Saturday, August 29, 2020

Price vs. Value: the Economics of Used Equipment

The amateur radio used equipment market is changing. It's changing because we're changing and the technology is changing. So-called "boat anchors" are increasingly becoming junk. However many seem not to have noticed. The matter is worth a deeper look.


Many of the premium tube rigs of decades past have maintained high prices on the used market. This is primarily driven by nostalgia. As we grow older (and the ham population is old!) we grow nostalgic about our youth and we have the money to relive those early experiences. 
It has become quite common to find some of these boat anchors alongside state-of-the-art equipment in our shacks. Whether the equipment is lovingly restored or left to look its age it is rarely used on air. It is there for the memories it elicits. Often it is equipment we never owned before because in our youth when we had little money. Now we can afford it and so we accumulate rigs we once admired from afar.

Nostalgia dies when the nostalgic ham dies. With the baby boomers aging out the demand for boat anchors is in decline. Estates are clearing out this equipment as are elderly hams compelled to clear out their homes for reasons of age and infirmity. We've arrived at the point when supply exceeds demand. As any economist will tell you that drives down prices.

Radio museums and public displays are no panacea. This is a small market itself destined to decline. Indeed many of these museums are owned and operated by older hams and they, too, have to clear the inventory. Soon the prices commanded by once top-of-the-line equipment from Drake, Collins and other revered names will enter their final decline and fall to zero.

The message is that if you own these boat anchors you should not long delay selling them. It is understandably difficult to let go but let go you must, willingly or unwillingly. At least if you sell now there will more funds to bequeath to your family. Don't bequeath the equipment because soon enough no one will want it and it will end up in the dumpster.

Nostalgia has a best before date so you should sell before the aroma grows too strong. Even $10 for a Heathkit HD1410 keyer is absurd. In 2020 its value beyond a lingering nostalgia is close to zero. Soon it will be zero.

Equipment of the previous generation

More pertinent to less aged hams is equipment one or two generations removed from current products. As each new generation of equipment arrives on the scene the value of older equipment declines. Again, not everyone seems to notice it happening. It has been the subject of conversations I've had with my contemporaries.
A good example of a generational discontinuity was the Icom IC7300. It's not the best rig on the market but for the price new it was exceptional value. Consequently the value of 10+ year old transceivers fell. Why buy a used FT2000 when you could buy a new IC7300 for about the same amount? The value falls as technology evolve and so must the price of older equipment. 
The market value of my FT950 dropped 20% almost overnight and it will continue to fall. It is interesting that the reason I traded a FT1000MP for the FT950 was to get 6 meters and the DSP. Prices for crystal filters for those old rigs are high and the value of the rig is low. It made no sense to spend $300+ on narrow CW filters that DSP does better at zero additional cost in more modern rigs. It was time to dispose of the 1000 while it still had a reasonably high market value.

Despite these generational effects it is common to see older generation rigs with high prices on the used market. Some hams fail to smell the coffee. When they don't lower the price the listings linger. There are few buyers at the high prices they commanded in the past.
Premium priced equipment
You will have noticed advertisements for equipment that is extremely expensive. However you rarely if ever see them in ham shacks. There are $10,000 transceivers and amplifiers, $3000 full globe wall displays, among other examples. These remind me of the old rule of buying new cars: the price drops 30% when you drive it off the dealer's lot. For these amateur radio products the depreciation can be far worse.

Premium products may have additional features and look impressive. What they typically don't do is outperform the second tier of equipment that is widely owned. Sticking with Yaesu for the moment consider the FTdx9000 versus the FTdx5000. I've heard of people owning the former but the latter is an excellent rig and widely owned and used, and is itself very expensive when new. I would never have purchased the 5000 new and the 9000 price was absurd. Some set a premium price for both these rigs on the used market and that is a mistake.

A common reason why those high priced premium product are bought is simply because the purchaser can afford it. The baby boom generation is the wealthiest in history and many of them have money to burn. So they burn it. The same mindset drives sales of gold-encrusted face masks. These hams build impressive looking shacks that would perform just as well or perhaps better for half or a third the price. If it makes them happy and it's their money that's their business and I wish them well.

The trouble comes when it comes time to sell. Buyers of a similar mindset buy new not used: the purchases are driven by status and emotion, not function. Everyone else assigns a low to middling value and are only willing to pay a fraction of the new price. You cannot justify a high price on the used market just because you paid a high price. You must price to value.

For a related and more common story consider Hy-Gain rotators. True, they're not premium products but they are priced that way. I've had several over the years and not one of them was purchased new. Their value is far less than the manufacturer's price. On the used market the price is usually less than half the new price. That is better aligned with the value.

Price vs. value: a lesson in economics

Price and value are distinct though related concepts. A transaction can occur when the price does not exceed the value of a product. Which brings us to an important rule:
The seller sets the price and the buyer sets the value.
Too many sellers believe that they set the value of used equipment. It is (or was) of high value to them so the resale price is set high. Buyers with the same history as the seller and, perhaps, the same sense of nostalgia may similarly value the equipment and proceed with the purchase. 
As I said earlier, that market is declining. Fewer and fewer buyers are of that ilk. Due to market forces the value of boat anchors and equipment of more recent though older generations is in a secular decline. Prices will only go lower; they will not bounce back.

Should you keep the old equipment simply because you like having it around and it later becomes part of your estate you ought to understand that it is more likely to end up in the dumpster than generate cash for your next of kin. The old stuff that comes up in the increasingly frequent estate sales is nearly without value. Sometimes you can't even give it away!

Many won't learn this lesson. They will be disappointed when there are no callers for that pristine set of Drake C-line equipment they value so highly. Yet they will not lower the price. Few others share their opinion of the value. 
Face facts: in 2020 it is little more than nostalgia fodder. By 2030 it will quite literally be trash.

Friday, August 21, 2020

L-network Stack Switch for 20 Meters

Not long ago I described a single-band stack switch utilizing an L-network to transform the 25 Ω parallel impedance of two yagis to 50 Ω. The network is bypassed when one of the yagis in the stack is selected. L-networks have advantages over the more traditional transmission line transformer on a ferrite toroid:
  • High efficiency
  • Ease of construction
  • Low cost
  • Small physical size Since it doubles as a filter it helps reduces inter-station interferenceNetwork can be dynamically altered to suit 3 or more yagis in the stack
However it is not without its disadvantages and challenges:
  • It is too narrow band to be used on more than one band
  • Coils and capacitors must be chosen and properly used to achieve the desired efficiency, stability and power handling
  • The network must be tuned (L and C) for the stack parallel impedance and frequency range
In other respects the stack switch design is quite similar for both broadband transformers and L-networks. I'll reproduce the generic schematic of the two antenna stack switch here from an earlier article for your convenience.

There are many software applications for designing L-networks. I use TLW that comes with the ARRL Antenna Book. It is very simple to design the "tuner" to match the 25 Ω parallel stack impedance.

I chose a low pass network because the coil is small and the capacitance easy attainable with parts on hand. It will modestly reject 10 meter band energy which may be helpful during a contest.

I left the coil Q at the default 200 even though a higher Q coil is easily attainable. Indeed I measured a Q of 330 on a VNA for the L-network coil (made of #12 wire). The reason for assuming a lower Q is that in a tightly packed enclosure it will inevitably degrade and I want to be conservative about the design. Recall that Q = X / R so a lower Q means greater power dissipation due to the ESR (equivalent series resistance).
Doubling the Q to 400 halves the dissipation from 6 to 3 watts at 1000 watts. However, average power dissipation is typically 50% less for CW and SSB, and another 50% less accounting for receive and transmit cycles. In theory the network will not get hot, and in practice? That requires measurement, which I will come to later.


I went for the smallest aluminum enclosure that would fit, was durable and had few seams. It's a Hammond cast aluminum enclosure approximately 4.5" × 3" × 2" (1590T). I drew the layout on paper and measured it, including the height and especially with room for the coil to stand free.

When the parts and enclosures arrived I laid the parts inside the enclosure, made a few measurements and started punching holes. First to go in were the UHF jacks. A few of the 12 screws support spacers for the L-network above the connectors and relays. K3 and K4 were attached first, using heavy wire from the connectors as mechanical support. 
All the relays are held in place by the wiring. A PCB is better for reducing stray capacitance and inductance and for reducing impedance "bumps" along the signal path wiring. For short wires and HF sloppiness does not cause serious problems so I went for the simplest construction method. I've done it before and it can work well despite the ugliness.

A spacer lifts the terminal strip and coil to mid-height. Strips of aluminum were cut and drilled to hold the 230 pf capacitor (100 + 100 + 30), with a spacer lifting them to the same height as the coil. The coil, capacitors, relays and wires are designed or selected to carry high power. The cold side of the capacitor assembly is adjacent to the enclosure walls so it is not at risk of shorting.

A terminal strip rather than a connector is used for the control cable. While less elegant I find this method easier to work with. When something goes wrong it is easy to repair on the tower without bringing it down to the ground. From left to right the connections are upper, lower, both and DC ground. Since both (BIP) is the default only 3 control wires are needed. 
The terminal strip supports the 1N4001 diodes for the switching matrix and relay coil back EMF protection, and capacitors (not attached in the photo) for bypass to prevent RF rectification by the diodes. The small hole on bottom left is for the control cable. Holes to weep moisture and to mount the tower bracket will be added later.

Adjustment and testing

A friend loaned me his VNWA3 for a different project. Since I had it I used it to test and adjust the stack switch. This is a very nice VNA that I like so much I went ahead and ordered one for myself. It was used to measure coil Q, adjust the L-network and to measure SWR and insertion loss. In the photo below it is configured to measure insertion loss, which requires a two port VNA. One yagi port has a 50 Ω load attached to ensure equal power division.

The coil turned out to have too much inductance so that the L-network centre was ~12 MHz. The stray inductance was larger than expected. On closer inspection the mounting of K1 and K2 on either side of the input port had interconnecting wires passing alongside the coil which, via the relays, seems to behave as an additional coil turn. As we'll see other than the stray inductance performance is still very good.

I cut a turn off the coil and tried again. This time its centre was 13.3 MHz. There was no need to remove and measure the inductance since a accurate calculation can be done based on its dimensions. The original inductance of 0.31 μH was reduced to about 0.2 μH. The stray inductance isn't much and wouldn't be an issue on, say, 80 meters, but as we go higher in frequency the stray inductance contribution to the total is significant.
I'll have to be more careful about the layout for the 15 meter stack switch and, eventually, the one for 10 meters. An alternative is to use a high pass L-network which requires a higher (shunt) inductance. On those two bands a high pass L-network would also be more useful to reduce inter-station interference.
With the cover in place the centre frequency rose from 13.3 to 13.8 MHz. Inductance is reduced for a coil near an aluminum sheet because the non-ferrous conductor "cuts" the field lines. I couldn't predict beforehand how great the effect would be. A toroidal shaped coil reduces the influence of the enclosure but this is difficult to shape without a form and a form would increase dissipation by dielectric heating.
Trial and error compressing and spreading of the coil turns raised the centre frequency to the low end of the band. Going further would require adjustng the shunt capacitor (not easy!) and since it performs well across the band I left it as is. 
Above is the VNA sweep of the input impedance and insertion loss. SWR is excellent across the band. Of course each port should ideally be -3.00 db since the power is being split between two 50 Ω loads in parallel. The additional -0.05 db is the insertion loss. It ranged as high as -0.07 db depending on port and load quality. The theoretical -0.03 db insertion loss calculated by TLW is difficult to achieve in a real device.

A -0.05 db insertion loss translates to a dissipation of 11.5 watts with 1000 watts of transmitter power. It is 16 watts for -0.07 db loss. This is very reasonable and better than a transformer.

Bypassing the L-network (selecting upper or lower yagi) unsurprisingly improves performance. Insertion loss is 0 db, which indicates that the wiring and relays are very low resistance. The SWR into the switch is 1.02 with a calibration quality 50 Ω load but a little higher when terminating to the VNA for the insertion loss measurement. That is, the S21 path is not quite the same as the through calibration done with a different cable arrangement.

Smoke test

The final test is to determine the true insertion loss by putting power through the stack switch. Since a 1% power drop is very difficult to measure with power meters I went with the finger test. This involves putting power through the stack switch for a while then opening it and touching the components to see how warm they are.

Two 20 meter antennas were used for the smoke test since I don't have two 500 watt dummy loads. I picked a frequency where the SWR on the high TH7 and low TH6 were low and close. I moved the coax from the two operating positions to the stack switch, selected the antennas on both sides of 2 8 switch and plugged the amplifier into the input port of the stack switch. A 9 volt battery and a toggle switch selected upper, lower or both in phase (BIP). Don't touch the selector when transmitting!

For the first test I transmitted a 100 watt carrier for 1 minute. I picked a time when 20 meters was dead to avoid any possibility of bothering others -- at this point in the solar cycle the test was easy to schedule! The coil was barely above room temperature after the test so I turned on the amplifier.

The Drake L7 does not have a heavy duty power supply so I have little difficulty staying within range of the Canadian legal limit with a pair of 3-500Z tubes. For this test I am transmitting as much as I can get out of the amp, which you may be able to see in the photo. Note that the picture was taken with the stack switch cover off but it was on for the actual tests.

I increased the power in steps to minimize the risk of mishaps. I tested the temperature of the stack switch components after each test. Finally I transmitted full power for 30 seconds (this amp isn't capable of continuous duty operation). The coil was warm to the touch but not at all hot. The transmit doorknob capacitors stayed perfectly cool, as was expected.
No smoke was emitted so the smoke test was successful. And please be careful trying this: you don't want the transmitter accidentally keyed while your fingers are inside the stack switch.

Parallel impedance

The efficiency and input SWR of any network or filter is dependent on the impedance presented on the other port(s). This stack switch is no different, whether it uses an LC circuit or a broadband transformer. Deviate from the nominal 50 Ω and its behaviour will be different. The question is how different.

Yagis stacked in the conventional fashion use in-phase parallel impedances. The calculation is straight-forward though not trivial. A calculator for impedances is useful. There are many online (this one for example, as shown below) and they can be implemented in a spreadsheet.

Play with them for a while and you'll discover that yagis with equal impedances and low SWR work best. Similar results can be had when the X values are opposite and equal or when the R values are geometrically opposite of 50 Ω, if the deviations are not large.

For the smoke test the measured yagi impedances are 39 + j9 and 44 + j2, with respective SWR of 1.38 and 1.15. The parallel impedance is 21 + j3, with an SWR of 1.25 relative to 25 Ω. The measured SWR at the input port of the stack switch is around 1.2 to 1.25, indicating that the 2:1 transformation ratio remains valid for these modest impedance deviations. That's good.
Although in this case the SWR was between that of the individual yagis that is usually not be the case when the SWR is high. Further, the network will not have a perfect 2:1 transformation ratio and insertion loss will rise for large deviations from the nominal 50 Ω impedance.. By how much depends on the network design and the impedances. 
Many hams don't realize that the broadband ferrite core transformer in commercial stack switches exhibits similar misbehaviour with deviant impedances. From talking to other hams it seems to be a common belief that transformers are better than LC networks. I don't understand why this is.
No matter the network you are well advised to stack yagis with matched impedance curves (ideally electrically identical antennas) with a low SWR across the band. That will also ensure equal power division which is required to realize the full gain potential of a stack.

For high Q antennas such as tri-band yagis take care that the impedance transformation network can handle the higher power dissipation and voltages at the band edges. The higher the power you run the more critical this becomes. The risk is greater for high duty cycle digital modes.

Future installation

Stack switches are pretty straight-forward devices. They contain no black magic, just a simple matching network and relays. Any ham can build one, even me!
It is unlikely that this stack switch will be put to use before mid-September. The upper 20 meter yagi is still on the ground, there are no phasing lines on the tower, routing of the control cable remains to be done and the rotator is in the workshop. While that work is being completed I will build and test the 15 meter stack switch so that it, too, is ready to go.

Assuming all goes well the 20 and 15 meter stacks will be fully operational in October. For me that is when the season will truly begin. Since there are few rare DXpeditions in the pipeline the stacks will see their first service in the fall and winter contests. I am looking forward to it.

Saturday, August 15, 2020

6 Meter Season: Pushing the Limits

I am declaring the end of the 2020 sporadic E season on 6 meters although there are still occasional DX openings. However those are marginal and deliver few results in my log. Indeed the last wide scale DX openings in this area were during the third week of July. Other parts of the North America and elsewhere are having better luck, but that doesn't help me.

In retrospect this year's sporadic E season was very good for DX despite starting late and fading early. There were a handful of spectacular openings and an irregular stream of interesting DX opportunities. My DXCC country count made a significant leap this season, more than I expected.
Later I'll discuss my future plans but first I'll recap the season that was, as I experienced it. Even between stations not far apart the experience can be surprisingly different due to spotlight propagation, station capability and the time one is willing to dedicate to the pursuit of 6 meter DX. I regularly checked in with friends during the season to compare notes.

DXCC progress

My worked country total on 6 meter FT8 stands at 89 and LoTW confirmations are 80. This is a big jump from 71 at the end of last year's sporadic E season and 56 after the 2018 season. I track my DXCC successes to measuring my progress, however for me it is not an operating objective. This atypical approach does not in any way limit my DXing enthusiasm.
Some countries that I almost snagged last year made it into the log this year: OH, LZ, etc. Others continued to elude me including many "easy" ones in Europe: 9H, LX, OM, etc. Some of my success is due to increasing activity from more stations in more countries. This was most noticable with stations in Central America, South America, the Caribbean and Africa. It is good to see them on 6. I hope to see even more of them next year.
Some of the activity is migration from HF due to the solar minimum and the support of 6 meters on most modern rigs. Since many of them will migrate back to HF when the sunspots return it is important to work them now and in 2021. Of course quite a few will stay and that's to the good. We must also tip our hats to FT8 which makes it easy for those without CW ability to get on the band and improve our DX prospects.
Notable openings

Among the notable openings this year were those of July 11. On that day I worked 60 stations in Europe and 30 in Japan. Numerous new countries were heard and a few were worked. Unlike the Asian opening of two weeks earlier that blessed those to the west and south there was just one HL (not worked). No other Asians were heard or worked.

A friend mentioned that he only worked north and central Japan. I must admit I'd never given much thought to Japanese call districts. When I checked my log I saw that he was right. I, too, worked nothing in southern Japan. This was despite HL being not far off their southwest coast and I did hear that and the southern districts. 
A few decibels difference in signal strength drew a boundary across the island nation. A kilowatt would have crossed that boundary.
Where the density of the ham population in the world is low the spotlight nature of long haul sporadic E illuminated little. When it does the results can be amazing.

In addition to those Africans noted in the earlier article there were CE, LU, CP, HC and others in South and Central America, UN in central Asia, KH6 in the middle of the Pacific Ocean, OX, JW, TF, Scandinavia and others in the Arctic. All were heard this season and some were worked. Despite their numbers Russia continued to elude me. That path is more northerly and longer than to eastern Europe.

Sometimes weak openings would last a long time. For example, an LU was weakly heard for almost an hour calling CQ and heard by few. Hearing him didn't help and I still need this one. Regular early morning openings to Europe are common but usually very weak and not workable with my station. 
A few nights I left WSJT-X running with the yagi pointed north since there are polar openings during the solstice period when the Arctic is in constant sunshine. There were a few pre-dawn surprises but little more than those always frustrating single decodes. Late in the evening a few signals are sometimes heard from the northern Europe coast and in turn they have success working Japan in their late evening.

Hawaii was a particular challenge. This year I heard KH6 stations on three separate dates. During the first I worked nothing for the few minutes it lasted. The second time I made my one and only Hawaii contact this year. In the third I had a partial QSO where I could not copy his RR73 message due to QRM. 
Indeed that's the big problem working Hawaii from here since signals must cross the North American continent and the FT8 window is filled edge to edge with signals. The QRMers are innocent since they do not hear the station I'm trying to work. Sporadic E is like that. Although more use made of the 50.323 MHz inter-continental FT8 window this year most operators prefer to endure the QRM on 50.313 MHz since that is where most of the action is.

Longtime 6 meter enthusiasts are, of course, not young. For many the transition to digital modes has been difficult or unwelcome. We all do it for one simple reason: it delivers the goods. The advantages of FT8 far outweigh its perceived negatives on 6 meters. What was once difficult to impossible is, well, not routine but doable. Previously unknown marginal paths are being discovered and exploited to make QSOs. 

That is why FT8 has not only been accepted on the magic band but, over time, enthusiastically embraced. I don't see that changing.

Operating aids

In addition to spotting networks and continuous monitoring of the FT8 window at 50.313 MHz two operating aids were added to my arsenal this year: PSK Reporter and ON4KST chat.

PSK Reporter turns yours digital mode receiver into a reporter of stations heard. These can be viewed by you (or anyone) to see where in the world you're being received, including the time and signal strength. It's a great way to uncover openings. There are often surprises, such as one morning I was heard in W6 before their dawn while I was beaming Europe.
WSJT-X has a setting to turn on PSK Reporter reports. It is a good idea to check the box. That way you contribute to the community in exchange for your use of the system.

ON4KST chat has been around a long time. Before the internet it was common for DX coordination to take place on 28.885 MHz (SSB). This was a good choice since the MUF would reach 10 meters before extending to 6 meters and higher. Like every aspect of amateur radio the internet changed everything and, in my opinion, made it easier to navigate the whims of fickle VHF propagation.

I used the chat myself this season to coordinate contacts. It adds an incentive to know that the DX station is listening specifically for you. When you call blind it can be a lonely and solitary pursuit. It is also a good place to learn what others are hearing and working so that you can predict (as much as you can!) when your turn will come. You learn a lot just by monitoring.

Pushing the limits: future station improvements

With a bigger signal I could have reached 100 this year. There were countless times I heard stations that couldn't hear me, that resulted in partial QSOs or signals both ways were too weak and the opening no longer than 1 or 2 minutes. Missed countries in this category ranged from central Asia (UN) to the Middle East (9K, 4Z, OD), Africa, South America (LU, CE), Asia (HL) and many countries in Europe. 

Despite excellent DX results this year I am running into the limits of my station capabily. Sporadic E on the longest DX paths are brief and signals are very weak. Every decibel counts. At the very least I need more power. This year my plan was to have a kilowatt and lower loss transmission line it was delayed (again) by tower, antenna and equipment projects for HF.

The lack of an amplifier will be dealt with before the 2021 season. I would have had it this year but for some doubts over which amplifier to purchase. It's primarily for HF contesting and those requirements are the priority. I am close to making a decision.

My roll of Andrew AVA7 is still unused. When I began work on it this spring I discovered that my connectors -- two DIN and one N -- are for LDF7. They are not compatible. The problem areas can be rectified with some metal work but I couldn't spare the time to get it done on time.

I will probably have the Heliax prepared this fall as part of other HF antenna work because the cables will share a trench. The change from LMR400 should deliver up to 2 db lower loss. With remote switching the Heliax will eventually be used for 2 meters as well, another band where I like to chase DX (of the less exotic variety).

After 2021 when work on the HF station will be largely complete I will investigate larger antennas. With 6 elements up 24 meters my present antenna is pretty good but it can always be better. Better means more gain with a longer boom and a stack. These will garner 4 to 5 db improvement. For sporadic E propagation antenna gain is more important than height.
To many this will seem excessive considering the short season and relatively small number of QSOs that a few extra decibels will garner. But nothing about this hobby is profit driven: you spend what you can to have the fun you want. No matter your operating interests and objectives every expense in your shack and in your antennas follows the same calculus.

In light of forecasts for solar cycle 25 there is little hope for F-layer propagation. We had none at this latitude in cycle 24. Cycle 21 was fabulous for 6 meters in 1989/1990 and to get there took one of the highest solar cycle peaks on record. A repeat performance is unlikely during my lifetime.
Sporadic E and other challenging modes are all we are likely to have for many years, and to squeeze DX from those with small antennas and low power is too limiting for my taste. In 2021 my 6 meter station will be more effective. By this time next year I expect to have reached 100 countries on 6 meters FT8.
Last word
It seems I lied earlier. While writing this article (August 15) I worked one more country. D4 makes 90.
Maybe this really is it, but on 6 meters you never know. That's a part of its allure.

Wednesday, August 5, 2020

Capacitance Hat Clamp

While building the 40 meter yagi element (recently described in a couple of articles) I ran into difficulty mounting the capacitance hats on the element. Rather than fuss with it I improvised a solution suitable for the experiment but not for the final antenna. For a permanent antenna I needed a better clamp.

Look back at that earlier article and you will see that the temporary joint ran the arms of a u-bolt through the ½" capacitance hat centre tubes and seated on the outer edge of the 1" element tube. A round tube crossing a round tube touches at a single point. This is electrically and mechanically unsound.

It is sure to malfunction, and that is unacceptable for an element of an antenna as difficult to access and repair as a 40 meter yagi. There are four of these on an element so the total number of these joints is 12 for a 3-element yagi. That's trouble with a capital "T".

This style of joint is common on yagis. You will find round tubes crossing in capacitance hats (see Cushcraft XM240 method at right), element-to-boom clamps (see Hy-Gain clamp below), among others. Small elements make do with curved clamps formed from aluminum sheet. Large elements use plates and u-bolts to clamp both tubes to either side of the plate or, in Hy-Gain's case, large formed sheets. My home brew HF yagis use plates and u-bolts for the element-to-boom and boom-to-mast clamps.

I browsed web sites and antenna manuals to find commercial clamps of the type I needed. That turned nothing suitable. Clamps were too big, too small or were not strong enough. Formed clamps only fit the specific tube diameters for which they are designed.

Making my own fitted clamps from aluminum alloy sheet was not an option since it is difficult to form with the tools found in a home shop. High tensile strength alloys require careful attention to the bend radius to avoid weakening or breakage. Soft alloys that are easier to form are too weak for 40 meter capacitance hats.

Plates and u-bolt clamps are easy to make but are too heavy for the outer reaches of a 40 meter element. Those used in the capacitance hats of the W6NL 40 meter Moxon and similar antennas require element trusses to survive severe weather.

My work shop has no shortage of tools or aluminum scraps of various shapes and sizes. So I pondered the problem while staring at boxes of metal scraps. Before long an idea leapt forth. I pursued it further and developed a mental picture of a design. On a rainy day I put it to the test in my work shop.

It's a simple device that anyone can make and I am documenting it since others might find it useful in their antenna projects. The 4 pieces I needed for one element took 1 hour to complete. I spent more time on the first of them as I fine tuned it to my satisfaction. I doubt that the design is original since, in retrospect, it's pretty obvious.

The custom tube saddle is made from a 1-⅜" length of ¾" OD × ⅛" wall 6061-T6 tube cut lengthwise. The only tools required are a hacksaw, vise and a set of flat, round and half-round files. You likely have all of these in your toolkit. The saddles are combined with the same 1" u-bolts I used in the experimental antenna to make the capacitance hat clamps.

The critical items are the half-round file and tube. Both must have dimensions that match the diameters of the crossing tubes. In my case the ID of the ¾" tube matches the ½" of the capacitance hat tube diameter. My set of half-round files -- coarse, medium and fine -- have a curvature equal to 1" diameter that is a perfect match for the 1" element tube. For files that don't match the tube curvature some finesse is required to shape the saddle.

The lengthwise cut of the tube should divided it into two equal halves or the larger of the pieces will not fit over the ½" tube or it will bite into it and be difficult to move or remove. A file can correct a small overbite but not a large error.

The chosen length allows indentations for the arms of the u-bolt. Thes hold the saddle in place without requiring precision drilling of holes for the u-bolt. This also shaves the weight by a few grams. Oversize indentations compensate for less than perfect machining to achieve a flush fit onto the 1" tube.

The central saddle (depression) was made with the aforementioned half-round files. It must be centred for the arms of the u-bolt not to interfere with a good fit to the 1" tube. It is acceptable to file the indentations deeper to compensate for being a little off centre. By using ⅛" wall tube for the saddle the file can cut deep and contact area maximized.

For thinner wall tubing you should stop filing when the centre of the saddle first cuts through the tube. This won't significantly weaken the saddle but it you file any deeper the saddle won't seat on the 1" tube. That would undo all the effort to have a large contact surface.

Because the filing action is mostly linear there are visible striations. This is not a problem since the texture can reduce slippage. I smoothed them a little with a fine half-round file. Conductive grease will protect against oxidation in the tiny gaps. Use the grease on the inner surface of the saddle as well. The u-bolt I show in the picture is plated steel. Although stainless is a better choice galvanic corrosion won't disturb the clamp's electrical bond provided by the aluminum saddle.

The result is a solid mechanical and electrical joint that resists slippage even with low torque on the nuts. I didn't fit washers or a top plate for the picture but you should do so to prevent abrasion and distortion of the smaller tube. Don't over tighten! Nylocs are recommended.

Don't be intimated from a little workshop improvisation. You can save money and enjoy the pride of making antenna parts yourself.

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