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.