Saturday, October 31, 2020

Prop Pitch Motor: Motor Repair

For your Halloween treat we're going to disembowel a prop pitch motor. Don't worry, it won't be too gory! We begin with the emergence of trouble atop the high tower one dark and stormy night...

When the prop pitch motor was first installed on the new tower with the 15/20 meter stacks I mentioned that it turned slowly. After doing resistance checks on the cable and the motor there appeared to be a faulty internal electrical connection. The problem was dirty contacts between the motor contacts and the receptacles on top of the gearbox. This I was able to fix with contact cleaner, tiny swabs and correctly torquing the bolts on the motor retaining ring.

A guide to the motor wiring can be found on K7NV's web site. There should be lettering on the motor housing denotes the motor as right hand or left hand, and the motor voltage.

Trouble signs

Unfortunately the trouble didn't end there. After several minutes of bench testing with a 13.8 VDC supply the motor noise I had already been aware of became worse and the motor repeatedly bogged down. When I connected it to the 24 VDC controller the deterioration accelerated and within two minutes the motor pretty well ground to a halt. This was accompanied by loud squeaking coming from the housing. For a motor running at close to 10,000 rpm any mechanical resistance can be catastrophic.

The failure was accompanied by excess current draw from the controller which, I discovered, is inadequately fused. The audible sizzling from the overheating was so apparent that I shut it down before the power supply failed. The ham who built the controller back in the deeps of time must have been an optimist.

With no backup on hand I had to take on the unwanted task of pulling the motor apart. I had some trepidation because these motors are becoming increasingly rare and expensive and I was unsure whether I'd be able to diagnose and repair it. With no readily available alternative I went to work.

Disassembly and testing

The prop pitch motors used as rotators were designed decades ago, with the originals dating to WW II. They were used on USAF bombers and other aircraft. The motor typically runs on 24 VDC, which is the aviation standard. We call it a motor but it is a motor plus a gearbox. It is the gearbox that enables it to be used as a heavy duty rotator.

DC motors with commutators and brushes are no longer common and parts are not always easy to find. Some hams have replaced it with an AC motor of similar RPM and power. Commutator motors will operate at lower voltage, but not too low or the motor can bog down and draw excess current. Luckily my motor is in good condition with none of the unique components requiring replacement.

The first stage of disassembly is to separate the motor and gearbox. The motor can be powered using the pins on its base. I used a high current 13.8 VDC supply on the workbench for testing during the repair process.

Removing the 3 bolts that hold together the two halves of the motor housing is not enough to pull them apart. The bearings at each end of the motor shaft are press fit into the housing halves. Axial force is needed to separate them. Levering too forcefully from one side can damage the shaft. Without a specialized tool for the job I used two pries between the upper housing and the shaft end to achieve a net axial (vertical) force. I moved the pries around the shaft in small steps as I worked the housing apart. Once separated the drive end of the shaft can be pushed out of the housing with a few taps of a cushioned mallet. Don't lose the thin shims that come tumbling out.

That done you have the armature and bearings removed and you can access to the brushes and field coils. Inspect the commutator and brushes for excess wear. If the brushes are excessively worn replacements can be had from a good quality motor repair shop. If the commutator is worn you may have no alternative other than to replace the motor.

The armature and field coil surfaces were pitted and coated with debris but otherwise sound. I cleaned them up to make myself feel better although it doesn't really accomplish much. Have a look at K7NV's web site to see what a pristine restoration looks like. Very pretty.

The brushes were inspected and found to have lots of life. I removed the easily dislodged debris (mostly carbon dust from the brushes) with a soft brush to avoid damaging the coils. The sides of the armature were cleaned with 400x sandpaper and then carefully brushed away the fine particles. Keep the motor interior clean of metal dust.

The brushes are bypassed with ceramic capacitor on bottom of the housing (not shown) which largely eliminate RFI generated by the rapid circuit interruption as the commutator rotates over the brushes. I cleaned and checked them. All were good.

The input shaft of the gearbox should turn easily by hand. Use a wide blade screwdriver to make testing easier. With a suitable coupler that doesn't damage the splines you can perform a higher speed test with a drill. I was satisfied by a hand test alone.


Now we come to the crux of the fault: bearings. In the picture above you can see the non-drive side bearing and in the picture below is the drive side bearing below the commutator. The upper bearing is a shielded (not sealed) 3201 (if I remember correctly) and the lower bearing is an open 6200.

Both bearings ought to be sealed. There is a lot of carbon dust and other particulate debris inside the motor. The shielded bearing tests good so I'll leave it there. The open bearing was dry and pitted and took force to turn. A little machine oil freed it but could not repair the pitting. 

This is not the original bearing and whoever put it in there made a poor choice. Sealed bearings cost only a few dollars more. It pays to check the specs since the RPM rating decreases going from open to shielded to sealed. The rating of 6200 shielded bearings from reputable manufacturers exceed 10,000 RPM, which is the minimum rating for this motor. Check the low temperature rating if, like me, you have cold winters.

Within days I had a new 6200 sealed and cold weather rated bearing for less than $10, including tax and shipping from an Ebay merchant located in VE2. The old bearing was given a shot of penetrating oil and pulled off without trouble. A little more oil and the new one was pressed on.

I should mention that both the 6200 and 3201 are shown with metric dimensions in all the catalogues I checked. This can be deceptive since they often round those with English dimensions to the nearest millimeter. A friend checked his US military parts list for the small prop pitch motor, dated 1944, to check the dimensions before I placed my order. I suspected an unsuitable substitution. It is very unlikely that the USAF specified a metric and Japanese bearing during WW II! Unfortunately the specified bearings were only listed with proprietary part numbers from long gone manufacturers. Further research into the bearings was not practical.

I reassembled the motor and powered it up. It spun nicely with 13.8 VDC and the current draw was 2 amps lower than with the bad bearing. With this positive sign I did another and longer test in the shack with the 24 VDC controller. Motor heating was nominal and the current draw was very good. Back on the gearbox the motor hummed along at a more typical 7.5 amps. The gearbox resistance (mechanical loss) increases motor current.

I have some concern about the grease in the gearbox despite it testing well in the cold and not having excess mechanical resistance. I may tear it apart next year if only to ease my mind. There is no time for this preventative maintenance before winter.

Current draw

Commutator DC motors are interesting devices. Their theory of operation is well outside of my areas of knowledge. For example, I hadn't realized that the starting current is quite high at around 20 A. It is a bad idea to operate the motor by switching high current DC, whether by manual switch or relay. The high starting current can and do weld together relay contacts. That can cause a lot of grief up the tower because the motor will not stop.

The controller I use switches the AC line, which is safer. The CW or CCW direction is selected by switch before activating the rotation lever which turns on the power supply. Commercial controllers designed for prop pitch motors use other means to deal with the starting current. I will have to keep this in mind when I design or purchase a better controller for my two prop pitch motor rotators.

As with any motor there is a kind of torque curve. The motor will operate quite well with less than 24 VDC, and even a little more if you're careful, but not at peak efficiency. The field coils and commutator are optimized for a smaller range of speeds. The current draw is not very different when driven with 13.8 or 24 VDC. Driving a load at the lower voltage will further decrease efficiency and increase motor heating. 

I suspect the greatest risk is to the bearing lubrication, brushes and commutator, though perhaps not in actual usage since on periods are typically under a minute. Risking motor failure is inadvisable unless you have a ready supply of spare parts or spare motors. They haven't been manufactured for decades.

One way to limit excess current draw is to not use the largest wire gauge you can afford. A modest amount of extra resistance puts a ceiling on the current draw even in the event of a dead short. I am using 125 meters of AWG 10 wire for the motor (3 conductors). The DC resistance of the 250 meter long return circuit is 0.8 Ω (confirmed with an ohmmeter). 

With a dead short at the motor the current draw at 24 VDC is limited to 30 A. This drops to 19 A for the 1.25 Ω of AWG 12 wire. These figures assume the power supply voltage doesn't drop under the high current draw. If it does the power supply can overheat and fail due to its internal resistance unless there is over current protection. Of course with greater wire resistance the voltage will drop at the motor terminals and mast rotation will be slower. 

There is no one correct answer to the system design. There are several parameters to consider.

Weather delay

It's cold, really cold. We are setting record lows. This is not weather in which I want to do tower work at great heights. Once the weather warms up again I will remount the prop pitch motor on the tower. That is also when I will deal with the upper 20 meter yagi of the new stack. Yes, it's up and mechanically secure though not tested because I had to remove an element during the lift. 

The weather is delaying a lot of work that needs to be done. I want the new stacks operational for CQ WW CW at the end of November. There is little chance they'll be ready much earlier. 

Being ready includes a working prop pitch motor for the rotator. At least it's working, and that's something. I would have preferred this lesson in prop pitch motor repair in warmer weather. However, when it comes to maintenance you rarely have a choice and most repairs are done in winter when the station is stressed by the activity spurred by the cold keeping hams indoors and by the schedule of major contests.

Thursday, October 29, 2020

More Thoughts on SSB Contests

When I got my phone license at 16 years of age I was understandably mic shy. Most everyone I spoke to was an adult and that was an unusual experience for a teenager. DXing and contests made it easier since there was no expectation of holding a conversation with all those "old" folks. As my comfort with the mic improved I quite enjoyed the ease of conversing on the air, even with the old folks. 

The popularity of SSB contests is in part due to that ease: almost every ham is able to speak. With short and often fixed exchanges language is a minor barrier for contests. And humans sure do like to talk!

Unfortunately SSB is a victim of its own popularity. With a 2.7 kHz bandwidth and thousands of stations piling into limited spectrum a popular SSB contest can cause quite a mess. CQ WW SSB, the most popular contest of all, is especially difficult. There's an endless number of stations to work but most are very difficult to hear, and the ones you hear often can't copy you. It is frustrating.

When you operate in CQ WW SSB with less than legal limit power and excellent antennas success can be elusive. This is not the case with CW and digital modes. Narrower modes allow some elbow room for less dominant signals to be heard well. You can even run with QRP. The same is only true of SSB during solar maxima when there enough open bands to allow everyone to spread out. Of course this is only partially true since you must eventually squeeze onto every band to work more stations and multipliers.

I have written about SSB contests before, more than just once, especially years ago when I did it with QRP.  Although I have two world #1 plaques for QRP in CQ WW SSB I gave it up because it was so difficult and because declining sunspot numbers forced me onto the low bands where SSB QRP is quite painful. Even during my winning years I spent very little time on 40, 80 and 160, only spending enough time to gather what multipliers I could and largely limited to trying my luck with the biggest of the big guns. It was futile to attempt anything more.

With this dismal introduction and another CQ WW SSB in the bag let's look at a few things to consider for those of us who are not the biggest of the big guns.


Power has an outsize impact on SSB. When you run low power you may be no more than 10 to 13 db below the high power stations yet it will seem far worse. Because the QRM and QRN are so fierce with the wide bandwidth of SSB the received SNR (signal to noise ratio) can be very poor with 100 watts. Solid copy with a kilowatt may be no copy at all with 10 db less power.

This is a threshold effect that I previously discussed with respect to QRP where the challenge is far greater. It can take a brave ham to venture into an SSB contest with QRP! Low power contest entries -- typically 100 watt maximum -- are somewhere in, what I've termed, the muddy middle. Many are surprised at how difficult SSB contests can be with low power when it can be very successful in daily use. Look at the QSO and multipliers of the top high and low power stations in a major SSB contest and the difference is stark. For the same power categories the spread is less in CW contests.

At least with QRP you know SSB contests will be difficult and slow going. Low power is deceptive. I am surprised by how many experienced contesters are surprised by their poor results with low power. Of course you can use big towers and antennas to compensate, and it does help to a degree. It isn't easy to boost your signal 10 to 13 db with antennas alone. An amplifier is far easier and cheaper.


This is another topic I've written about before. On SSB there are language and accent differences that can make copy difficult. Although English is in many respect the universal language on SSB many hams have only a rudimentary grasp of it. Their English may be limited to phonetics and a handful of useful phrases. It is perfectly possible for a non-English speaker to be successful in an SSB contest, especially CQ WW with its simple and (to be honest) predictable exchange.

For English speakers like myself -- which probably includes you if you are reading this -- there is an obligation to make it easier for others. Use standard phonetics or common variations that in your experience are successfully understood by all. Learn the numerals of several common languages and you'll complete contest contest exchanges faster and more accurately. 

Spanish is the one I find most useful, and there are others. Don't be afraid to experiment with others! There are only 10 numerals to learn and your effort will be appreciated by other hams. Try not to be too glib with more words and phrases or the other operator may believe you speak their language and will reply with full sentences and then you'll be lost. Stick to numbers and you'll do just fine.

Phonetic usage

Months ago I shortened one of my standard voice messages. If you're familiar with N1MM Logger+, this is the message that goes in F3 when you run using ESM (enter sends message). This is the message that logs the QSO, confirms completion to the other station, and solicits the next contact.

"Thanks! Victor Echo Three Victor Norway"

Simple, short and it does the job exceptionally well. All I had done was to change "Victor Echo" to "V E". It caused me a lot of grief. The problem was due to using high power and a good antenna. When you do this the runs continue without a break. As a result I rarely sent a CQ. The CQ uses complete phonetics for my call.

Some of those who tuned across me heard "V A". They sound much alike on SSB where fidelity is less than perfect and is worsened by QRM and poor SNR. Indeed, that's why we use phonetics. The miscopied call was occasionally spotted and I would get a run of dupes. My temporary workaround was to interrupt F3 and speak my call with full phonetics.

My reasoning for the shortening was to save time by cutting two syllables. While running low power it worked well since my runs frequently dried up and I'd CQ with full phonetics. Time was saved and there was little risk of a misunderstanding. I'll have to revert to using full phonetics. Becoming a big gun requires a different approach to SSB.

Vocal endurance

Many hams in their eighties have no difficulty sending CW at high speed. A modest loss of manual dexterity is compensated by decades of practice. With the extensive contest use of software generated CW even that may not be necessary. The same cannot be said of SSB, and that's a problem because as we age it can be difficult to impossible to keep our voices going for 48 hours.

Voice memories reduce the need for vocal endurance. Unlike CW, unfortunately, speaking into the mic is difficult to avoid. In CQ WW there is the need to speak the other person's call. In other contests there are serial numbers and other exchange information that is not constant throughout the contest. When a repeat is requested alternative phonetics and syllabic emphasis are helpful, as is using the other operator's language for critical items, especially numerals as mentioned earlier.

For those who have lost their vocal endurance the alternatives are multi-ops, so that they operate fewer hours, or to use technology to voice serial number and call signs. The latter is improving and no longer has to sound like a freaky robot. Expect to hear more of it in future. 

Although I've not reached the age of lost vocal endurance I am beginning to think about the aids I'll eventually need. It is funny to realize that as a little pistol I could operate CQ WW without using the mic for long stretches. In search and pounce (S & P) you can get by without the mic by recording just two messages: call sign and exchange. You rarely need to speak other than to vary phonetics or syllabic emphasis for difficult contacts.

40 meters

The globally common portion of the band is only 75 kHz: 7.125 to 7.200 MHz. That's a problem. With a 2.7 kHz bandwidth only 30 or so SSB signals (and clean ones at that!) can coexist without overlap. Yet there are thousands active in CQ WW. When the sun sets this band segment is a wall of QRM. Big guns are rattling out a constant stream of CQs and answering almost no one because only the strongest signals can be reliably copied. Other CQers are deep inside the noise, mostly unheard.

I have worked DX multipliers in CQ WW SSB with QRP, although not many. I would wait for the second evening when rates declined and I had less competition. Even so I needed excellent propagation to have a chance with a wire dipole or loop for an antenna. At sunrise, if I got lucky, I'd work a KH6 and a W6/7 in zone 3 and that would be it.

To deal with the narrow spectrum there are ways for a Canadian to do better. One is to operate below 7.125 MHz where there are no Americans; our mode permissions are less strict than in the US. This allows us to work more DX with less QRM. To work American stations and other North and South American countries you operate above 7.200 MHz. Americans and Europeans will often work split, with Americans calling above 7.2 MHz and announcing a listening frequency below 7.1 MHz. Europeans do the reverse.

Compared to years past when broadcasters outside the Americas were to be found down to 7.1 MHz and sometimes lower (remember Radio Tiranha?) it's actually better today! That isn't saying much.

160 meters

This band is like 40 meters for SSB, except worse. All modes must coexist so when there's a major SSB contest on 160 it can be impossible to have non-contest QSOs on CW, digital and SSB. Antennas are often narrow bandwidth because they're small and that restricts operating frequency. Many sponsors of SSB contests exclude 160 meters because of these problems.

Amateur radio does not have exclusive access from 1.8 to 2.0 MHz in many parts of the world. Although not as restrictive as in the past due to LORAN and other services moving away it is still helpful to be aware of the restrictions some countries face. A notable example is Japan which, until recently, had strictly limited access. Now it's a little better and JA stations have allocations where we like to operate in the lower part of the band so that we don't have to work them split.

The high atmospheric noise on 160 meters makes high RDF receive antennas almost mandatory on SSB. You can only narrow your receiver bandwidth so much before phone signals become unintelligible. The only resort is a directional receive antenna to improve SNR. SSB DXing on 160 with an omni-directional antenna is neither enjoyable nor productive. 

Yet the big guns keep CQing, as they must, to make all the contacts they can. The rest of us can at least work them if nothing else.

Casual operators

There are more casual contest operators to be found on SSB than on CW. Make an effort to sound welcoming on the bands and at the times when many hams habitually turn on their rigs and they will call you. They may not know the contest or the exchange but they are happy to oblige you with a brief QSO. 

Succinctly explain what information they need to exchange and you'll both benefit from the experience. However you will need to know enough to help them out. For example, when a ham can only tell you they're in Ohio, do you know whether they're in CQ zone 4 or 5? If you can't tell them the correct zone they can't tell you, and that means you don't have a valid QSO. Be prepared.

Another advantage of giving a quick lesson on the contest exchange is that there are often lurkers listening. It is quite common that I get a few casual operators calling me after I explain the exchange to someone. A helpful and friendly SSB contester is a successful SSB contester.

Opting out

Although I don't dislike SSB contests they are not favourites. With my summer antenna work incomplete and major tower work in the immediately preceding days the CQ WW SSB contest was far from a priority despite its importance on the contest calendar. I was too tired and my score potential too poor to be bothered.

I took it easy and got on when I was so inclined. A geomagnetic disturbance made the contest a challenge on the low bands. For me the contest was an opportunity to practice running and observe propagation under unusual conditions. Openings on 15 meters were spectacular and running Europe was quite easy. Even 10 meters made itself felt. In contrast 80 and 160 were poor, and without a 160 meter antenna at present the latter band was irrelevant to me even had conditions been good.

It isn't necessary to operate every major contest or to do so competitively just because contests are one's primary interest in amateur radio. If you do it because you feel you must it becomes more of a job than a hobby. That's a recipe for losing interest in what you love to do. When the feeling isn't there, there is no shame in stepping back and being a spectator rather than a competitor. 

For me SSB contests are best enjoyed with others as part of a multi-op team, something that is difficult to impossible at present. As my station evolves I plan to host multi-ops for contests like these. By then the pandemic will be over and we can once again gather with friends for a weekend of SSB fun.

Sunday, October 18, 2020

Presentation: DX'ing on 6 Meters With FT8

Those of you who have followed this blog for a while will know that I have a special passion for 6 meters. It has been so for many decades. Nowadays my primary focus is on 6 meter DX. When sporadic E season rolls around each year you'll find me on 6 meters, and not on HF.

In early 2019 I did a presentation about 6 meter DX'ing using FT8 to a local group of contesters and DX enthusiasts. I was subsequently invited to repeat the presentation to several local clubs. If you've heard more 6 meter activity from FN25 and surrounding grids I may be partly responsible.

For the RAC AGM this fall they took the advantage of it being done by video conference to organize a few hours of presentations preceding the business segment of the AGM. It was a learning experience for all and for the most part it went well. There were a few technical problems related to running several concurrent webinars using Zoom. My webinar was abruptly interrupted halfway through.

I recorded the audio of the second half of the presentation which was coordinated with the slides by one of RAC's technical gurus and spliced onto the first half video. It is now available for viewing on YouTube. For those interested it is embedded below, or you can find it on the RAC YouTube channel. It's just over 1 hour long.

Since I rarely put my picture anywhere on this blog it is an opportunity to see what I look like, if you care. I deliberately avoid including pictures of myself on the blog so that the focus is on the topics being discussed and not on myself. Too many people, hams and others, turn blogs and their social media channels into vanity projects. That I disdain.

Be sure to watch some of the other interesting presentations from the conference. One in particular that is especially interesting to me is by my friend Chris VE3FU/VO2AC about their DXpedition to VO2 for the CQ WW 160 meter contest. That was quite the adventure, and they didn't do it just once. Expect to hear them on again, if not this season then perhaps next.

The Zoom conference was open to all and there were participants from at least several countries outside Canada. A benefit of virtual conferences, because there is no travel required, is that every conference can be global. Most are of course of local interest only, such as the virtual meetings by a large number of clubs. 

I see no reason why the trend won't continue when the pandemic is over and hamfests return. The one thing I miss from hamfests are the flea markets and those don't work well virtually. Online swap shops are poor venues for buying and selling small items and browsing for the unexpected gem that you absolutely must have.

With regard to 6 meters, we are approaching peak sporadic E season in the southern hemisphere. Don't spurn the small winter peak north of the equator since, although DX opportunities are uncommon, the possibility of TEP to connect to southern stations is ever present. You should at least watch the DX spots for signs if you prefer not to monitor 50.313 MHz. You may be surprised.

Thursday, October 15, 2020

80 & 160 Meter Verticals and the Tower Forest

An antenna farm is replete with interactions. Most are unwanted and detrimental. It isn't enough to deal with harmonically related bands since non-resonant antennas, guys, coax, control cables, utility lines and building metals will affect antennas when they are an appreciable fraction of a wavelength or longer, or if they are short and close.

Interactions can be eliminated if the offending conductors are within your control. Unfortunately the effort and expense may not always be worth it. Many hams are unaware of interaction problems or, unless it is especially serious, live with it. A few choose to pretend they doesn't exist. 
For those of us invested in high performance antennas those interactions can remove a lot of that performance and therefore require attention. It may not be possible to eliminate every problem but that is not an excuse to do nothing. The answer is to focus our efforts on the worst of the lot and, perhaps, deal with the rest later.
This article is about vertical antennas for 80 and 160 meters since the best for DX are verticals and towers are also verticals. That is, towers are inevitably a part of the antenna system unless they are exceptionally distant. Recall that "distant" is with respect to wavelength and a wavelength on these bands is larger than the vast majority of hams' properties.
There are several common methods for dealing with tower interactions: 
  • Locate the vertical antennas as far as possible from towers. 
  • Place verticals arrays and towers so that no tower is in the direction a directional vertical array is pointing.
  • Have more than one vertical antenna for the low bands so that the directional deficit of one is covered by another.
  • Detune towers so that they don't resonate on 80 or 160 meters.
Methods can be combined when helpful. For my antennas I placed the 80 meter array far but not too far from the towers. I wanted to keep the array close for unrelated reasons and the placement only disfavours one direction (southeast) which is not the most productive for DX paths in contests. Other directions are unaffected.
The situation on 160 meters is more problematic. The towers are supports for 160 meter antennas for the foreseeable future so the interactions must be dealt with in a manner that is most advantageous or, failing that, least deleterious. That can be difficult to achieve due to their large electrical length. As we'll see detuning is not always necessary.
You will not find many satisfying conclusions in this article. It's all part of my learning process. Expect food for thought rather than clearly defined antenna ideas.
Site plan
For this exercise I will use my own antenna farm since that's my immediate interest. Lessons should be applicable to other stations, both existing and planned. For those who cannot change what they have, whether due to lot size or other reasons, there is an opportunity to understand what is going on and to use methods such as tower detuning to deal with the most severe interactions.

The 80 meter 3-element vertical yagi is in the north field and the two big towers are in the field to the east and south. The northernmost is 150' and the other is 140'. The heights are nominal due to section overlap and masts. Actual heights to the top of the mast are approximately 46 meters and 43.5 meters, respectively. The Trylon tower near the house is 24 meters to the top of the mast.

The 140' tower is marked with a red dot since the Google Maps image predates it. The distance from the 80 meter central tower and driven element to the 150' tower and Trylon is ~60 meters, as is the distant between the big towers. 
Model notes
Since the Beverage antennas to the northeast of the image appear to suffer no pattern distortion due to these towers and antennas they will be omitted from the models. That is not always true, especially with vertical receive arrays, and those interactions must also be dealt with.
For the 80 meter case the 140' tower and Trylon will also be omitted, the first due to the distance and the second due its not being directly in one the array's four directions. That is not to say they have no effect but the effect is small enough that I am not unduly concerned. For the 160 meter case the Trylon and 80 meter array will be ignored due to their short heights.

The big towers will be modelled as 40 meter high thick wires with 2 capacitance hat arms at the top representing the top yagis. This is not exact and is in fact not really even very close to reality. The side mount yagis and the mess of control wires and coax are difficult to model and so they aren't. 
To see what the impedance might really be I plotted the R and X of both towers with an antenna analyzer, placing it in line with the wire to the ground rod. The galvanized ground rods are ⅝" × 10' and are located just beyond the subsurface reinforced concrete platform. The tower and pier pin are not bonded to the the re-rod so the bases do not behave as Ufer grounds.
As you can see it's a bit of a puzzle. The only recognizable resonance is around 1.2 MHz, which is not out of line with expectations. Overall the impedance is dominated by the ground resistance. All I can say is that the ground resistance (loss) at these frequencies is probably in the range 25 Ω to 40 Ω. The other tower has a similar profile.
At 1.83 MHz the impedance is 79 + j20 Ω, which is equivalent to a reflector with a lot of ground loss. This is not too unlike the model of an electrically long wire on 160 meter, and is an approximation I've used before. However the impedance is about the same at 3.5 MHz. Who knows, had I inserted the analyzer on a feed line or control cable the result could be very different. 
Modelling the tower as a simple thick conductor with yagis as capacitance hats and a resistance load (ground loss) at the bottom has no resemblance to the real world measurement. There is no simple model that mimics the measurement and I decided it would be foolhardy to make further attempts.
As a consequence, what a distant antenna "sees" is exceedingly difficult to know. This complexity limits what I can say about interactions, more so on 80 than on 160 meters. At least on 160 meters the measured impedance is likely closer to what a 160 meter vertical antenna would see.

MININEC ground is specified in the model with resistance loads at the bases of the towers and vertical antenna elements. These can be set to values approximating the ground rods and radial systems. We don't need to be very accurate for this overly rough analysis.
80 meter array
From the foregoing model discussion it should be evident that modelling the interaction is quite a challenge! So I won't try. What I did was try various electrical lengths of the 150' tower that is 60 meters distance.

Regardless of tower tuning the SWR of the 80 meter array does not measurably change. This is not surprising. Feed point impedance is the last thing to be affected by interactions. In order, the effects are felt in directivity and gain. It takes a far tighter interaction to alter the SWR.

Depending on tower tuning, with the 80 meter array pointed at the tower (southeast) gain varies by ±1 db and F/B is reduced by 3 to 10 db. Yes, the tower can act as a wide spaced director and increase gain. From my measurements, however, that is unlikely. From on air use I know there is gain southeast and a small degradation in directivity. That is, there is interaction but nothing too concerning.

When the tower is placed to the side of the 80 meter array, as it would be for northeast and southwest directions, the influence of the tower is small. Recall from the pattern charts for the 80 meter array the F/S is less than 10 db so there is just enough radiation towards the tower to have some effect. Pointed northwest, where the tower is behind the array, there is no interaction, and that is expected.

A more detailed analysis is pointless because of the discussed uncertainty of the tower's true effect on 80 meters. I feel confident that I can ignore it, just as I intended when I chose the location for the array. When directivity is insufficient to copy a weak station I can listen on the Beverages.
160 vertical centred between the towers
For the model I adjusted the towers to assign them the approximate measured impedance. This may not be representative of the situation at other stations. Despite that there are qualitative results that are broadly applicable. 

Assume a rope catenary from tower top to tower top supporting a vertical wire. Were I to build it that is how it would be done. This is not arbitrary since I planned to use this method to build a reversible 3-element vertical yagi by tuning the towers (LC networks) to act as directors and reflectors (see below for references). However in this case we take the towers as they are with their measured resonance and ground rods. This is an antenna I've looked at before but without the tower impedance data known.

The current magnitude and phase on the towers are equal when their impedances are equal, and that should result in a symmetric azimuth pattern. In other cases expect the pattern to be asymmetric. The feed point impedance at resonance is 28 Ω assuming a radial system with an equivalent ground resistance of 10 Ω, typical of no more than 8 radials. With a matching network the 2:1 SWR bandwidth is 70 kHz.
Bandwidth is not great and neither is the efficiency. Both towers together have about the same ground loss as the antenna itself. To improve efficiency you'd have to put radials on the towers since adding radials to the vertical won't fix the tower loss. The pattern is a problem if you have no other 160 meter antenna to cover the broadside directions.

This is not a very good antenna. It would help to detune the towers, not only to reduce tower currents and loss, but also to circularize the azimuth pattern. 
You may not know you have a problem unless you measure the impedance and resonance of nearby tall towers. Ignorance is not bliss.

160 bent vertical closer to one tower than the other

This antenna requires just one tower for the upper support. A rope from the bend is anchored on the ground to give the antenna its shape. The configuration of the towers is identical, electrically and physically, to what is described above. The vertical is 20 meters from one tower and 40 meters from the other. 
The antenna is a little like an inverted-L but with efficiency and bandwidth closer to that of a straight vertical. The angle of the bend is 45°. The radiation resistance is higher than the T wire vertical I've used for the past few years.

As you can gather from the EZNEC antenna view, including currents, the left tower's induced current is lower than for the centred vertical wire. Loss in that tower's ground is therefore lower. Current and loss in the right tower is about the same as for the centred vertical wire. Overall loss is lower so the antenna is more efficient. Rather than resulting in higher gain the power appears in the broadside directions so that the pattern is more omni-directional.
As you would expect from the asymmetric currents and the wire being closer to one of the towers the pattern is asymmetric. Before the second tower was built I knew from models and from operating that the pattern of the T wire vertical was directional by a few decibels, with the tower acting as a reflector. The result was a small deficit towards Europe. Even so it worked well in that direction.

With the second tower on the scene the deficit in that direction is reduced (on the plot left is northeast). Compared to the centred wire vertical the broadside deficit is reduced and is acceptable. The modelled 2:1 SWR bandwidth is 80 kHz, which while not great is good enough for my operating interests.

Here I have to use the term "good enough" since this is almost certainly the antenna configuration I will use this winter. It's simple and effective despite its imperfections. 
Actually the wire will be offset a few meters to the east so that two of the radials don't run into the stone wall separating the yard and the hay field (see satellite view above). That will skew the pattern so that the east (up) and west (down) directions will differ by about 2 db.

Efficiency of the antenna can be improved with a few radials on just the right tower. That is a feature I will consider. The antenna is simple enough that I could put up another to better cover the east and west directions. That is not a feature I am considering for this winter. I have the 160 meter mode on the 80 meter array available to fill pattern holes since it is more omni-directional although it may be as much as -6 db worse than the full size wire vertical.

160 vertical alongside the tower

A quite common style of wire vertical is to run it alongside a tall tower. It is given its own radial system so that there is no direct connection to the tower and lightning ground rod. Of course the wire couples strongly to the tower and its many cables, and ultimately the ground rod as well. This is not a shunt fed tower. Many report low to no deleterious RFI to antennas and connected equipment while others report severe problems. It seems to depend on details of cabling and yagi feed systems and the resonances therein.

I used the previously measured impedance of the tower at 1.830 MHz to model a grounded thick wire model of the tower and put a vertical AWG 14 wire separated by 2 meters, which seems to be a popular choice among hams who have tried this antenna. Again I use a MININEC ground with resistance loads at the base of the wire and vertical to mimic the loss of radials and ground rods.

The feed point impedance is around 25 Ω and with a 2:1 transformer (another popular choice) has an SWR 2:1 bandwidth of 80 kHz (say, from 1810 to 1890). That's pretty good. The pattern is skewed somewhat and the efficiency is poor. Current in the tower is high (as you can see in the plot at right) so even with a good radial system for the vertical wire about 40% of the power is dissipated in the tower's ground resistance via the ground rod.

Despite the gross simplification of the model this appears to be a risky design. Some of the loss can be mitigated by tying the radial system to the ground rod at the further risk of more serious RFI. It may be little better than a shunt fed tower, if at all. But, again, there is a lot of uncertainty in the model due to the unpredictable behaviour of the many cables on the tower and running toward the shack, whether over or under the ground.
Future 160 meter directional antennas
In an earlier article I explored recruiting the (unavoidable) towers as parasitic elements to make a reversible 3-element vertical yagi. Tuning the elements appropriately will be more difficult than in the simple models (again, all those cables) but it does get useful work out of towers interactions. Radials on the towers reduce the loss due to coupled current on the towers and, to a degree, may reduce the impact of the cables on the tower.

That only provides two directions since, as shown earlier, when the towers are somehow detuned or decoupled they are still present as non-resonant elements that will distort the otherwise omni-directional pattern of the driven element alone. Switchable detuning networks are needed to have an omni-directional mode.

In the future I will explore this antenna further and perhaps attempt an experimental tuning of the towers to see how well they can be adjusted to give the desired effect. I am less hopeful now than I was when I wrote the article that it will work well in practice.
Final notes on mitigation
Towers with resonances on bands with nearby antennas is a problem, even if you aren't fully aware of it in your operating. Some mitigation strategies were discussed but not analyzed in depth. For me these can be summarized as follows:
  • Detune towers: This can be more difficult than it sounds. The problem lies with all the various cables alongside the tower. Placing an analyzer at the tower base can be misleading. A field strength measurement may be necessary to confirm that the pattern is circularized.
  • Improve tower ground: Towers with substantial induced current and a lightning ground will decrease vertical antenna efficiency. Although it won't stop the interaction, lowering the ground resistance will improve efficiency. Several radials attached to the tower base may be all that's needed. The specifics are not described in this article but were confirmed in the model.
  • Recruit the tower: Tall towers often act as reflectors quite naturally. Place your vertical antennas to take advantage of that and thereby improve your signal. Unfortunately you will then need at least one more wire verticals to cover all compass directions. With more effort the tower can be tuned as a director or reflector and that can be a great antenna.
Another technique that can work well in many circumstances is to disconnect the tower from the ground rod and replace it with a high value resistor and spark gap. Think of it as a more radical method of detuning the tower. This technique is typically only used on tower verticals that must be isolated from ground. Its value for a tower supporting multiple yagis and cables is less certain and introduces serious safety concerns. To be complete I want to mention it even though I recommend against it.
This is not the final word. My hope is that this limited analysis of potentially destructive interactions for low band vertical antennas will open eyes. Only then can corrective action be taken. It's food for thought that I will consider for my future 160 meter antenna plans.

160 meter antenna for this winter

As I write this I am not able to operate 160 meters. There is a fault in the 160 meter mode of the 80 meter vertical yagi. I have had little time to work on the problem due to more important tasks. The full size wire vertical can't be deployed until work on the 15 and 20 meter stacks is complete.

I will use a bent vertical described above rather than the sloped T used for the past few years. Interactions with the stacked yagis has not yet been modelled but I expect it to be minimized by locating the wire directly ahead of the lower, fixed yagis. 
Next year I may try something different. I'm out of energy and time to be creative with a 160 meter antenna for this winter. Unravelling those tower interactions is an intriguing puzzle that I am sure to return to.

Tuesday, October 6, 2020

Stairways to Heaven: Ladders, Towers and Masts

Not all climbing is alike. I was recently reminded of this when I painted the exterior trim of my house. Ladders are not towers, and not in a good way. Most people will climb a ladder while few will climb a tower. I find that interesting because towers are safer than ladders. For this discussion, I mean a high ladder and not a small one.

At the risk of irritating those of you suffering from vertigo I offer a view looking down from only a little more than 20' (6 m) while painting the apex of a two-floor high cathedral ceiling exterior. Look like fun? It isn't.

Unlike towers, ladders are not firmly attached to the ground. Safety lines and temporary anchors are available if rarely used by the average homeowner. When ladders are handled carelessly they can slip, topple or break. In most cases you will survive though you may wish you hadn't.

As you may have guessed I don't like climbing tall ladders. I was sorely tempted to hire the job out to a professional. However, as with my amateur radio duties I am very much a do-it-yourself kind of person. I can do the work despite not enjoying it.

One advantage of a ladder is that you don't need a safety harness to stay on it. You can stand vertically and you can lean against the ladder. Just don't lean too far backward or to the side! With suitable peripherals you can hang paint cans and other tools to free your hands for the work and to hold on.

The perceived threat of towers is their height. Get over that and they are safe. They can be more tiring since it takes a lot of effort to climb up and down and your body will ache at the places that bear your weight or the force of applying torque to a wrench. Rigid sole boots are used to avoid foot injuries and aches. Lifting stuff from the ground can be a lot of work unless you have ground crew so don't forget a tool or part that you'll have to climb down to retrieve. You will drop stuff so carry spares.

The view from a tower is far better than from a ladder. As you climb past 100' (30 m) the scenery is similar to what you see when flying, except quieter with a bigger window and no fighting for the arm rests. In my experience you don't really notice the ground when you're up that high since it looks so remote. In contrast the ground appears terrifyingly close, vicious and unyielding from the top of a ladder.
Over the years I've offered visiting hams showing a curiosity about climbing the opportunity to try it out. Climbing 20' or so requires no more effort than a ladder. Very few have taken me up on the offer. I was no different the first time I took the challenge when my Elmer told me to try out his tower when I was 15 years old. I only made it to 20', the level of the eaves. It terrified me. 
Fast forward one year and I was happily climbing 50' towers for friends and by the age of 17 I had my own. You grow accustomed to it. The ambition for more DX and higher contest scores is a fantastic motivator. The terror gradually recedes to a level one can live with. It never departs completely, and that's a good thing since fear keeps you wary and safe.

As we all know the top of a tower is not actually the top. Antennas are mounted on a mast and the mast sticks up out of the tower, sometimes for an appreciable distance. How do the antennas get up there? You must mount upper antennas on the mast when it is retracted within the tower and push it up, or you must climb the mast and do the work up there. In my early tower days we always did the former. That can involve some heavy lifting and the resulting installation is difficult to service. So I learned to climb masts.
The first time it wasn't planned, it simply had to be done. Since I'm lightweight and most people are normal weight or overweight my friends all looked expectantly at me. I mumbled a few unmentionable words and started climbing. 
Getting on top of a tower is awkward. Holding a narrow pipe as you push up is difficult to do without your body swinging outward. It helps if there's a boom or boom truss to grab.
Finding useful places to attach your safety equipment isn't always possible so you wrap everything around the mast. This makes vertical motion difficult since you must slide the straps as you rise and they hold the mast tight if you put weight on the harness, as you often must. Every obstacle along the way requires detaching and reattaching each strap (fall arrest and positioning).

On the bright side a mast, like the tower holding it, is securely held. In that way it is better than a ladder. When the mast is engineered to support an HF yagi at the top it will support your weight, whether pipe or tube, high strength or moderate strength. However, it will wobble a bit and that may be disconcerting.
Mast climbing is tedious and slow work, and you must do it again on way down. You are so well tied in you won't fall. The temptation to reduce the number of connection points can be deadly. In my immortal youth I did it free climbing with just a lineman's belt. After stepping onto the top plate I used boom, clamp and truss supports as steps. Yes, that is a little crazy. Even so it was usually enough to get the job done. 
These days I do it differently. There are professionally made mast steps that are beautiful to behold and strong. Mine are not. They are home brew and barely adequate to support my weight. Someone of normal weight would be unwise to rely on them. I know hams that weld steps to the mast for added strength and permanence.
In the picture you can see what I call my stairway to heaven. It takes 3 steps for my hands to comfortably reach the top of a 12' mast to work on the upper antenna. The antenna you see above is the upper 5 element 15 meter yagi of my stacking project. The upper 20 meter yagi will be located at the bottom of the mast.

To lift the antenna after it is trammed to the top of the tower the tram is removed, a pulley is attached to the mast top and the mast clamp bolts are loosened. Ground crew pulls as I slide the yagi upward. As it rises I follow it up, installing steps along the way. Finally I am at the top of the stairway bolting down the yagi. The pulley is removed, I climb down to the tower and remove the steps. Simple and reasonably safe if somewhat slow. I recommend a quick check with an antenna analyzer before sliding the antenna up the mast.

The view is pretty good from up there! Looking down is also interesting. Instead of doing that I'll direct your attention to an earlier article with just such a view from the mast of the first big tower (150') when I first filled it with antennas. 

How does climbing a ladder compare to climbing a mast? I favour the ladder. Although less safe the ladder is far easier to climb and work on. Climbing masts drives me crazy, mostly due to the time consuming safety procedures. The temptation to be less safe and work faster is ever present.
Most people are comfortable on short ladders and fewer are comfortable on tall ones. If you're a ham and you can safely use a tall ladder you have potential as a tower climber. Over the years I've met countless people, hams and non-hams, who are convinced towers are extremely dangerous. Yet they are willing to climb ladders and work on sloping roofs, and are comfortable operating heavy machinery and chainsaws.
Those with the discipline to follow safety procedures in one area can do it on a tower. Masts, however, are another thing entirely. I don't blame anyone for thinking that's crazy. That and ladders.