Sunday, August 2, 2020

Experimental 40 Meter Yagi Element

A 3-element rotatable 40 meter yagi is big, really big. It is not an antenna that accommodates repeated adjustments since it is far too large and heavy for repeated lifting, unless you own a crane. Ideally you want to get it right the first time.

With modern software tools such as NEC2 it is entirely feasible to do so. However that is only true for unloaded elements to which you can reliably apply SDC (stepped diameter correction). Otherwise your options are more limited: NEC4 (expensive); or, purchase a commercial yagi (also expensive) from a company that has (hopefully) designed it properly. EZNEC version 6 supports SDC when using discrete loads (e.g. coils) but not appurtenances like capacitance hats.

If you have a friend with NEC4 you can ask for their assistance. I don't and in any case I would like to do the entire job myself for the challenge and for the education. Therefore another option is required.

In an earlier article I went through the mechanical challenges of building a 40 meter element. The small capacitance hat reduces the length to approximately 90% of full size and shifts the third harmonic resonance well outside the 15 meter band. This element is the basis for a 3-element 40 meter yagi.

Before liftoff: model calibration

I am using EZNEC with the NEC2 engine and using a combination of model extrapolations and in-the-field experimentation to generate the required data. The experimental element is critical to the process. It is split for dipole feed to allow accurate measurement with an antenna analyzer.

Although NEC2 cannot model this antenna it does inform the design. It is desirable to get close to the target frequency before the antenna is lifted into the air. Relying solely on trial and error will result in a lot of extra physical work which I want to avoid!

The first task is to estimate the error due to the lack of SDC. To do this I modelled the element without capacitance hats and plotted the impedance in free space with and without SDC enabled. The difference is stark.

For SDC enabled the antenna resonates (X = 0) at 8.03 MHz and it drops to 7.62 MHz when SDC is disabled. That's a shift of 410 kHz or 5%. This is an effect that should never be ignored or guessed at.
Note: For shifts greater than 2% or so it is better to use a ratio rather than an absolute (linear) frequency offset as an accurate scaling factor. Yagi parasitic elements are almost always offset more than that relative to the design (centre) frequency.
The other important effect is that of ground. A 40 meter dipole up 30 meters (¾λ) is strongly influenced by the presence of ground. Therefore we must inspect the R and X components of the impedance; however we need not be concerned about the effect of ground on the far field pattern.

For a yagi the effect of ground is small at this height so we need to know how much the experimental antenna is effected by ground and subtract the difference to know how it will behave as a yagi parasitic element. That is also why it is routine to model a yagi in free space and expect it to work the same on the tower.

For the dipole at this height the model calculates that resonance is lowered by 45 kHz (0.7%) compared to free space -- from 6.730 MHz to 6.685 MHz. For example, the actual measurement of the antenna as modelled in EZNEC is resonant at 7.110 MHz. We add 0.7% kHz to get 7.160 MHz, which more accurately describes its behaviour within a yagi. The adjustment to free space is desirable since a 45 kHz frequency shift of a 40 meter yagi's optimum pass band is quite large.

Also notice that the absence of SDC lowers the resonant frequency from the measured 7.110 MHz to 6.685 MHz in the model, or 6% lower. This is almost equal to the modelled 5% done without capacitance hats (see discussion above). The large difference exemplifies how challenging it is to calibrate for loss of SDC. Measurements are superior to relying solely on model calibration for complex elements.

As we'll see in the measurements the model's R value is also incorrect when SDC is disabled. It is more than 20 Ω lower than the correct value. The radiation resistance is less affected by the slight shortening of the element than the model calculates, which is important for antenna efficiency and gain.

With these model predictions in hand we can proceed to finalize construction of the experimental element and lift it into the air.


The element is as described in the previously provided link. One additional style of joint was added to extend element tips and capacitance hats with ¼" aluminum rod: a set screw. It is only suitable for experimentation since it is not mechanically sound. A slot and gear clamp or compression clamp are recommended for a permanent joint.

A hole is drilled near the outer end of the ⅜" tube and tapped for a 6-32 screw. It is a delicate operation since just 2 threads fit within the 0.058" tube wall (thread pitch is ~0.03"). I used ½" long screws since I had them but ¼" are better. The nut locks the screw after it is tightened against the rod. Since the fit is sloppy the nut should be little more than finger tight to avoid damaging the threads or lifting the screw off the rod.

The ¼" rod was the easiest and cheapest way to extend and adjust the element and hats. The rod is cheap and I have a bunch of it from a previous antenna project so there was no need to sacrifice more expensive tubing for the experiment. I hadn't expected to extend the capacity hats until I ran into an issue that I'll describe later in the article.

Rigging and lifting

The size of the antenna made for a cumbersome lift. I needed a location in the hay field with a clear path by tram line to 105' (34 m) that would keep the tips and capacitance hats from fouling in the guys of both tall towers, antennas and tree and with a convenient anchor for the tram line, ropes and related equipment. I chose the best location that didn't require the effort of screwing an augur style anchor into the ground

You can see in the above picture (with Alan VE3KAE steering the antenna) that the tram line anchor is the base of the 140' (40 m) tower with the 15 and 20 meter stacks. The tram is pulled tight with a winch and the antenna is manually hauled up by rope. It isn't too heavy at 52 lb (plus rigging) but it is tiring for one person. For the second session we had a larger crew to share the work.

Tag lines were added to steer the antenna around the guys you can see on the right. To protect them the capacitance hats were spread when the antenna was lifted a few meters off the ground and folded before touching down. I do the same for the XM240 and its smaller though more fragile capacitance hats.

We had minor tangles with the guy set whose attachment point was just a few feet below the top of the tram. Had we gone any higher the top guys would have been a more serious hazard.

It was awkward but worked surprisingly well. Tangling was easy to rectify without damaging the antenna. Chains and shackles connected the centre plate to the tram line pulley and haul rope.

The binding posts were attached to the feed point on the ground since it is an easy item to drop from the tower. The more fragile analyzer was carried up with me. It's amusing to see a tiny feed point on the fat pipes of the antenna.

The antenna survived multiple ascents and descents despite some guy tangling and bouncing on the tram as the haul rope was pulled. The improvised split and insulated element centre withstood the abuse without any problems. The antenna was blown off its ground supports twice during high winds between the two trial dates and suffered no damage, not even to the relatively fragile element tips and capacitance hats. That bodes well.

Trials, measurements and implications

In total we did 6 lifts of the antenna to gather the required data. That should be enough to confidently interpolate and extrapolate the dimensions of the 3 yagi elements using EZNEC without benefit of SDC.

The first measurement was to set a base line. From a rough extrapolation from the software model I expected the antenna to resonate higher than band centre. In that I was correct since it resonated at precisely 7.300 MHz. Correcting for the modelled ground effect the free space resonance should be close to 7.350 MHz.

For each trial I determined the resonant frequency, the impedance ±350 kHz and the frequency of minimum SWR for the third harmonic. The 5 subsequent trials explored variations of the following parameters:
  • Length of the inner (largest) diameter section, which also shifted the position of the capacitance hats
  • Length of the outer (smallest) diameter section (element tips)
  • Length of the capacitance hat arms
  • Test approximate dimensions for director and reflector elements
Only one variable was changed at each trial. This is necessary to avoid confounding factors that will leave you wondering the magnitude of two or more changes. You may think you're saving time but you are in fact creating doubt and ultimately making more work for yourself.

I was careful to orient the antenna so that there was minimal interaction with the guys above and below the antenna. The measured difference was only a few ohms yet that can make a difference for optimum yagi performance. You can get away with inaccuracies for a single element antenna that you cannot for a yagi. Don't deceive yourself into believing you can take shortcuts.

For the second trial the 1.9" pipe was extended outward 6" on each half-element. This simulates a 5' centre section of 2.375" pipe. The modelled difference of extending either diameter of pipe is only a few kHz using the baseline SDC model. The longer centre section may only be required for the reflector element. The third trial extended the tips by 6" using ¼" rods; there was not enough spare length in the outermost tube sections and I didn't want to unnecessarily waste new tubes cut longer.

As expected the rate of resonance shift differed for equal length changes at the centre (large diameter) and tips (small diameter). Within the 40 meter band a 6" length adjustment shifted resonance by 90 and 100 kHz at the tips and centre, respectively.

What surprised me was the large effect of small changes to the lengths of the capacitance hat arms and (per the model) their diameter. With the arms of the capacitance hats increased from 43" to 48" with ¼" rods resonance in the fourth trial was 155 kHz (2.2%) lower and the resistance (R) dropped from 76 Ω to 69 Ω (9%). I did not confirm the modelled effect of different diameter tubes in the hats.

The resistance drop indicates that, as expected, the more an element is loaded the lower its radiation resistance. In this case the decrease is not enough to noticably effect antenna efficiency. It can be substantial for elements significantly shorter than 90% full size, and will vary with the loading method.

When tuned as a reflector below 6.8 MHz and the original 43" hats the 22 MHz third harmonic was close to the 21 meter band. The additional hat length pushed it close to 22.5 MHz, which is acceptable. The third harmonic went as high as 25 MHz for a fundamental resonance of 7.7 MHz. Since I am unconcerned about pattern degradation of nearby 12 meter antennas this is fine.

My conclusion from the data is that the capacitance hats perform as expected to eliminate the destructive 15 meter interaction. Further modelling is required to confirm that this result applies to the yagi. The longer 48" hats will likely only be used for the reflector.

Length choices for the frequency extremes are greater than required for the director and reflector. These were deliberately chosen to bracket the targets during the trial to allow more accurate interpolation versus extrapolation. I am now confident, for example, that tip and hat extensions are only required for the reflector element. I prefer not to use ¼" rods even though I know others do so successfully in similar climates.

The ±350 kHz impedance measurements were approximately 45 - j35 Ω and 120 + j35 Ω. The reactance values were a little higher at the highest resonant frequency tested. These values were expected at the ±5% frequencies and their confirmation will help with designing the yagi.

Recall that tuning of the parasitic elements is to achieve the correct phase, not resonant frequency, since that and the element spacing determines the mutual impedance and the far field pattern. The reactance determines the phase shift. Tuning by resonant frequency is an heuristic that is only valid for unloaded dipole elements; it is not a general solution. The reactance measurements are critical.

Next Steps

I may conduct one or two more trials, but not immediately. It would be helpful to confirm the effect of tube diameters on the 48" capacitance hat arms since they are acutely sensitive to the diameter. These will have longer ½" tube centres and not ¼" tips to keep the joint count to a minimum. Other than this case I expect to rely on interpolation from the current set of measurements.

I will build an element resonant near 7.1 MHz with a continuous 2.375" centre pipe and gamma match. Although gamma matches are not usually employed on simple dipoles it avoids splitting the element for dipole feed (also required for a beta match) for a robust mechanical design.

Modelling the 3-element yagi has already begun. However there is no rush since I don't intend to build it this year. For now it is enough to generally characterize element parameters to achieve the yagi design I want. I am encouraged that the modelled performance parameters -- gain, F/B, bandwidth and SWR -- are close to that of a 3-element yagi employing full size elements.

Proceeding in small steps is a sensible way to custom design and build an antenna of this size. Later this year I will lift the dipole to the top of the mast of the 150' tower, where the XM240 was once located and performed well. This will give me a high 40 meter antenna until I am ready to build the yagi, albeit one with no F/B and less gain than a yagi. The yagi will be my major antenna project in 2021.

The dipole will test the robustness of the element design in severe weather through a cycle of seasons. Intensity of wind, electrical discharges and ice are greater at that height and I want to be certain that the element is equal to the challenging conditions. A 3-element yagi made from these elements will experience even greater mechanical stress.

I want to build this antenna well. It has to last a lifetime: mine.

Heading down after a job well done

Thursday, July 23, 2020

Phasing Stacked Yagis with Coax

In the most common configuration stacked yagis are fed in phase to develop maximum gain in the forward direction. Reverse phase feeding is a less common option that is used to create high angle lobes. The latter is not my concern since I have a low tri-bander (or two) for that and I can keep the stack switching uncomplicated.

The home brew stack switches I am now building transform the 25 Ω of the two 15 and two 20 meter yagis connected to 50 Ω. The yagis are connected in parallel at the switch. Control lines and relays allow selection of either the upper or lower yagi, and to bypass the impedance transformation network.

When the upper or lower yagi is selected the length of coax from the switch to each yagi is unimportant. It simply has to be long enough to span the distance and minimize transmission line loss. When both yagis are connected in phase the coax to each yagi must be the same electrical length.

There are installations where it is helpful to have different lengths, but in those the difference must be an integral number of electrical wavelengths. To stack multi-band antennas such as tri-band yagis you must use equal lengths of coax since you cannot preserve phase on all 3 bands with any other combination of lengths. However you could do it for just 20 and 10 meters since they are harmonically related.

For my 15 and 20 meter stacks the coax lengths will be equal. This is not as simple as it sounds. In addition to the physical length there are a few factors that must be taken into account:
  • Coax contained within the common mode chokes: coax in the coil is part of the total length for phasing purposes; when current baluns are used they must be identical and identically wired
  • Polarity of the yagi feed points: it is almost always better to place (in my case) the gamma match on the same side of the boom rather than having to compensate with a 180° phase shift (λ/2) for correct phasing
  • Rotation loops for rotatable yagis: the switch box will have to be closer to the rotating yagi (if there's just one it's usually the upper) to equalize coax length
  • Velocity factor of the coax where more than one type is employed: I use a combination of RG213 (rotation loops), LMR400 (chokes and between the yagis and switch box) and Heliax (low loss runs affixed to the tower)
  • Connectors and adaptors: care is needed to include these in the total length of the coax
There is also the matter of accuracy which effects phase and therefore stack performance. This is a separate issue that I'll return to later in the article.

Accounting for the lengths of every section of coax and is tedious work, but it must be done. Although my exact requirements were unknown when I raised the lower yagis this winter I was careful to measure out the length of the coax in the chokes and along the boom so that they are within easy reach on the tower.

For the upper 15 meter yagi the coax is longer so that it can be reached from the tower without climbing the mast. For the upper 20 meter yagi the coax must end well before the rotation loop so that the connection can be secured to prevent flexing of the joint during rotation.

The picture shows the current state of the stacking project, with 3 of 4 yagis in place. Annotations highlight where the components of the phasing system will be approximately located.

The only parts installed right now are those coloured red and white, with the exception of the jumper from the transmission line. Those lengths of coax are otherwise difficult to see clearly in the photograph.

Calculation aid

To plan it all I developed a spreadsheet where I can enter the length and velocity factor of every section of coax. The electrical lengths are calculated and summed. The lengths can then be adjusted until the they are equal. It is important to ensure there is enough coax to reach between adjacent sections, and that calculation must be done manually.

Make the tower runs a little longer than necessary to guarantee that they reach. I included an "extra" length which I didn't need but which may yet prove useful for lengthening the sections.

This is not the final version of the phasing harness calculations. During a tower climb this week I measured other rotation loops to for planning data. Ordinarily I don't record these lengths since they are not critical and every tower has different requirement. For stacking the length is critical. Exact spans to the switch box are pending final measurement. Wherever possible the lengths are an integral number of feet to keep it simple, and that helps to avoid mistakes later.

The velocity factors are those for the types of coax to be used. It is 0.66 for solid polyethylene dielectric RG213 and equivalents (rotation loops), and 0.84 for the foamed PE found in variants of LMR400. Remember to confirm the velocity factor for the coax you choose. Heliax VF is 0.89 or 0.90, which I won't be using for phasing these stacks.

I am planning on LMR400 for the tower runs since at the moment I've run low on Heliax connectors and my usual sources of surplus connectors have run out. The additional loss on 20 and 15 meters for these relatively short lengths is quite small. The approximately 100 meter runs from the remote antenna switch across the field and up the tower to the switch box are Andrew LDF5-50A Heliax.

I now need to order more PL259 UHF connectors. Although every past order includes extras I always run out sooner than expected. I have lots of soldering ahead of me. Others prefer crimp connectors or purchasing custom lengths of coax with connectors. I don't mind the few minutes to solder connectors, and it isn't difficult with silver plated connectors available at good prices from many outlets. For repairs on the tower on coax that cannot be lowered to the ground crimp or compression connectors are avoid soldering connectors on the tower which, believe me, is difficult.


With so much discussion about getting the lengths of the coax exactly equal for in-phase stacking you might believe this is critically important. It really isn't. A sensitivity analysis with software is enlightening. In EZNEC I varied the lengths in the two runs of coax to the individual yagis to determine how close they need to be before performance is noticably degraded. I've done similar exercises in the past, including one many years ago for quarter-wave transmission line transformers.

My chosen criterion is a reduction of -0.5 db forward gain. This is arbitrary in that 0.5 is almost negligible but might make a difference in practice.

With respect to the phasing coax you don't need to be concerned about is F/B, F/S and SWR. When the separation between yagis is large enough the mutual coupling too small to appreciably affect each yagi's individual behaviour. That is the case for my 15 and 20 meter stacks. Although there exist heuristics for this measurement it is better to do a software model.

F/B and F/S are little affected since a summing of fields which are already very small will remain small, and poor coupling limits impedance effects; they really cannot degrade. Guy wire interactions are a greater concern with respect to pattern and SWR.

What can degrade due to phase error are the minor lobes and the nulls between lobes. Those are directly determined by yagi phasing as we are about to see.

The model held a surprise. The coax (LMR400, VF 0.84) to one of the yagis had to be shortened 4' (1.2 m) for the gain of the main forward lobe to drop by -0.5 db. (The absolute gain in the above chart is understated due to an average gain error caused by the NEC2 model of the gamma match.)

That 4' difference is an error of just under 0.1λ (35°) on the 15 meter band. For one quarter the error (1' or 30 cm) the effects on pattern and gain are negligible. A few inches (cm) of accumulated error is not a concern. Although you clearly needn't obsess over every millimeter don't use this as an excuse to become reckless. As you go up in frequency length errors become increasing important, and are critical at VHF and above.

More significant that the gain is the filling of the nulls, especially the large one between 30° and 40° elevation. This is expected. Indeed, feeding both yagis 180° out of phase (BOP) is a common way of increasing the elevation angle, at the expense of the low angle main lobe. Many commercial stack switches have this feature.

Next steps

A few days ago I took delivery of the parts needed to build the stack switches. In the coming weeks I will build and tune the switches for the 15 meter and 20 meter stacks. Assuming other projects don't clog my schedule I should have both stacks fully operational by late September.

Other than the phasing and switching the upper 20 meter yagi has yet to be raised (it's a monster). The prop pitch rotator has to be installed and there is a lot of cabling to be done and a temporary control system built for the operating desk. All of this will take time.

Saturday, July 18, 2020

6 Meters: Waiting for the Big One

6 meters is not for every DXer. The bulk of the openings are sporadic E and it really is sporadic, even during the peak season. Openings can occur when it's inconvenient, such as meal times, while you are at work or you are simply busy with life. HF openings are far more predictable, so much more that you can schedule your operating time and expect to meet with success.

It doesn't help that sporadic E peaks during the warmest months and mostly when the sun is shining, and in this climate you want to get out and enjoy the nice weather while it lasts. Turning on an amp in July can make the shack uncomfortable even with air conditioning. Monitoring the band or the spotting networks is surprisingly difficult on an ongoing basis. You're bound to be distracted or forget, and that's when that fantastic 5 minute once a year opening occurs.

When the band opens you are there or you lose out: you snooze, you lose. For the difficult long DX paths there may be just one of those each year, or every 2 or 3 years. Being tied to the shack for 2 to 3 months of the year when the big one might occur can be exciting in May but by July the shack feels like a prison. You become eager to discard the ball and chain.

Is it worth it?

In my experience it most definitely is not for most hams, including those with a love for the magic band. They will not adjust their schedules to monitor or to be active during times of known or likely openings. Even when the big one arrives they will exit the shack to honour their obligations to work, family and community. They may do it with regret but they will do it. They'll rue the loss and hope for better luck next time as they listen to the tale of what they missed from others with different priorities.

Last year I wrote of my small obsession about not missing an elusive opening to Japan. When the conditions were ripe I planted myself in front of the radio, with my evening meal. When the brief opening did arrive I was there. Others were not. Was it worth it? Perhaps I should have waited a year, because on July 11 this happened:

Every signal is a Japanese station. You don't see the other side of the QSOs because I am transmitting on the 00/30 intervals. During the 1 hour opening I worked 30 stations: that's a high rate for FT8! I heard South Korea but failed to work that country. Earlier in the day I worked 60 European stations. That was my best DX day ever on 6 meters.

Almost certainly it won't happen again this year, or next. This is rare and the rarity is what the magic is all about. Eventually the openings will return, hopefully when it's convenient and not when I have to rearrange my life to be there. Does waiting another year or two matter? There is no simple answer.

In contrast to that success this is what usually happens when you do choose to stay glued to the radio:

This was a fantastic opening to Europe -- for others. Notice the absence of signals in the 00/30 intervals, other than a few North American stations focused on domestic activity. Waiting for the band to open was fruitless. Patience is rarely rewarded.

The big guns on the band go one further: regular and intense sessions of calling CQ DX into a dead band to hunt for elusive patches of ionization. Sometimes someone will hear and respond and a QSO may ensue. Other times a "flag" will be raised on PSK Reporter from an unattended station on the other side of the world or just a few thousand kilometers away. The vast majority of the time the result is nothing at all. Rinse and repeat.

Yet it can be intriguing. One night I left my FT8 station running with the antenna pointed over the pole. Polar openings are not uncommon during sporadic E season since the Arctic is in constant sunlight. Some Europeans were decoded during the hour surrounding sunrise. I slept through it. A couple of other nights I nothing was received. One morning while I was calling CQ towards Europe I raised a flag in California a full hour before sunrise, off the back of the yagi. You just never know.

I sometimes wonder if FT8 and tools like PSK Reporter are fuelling our 6 meter addiction with these enticing lures. I wonder if it adds much to our QSO and country totals.

FT8 and constant monitoring and CQing is exposing the existence of unexpected clouds of E-layer ionization. For the scientifically inclined the data is fascinating. But only rarely does it put contacts in the log. Waiting for a momentary pulse of ionization and a similarly diligent ham at the other end of the path can deliver dividends. Is the constant vigilance worth it? Is it enthusiasm, dedication or stupidity?

With my modest station I get frustrated since so many of the DX openings are marginal. Despite having 6 elements up 24 meters my 200 watts often fails to make the grade. I am plagued by partial QSOs or not being heard at all. The wait for the big one is longer the smaller your station. Do you accept the longer wait or do you scale up your antenna and power? How far will you go, and how far will you push your family's tolerance?

I saw a new technique for "having it all" recently. During that astounding July 11 opening to Europe a ham of my acquaintance was full time SO2R in the IARU Radiosport contest. Yet there he was working Europe on 6 meters at the same time. Is that SO3R? Call it a curse or a blessing that this is possible with FT8. Watching him work stations brought a knowing smile to my face.

I was in the contest part time but it was enough for me to do one or the other and not both at the same time. When the MUF dropped below 50 MHz I had fun on CW working hundreds of Europeans on 10 and 15 meters which were also open at the time. As the MUF rose I would return to chasing DX on 6 meters.

Now that it's mid-July the season is growing old and the frequency of openings is tapering. I am fatigued enough that I am looking forward to the end of the sporadic E season. Rare openings do occur throughout the year and the true addicts will keep going. I won't.

If 6 meters is not your passion it is likely that some other aspect of amateur radio is. In that way some of what you've read here may strike a cord. The intensity of your involvement is your choice. As the saying goes: time to do a thing is never given; it must be taken. How much time you take to pursue your passion is up to you.

In a few weeks I will sum up my 6 meter season, the same as I've done the previous few years. It has already been quite successful. With a little luck, and obsession, more rare DX will yet find its way into my log. Last year the biggest European opening was in early August, so perhaps another big one is just around the corner. I can only hope, watch and wait.

Monday, July 13, 2020

Choosing Band Pass Filters

SO2R and multi-op contesters know that BPF (band pass filters) at each operating position are mandatory. On receive the BPF reduces interference from other transmitters and on transmit it reduces out of band noise and harmonics.

Although it is possible to forgo BPF (and I have!) the benefits far outweigh the expense. With low power the interference is so objectionable that many band combinations are unworkable, especially in small stations where the antennas are close together. High power exacerbates the problem and presents severe risk of receiver damage.

The latter occurred a few times in my own experience with high power multi-op contesting decades ago when BPF usage was uncommon. The lamp fuse in the pre-amps of some rigs of that era saved many contest efforts (and bank accounts). We always kept one or two spare transceivers on hand for emergencies. Technically adept team members replaced lamp fuses during their off time.

Today there are BPF with excellent performance:
  • High return loss (low attenuation and SWR) across the pass band
  • High out of band rejection (low return loss), and notch filters for adjacent bands, including the all important second harmonic
  • High efficiency (low power dissipation within the pass band)
  • Automatic band switching by transceiver band data, CAT or software control
  • Bypass when not needed and for non-contest bands (leave them inline between contests)
  • Power rating available from 100 watts to legal limit (ICAS or continuous duty)
A full set of BPF can be expensive. Strangely enough the big gun multi-multi stations have an easier time of it because typically there is a dedicated operating position per band which greatly reduces the switching requirements and thus can lower the cost compared to smaller stations while maximizing performance.

Because they are a specialty product commercial choices are limited and prices can be quite high. Used products sell quickly because they are in demand by the budget conscious. For low cost there are kits or just PCBs with instructions for building your own. You trade your time for money saved. Tuning these high performance devices requires a VNA and knowing how to use it.

Until now I have operated SO2R at my own station without BPF. This limits me to low power or QRP, and even then there are frequency and antenna combinations that must be avoided. Since my objective is to do better with SO2R and to host multi-op contest operations with legal limit power the time has come to add BPF to my station.

For my station, antenna system and contest objectives I can trim the tree of possibilities to find the optimum solution. Before I come to that it will help to discuss the several alternatives. Each has its pros and cons and implementation requirements. I've condensed the alternatives to just 3 and there are variations within each.

There is more than one way to implement BPF in a station. Perhaps the most flexible option (1) is a switchable 6-band BPF -- the HF contest bands are 160, 80, 40, 20, 15 and 10 meters -- plugged into the transceiver. The BPF need only handle the transmitter power for low power or QRP contest entries, or the even lower drive power for a high power amplifier. Oddly enough you can get by with BPF rated for 100 watts if you are high power, yet you need the 200 watt rating for low power.

The transceiver or other control system selects the appropriate BPF for the band, or bypass the BPF for non-contest operation and for non-contest bands. Manual switching is not recommended since you're sure to forget and that can damage the BPF if the transmitter does not fold back power fast enough due to the high SWR. If you hear silence from the headphones check which BPF is selected! Even so it is a reasonable way to start and automation can be added later.

BPF can be placed between the amplifier and the antenna switch (2). This is a more expensive solution since the BPF must dissipate perhaps 10x to 20x the heat compared to low power and the design to achieve high performance is more challenging.

A benefit is that harmonics and other out-of-band noise generated by the amplifier -- all amplifiers do this so transmitter quality alone is insufficient -- are removed by the high power BPF, something that is not possible for low power BPF. Many contest stations with low power BPF compensate with switchable coax stubs after the amplifier to notch the second harmonic. Although the stubs are inexpensive it takes time to design and build them, and to place them correctly.

The high power BPF switching system must handle full legal power. It most always must be home brew since high power commercial filters are sold individually. Power lost in the BPF cannot be recovered; in (1) you can increase amplifier drive power to compensate.

Many high power BPF require cooling fans and that increases shack noise unless they are located remotely. Automation to select the correct filter is more critical than with low power. If the filters are outdoors power must be provided for the fans. In fact, all BPF can be remote if you have confidence in your automated switching system.

Switching and harmonic stubs can be avoided entirely by placing the filters after the antenna switch (3). This can be complicated in stations where one or more of the antennas are multi-band. A common example is tri-band yagis. However when all your antennas are mono-band this is a good choice. It can be economical since only one high power filter per band is needed since, other than in rare cases, no two operating positions are on the same band at the same time. That is, you need only one set of BPF.

A little deeper

No filter is perfect. There is attenuation in the pass band and on transmit that means heat. Filter components must withstand the RF currents and voltages and safely dissipate the heat. At 1 kilowatt a pass band attenuation of -0.3 db equates to 70 watts of heat. That's a lot to dissipate from within a small metal box!

The adjacent band notch can be -60 to -90 db. That's excellent. However even the slightest leakage elsewhere in the system can render this spec meaningless. Relays are the most common bugaboo. The typical small PCB mount relays used in antenna switches can limit cross-talk across the contacts of a SPDT relay to no better than -50 db, and in some cases is no better than -30 db on the highest contest band: 10 meters.

Performance worsens with frequency since the capacitance across the relay is constant but the reactance is frequency dependent. I referred to this problem in my Beverage switch by recommending against the simple reed relay setup for other than low frequency use.

Since antenna switch isolation is not ideal it is helpful that the BPF be between it and the receiver. Out-of-band RF from the other transmitter that leaks through can then be removed by the BPF. However, this does not work for harmonics since, for example, the second harmonic of a 40 meter transmitter is not attenuated by a 20 meter BPF.

That 20 meter harmonic must be removed by a filter on the 40 meter signal path after the amplifier (or transmitter if there is no amplifier). The filter must be a high pass BPF, (2) or (3), or (1) with a supplemental coaxial stub notch filter on the amplifier output. Of course this is not mandatory since you may be able to live with a stronger harmonic on 20 meters by avoiding frequencies close to it. The avoidance area is wider with SSB (and RTTY to a lesser extent) since the second harmonic is 6 kHz wide.

This should also be an incentive to clean up your transmitter since SSB splatter and CW key clicks will wreak havoc on other bands, annoying you and your multi-op team members in addition to everyone else on the band. It is poetic justice should you suffer from your own poor signal.

I'll stop here; I said a "little deeper" not a lot deeper. Filters are not my expertise so I will leave further technical detail alone. Diving into the weeds of Cauer and Chebyshev filters and multi-filar coil windings is for others to deal with. Luckily the ham community includes people with the knowledge to design high performance BPF and we can benefit from their work, only needing to pay attention to the metrics and correct application in our stations.

No product recommendation

I deliberately avoided recommending specific products. I don't have sufficient experience with the many available alternatives. To ease your product search here are links to a few BPF commercial products. There are others, including kits. These are not recommendations and the order is random:
There do exist technical and comparative reviews of some commercial products. All that I've seen are of uncertain quality or are good but not up to date and therefore obsolete, so I won't link to any. It is better to consider products used by successful contesters and not the opinions of a friend or the ham you happen to share a coffee with. Talk to them.

Many contesters build their own BPF from kits and the performance depends on how well they did it, including proper use of a VNA to optimize them. As noted earlier, antenna switches can compromise performance so don't be seduced by high out-of-band rejection specs that may not be achievable in your station configuration. You must carefully design your station to realize the full benefits of a high performance BPF.

I do recommend BPF with notch filters for adjacent bands. The ultimate rejection of a BPF is not enough to avoid noticable interference on portions of adjacent bands. They cost more but this is not the place to choose economy over performance. Your contest scores will reflect your choice, good or bad.

My choice

Although an indoor task I want to work on my BPF solution this summer and early fall rather than leave it for the winter. With tower and antenna work progressing well I believe I can free up enough time to get it done. Well, that's my objective but my track record of keeping to schedule is not good. This is a hobby and I do what I can.

My original plan was to buy commercial products rather than build my own from bare PCBs that are available. Some are very good, but they are not inexpensive. Although I built this into my budget I can find uses for the money saved by going with kits. I do not have a VNA and I my skill at tuning these filters is untested. The filters are mechanically and electrically straight-forward but precision is paramount to achieving ultimate rejection and low pass band attenuation.

I researched options and spoke to a few hams I respect and I decided to build the BPF from kits. The precision tuning should prove interesting. If I have to buy a VNA I will do so. The NanoVNA is inexpensive and widely available from many manufacturers. Several friends offered to lend me theirs.

I now have a kit in hand for a single band low power BPF. If the build is successful I will continue along my chosen path and build the other 11 kits (!) and the switches for selecting among the 6 BPF in each unit. I'll say more about the kit after it's built and tested. Some readers may recognize it from the picture.

This choice (1) will give me the flexibility to handle multi-band antennas, reduce interference due to switch cross-talk and alleviate concerns with high power filters, which would have to be outdoors with the antenna switch. Time is marching onward and I need to be ready for the coming contest season.

Wednesday, July 1, 2020

Joining Pipes & Tubes: 40 Meters

In the midst of the many antenna and other projects going on in parallel I have taken the first steps towards building a 3-element yagi for 40 meters. This is a very large antenna with many challenges. After reviewing the many different techniques I used to join the pipes and tubes I realized it was an almost complete catalogue of the common (and some uncommon) techniques that may be of interest to others.

I built a single dipole-fed element to prototype the mechanical and electrical design of a yagi element. The split feed allows connection of an antenna analyzer. The final antenna will use a continuous conductor on all elements for mechanical robustness and the driven element will use a gamma match. Since a continuous element cannot be precisely tuned with an analyzer this is how I will determine dimensions of the 3 elements.

Precise tuning is critical for a yagi. NEC2 supplemented with SDC (stepped diameter correction) is accurate but is only reliable for unloaded elements. My prototype element is ~90% full size with a modest size capacitance hat.

Although the loading reduces weight a small amount the primary reason is to eliminate the third harmonic resonance on 15 meters. That resonance, if allowed, can have a deleterious impact on my 15 meter stacked yagis. Modelling indicates that a small hat will move the harmonic resonance far enough above 21.5 MHz to maintain the excellent pattern of the 15 meter yagis when pointed towards Europe. The planned 40 meter yagi is in the same direction.

That is all I will say at the moment about the electrical design. It is enough background for readers to understand what I am trying to accomplish. There is much yet to do on that front before I am ready to build the full yagi and lift it to the top of the 150' tower. You don't want to do that job more than once so it pays to get it right!

Taper schedule

For the following discussion you can refer back to the following list of pipes and tubes. All pipes are structural aluminum alloy 6061-T6 or 6061-T6511, or aerospace alloy 6063-T832. Tensile strengths of the alloys are comparable. English units are used since that is how our pipes and tubes are sized.
  • 2.375" OD and 2.067" ID schedule 40 pipe
  • 1.9" OD and 1.61" ID schedule 40 pipe
  • 1.315" OD and 1.049" ID schedule 40 pipe
  • 1" OD and 0.120" wall tube
  • ⅝" OD and 0.058" wall tube
  • ½" OD and 0.065" wall tube
  • ⅜" OD and 0.058" wall tube
The largest pipe's ID leaves a wide gap for the 1.9" pipe. The final element will use schedule 80 pipe which is a close fit to the 1.9" pipe. I am using short scraps for the prototype to permit a centre gap and dipole feed point and those happen to be schedule 40.

The capacitance hats are made from the two smallest tube sizes and attached to the outer end of the 1" OD tube.

Ordinary joints

There are two methods of joining tubes that are common enough that I will not discuss them in detail. The latter of the following two methods was previously described for my 15 and 20 meter yagis.
  • Tube slit at one end with the smaller tube slipped inside and held with a gear clamp (commonly called a hose clamp). Length of the smaller tube (portion that protrudes from the larger one) may be adjustable.
  • Tubes drilled through at the overlap and secured with fasteners. Length adjustment isn't possible.
Both methods require that the two tubes are a close fit but not so tight that it is a press fit. This is where 0.058" aerospace tubing in ⅛" diameter increments comes in handy. They telescope perfectly for making stepped diameter yagi elements. The only tubes of this type in the element are ⅜" and ⅝" OD.

The ⅜" tube can be slit to take a ¼" rod to extend the tip, but I have not done so. It will depend on the results of field testing and the amount of adjustment room I've designed in.


The dipole feed has the following requirements. 
  • Interior non-conducting rigid material to mechanically couple the two halves of the element.
  • Backing plate to support the element halves and for attachment to the tower.
  • Element insulated from the backing plate.
It must be sufficiently robust to survive numerous tram trips up and down the tower for repeated measurements and adjustments. Since it's temporary I didn't want to overdo it or spend any money building it. So I looked through my junk pile and then took a drive to look through someone else's junk pile.

The backing plate is 6" × 27" ¼" steel plate. I cut and drilled it after grinding off the surface rust. It will not be painted and is it already forming a new layer of rust. That's okay since it doesn't need to conduct and it's temporary.

Ordinary muffler clamps attach the element halves to the plate. Each clamp is wrapped with a thin, flexible material that I thought was rubber but turned out to be some kind of plastic. I inserted strips of galvanized sheet on bottom and top to prevent the tightened clamps from splitting the stuff.

An old piece of 2×2 seasoned maple joins the element halves. It can support the full element weight but it is not a perfect fit so there is droop. The droop is partially remedied by the backing plate, as we'll see later. I will tape the gap so that rainwater doesn't reduce its insulating properties.

The pipes are tapped for #8 stainless screws to connect the element to the analyzer or coax. The nuts under the heads are needed to prevent the screws from striking the wood since I didn't have shorter screws at hand.

Screw pressure clamp

The 1.9" OD pipe fits very loosely in the larger pipe with its 2.067" ID. Since this is a temporary joint to be replaced with a pipe with thicker walls I opted for a pressure clamp using screw. It's simple and robust enough for the prototype.

There are four ¼" stainless hex head bolts in tapped holes on the 2.375" pipe. Screwing them down presses the smaller pipe against the opposite wall of the larger pipe. The bolts double as set screws to prevent slippage. Nuts can be added under the bolt heads and tightened onto the pipe to avoid accidental loosening.

This method is not recommended for a permanent installation yet it seems to be reliable. The pipe overlap is 12", the maple insert is 24" long and each 2.375" pipe is 24" long. Thus the smaller pipe and maple insert just about touch. This was done to maximize the overlap and strength of the joints.

Slit tube reducer

Element taper does not require using every available step size. For an optimum balance between weight, cost and strength 40 meter elements have a strong centre and step down to much smaller diameter tubing. A reducer is needed for the larger steps. Often these can be made from short sections of intermediate tube diameters. This may not be possible when pipes are used because NPS pipes often don't telescope well with other pipes and tubes.

The first large step is between 1.9" and 1.315" OD pipes. The radial gap is (1.61 - 1.315) ÷ 2 = 0.1475". A tube or pipe of this non-standard wall thickness must be fabricated. Rather than machining a reducer I fabricated a two-step reducer using material on hand. The reducer is 6" long to match the pipe overlap.

A 1.5" OD tube with a wall of ~0.095" was cut to size and slit with a hacksaw. When the slit is slightly spread it fits tightly over the smaller pipe. Another 1.5" pipe of 0.065" wall thickness was cut into 3 lengthwise pieces and one is used to fill the remaining gap. The fit is tight. All of this was determined by calculation and experiment.

When assembled two holes at right angles are drilled through the pipes and reducer for ¼" bolts and nylocs. One of those bolts is centred on the double thickness of the reducers. Both pipes must be aligned so that the element is straight. I moved the drill press to the floor and levelled the pipes with the drill press's work surface.

The finished joint is strong and straight. I don't anticipate any problems. As with all joints the mating surfaces are sanded to remove oxide and then coated with conductive grease. I have always had good success with Noalox but there are alternative products available from electrical suppliers.

Flashing reducer

Sometimes the fit is close but not close enough. There is a tight fit is between the 1" OD tube and the 1.315" OD pipe with its 1.049" ID. There's a radial gap of 0.0245". I keep a roll of aluminum flashing handy for these situations.

One layer of flashing made a reasonably tight fit. I have long lost the packaging and I don't recall the flashing thickness. Since a second layer made it too thick the flashing may be ~0.015". I used #10 screws (3/16") to secure the joint.

Because of the large stress on the joint at this mid-point location on the element I opened up just one of the holes for the screw head to press against the flashing and 1" tube. This ensures a solid electrical connection.

Both sides of the flashing are coated with conductive grease. The overlap is 4".

Closed tube reducer

For ⅛" step sizes and 0.058" walls it is easy to make a reducer. All you need is a short length of the missing step size(s). This is what I needed to make the step from 1" to ⅝". The 0.120" wall thickness of the 1" tube is 2 steps on it own so a ¾" tube fits nicely. Since I didn't have that size with a 0.058" wall on hand I made one from 0.125" wall thickness tube.

I drilled 3" lengths of the ¾" tube as described in the (previously linked) article about building the 15 and 20 meter yagis. I then reamed the reducers to 0.627". The resulting reducers are an excellent fit. With the sunk screw heads the electrical and mechanical bond is strong.

Element tips

The ½" tube is joined to the ⅝" tube with a slit and gear clamp. This joint is the primary one for adjusting element length for tuning so the ½" tube is longer than required for a solid mechanical joint.

The 0.065" wall of the ½" tube must be reamed to accept a ⅜" tube. The few thousandths reduction of wall thickness is easily accomplished with a hand drill and ⅜" and fluted bit. It does not bind or wander off centre, difficulties that can arise when more material must be removed. A reamer is a better choice than a fluted bit for this job, but most ham workshops may not have these reamers.

The job was done in a minute. Unfortunately there is little adjustment room in this joint, perhaps 2". Should more length be needed I will use longer ½" tubes or insert a ¼" rod into the ⅜" tube with a slit and gear clamp.

Capacitance hat clamps

The capacitance hats have a ½" centre and ⅜" tips. They are joined as described above. There is little room for adjustment and that is acceptable. For now I am using fixed length hats and tuning the element by adjusting the element tips.

The 4 arms of each hats are made of two of these assemblies. Each arm is 43" long (1.1 m). The ½" tube is drilled with two ¼" holes (reamed slightly larger) to fit a 1" u-bolt.

While strong enough for the prototype this is a poor joining method. Those ¼" holes weaken the ½" tube, and greater contact area is needed between the tubes for reliable electrical contact and to prevent bending and slippage.

A cursory search did not find turn up suitable commercial clamps with the attributes I need so I have a few design ideas that I will experiment with in my workshop in the coming weeks.

This is the furthest outboard I am willing to mount the capacitance hat. Loading increases (element shortens) the closer the capacitance hats are to the tips. However the tubes further out have narrower walls and the weight would increase droop and reduce survival from ice loads. Since my primary objective is to tame the third harmonic and not to greatly reduce element length the chosen position is an acceptable compromise.

Fully assembled element

The assembled element weighs in at 42 lb (19 kg). The steel backing plate and clamps are a further 10 lb (4.5 kg). I expect the final weight with an aluminum element-to-boom clamp to weigh ~48 lb (22 kg). That is a typical weight for 40 meter yagi elements. The capacitance hats added back most of the weight saved by the 10% length reduction.

Droop and flexing is not as bad as it appears in the photo! It's due to a combination of perspective and flexing at the improvised centre joint. The tips are not touching the ground. Using the 2.375" pipe as a guide the droop to the element tips is only 2'. With a continuous pipe at the centre the droop is expected to increase a small amount, perhaps to as much as 3' at the tips.

That is quite good for an element that is 62' (19 m) long. Ice loading is a greater danger than wind at this QTH and I still need to do the calculations to confirm that it will survive our weather. Similarly constructed 40 meter yagis have successfully survived severe weather, albeit with substantial bending.

Above is a close up of the element centre so that you can better see the causes of the additional flexing. The ¼" steel backing plate is bending more than I expected. However it can withstand the abuse. Compression of the wood fibre of the maple causes misalignment of the pair of 2.375" pipes. Despite the bending and misalignment I have little doubt that it will survive repeated trips on the tram.

Not shown are holes near the top edge of the plate that are for  bolting the antenna to the tower. The orientation of the backing plate will be vertical.

Coming up: lifting and tuning

I am planning to test and tune of the 40 meter element in mid-July, once I get a few other projects out of the way. I expect it to be an interesting exercise, figuratively and literally.

When I have collected all the data that I need to design and build a 3-element yagi based on this element design I will build an element with a continuous centre pipe. It will be fed by a gamma match, in keeping the the "plumber's delight" construction.

I will either side mount it at 100' on the 150' tower or (if I'm brave) raise it to the top of mast. The latter position will produce better data on its robustness and give me a temporary high 40 meter antenna to work distant DX. This could be valuable is the (likely) case that the yagi can't be completed in 2020.

Tuesday, June 23, 2020

Spotlight Propagation on 6 Meters

A long time ago I was driving through the city with one of my siblings. The sky was uniformly gray and a steady rain was falling. Further down the road the rain abruptly stopped within less than one block. Two blocks later the pavement was dry and soon thereafter the clouds parted and sunshine blazed. It had become a beautiful day.

She expressed surprise that the rain could disappear so abruptly, with such a sharp dividing line. I answered that the rain has to end somewhere so why not here. Of course this happens all the time but our perspective is different when we are standing still and the weather moves over us rather than the opposite.

Radio propagation can behave the same. For sporadic E on 6 meters we are almost always riding the MUF, the frequency above which the signals don't reach the ground. The E-layer regions of unusually intense ionization are small and typically only support propagation between relatively small areas, except in exceptional cases. We call it spotlight propagation and like a spotlight the edges are often sharply defined.

A shift of a few kilometers can be the difference between the opening of a lifetime and an empty log. I experienced this phenomenon twice in the past week. With the arrival of the summer solstice the sporadic E season is at its annual peak 6 meters is consuming all the time I have available for operating and makes me all too aware of the joys and frustrations of DXing on the magic band.

I will give you two examples that are illustrative of the phenomenon of a bright line between the haves and have nots. These may help to explain why you are in a DX desert or the land o' plenty.

About a week ago 6 meters was wide open almost everywhere at the same time. Within minutes signals could be heard here from Europe, Africa, South America, the Pacific and the Far East. I didn't know which way to turn the yagi. The rotator had a real workout that afternoon.

I worked a lot of amazing DX, including my first KH6 via sporadic E. The spotlight roamed in the Pacific, eventually striking the sweet spot for working Hawaii. In other places the spotlight stood still, such as in South America where I only heard Ecuador and proceeded to work what may be every HC active on 6 meters. What I didn't work was the Far East.

It was aggravating. Just 300 km to the west many hams I know in FN03, EN93 and nearby grids (and eventually extending west right across the country) were working a steady stream of JA, HL, DU and BY. Here is FN24 I decoded a total of 3 FT8 messages, and that was from a single JA. That's it. While it was going on I was exchanging messages with a couple of friends in FN14, about 100 km to the west, and they didn't hear anything at all (my antenna is better).

In the map below you can see that the beginning of the peak probability for the path to Japan and the vicinity is 2200Z when the sun is midway between here and there, and therefore the greatest insolation on the northern edge of the path. Sporadic E is sporadic but it needs energy input.

This was my second near miss with JA this year. It's a difficult path and the rain has to end somewhere. That somewhere was near Toronto. The good fortune did extend further south, along the same path from the northwest that favoured southern Ontario. You just had to be in the right place; FN24 was in the rain and FN03 was in the sunshine.

I shouldn't complain too much. I had my day with working Japan last year, and there is reason to hope for more this year. To the east, further from the line of propagation, the big gun 6 meter stations heard nothing. It's a difficult path since a short move east brings the path from the Far East closer to the north pole. The benefit of 24 hour daylight in the Arctic at the solstice is rarely sufficient.

The next example of spotlight propagation is one that favoured me and few others. During a marginal opening to Europe I saw 6W1TA calling CQ on FT8. I turned the beam east and his signal came up nicely. Since I worked him earlier this year I let him be. I tend not to make duplicate contacts since you never know if the person will be annoyed or glad to know he's getting through. However I am always happy to respond to duplicate callers.

This went on for some time. Was this a spotlight opening and only I could hear him? There could have been other factors like yagis pointed to Europe or the Caribbean which would significantly attenuate west Africa signals. Perhaps I should have spotted him although that would have required starting other software and I rarely spot FT8 stations since anyone can see them themselves if they have propagation.

It isn't every day you are called by a moderately rare station and especially on 6 meters. I suppose he heard little other than me since the CQing and I didn't notice him call anyone else. Those in rare locales are hams just like everyone else and they want to communicate. I was happy to make the duplicate contact.

It's funny that not long ago I chased him hard to earn the new 6 meter country. Last year I was unlucky and missed him because although the opening was superb it was also superb for everyone else in North America.

Eventually he did work at least one or two other stations. I was amused to later discover that he had spotted me. You need propagation to benefit from being spotted and 6 meter sporadic E long DX paths and this was no different. No one scrambled to call me or they did and heard nothing.

I was going to end the article here when the very same thing happened again the next day.

It is difficult to believe that only I heard him or that everyone in this region has worked him. I monitored 30 minutes of CQ with no apparent callers. His signal was weak but steady all that time. There are a few others active in Africa so hopefully one day I'll work them. But I worked TT8SN already and elected not to bother him.

I will stop now. Should you be wondering what happened to all the technical articles, fear not. I have several projects underway and I won't write about them until they're done. Pictures and words are accumulating and the articles will eventually be published.

While that work continues in the background and with sporadic E in full swing these little articles almost write themselves. I suspect my slow summer blogging pace won't return this year.

Wednesday, June 17, 2020

Don't Be a Curmudgeon

"I've come up with a set of rules that describe our reactions to technologies:
  1. Anything that is in the world when you’re born is normal and ordinary and is just a natural part of the way the world works.
  2. Anything that's invented between when you’re 15 and 35 is new and exciting and revolutionary and you can probably get a career in it.
  3. Anything invented after you're thirty-five is against the natural order of things." 
The quote is by the British comedy writer Douglas Adams. For brevity we'll call the last point the curmudgeon's credo. It applies to pretty much everything. Yes, that includes amateur radio. The demographics of our hobby are such that over 90% fall into the third category. I am and it's very likely that you are, too.

As we grow older not only do we because suspicious of new technology and ways of doing things we are more likely to say it. The social governor we grew in adolescence begins to malfunction. We say what we want when we want to everyone and anyone. We ignore or fail to see the reactions of others. Physical presence is unnecessary: we say it on air or by pounding on a keyboard.

When I was a new ham in the 1970s quite a few of the older generations remained dissatisfied with SSB and transistors -- "real radios glow in the dark" -- and made sure you knew it. Contests and DXpeditions were frequent targets. For my operating interests I endured many derisory comments from the gray haired crowd. I quickly lost my respect for these elder hams. My teenager friends and I sneered and laughed at them and finally ignored the curmudgeons altogether.

Yesteryear's adolescent hams are today's curmudgeons. The passage of time does that. Not all of us succumb though far too many do. They're everywhere. Indeed, you may be one and not realize it.

Do you believe that the quality of hams declined when CW was removed from the license exams? Is FT8 not real ham radio because it's a machine talking to another machine? Do you demean those who use modelling software to develop and optimize their antennas because you know that any old wire tossed into a tree works just fine? Do you lament the endless cries of "599 04" filling the bands during contest weekends despite your not having turned on a rig other than a 2 meter handheld for the past 3 months?

If you answered "yes" to any of those questions you may be a curmudgeon. Even if you thought to yourself, "no, that's not me," there is likely another similar question out there that will tempt you to answer "yes." I consider myself pretty adaptable to the progress of technology and operating practices and yet I occasionally get caught. The certainty of the old is insidious.

There is an antidote. There is a cardinal rule to remember to test yourself for curmudgeonly behaviour. I don't know the original source though variations are widely quoted. It can be a tough pill to swallow.
Never mistake a personal preference for a universal truth.
You see this play out daily in the news, infecting politics, religion, our families and our jobs. The tide against this style of thinking currently fills the streets with protests. Hams are not immune to the same deep misunderstanding. Escaping the trap of "universal truth" requires a leap of perspective.

No one is beneath you because they make different choices. Time changes everything. What was once common is now rare and what was once impossible is now routine. Technological progress changes our culture and especially the interests of the younger generations of hams. There is no right and wrong about it, just that we are most comfortable with what we know and believe.

Remember that and you will avoid becoming a curmudgeon. If you discover that you have become a curmudgeon it is never too late to change. Kick yourself out of your comfort zone and try something new. You have nothing to lose and you may find a new way to enjoy amateur radio and gain new friends.

Sunday, June 7, 2020

L-network for Stacked Yagis

An important component of switching stacks of yagis is an impedance matching network. When yagis are connected together their impedances are in parallel. Two 50 Ω yagis present a 25 Ω load to the transmission line. The parallel impedance should be transformed to 50 Ω to lower transmission line loss and to keep the transmitter happy. The network is switched out of circuit when only one of the yagis in the stack is selected.

The network is 2:1 for a two yagi stack and higher ratios should be used for 3 or more yagis. More than one network may be needed if a variable number of yagis can be selected in stacks with 3 or more antennas. The 15 and 20 meter stacks I am building have two yagis each so that is the antenna system I will primarily address in this article.

The simplified schematic shows the topology of a stack switch for two yagis. The options are upper, lower or both in phase (BIP). Switches for both out of phase (BOP) and 3 or more yagis are similar but more complex. I am sticking with the simplest version since it helps with the explanation and is what is needed for the 15 and 20 meter stacks I am building. 'N' is the matching network.

Most stack switches I've looked at typically have all yagis connected and in phase, therefore the network is in line by default. When only one of a two yagi stack is selected the network is bypassed and that yagi is connected to the input port. The basic two yagi circuit requires 4 SPDT relays rated for the RF power. The diodes isolate the control lines from each other since both must power K1 and K2.

The unused yagi(s) can be left connected to the network under the assumption that the yagis are far enough apart that their mutual impedance is low. This is a fair assumption since otherwise the network would require customization to accommodate impedances that are neither 50 Ω individually nor 25 Ω for two together.

With DC injectors, reverse polarity for one the selections and several diodes the transmission line can be used in lieu of separate control lines (3 wires for the above circuit). For the design of my station it is more convenient to use a dedicated control cable. With a common DC ground a total of 5 wires are required for the 15 and 20 meter stack switches (they are on the same tower).

There are a variety of networks that can be used for the impedance transformation. The most popular for HF are:
  • Broadband transformer: Typically a transmission line transformer with trifilar windings on a ferrite toroid.
  • Transmission line section: ¼λ transmission line to transform the 50 Ω of each yagi to a higher value so that the parallel impedance is 50 Ω or, alternatively, to transform the parallel impedance to 50 Ω.
  • LC network: An L-network or similar network comprise of discrete inductors and capacitors.
Commercial products are almost all of the first type (example). These can be made to work on all bands from 80 through 10 meters with high efficiency and so a single product can suit many applications. Many home brew networks for mono-band stacks use the second type with 70 Ω coax (e.g. RG11) switched into both yagi ports. In all cases the electrical lengths of 50 Ω coax from the switch to each yagi must be equal to phase the yagi feed points for maximum gain.

Although they look simple enough building your own broadband transformer is not without its challenges. These are not conventional transformers but transmission line transformers. These are variations of the excellent design to be found in Sevick's Transmission Line Transformers book. Modern versions use Fair-Rite 61 mix 2.4" OD ferrite toroids.

The number and arrangement of the trifilar windings has a significant effect on efficiency, optimum port impedance and impedance ratio. Get it wrong and the heat generated at maximum legal limit, especially on 10 meters, can destroy the transformer. Compensation for stray capacitance at the highest bands may be required. When properly designed and built they perform very well, with a loss better than -0.1 db (20 watts dissipation for a 1000 watt transmitter) from 80 through 10 meters.

The transformer, and any matching network for that matter, can exhibit different behaviour with high Q yagis (e.g. most tri-band yagis) at the band edges where the SWR is high. Deviations of the impedance ratio and efficiency in these situations can become a serious problem with high power. Regardless of the matching network and SWR it is important that the yagi impedances are near equal at all frequencies of operation to achieve equal power division. Special design considerations for stacking dissimilar antennas are beyond the scope of this article.

For a multi-band yagi a broadband transformer is the best choice since the others have a narrower bandwidth and are only suitable for a mono-band stack, with narrow exceptions as we'll see. Since most stacks are mono-band the narrow band choices deserve a close look. I plan to build my own stack switching systems since they are not complex and I can put the money saved into other projects. The learning experience is another benefit. But I would like to keep it simple, hence the motivation for this article.

Since transmission line sections require more extensive switching systems and I don't have a ready supply of RG11 I pivoted to L-networks. Both can be very efficient for the broadband yagis I've built since the impedance is close to the ideal 50 + j0 Ω across the band. TLW produced the following design (with a low pass network topology) of a 15 meter L-network for a 2-yagi stack.

The L and C values are easily attainable. For stability the capacitor should be one that is not temperature sensitive and must have a low ESR (equivalent series resistance) for high efficiency and large enough to safely dissipate the heat. The coil will be physically small and with an easily attainable Q of 400 will only shed 3 watts at a power level of 1000 watts. Efficiency is worse for large deviations from 50 + j0 Ω so design the network accordingly.

While this is a simple and efficient way to match the stack parallel impedance there are a few issues to be considered:
  • Tuning: Unlike a broadband transformer the L and C values must be carefully adjusted. A small variable capacitor with a rating of at least 1000 volts in parallel with a similar fixed capacitor is a good choice. The coil can be tapped and once the correct value is found the tap can be permanently bonded.
  • Bandwidth: The design is for a single frequency near band centre. The network must work across the entire band and behave well when the yagi SWR is high. Yagis optimized for gain can have a high Q and therefore high SWR at the band edges.
  • Field management: Within a metal box the value of a coil is different due to the field intersecting the enclosure, either increasing (steel) or decreasing (aluminum). Toroidal coils are mostly immune to this effect. Variable capacitors, the coil and wiring will exhibit stray capacitance with each other and the enclosure walls.
On the positive side the network is cheap and efficient and easy to adapt to bigger stacks by switching in one or more capacitors. For a 3-stack C is 210 pf and 260 pf for a 4-stack, while L decreases very little and can be left alone. An intermediate capacitor value can give a good match to 2 or 3 yagis in the stack without the need for switching.

The tuning process is not too demanding. To deal with enclosure effects simply cover the box after each adjustment. Broadband transformers and transmission line sections have similar issues that, although smaller, can be more difficult to compensate.

For me the critical issue is whether the L-network is broadband enough to use across the typical amateur band without additional tuning elements that must be dynamically switched. To test the concept I used EZNEC to model the L-network. As a first step I simulated the 2-stack 25 Ω parallel impedance with a long lossy transmission line. I have found that this is a good technique to emulate a resistive load in EZNEC. Although only virtual wires are needed the model requires a real wire so I specified one but didn't use it.

That's excellent. The match is even broader than the 15 meter band. I developed similar L-networks for 20 and 40 meters and achieved the same result. L-networks for a 3-stack and 4-stack were equally good across the entire band despite the higher transformation ratio.

Real antennas do not have a perfect 50 Ω impedance across the entire band. I did not explore the L-network's performance with high SWRs since my 15 and 20 meter yagis have low SWR (below 1.5) across each band. Besides, the other impedance matching alternatives would fare no better. With EZNEC I stacked the 5-element 15 meter yagis at 100' and 150' (close to the actual heights of my antennas), including the gamma matches previously modelled.

The match is perfect, barely deviating from the SWR curve for an individual yagi. The lengths of the phasing harnesses are nominal and close to the actuality but would only have a significant effect on the match were the SWR high, which it isn't in this case. To be fair the match is also very good for the broadband transformer despite its 22.25 Ω antenna port impedance (shown below). It is common for the same transformer to be used in a 3-stack, with 2 or 3 yagis selected, since the SWR is moderately good for a 17 Ω parallel impedance.

Having reached this point there is one important question to be explored: does a real L-network live up to the promise of the theory and model? Happily enough the answer is yes. I bread boarded the 15 meter L-network with little regard to good layout. The many sources of stray L and C tune out during adjustment of the network. The yagis are simulated by two parallel 51 Ω carbon composition resistors.

I measured the same excellent result with the L-network tuned for 20 meters. The only difficulty with the tuning was moving the coil tap with this fragile setup. Squeezing and spreading the coil turns does not allow a wide enough adjustment range.

To give an idea of how far the network can be pushed I modelled a L-network centred midway between our two closest (by percent) HF contest bands: 10 and 15 meters.

Although it does reasonably well it is marginal. It certainly cannot be used for a stack of tri-band yagis or for any other pair of adjacent bands. To test the model I measure the SWR of the 15 meter L-network prototype from 14 to 30 MHz. It does better than I expected for 20 meters but is unacceptable on 10. There may be unexplored loss in the network at the frequency extremes that damp the measured SWR and therefore overstate its actual performance.

Building it

The upper 15 meter yagi was raised last week. I'll have more to say about it in a forthcoming article. Once the yagi is in position at the top of the mast and the phasing harness installed I will build a stack switch using the L-network discussed in this article. If all goes well I will do the same for the 20 meter stack. The upper 20 meter yagi will not be raised before late summer.

Sometimes it feels like progress on the station is glacial. That is unavoidable since I do most of the work myself and rely on friends to help out with the big jobs. But I would not experience the same sense of accomplishment by hiring out the work and only using commercial products.

If all goes well I am going to have a lot of fun during the upcoming winter contest season.