FT8. Trying CW in a recent opening saw me through only 3 QSOs, including one Caribbean station I have not heard on FT8. SSB activity was a little better.

TEP, tropospheric ducting and (we hope) F₂ benefit less from the speed of FT4. FT8 is fine but then so are CW and SSB for these propagation modes. When signals persist the comparison between digital and traditional modes is little different than for HF. To give you my take on this I will merely state that I do not use FT8 on HF.

Expectations

I am intrigued by FT4. As of its latest incarnation the time slots are half that of FT8 (7.5 seconds), promising QSOs in half the time. Indeed it may be even faster since there is the possibility of fewer message repeats due to QSB during a lengthy FT8 QSO.

Despite not having used FT4 and the few reports I've heard from those using it I am comfortable predicting that FT4 most likely will be a good fit for 6 meters. In particular to E_s DX openings.

• Loss of a few db of sensitivity is more than compensated for by speed
• Ability to squeeze more QSOs out of brief DX openings
• Favours the small station better than FT8; while this may seem surprising consider the ability to better exploit brief propagation peaks

I have not yet used FT4. I am content to wait for it to stabilize. This is scheduled for July. That leaves me with enough time to play with it at the tail end of this E_s season. Others are using the beta software on 6 meters at its proposed slot of 50.318 MHz.

But more than a choice of mode my main concern right now is propagation. DX opportunities have been slim lately on 6 meters despite a promising beginning in mid-May. DX has been worked though nothing new or of especial note.

I listen most days with WSJT-X monitoring 50.313 MHz while I go about other activities. Between 6 meters and chasing 3D2CR there is enough DXing to fill the gaps in my busy warm weather schedule.

Fallow Summer

My on air activity is limited at the moment. This is partly due to the arrival (finally!) of warm spring weather. There is so much to do that the weather makes attractive, both radio and non-radio related. The other part that makes me relatively inactive is a lack of antennas. That may sound odd when you consider the number of towers and antennas that I appear to have. But there are reasons.

The new big tower was raised to 120' in early May with the help of Eric VA3AMX and my regular assistant Don VE3DQN. The top 20' are waiting for my decision on a rotator. I will then fabricate plates, machine the mast and select bearings. There is no rush since the stacked yagis for 20 and 15 meters that will go on it are not ready.

In the background you see the 150' tower. The TH6 and XM240 up top have intermittent problems and so they must come down. I wanted to do this before the hay got too high (and the ticks emerged) but is delayed until the hay is harvested. I could not arrange a full ground crew and rig the tram line in the short window after the new tower work was completed.

Once they are down and repaired the XM240 may end up in its original position atop the Trylon tower at 21 meters height, below the 6 meter yagi. However this depends on whether I can build and raise a new 40 meter yagi onto the big tower before winter. It will be a challenge. The TH6 and TH7 will be stacked at a low height on one of the big towers to cover North America.

The 160 meter vertical is disconnected and the radials rolled up, again in consideration of the haying. To simplify redeployment this year I left it tied to the tower top and secured the wires to a guy anchor to keep them from tangling with farm equipment.

My 80 meter yagi project suffered a setback when the stinger for the driven element broke and the top half speared into the ground. After putting it up I discovered that the column (compression) strength of the stinger was insufficient to withstand the tension on the rope and wire supporting the parasitic elements.

My hope was to delay replacement, but it must now be dealt with before I can resume work on the yagi switching system. The job is not difficult just annoying, especially since I was making good progress on the direction switching system construction and installation. In any case the low bands are not attractive during summer due to the high noise level.

Inside the shack there are also problems. One is that the recently acquired Drake L7 amplifier has a couple of faults. One is the T/R relay (a common failure point) which has intermittent high contact resistance on receive. My gentle cleaning temporarily cured the problem but now I must take more aggressive action. Eventually I will replace the open frame relay with something faster and quieter.

The second fault is the loading variable capacitor. Occasional arcing on the low bands led me to discover that several rotor plates are not straight and get too close to the adjacent stator plates. With the capacitor pulled pulled from the chassis I found that it had been previously worked on, and not very well. The repair is easy but time consuming. Again, there is no urgency since I am relatively inactive during the summer.

Last weekend was CQ WPX CW. Although it is not one of my favourites I wanted to play around for a while with high power. Since that wasn't possible and because of the antenna situation I opted to enter as 20 meter low power. It is perfectly possible to have a few hours of fun with a crippled station.

The one antenna I do care about right now is the 6 meter yagi. At least that antenna is trouble free. If only DX conditions were better. Sporadic E season started well but has entered a lull in this part of the continent.

I'll have more to say about yagi construction when I'm further along in that project. The bulk of the required aluminum is in hand and machining of the tubes has begun. I want the yagis ready to fly in August for testing and adjustment after the hay is harvested and it is easy to move around the fields. This project will keep me busy since with my lack of antennas (and sunspots) there is little incentive to spend time in the shack. Well, at least when 6 meters isn't hopping.

Reflections on Doing Business

Building a middling large contest station necessarily involves lots of spending. It adds up quickly. Consider the following (partial) list:

• Station equipment: rigs, amplifiers, switching, rotators, antennas
• Electronic components
• Materials: steel, aluminum, concrete, wood, PVC
• Fasteners: clamps, screws, guy hardware
• Services: welding, machining, heavy equipment, steel work
• Cables: coax, wire, electrical

That's a lot of business! Even when you build much of it yourself as I am doing there is an endless list of items to be purchased. For most of us who are not wealthy it is important to develop and work to a budget to avoid frustrated ambition.

Let's look at a few examples of doing business while building my station. There are lessons for us all, positive and negative.

Tower services

The commercial tower business is thriving. Most is done by large enterprises that have multiple locations and remote management. They are the least flexible in working with hams. Small local outfits are far easier to deal with. When I first moved to this QTH I made a point of dropping in to see the local tower service company to get to know them and see where they might be able to help me with my station. I knew a few hams who had hired them and were happy with their performance.

You might not be so lucky but unless you take the trouble to look them up you'll never know. They put in the foundations for both my big towers. Although there were a few problems they took the extra effort to resolve them. Some of these were due to miscommunications, such as their expectation that I knew more about dealing with heavy equipment better than was the case.

They have become a good source of surplus equipment and material from decommissioned towers and communications systems. I make a point of recommending them to others and I continue to give them them my business.

Aluminum

Buying antenna aluminum in Canada is a challenge. Although we have a huge aluminum industry the selection and availability of many types of aluminum tubing is often not friendly to hams. The 0.058" wall tubes that assist with telescoping yagi elements are almost unheard of despite being widely available in the US. These are considered aerospace material. Importing long tubes is far from economical.

On the other hand aluminum tubes and other shapes are widely available and economical if you are willing to improvise with more common sizes. I've modified my yagi designs in accord with local availability. All I'm then left with is finding the best company to work with.

My first experience several years ago was barely adequate. The company that seemed best had a limited selection and they only dealt in full lengths, typically 20'. Cutting to size was expensive. They catered to industry and barely tolerated retail customers. But the prices were excellent.

Several months ago I went to another firm that makes a big deal of their selection, cutting to small quantities and walk in retail customers. Again, the result was not great. For my second order I selected a small quantity of short tubes to test in my workshop for suitability to build yagis. Every tube in that order was wrong, either OD, ID or alloy. I gave up on them for good.

I returned to the first company. On the phone they were far more receptive than I remembered so I dropped in to see them. What a change. They worked hard for my business including hunting down non-stock items and discounting the price for the quantities I needed. The order was filled to perfection.

When I had difficulty loading the tubes into my car the company president offered to deliver it to my place, despite the bother and the 100 km distance. It was delivered in good order the very next day. On being asked to think of them in future I told him I certainly would.

Like people, companies change. Don't be afraid to try again long after a poor experience. Companies that survive must change to stay competitive.

Wire and cable

In my station there are ~500 meters of coax, 2000 meters of wire, 1300 meters of guy strand, 500+ meters of control cable, plus guy grips, junction boxes and much more. I buy very little of it at the better known retail outlets. Some was bought used, most was bought new and I've acquired quite a lot for free or close to free.

After shopping around I do most of my buying at a local branch of a province wide supplier of electrical equipment to trades and industry, including power companies. Their prices and selection are good and they make an effort to special order what they don't stock locally or their central warehouse. They are happy to sell small quantities to retail customers, although they prefer large quantities.

When there have been errors and other problems they always try to make good, albeit grudgingly at times. I'll keep going there for as long as the good experiences predominate.

Welding and machining

There are welders everywhere. In rural areas like mine there is a sign hanging out front every few kilometers where small operators work out of their homes. There are almost as many commercial enterprises with 2 or 3 people. Then there are the large companies that cater to industry. Machinists tend to be collocated with welding shops since they are closely associated despite being distinct trades.

I went by recommendation rather than hunt one on my own, choosing the shop that did the welding for the guy anchors on my first big tower. The proprietor is a machinist and his partner is a welder. This is where I went for my custom guy yokes, tower load bearing plate and trimming aluminum pipe for booms.

A misunderstanding led to incorrectly bent plates for the yokes, which they corrected by making new ones from their own metal stock. Aside from that error the quality of the work is excellent and the prices reasonable. I intend to return to them to fabricate steel plates for the mast bearings and rotator needed for the new tower.

Specialty markets: retail and swap nets

The specialty amateur radio market in North America is going through a secular shift as older generations age out and the type and quantity of business changes. In Canada several ham market retailers have closed down in recent years and the ones that continue have branched out into other lines of business to survive. It's a tough sector to be in.

At a flea market this spring it was obvious that our hobby is approaching a crisis. A friend and I surveyed the crowd and saw an almost uninterrupted panorama of male grey heads. That isn't sustainable. Among the items on my table the old stuff interested old people. More recent technology drew in younger hams. By younger I mean in their middle years.

It won't be long before the smaller ham flea markets fade away. That's a shame since they can be very enjoyable events.

Online swap shops suffer from the same ills of all online fora: swindlers. This was far less common when swap shops were in print or on air. The bad actors are mostly small time criminals and scammers that infest all branches of online person-to-person commerce. A few are hams. Due diligence is required on every deal, whether you are a buyer or a seller.

Lessons learned

It is well worth the effort to find and stay with companies that are reliable, fair and willing to take the trouble to keep customers happy. For most of these companies the business from a ham is small in comparison to their industrial clients. Since many can't be bothered with small retail customers it is important to appreciate the one that do.

Although ugly stories abound most people are reasonable and reliable. Doing business can be enjoyable with the right person or company on the other side of the deal. Don't let yourself be ruled by cynicism: many people are their own worst enemies. When you mostly have bad experiences doing business it's probably you not them. Don't be one of "those" customers.

Strive to build strong long-lasting business relationships. Treat a first deal like a first date. Trust builds over time. Excuse a few lapses since we all make mistakes. But move on when it becomes a habit.

Do you need a recommendation for a supplier or service? Ask the people you've come to trust doing business with. One of the reasons they're good is that they rely on other good companies. They'll lead you to the good ones. That's how I've had success finding the dealers and services I've come to rely on.

Prime Time 6 Meters

Summer is coming.

The solstice is less than 6 weeks away and E_s (sporadic E) season is well underway. As I tell everyone I know with a passing interest in 6 meters mid-May is when the DX starts to roll in. Indeed the DX has been heard here since early in the month.

This is the DX I've heard so far on 50.313 MHz and 50.323 MHz FT8:

• Europe: EA, F, CT
• Africa: EA8, 5B
• South America: PY, YV
• North America: VP9, J8, KP4

Some signals have been excellent and in for several minutes at a time. Here is one example:

I have not worked any DX as yet. The interesting stations have been weak or fleeting while the strong ones I've worked before and did not want to get in the way of the many trying to work them, perhaps for a new one. Further east in W1, VE1 and VO1 the openings have been better than ours.

Chances are you have an HF transceiver with 6 meters. Try loading up any HF antenna with the ATU and give it a try. It doesn't take a lot of power or a lot of antenna to have some fun. This is prime time 6 meters. It will continue until early August, peaking in late June. Go ahead and give it a try. If you are already active on 6 meters be on the lookout for those elusive DX openings.

Have fun and I hope to see you on the magic band.

Yagi Elevation Angle Nulls: Tilt & Fill

A common source of ideas for this blog is other hams. When a recurring question came up recently I thought that it would make for an interesting article. The question is: can you fill in those elevation angle nulls in a yagi pattern by tilting the antenna up? The short answer: no.

More interesting is why it is true since it can teach us about how antenna patterns are what they are. In this article I'll dig a little deeper and discuss its implications, including how to go about filling those nulls. I believe it is worth the effort since you can be held back in the achievement of your operating goals if you ignore pattern nulls.

Yagi tilt in free space

Ground is responsible for the existence of elevation nulls. Before we go into that we should have an understanding of a yagi's free space pattern where ground plays no role. Only then should we bring ground into the picture.

I am using EZNEC to do the models and pattern generation. I am use its features to easily rotate antennas and to compare patterns by overlaying them on one plot. Since I have 6 meters on my mind these days I returned to my optimized A50-6 model to provide the examples. It has enough gain that the main lobe is not too narrow nor too wide, helpful in illuminating the current topic.

In free space the main lobe (looking forward along the boom) is a bulb shape. With no ground reference the 15° tilt has no effect on the pattern. Of course the same is true for any amount of tilt, for any direction or boom rotation. Just as for astronauts in space there is no up or down.

Ground reflection

The pattern of a horizontally polarized antenna over ground is the sum (interference pattern) of the sky wave and the ground reflection. The relative phase and amplitude of the two determines the flux in every direction. Since we cannot transmit a signal into the ground we can safely ignore the half sphere of the pattern below a plane tangent to the ground. EZNEC automatically trims it.

We can now return to a comparison of the two antenna patterns. Because height affects how the ground reflections sums with the sky wave I am using the actual height of my antenna, which is 24 meters (80'). I've made the elevation plot large and used a 1° step size so that detail can be seen.

There are a few features of the pattern comparison that stand out. First, gain of the tilted yagi decreases at low elevation and increases at high elevation. This should not be a surprise. The antennas have equal gain at 35°. The second and perhaps most important feature is that the elevation angles of the nulls are identical. All that has changed is the depth of the nulls, which are shallower for the tilted yagi. The third and equally important feature is that the nulls of the tilted yagi are increasingly shallow at high angles.

To understand what's going on we need to refer back to the free space patterns. Gain is equal for equal positive and negative elevation angle deviations from 0° elevation due to pattern symmetry. Therefore when ground is parallel to the yagi ground reflections are therefore of similar amplitude for equal positive and negative angles. Reflection gain (nominally 6 db) and null depth depends on ground quality which determines reflection loss and, at low elevation angles, reflection phase shift.

Maximum gain and maximum null depth occur when the amplitude of the reflected wave is equal to the sky wave. Lobe peaks are at elevation angles where the phase difference is 0° and nulls occur where the difference is 180°. The higher the antenna is above ground, as measured in wavelengths, the greater the number of minor lobes and nulls. The 0° null is due to phase reversal of reflections at low incidence angles over imperfect ground.

Tilting the antenna does not affect behaviour of ground reflections. What does change is the amplitude of the reflections. Since the yagi main lobe is no longer symmetric with respect to 0° elevation the sky wave and ground reflection amplitudes are no longer equal for equal positive and negative elevation angles. For the modest 15° tilt being examined the inequality is greatest at high elevation angles.

Suffice to say this is not what we want. Tilting a yagi upward not only doesn't fill the nulls, except at the less useful high elevation angles, the gain at low angles is reduced.

Terrain

This analysis assumes flat, uniform ground. Complex terrain introduces complex ground reflections that shift the positions of the lobes and nulls, and their heights and depths, respectively. However, tilt still has no particular advantage since the lobe and null elevation angles remain as they are. Ray tracing tools such as HFTA can provide insight into how the terrain affects the elevation pattern.

Ground quality determines the amplitude of reflections, including the elevation angle below which reflections are phase reversed. The latter is why gain is zero at 0° elevation. For horizontal yagis like yagis the reflection amplitudes even over poor ground are reliably strong. Not so for vertically polarized antennas, but we're restricting this discussion to horizontal yagis.

Stacking

A common technique for increasing gain is to stack two or more yagis. Power is split among the antennas with phase set to achieve the operating objective. In almost all cases the yagis are fed in phase.

Another desirable characteristic of stacks is to move or reduce elevation pattern nulls. This is done by taking advantage of the different heights of the yagis (not applicable to side-by-side yagis). The lobes and nulls are different for each yagi alone and when two or more yagis are used. We'll keep it simple by restricting the discussion to two yagis in a vertical stack.

I've kept the first yagi at 24 meters height and added an identical one at 18 meters height, which is 1λ separation, the same as the boom length. Equal separation and boom length usually works well, and in any case we are interested in the general pattern rather than maximum forward gain.

Green is the upper yagi, red is the lower yagi and black is the stack (BIP: both in phase). Stack gain is as expected at about 3 db. The higher yagi has more minor lobes due to the greater height. For the lowest inter-lobe null (marked) the difference among the three is only very small at a little over 1°. The difference increases at higher angles. Unfortunately that first null is pretty stable for all configurations of the stack, so our objective is not met since the lowest null is the most critical for DXing.

The reason for the lack of movement of the lowest null is that the ratio of yagi heights is only 1.3. Small height ratios are typical at VHF and above, while at HF larger differences are the norm. The small ratio is good for gain but not for moving nulls. If this is important at VHF it is desirable to have a second antenna at a lower height. For example, to work single hop E_s at high angles and DX at low angles, as covered in a previous article.

Space

Now then, what about antennas that must tilt upward to target satellites and the Moon? I rarely hear talk about the effects of ground reflections in these communications modes, yet they can be important in some situations.

If you point a high horizontal yagi well above the horizon there is no problem. Ground reflections are negligible because the main lobe has a narrow beam width and little radiation is directed towards the ground. However there are ground reflections for moderate and low gain yagis that enter the picture for satellite passes less than ~30° above the horizon.

Circular and vertical polarization are immune from deep nulls. But many small satellite antennas are linearly polarized, especially those that are hand held. Hand held horizontal yagis are actually less of a problem on low passes because they are close to the ground and the nulls are at higher elevation angles.

So, not a big problem overall. When a null is encountered it tends to be a transitory phenomenon (the satellite is in motion) that may be mistaken for bad aim or incorrect polarization. By the time you adjust the antenna the satellite would have already moved out of the null.

Anyway...

Tilt seems a simple solution to dealing with nulls. Like many simple solutions to difficult problems it does not work. Besides which it isn't easy to accomplish. Do you really want to spec the mechanical design of an elevation rotator for a long boom 20 meter yagi? Leave those elevation rotators where they do something useful: satellite and space communication.