3-element Trap Tri-bander on 50 MHz

Not long ago a friend asked me about using his short boom 3-element tri-band yagi on 6 meter. Apparently the idea is more popular than I expected. But it makes sense: it's already there, it's rotatable and it's high on a tower, so maybe it's worth a shot. He wanted to try it to see if he could do better than with his wire dipole.

When loading any antenna on a band for which it was not designed there are two critical metrics that must be addressed:

• Efficiency
• Effectiveness

Unless you're very lucky the SWR will be high. That has a strong impact on the efficiency of the antenna system. Elements far longer than that of a half-wave dipole can have peculiar patterns with multiple lobes and nulls. That can be a problem if the antenna is not rotatable.

I gave him my opinions solely based on my experience and thinking through the issues. Out of curiosity I followed up with a software analysis. It wasn't difficult to do since I have a EZNEC models of a tri-band dipole and small 3-element tri-band yagi in my files, and TLW easily analyzed efficiency and matching of the transmission line and tuner (matching network). The dipole is the driven element of the yagi without the beta match. The element model utilizes traps similar to that for common Hy-Gain antennas like the TH3.

It turns out that what I told my friend was partly right and partly wrong. While an educated guess is often good enough for ham work, a model and measurements will do far better for a modest amount of effort. Even experienced hams can be wrong so be very careful whose opinion you solicit. I don't mind being wrong since it's an opportunity to learn. Hence this article.

Above is my EZNEC model of a tri-band yagi fed on 50 MHz, showing the element currents. It is no surprise that the reflector and director currents are low since the spacing is almost double that on 10 meters (by wavelength) and the elements are not resonant on 6 meters. The low impact of the parasitic elements is apparent when they are removed from the model, since the pattern and impedance are little different (see below).

There are current "bumps" at the beta match and traps where there are impedance discontinuities. The traps have a net capacitive reactance at frequencies higher than their resonant frequencies. The beta match is essentially a dice roll since it is arbitrarily transforming a high SWR to a different but also high SWR

The azimuth pattern is pretty good. It's bidirectional with modest gain due to the narrowing of the lobes. The narrow lobes are due to the long length of the yagi driven element; as already noted, the parasitic elements have little effect so that the pattern of the tri-band dipole is almost the same. I wouldn't read too much into the higher free space gain of the dipole (red) since the difference is small and not easily predictable for the variety of commercial antennas of this type.

When I inspected the behaviour of the traps in EZNEC I was surprised to find that their loss was negligible even for realistic Q values of the constituent coil and capacitor. I expected the loss to be low but significant. I haven't yet looked into the details of what was to me an unexpected result. I have an idea how it comes about, but rather than present a guess let's move along.

Well, that was the good news. Now it's time for the bad news.

The SWR is very high, and that's bad. It is similarly high for the tri-band dipole, except that the R and X values are transformed to a high R and negative X by the beta match. I used a beta match in the model due to its use on Hy-Gain yagis and some other tri-band yagis. With other yagi matching networks the effect will be different but perhaps not so much in its behaviour on 50 MHz.

The SWR can be corrected in the shack using a tuner. The ATU in our transceivers cannot cope with an SWR of 30. In actuality, the mismatch loss on the coax is so high that the SWR in the shack will be far lower than 30. TLW is my tool of choice to model the effect of the transmission line.

Many hams use RG213 class coax in their HF antenna systems so that's what I started with. I further assumed 100' (30 m) of coax length, which is typical for a yagi on a small tower located a short distance from the house. The matched loss of RG213 is quite high at -1.6 db, and it balloons to -8.2 db due to the extreme mismatch. With LMR400 the loss is "only" around -6 db. Even with pricier LDF5 the loss is still -2.7 db. 

Notice that the SWR of 5 in the shack is almost within reach of a transceiver's ATU, and it might even manage it depending on the R and X values. That will depend on the length of the coax.

It should be obvious that to reduce the loss the transformation network should be placed at the feed point. It will have to switched by relays (one at each port) so that it is bypassed on HF. With fixed C and L an L-network can be very efficient. TLW has the following to say based on average quality components.

To connect all of this to the real world, I measured the impedance of my TH6 on 50 MHz. Of course there are differences between a real 6-element tri-band yagi and the 3-element tri-band model, but they're both trapped and the driven element of the TH6 has a close resemblance to the model. That was my intention when I originally developed the model years ago.

The impedance was measured in the shack with a RigExpert AA-54, which you've probably seen in many other articles on the blog. I could have used a VNA for improved accuracy, however that really isn't needed for this exercise. The transmission line has a few short sections of RG213 and LMR400, so I mentally estimated how that translates as an extension to the LDF5-50A Heliax transmission line I'm using. I only needed the approximate SWR, not the precise impedance at the antenna feed point.

The measured SWR is 5, which is about 8.5 at the feed point. Although that seems significantly less than 30 for the model it really isn't. Very high SWR is acutely sensitive to small details of the load and transmission line. For example, when I substituted RG213 in TLW the result was nonsensical because the load would have to have a negative impedance. The reason is that you could never read an SWR as high as 5 since that requires a transmission line with loss better that of RG213.

Suffice to say that the SWR is high and the mismatch loss can be very high. Few hams in the situation described in this article are likely to be using LDF5 Heliax!

For hams looking for a simple and effective 6 meter antenna, a switched network on top of the tower is probably not what they are looking for. A dipole mounted above the yagi and an inexpensive remote coax switch (or separate feed line) are probably a preferable alternative. 

A 2-element yagi is very small and has significantly better uni-directional gain. Hams of my acquaintance who get on 6 meters with their 20 to 6 meter hex beams seem to be satisfied with the performance. It isn't what I would choose for myself  but few hams have the towers and property that I do.

The northern hemisphere's summer sporadic E season is now finished. For those planning to get active on the band in 2023 this article may motivate you to put up a resonant antenna rather than try to get by loading an existing HF antenna. The improvement will have a remarkable impact on your success.

6 Meter Season: Diminishing Returns

I am reluctant to write this article. Although we are now 2 months past the peak of the summer sporadic E season the band is not quite dead. But I have to accept that we are about done and this is a good time to reflect on the season that was. E season wrap ups have become a regular feature of the blog; for example, here is last year's summary.

The 2022 season was a mixed bag. Propagation started slow on 6 meters, petered out right at the solstice peak, then July delivered a multitude of openings. The magic band likes to keep us guessing.

DXCC

On the DX front my DXCC count changed little. At the end of last year I had 111 worked and now the count is 120. Of the 9 new entities, a maximum of 7 can be attributed to sporadic E. The others were F-layer or TEP deep into South America to work CE and CX. The opening to the Pacific that netted E51WL and 3D2AG likely had a sporadic E component at this end of the path.

Considering that I logged 900 QSOs on 6 meters this season (over 700 of which are DX) that is one new country per 100 QSOs. The reach of sporadic E is limited and the further the path the lower the probability of an opening, especially at higher latitudes. Were there more activity from Africa the DX potential would be greater. Working DX on 6 meters is rarely easy, and if it was it would be less rewarding. I am not complaining, just relating the facts.

Operator duty cycle

One of the questions I am regularly asked about DXing on 6 meters is: when should I listen for which DX? Sporadic E is called sporadic for a reason, so I have to disappoint them. There are times of the day among a multitude of other indicators with an increased probability of DX over particular paths. Unfortunately for us, good and bad surprises are the norm.

There is no substitute for dedication and listening. At the very least, monitor DX spotting networks. During sporadic E season my rig is regularly monitoring 50.313 MHz even when the band is dead, just in case. I leave it on when I am out of the shack or away on other business so that I can review the sometimes long list of decoded FT8 messages.

The point is, you can't be sitting in front of the rig 24×7 for 4 months straight. Surprise openings are surprises (duh!) so you have to accept that you will miss some or many. When I miss openings I am able to learn likely propagation patterns by reviewing what I missed. That's the point of leaving the rig on when I can't be there.

There are times when I fully expect the band to open and I leave anyway. We all have responsibilities that brook no delay or rescheduling. During summer I also have non-ham interests that I attend to. Life is more than ham radio.

Here are many of the DXCC countries I missed working this year because I had to be away or I chose to be away: Z6, EY, ZL7, OA, V4, TR8, TT. There were also the inevitable almost new ones from partial contacts that could not be completed due to the rapid rise and fall of DX paths. One in particular that comes to mind is OD. 

Several others new ones were heard that did not hear me. That list is longer. Few hams have the quiet location that I have and it takes a lot of ERP and luck for be heard by them.

Rather than dwell of the disappointments, I prefer to focus on the future: the anticipation of next year's opportunities and the coming F-layer openings. I relish the challenges ahead.

Notable openings

There is an unfortunate habit for hams (just like regular folks) to brag. The presence of examples in this section is not intended that way, although I have to admit to being pleased with my successes. If this hobby doesn't give us pleasure, well, what's the point? With that in mind let's continue.

I already spoke of the fantastic Pacific opening that netted E51WL and 3D2AG early one evening so I'll simply link to that article. There you will also find a screenshot from a friend who copied ZL7DX. I was not present for that one and I don't know if I'd have copied the signal or made a QSO.

It is not always that way. My antenna system and low noise QTH often make the difference between my success and that of those nearby. I took the adjacent screenshot to prove to several friends that I was hearing Hawaii very well one evening in early June. Despite being within 100 km of me and with only slightly less effective antenna systems, they had no decodes of KH6HI or the other Hawaii stations I copied.

It's impossible to say whether the difference was propagation or our stations. Nevertheless, it is a fact that incremental improvements in transmission lines, power, antenna gain and antenna height are in your best interest. Small differences add up and will improve your success rate.

Last year I copied UN stations during a brief early morning opening. The amp was off and I failed to make a QSO. That isn't unusual for sporadic E.  I was therefore happily surprised when UN3G answered my CQ in early July. As you can see the signal reports are painfully low, but all that matters is that the QSO is good.

Much to my surprise there was more to come. On at least 3 further days in July we had openings to UN. In one case it lasted close to an hour. Now I have 6 contacts with Kazakhstan on 6 meters. Although the latter 5 are superfluous for DXCC, the joy of contacts over such a long path is never uninteresting. I almost don't mind that I missed EY8MM. There is always next year.

Despite the frequency of openings and large ham population, there are countries in Europe I have yet to work. Some have only occasional activity or their station conditions are poor. Two that I added were ER and ZA. The Italian station in Albania was periodically heard by me and my friends but never strong enough or in for long enough to allow for a QSO. 

During one excellent opening he was quite strong on 50.323 MHz and, as you can see, the new one was rapidly logged. It's always fun to convert a purple CQ to green. Signals improved and the friends I notified were also able to add ZA to their 6 meter DXCC total. The buddy system works.

I finally managed to work HL this year, and 4 of them for good measure. The bearing as you travel the short distance from JA6 to HL to BY shifts alarmingly north due to the long path length. That's what makes them so difficult from this part of the world.

The final South Korean station I worked occurred unexpectedly late in the evening (July 7, UTC). It took several tries with a kilowatt to finally be heard. Unexpected DX is always a pleasure. After our QSO he began calling CQ NA, probably similarly surprised by the opening to eastern NA.

Among my misses this year was China. As already mentioned, China is difficult due to the northern bearing of about 345° to 0° for the densely populated eastern half of the country. Compare that to 330° to 335° bearing range for Japan, which is a far easier path. Indeed, I worked dozens of new Japanese stations this year over several openings.

As you can see from the screenshot, I certainly tried to work China! BA4SI appears to be in the Shanghai region, which is about the least northerly path from here. Perhaps like many others they have a QRN problem. I heard them and other Chinese stations on at least 3 evenings in July. None responded to my calls. Other than BA4SI none of them were in for more than 3 consecutive FT8 intervals, which is barely sufficient for an QSO. I'll try again next year.

A notable opening that almost but didn't quite reach me was VK4. That's an extraordinary long path! It is a little north of west for us to those stations located in eastern VK4. That is about as far to the south the great circle path goes from here to anywhere in Australia, and that makes it about the easiest.

Many stations not far to the west and south of us were successful working the several VK4 stations that participated in the opening. I and my friends had not a single decode. Some things were never meant to be. This one may have to wait for an F-layer opening.

Pros and cons of increased activity

When the band is open it's busy, very busy. An increasing number of hams are discovering 6 meters and they are active. This is fantastic but it does add to the QRM and QRN. 

The increase is not only local so there are more DX stations to work and more countries to be worked. Cutting through the QRM to do so is unfortunately necessary. I prefer it this way compared to a quiet band with fewer stations to work. The intercontinental window at 50.323 MHz is less busy yet relatively few stations QSY.

Some of the QRM is due to improperly adjusted transmitters and amplifiers. Although we use the SSB mode on our equipment, digital is not phone. On SSB we "own" the full 2.7 kHz of spectrum we occupy, which is necessary to contain enough of the human voice to enable legibility. It is therefore not an inconvenience to others that we use non-linear techniques such as clipping and compression to increase average power. The distortion due to non-linear processing improves communications when judiciously applied.

Try this on digital modes and bedlam ensues. The 3 kHz window typically employed for FT8 requires that each occupant strictly limits their signals to the approximate 50 Hz audio bandwidth required. To do so requires audio linearity and RF linearity. 

Unfortunately our rigs often make this difficult since the non-linear features used for SSB must be manually disabled when switching to digital. Many hams forget or don't realize its importance. I know and yet I occasionally make mistakes. With a constant probability for each operator to make a "mistake", the probability of QRM due to these mistakes increases in proportion to activity. Then there are the anonymous "policemen" who add to the QRM with directed custom messages to harass the guilty parties. 

Expect problems to continue until we have a new generation of equipment that incorporates improved digital features and abandon use of the SSB mode. Hopefully that will also reduce distortion, audible computer sounds and background noise from live mics. For example, in the FTdx5000 that I use the mic is always live despite using the rear audio jacks to connect the PC. I have to remember to unplug the mic when switching to FT8. It's easy to forget.

Another notable behaviour is the increasing presence of robots. For those not familiar with digital robots, they are typically made from forks of the open source WSJT-X software. Robots are in violations of the open source license. It is not easy to get them taken down and there are numerous applications proliferating in the wild. I won't provide specific examples but the curious will not find them difficult to locate on the internet.

Robots will automatically CQ or call CQ'ers, work stations and log the QSOs. Some do not discriminate, calling everyone they hear, often progressing up and down the FT8 window in a easily identified pattern. Others are more particular about their behaviour and use filters to limit who they call. 

Most robot software users appear to have no evil intent. They may simply enjoy the novelty of the technology. Others like the idea of logging lots of stations while they at work or doing other things. A few do it to demonstrate (mostly to themselves) that digital modes are not "real" amateur radio. I will not work robots that I can recognize as robots. Persistent bad actors are particularly obvious and they have a well-earned poor reputation. 

Most on the band don't care or are even aware that they are working robots. It was inevitable that robot operators would migrate from HF to 6 meters along with everyone else.

Propagation lessons

It's impossible to be passionate about DXing on 6 meters without developing a keen appreciation of the underlying propagation science. Despite the vastly improved understanding of the mechanisms underlying sporadic E, TEP, scattering and other propagation modes, predictability remains elusive. 

We do far better with the weather since measurements that feed computer models are easier and more extensive. There are also strong economic incentives to improve predictions. Radio propagation lacks the economic incentive in the 21st century and there is no good way to install measurement instruments at multiple points in the outer reaches of the atmosphere. Remote observations are a poor substitute. 

Predictability will therefore remain a challenge. Those seeking certainty will continue to be disappointed with the poor predictability of openings on 6 meters.

Despite the challenges, the increased level of 6 meter activity, prevalence of digital modes and the broad use of internet tools are giving us a better understanding of sporadic E propagation. Although prediction is poor, we can do a lot better than in past decades. Here is a short list of what has become apparent to me (and others):

• Weak and brief sporadic E propagation over both short and long paths are more frequent than has been previously believed. Single decodes of distant stations on a "dead" band are fairly common. The proliferation of big stations continuously transmitting CQs makes this possible; beacons are too weak and require effort to individually monitor. I also will periodically rattle off CQs on a dead band, and in combination with PSK Reporter I see flags of my transmissions being decoded in unexpected places more than I would have once believed possible.
• Overnight DX openings during sporadic E season are more common than expected. I have taken to leaving WSJT-X running overnight many times this year, with the yagi pointed north into the perpetual Arctic daylight. The log of decoded messages that I review in the morning contains notable surprises. I have also been surprised by how often I will get a flag in western NA during our mornings, well before their sunrise, while I'm beaming to Europe or west Asia.
• Brief long distance openings to eastern Europe and Asia are common in the hour or two after sunrise. These are easy to miss since I am not an early riser and sunrise in the weeks surrounding the solstice is 5:15 AM.
• Conditions that result in an opening today don't immediately dissipate. There is an increased probability of a similar opening, perhaps weaker or stronger, the next day. I have noticed this on almost every path: east Asia, central Asia, east/west/south Europe, NA west coast, and South America. The same may be true of the Pacific, but with sparse activity it is difficult to know.
  

While scientifically interesting, some of the above phenomena do not translate into QSOs. The openings are too transitory and digital modes too slow to take good advantage. But they are there, and that's good to know. We may develop technological aids to turn those opportunities into results in the future. Being a ham involves progressing the art and not twiddling the VFO knob as we had to do in days of yore.

Looking ahead

We can expect a return of north-south propagation as we approach the fall equinox in late September. That can happen with modest solar activity if other factors are favourable. There are a few more South Americans that I would like an opportunity to work. Beyond that, we'll need a higher solar flux. 

When the solar flux rises above 150 the propagation, as seen from here, will spread east and west across the tropics and to the southern hemisphere. Going by my experience from the 1989/90 solar maximum, openings to South America will expand to include southern Africa and equatorial Africa. It is almost certainly present to the eastern part of the South Pacific, but there is almost no one there. The sparsity of land and hams is at fault.

Eventually, but probably not this year, we'll see east-west propagation within the northern hemisphere. The solar flux would have to hold above 180 for that to happen, and it would have to happen during the fall and spring for best results. Deeper into winter the northerly paths are less likely to open since solar insolation at high latitudes is weak. Winter sporadic E will occur but it is poor for DX at my location.

Despite the title of this article, I am hopeful that 2023 will be a good year for 6 meter DXing. We can count on a higher solar flux, increased activity and the return of sporadic E. More paths will open and more of us will be there to take advantage. With the diminishing returns of sporadic E propagation at my DXCC level, those additional propagation modes are needed to eke out more than a handful of new countries each year. 

Although the 1989/90 solar maximum was exceptionally good, I could not get past 70 countries because so few countries had access to 6 meters. It was frustrating to have great propagation to Europe and only be able to work a handful of countries. The majority had no access to the band. Those barriers are almost entirely down so if the present solar cycle is a good one there will be no shortage of DX to work, including with small stations. We have a lot to look forward to.

For the present my attention has shifted back to HF and preparing my station for the fall and winter contest season. I still have much to do.

Analyzing an Absurd Attenuator

I made a last minute decision to enter the CW NAQP (North American QSO Party) this past weekend. My contesting ambitions are muted during the warm summer months and my station is a shambles, as those of you following the blog will know. The weather on the weekend was so hot and humid that staying indoors with air conditioning was too tempting.

With several antennas not available, a competitive effort was off the table. I am also several months out of SO2R practice. On a whim I decided to enter in the QRP class. By thus lowering my expectations I could relax and enjoy 10 hours of summer contesting.

Choosing QRP caused one small problem. The primary radio, a FTdx5000, can only be lowered to 10 watts. I ran into this once before and I built a 3 db attenuator capable of dissipating 5 watts of CW or SSB; it wasn't robust enough for key down, and that isn't necessary. Not expecting to do QRP again with that rig I repurposed the aluminum enclosure for another project. 

I did keep the core of the attenuator -- a π resistor network -- storing it in a small plastic bag. I should get rid of lots of junk I've accumulated over the years, but the attenuator hardly takes up any room. The greater danger was misplacing or losing it because it's so small. But there it was in a drawer of the operating desk and I pondered what to do. 

I was in no mood to spend the time punching holes in another enclosure for a one-time use. So I improvised. With a couple of SO239 jacks, wire and a few minutes solder, I produced the monstrosity below.

No, you should never do this! RF circuits need be wrapped in a conductive enclosure to keep RF in and to keep RF out. But it worked after a fashion. With a calibrated load on one end the SWR gradually rose with frequency, reaching about 1.15 at 30 MHz. That's pretty good, and better than I had any right to expect. The power meter read about 5 watts, so the primary objective was met.

The 3 db of attenuation on receive was no inconvenience: any signal so weak that 3 db would make it inaudible is someone I'd never be able to work with QRP anyway. On bands below 10 meters even that is irrelevant since atmospheric noise dominates the SNR.

Knowledgable readers will see that there is are potentially serious drawbacks of the open air design, related to those already mentioned. Any nearby noise source would leak into the receive path, and on transmit the leakage could affect the receiver of the second radio (for SO2R).

Tuning across the 10 meter band where the problem should be most severe, I heard a number of electronic noises that are likely coming from computers in the shack and other electronics in the house. The spurious signals were strong enough that I decided it would be worthwhile to deal with it. After about 30 seconds of not-so-serious thought I invented version 2. It took another 5 minutes of my time.

This proves that monstrosities can be made into truly absurd monstrosities. A strip of aluminum foil stolen from the kitchen was spirally wrapped around the attenuator and firmly taped to the SO239 jacks for a conductive seal. The exposed leads and resistors connecting the centre pins were taped to avoid shorts when the foil was wrapped.

Yes, this is a monstrosity but it is a better performing monstrosity. The spurious signals dropped at least 20 db. The SWR also improved, though only slightly. The contest start was rapidly approaching so I declared the project a success and spent several minutes readying the rest of the station. The attenuator worked flawlessly during the contest.

A couple of days later I contemplated what to do with it. I can spare the connectors so it could go back into storage. I didn't expect the foil to survive storage so I prepared to remove it. I stopped and wondered how it truly performs. I pulled out my VNA and put it to the test before ripping off the foil.

I measured S11 (SWR) and S21 (insertion loss). I did the S11 measurements with a calibrated load. The result changed a small amount when I added a coax jumper to facilitate the S21 measurements. The test setup is shown above. I did the measurements with the foil covering first to best reflect its performance during the contest, but I'll show the VNA plots in the order I built the attenuator. The foil is too fragile to survive a second wrapping.

The coax jumper is not the best, which explains the higher SWR than what I measured when it was first built. The calibrated coax tails from the VNA are too stiff and short to connect to both ends of the attenuator without using the jumper. An SWR of 1.2 on 10 meters is pretty good. No tuner is necessary to prevent the transmitter from folding back power, and in any case the antennas are responsible for most of the higher SWR where it does occur.

Insertion loss is slightly less than the required -3 db to drop 10 watts down to 5 watts. The power meter read very close to 5 watts on both 20 and 10 meters, so either the meter is misreading or the transmitter is not quite accurate on the power setting. Setting and measuring RF power is something of a black art and accuracy requires careful test methodology and calibrated instruments.

My measurement of the RF power is good enough for amateur work, and for a one-off application at that. In the worst case the BPF add an insertion loss of from -0.2 to -0.5 db, depending on band, so I was easily at or below 5 watts even if I started with 5.5 watts.

This plot is with the aluminum foil wrapping. The foil has a negligible effect on the insertion loss, which is no surprise. The improvement of the port impedance is more significant. The reduction of common mode and radiation into and out of the attenuator are likely responsible. The measurement data support what was already evident from the sharp reduction of spurious signal reception when the foil was added.

Many readers will have no doubt thought up ways to improve the attenuator. Well, so have I, but that was not the point of the exercise. I wanted it to be quick and functional for a single operating event. For example, with 15 minutes of work I could have run wires between all 4 corners of the jacks to improve it mechanically and electrically. I deliberately avoided doing that.

Finding the optimum balance between excess effort and poor performance is never easy. Others in the same predicament might lean either toward higher quality or shoddier construction. I simply find it interesting how much can be accomplished with a minimum of effort when you understand the underlying physics and mechanics of what you're doing. 

The less you understand the more you tend to overbuild or underbuild. The same is true of almost everything in our stations: antennas, towers, small accessories and more. A little knowledge goes a long way. Amateur radio offers an ideal playground to learn, and to practice what you learn. 

I hope you enjoyed this (slightly) silly midsummer distraction.

Frankenstein's Prop Pitch Motor

When we last visited my malfunctioning prop pitch motor, a variety of problems were evident. After that article was published I proceeded with a complete disassembly of the gearbox. That uncovered more problems; poor low-temperature performance was the least of them. 

Several bearings were in poor shape. Among those are the 6 bearings supporting the 3 low-speed planetary gears. According to K7NV, perhaps the foremost expert on prop pitch motors, they have no modern equivalent. I was able to confirm that with extensive catalogue searches. Note that I'll be citing the K7NV web site numerous times in this article. If you have an interest in prop pitch motors you'll enjoy browsing his web pages.

I can try to repair the bearings or I can fabricate shims to accommodate the closest alternative. With the oil and grease removed in a solvent bath there are two of the group that exhibit damage to the races and/or balls serious enough to warrant repair or replacement.

Another difficulty is the sleeve on the motor end of the gearbox axle. It has splines that match the motor shaft splines and the sleeve cannot be removed with any jig I could cobble together in the workshop. I'll have to take it to a professional.

None of these are insurmountable problems, but it will take time. Summer is running away from me and I need the 15 and 20 meter stacks to be fully operational before the fall contest season. There are many other jobs to be done and I can't spend all my time on one motor. 

In the midst of these woes the elderly father of a friend of a friend was downsizing and had a small prop pitch motor for sale. After speaking on the phone and reviewing pictures of it I went ahead and bought it, sight unseen. The price was attractive, especially since it came with a DC power supply and rudimentary direction indicator. A friend picked it up and saved me a long drive.

When it arrived I connected it to my existing controller and got a big surprise. The motor turned at 4 or 5 rpm. That's much faster than the more typical rotation speed of 0.6 to 0.75 rpm. I called the seller and he, too, was surprised.

He had never opened the gearbox and had no idea that it was performing in any way that was unusual. The motor has a long pedigree of many owners, lost in the mists of time. Like those earlier owners, he controlled the speed with a Variac or similar voltage rheostat; that is; with a voltage below the nominal 24 VDC. Not everyone is a prop pitch motor expert -- it works to their expectations and that's that!

I refused his offer to refund my money. The spare parts and electronics are worth the price. However there was still a mystery to be solved. I had a vague recollection that some early owners of prop pitch motors modified them to run at a higher speed, or lower voltage. I had never come across one before and I was curious how it was done.

It turns out that the way it is done is horrifying. A friend dug through his extensive prop pitch motor library and found an article from a 1949 issue of CQ magazine that described the mod. It's a non-reversible mod that destroys a critical component of the gearbox. 

With trepidation I opened the gearbox. I could have been more careful about it because a quantity of machine oil poured out and splashed over the workshop bench and floor. Almost everyone who converts these motors for rotator service drains the oil and does not replace it. The oil is removed to prevent fouling the motor when it is mounted upside down. The oil seals are not perfect and the original bearings are open and need to splash through the oil reservoir. 

The usual procedure is to replace the original bearings with modern sealed bearings and to grease the gears. That wasn't done for this motor. However, the oil seals seemed to be working well.

After cleaning the mess I discovered that the motor was indeed modified per that 1949 article. In the picture I am pointing at the cut edge of the large bell gear. The bell gear of the motor being repaired is shown for comparison. 

The smaller diameter gear of the low-speed planetary gears have nothing to mesh with. Steel plugs (one is shown) directly couple the oil channels of the bell gear and the carrier for the low-speed planetary gears. One stage of speed reduction is thereby eliminated.

It is an atrocity to do this to a perfectly good prop pitch motor. Or is it? A digression to review the historical context is enlightening. 

After WW II, these motors were often easily and cheaply acquired on the US military surplus market. Indeed, many ham shacks of the late 1940s and 1950s were filled with war surplus transmitters, receivers, cables and other odds and ends. Few hams had towers and yagis, and large rotatable HF yagis were very rare indeed. It was still the early days of broadcast television and the mass market for rotatable TV antennas and rotators. Many hams made their own rotators.

A 24 VDC power supply was easily built, and everyone knew someone who could do a little machining and welding to adapt the motor to a tower plate and mast. But the motor turns slowly and it makes quite a lot of noise for a suburban neighbourhood. Bypassing one stage of reduction in the gearbox speeds up rotation too much, so the motor voltage is reduced to compensate. The motor is much quieter when run at only 1000 to 2000 rpm. Problem solved. That is, until a ham like me comes along decades later.

So, the modification is not an atrocity, just a disappointment. I considered options. The motor mounts and housings are of a different design and it was no simple matter to substitute components from one motor housing to the other. Over the ~25 years these motors were produced there were a variety of engineering changes, not all of which were backward compatible. The mast drive system on the new motor is well done (see pic at the bottom of the article) but it, too, is not easily retrofit to my mast and I am not going to take down the 15 and 20 meter yagis and mast to do it.

I pulled the planetary drive assembly from the gearbox to compare it with the malfunctioning unit. They are identical, right down to the part numbers stamped or printed on the components. All the bearing and gears were working properly.

I decided to mix parts from the two dead or half-dead prop pitch motors. Hence the reference to Frankenstein in the article title. 

The critical step was to drop the newly acquired planetary drive assembly into the old housing. That sounds straight-forward, except that I ran into a few complications.

The first thing I did was a cold temperature test of the assembly. As I did to diagnose the problems with the one I pulled down from the tower, I placed it in the freezer overnight. Not all oils and greases have a temperature range to match our climate.

The next morning I checked all the bearings and they spun freely. The oil didn't noticably thicken. Condensation moisture gradually evaporated and did not appear to foul the oil residue. This is important since despite the best waterproofing moisture will condense inside the housing at night when the relative humidity is high. Condensation and freezing fouled a couple of the open and improperly greased bearings in the old planetary drive assembly, which contributed to its poor low-temperature performance

Without the oil bath to keep the bearings and gears lubricated, grease would have to be applied. I was not prepared to fully disassemble the planetary drives to replace the bearings with sealed units. It would take too long and there was a risk of damaging something.

I may pull the motor down again next year to inspect the durability of the lubricants and bearings. At this point I simply need it to survive the coming winter and contest season.

I bought suitable wide-temperature range oil and grease, oiled the bearings and packed grease where I could. Gear cogs were heavily coated with a water-resistant grease as recommended by K7NV.

In the picture the oiled and greased planetary gear assembly is pressed into the motor side of the old housing, along with the ring gear that is sandwiched between the halves of the housing. The only thing left is to press the large bell gear onto the assembly and the gears of the low-speed planetary drive. That did not go well.

There is a trick to mounting the bell gear that is due to the asymmetry between the gear cogs that engage the ring gear and those that engage the bell gear. I assumed they were aligned since the assembly showed no obvious sign of having been taking apart in the past. No matter what I did the bell gear would not fit.

Checking the alignment was a messy job since the gears were heavily greased. There are alignment marks on the 3 low-speed planetary gears that must all be oriented outward. I posed one of these gears of the disassembled planetary system to illustrate the procedure.

I removed the ring gear and rotated the planetary drive while wiping grease off the upper surface, searching for the alignment marks. Two of the gears were aligned but not the third. It had been disassembled in the past and not correctly reassembled. You can do that with the modified gearbox because the ring gear will engage the planetary gears no matter how they're oriented.

With a long sigh I removed the nut from the axle and pried off the top bearing and low-speed planetary drive. With the cogs disengaged from the gear on the bell gear for the high-speed planetary drive, the gears were aligned and then carefully pressed back onto the axle and spur gear. Despite being covered in grease up to my wrists I grinned when the bell gear easily dropped onto the planetary drive gears. 

I repacked the grease and proceeded to finish the job. Mostly this consisted of cleaning the remaining half of the housing and renewing the seals. Waterproofing the coupling to the mast is a separate task that I've already begun. I temporarily used a small number of bolts to hold the housing together and secure the motor to the housing. There were two tests to be done before the rebuild could be considered complete.

It was more convenient to take the motor to the controller than the opposite. Well, sporadic E season isn't quite over and it's easy to monitor or operate digital modes on 6 meters while testing the prop pitch motor. I gave it a lengthy spin in both directions and it behaved as it should. Rotation speed is the same as before, which is about 100 seconds to turn 360°. The motor on the other tower does the same in 80 seconds. I have an idea why it's slower but that will have to wait for another time, perhaps in 2023.

I removed the motor and freezer tested the reassembled gearbox for low-temperature performance. That went well so I remounted the motor and installed all the bolts holding the components together.

Assuming no new obstacles intervene, the motor should be back in service by mid-August. I am making a few changes to the mast coupling system to better centre the mast coupling and to keep water out of the motor. Although the drive system appears to be well sealed there's clear evidence that some water is leaking into the crown gear and from there into the gearbox. That's another good reason to use sealed bearings throughout.

I cleaned the waterborne rust that coated the inside of the hub. It was washed down from the unpainted coupling pipe. The high quality steel comprising the prop pitch motor housing and parts is almost immune from rusting.

I'll resume work on the disassembled gearbox when I have time this winter. I plan to rebuild it with sealed bearings and, hopefully, repair the damaged bearings on the low-speed planetary drive. That will give me more prop pitch motor service options for next year and beyond.

One final point worth mentioning is that the recently acquired motor has an adapter plate. This is ideal for keeping water out when mounted upside down for rotator service. Unfortunately I had to set it aside since the mast coupling system I built isn't compatible with it. It seems a shame to waste so I may try a retrofit in future.

I've learned a lot about prop pitch motors while doing this repair job. I am no longer shy about cracking them open and getting my hands dirty, literally! They are truly impressive devices.