Living Without the Internet

The appearance of articles on the blog slowed this month for a variety of reasons. Perhaps the most significant was a total internet outage. My terrestrial home internet service died due to obsolete technology, which required an upgrade rather than repair. I am too far from town to use my mobile data service. 

Living in the middle of nowhere is great for ham radio and not so much for connecting to the rest of humanity. Whether in the city or the country we've all leaned heavily on the internet during the pandemic. The recent loss of connectivity impacted my life in several ways, not least of which was amateur radio. Leaving everything else aside, I'll speak a little about the latter impacts.

Of course I could not access this account to manage the blog or check my email. Hence no articles. I did draft a couple of articles using an ordinary text editor, which allowed me to publish the most recent one less than 24 hours after being reconnected. All I had to do was copy the text over, include the pictures and embed links. It was quickly published.

Perhaps the oddest thing I worried about was this question: how will I know what's on the bands? No internet means no DX spots. I was also less aware of conditions since I couldn't check the solar and geomagnetic indices. That was when I rediscovered a feature all my rigs have. You may have come across it as well.

It works well, although it took some time to populate the band map. A better alternative was to flip on the amplifier and call CQ. Soon I was merrily making QSOs without the burden of spinning the big knob.

A more serious problem was my promise to give a Zoom presentation midweek. I could neither do the talk from home nor send the presentation out in advance. I resorted to the old sneaker net technology. I loaded several versions of the slides (PDF, PPT, etc.) onto two flash drives (for redundancy), filled a bag with A/V peripherals and drove to a friend's home with high speed internet. 

It didn't go well at first. Zoom is a resource hog and the laptop was page thrashing itself into oblivion. Due to physical distance imperatives only one of us at a time (both with masks) could hover over the computer to diagnose the problem. The rest of the time I was isolated in the room. 

We eventually gave up on using Zoom in the usual way. I connected to the conference by phone and one of the organizers flipped the slides while I spoke. That was a long 2 hours! Luckily I had emailed the presentation in advance to the organizers the moment I had an internet connection.

In case you're wondering, the presentation was about the design and build of my station. Above is an annotated panorama shot of the antenna farm that was taken this month and used in the talk. As the world's worst photographer I was lucky to snap this adequate shot. We are looking south from a point north of the 80 meter vertical yagi. I decided it was good enough that I uploaded it to QRZ.com.

I'll point you to the slides (PDF, 25 MB) with the warning that the presentation was not recorded and the slides may be cryptic on their own. I relied on maximum pictures and minimum words since that works best.

Installing the new internet service was a challenge. The installer's "tower guy" had quit and the subcontracted rigger wasn't immediately available. Hungering for connectivity I told him I'd do it myself. He agreed and drove over on Friday afternoon.

Unfortunately the small tower I bracketed to the side of the house a few years ago didn't give us enough height to reach my contracted 5 Mbps download speed. With extension pipes I had on hand I could raise the radio/antenna unit to no more than 45' (14 m). The 6 nearest towers are approximately 20 km distant, and only 2 or 3 of those had acceptable signals due to terrain and foliage.

Per Shannon, bandwidth is proportional to data rate. However as you increase bandwidth the SNR declines. Although I am a little out of date with my telecommunications technology knowledge I do recall enough about LTE to know how it dynamically trades off data rate (bandwidth) and reliable connectivity.

We needed more height. I reluctantly agreed to put the radio/antenna on the Trylon tower behind the house. The maximum height I was willing to risk was 60', a little below the rotator plate. Any higher would create a problem for operation and maintenance of the rotator and antennas above.

That worked. He patiently hunted for the best tower, with me adjusting the azimuth and (surprisingly) elevation. He got me more than 7 Mbps download speed, and about 1.5 Mbps upload speed. I will no longer have to leave home to use Zoom. With the tools and equipment he sent up the tower I completed the installation while he confirmed signal stability. He connected the surge protector to the tower ground.

The cable from the tower base to the house was buried two days later. Instead of a trench I cut a slit through the sod and pushed the cable down about 3" into the slit. That's a third cable run from the Trylon to the house. 

I fed the cable through the second hole into the house that I made for the second trench and the 6 meter transmission line. It's a very neat installation.

Worried about EMI I asked about the cable he was using. He reassured me that he used Cat6 shielded cable throughout. As of this writing I have not noticed any noise problems while receiving with the antennas on the tower. I will have to do a kilowatt test to ensure that the internet service is not disrupted by my transmissions.

I am considering the installation of two lengths of steel stock on the tower to protect the internet equipment from accidental bumps during antenna and tower work. Damage could be costly and inconvenient. The pile of ferrites I distributed along the unshielded Cat5 cable for the previous service will be reclaimed. They appear to have survived several years of UV and weather.

One remaining question is what to do with that small house-bracketed tower? I'll leave it where it is until I can come up with a plan. It would be a shame to see a perfectly good tower go unused.

Station Improvements for 6 Meters

After the 2020 sporadic E season I planned improvements to my 6 meter capabilities in 2021. Although the antenna is unchanged there are several items recently completed that should make the upcoming sporadic E season a successful one.

• Adjust the yagi
• New low loss feed line
• Power

The redesigned Cushcraft A50-6 continues to perform well. However when it was raised a few years ago the carefully adjusted gamma match did not escape unscathed. As a result the SWR at the shack end of the coax ranged between 1.5 and 1.8 over the important band segment of 50.0 to 50.5 MHz. The rig's ATU handles this well, though at a cost.

First, there is additional transmission line loss due to the mismatch. That -0.3 db is in addition to the -1.3 db matched loss of the 40+ meter run of LMR400, RG213 and some LDF5. The calculated total loss is therefore about -1.6 db. 

The ATU is not loss free. An estimated ATU loss of -0.2 to -0.5 db is as bad as the line loss due to the mismatch. HF rig ATUs tend to have high loss at 50 MHz so my estimate is within reason. Let's guess that it's -0.3 db, thus bringing the total mismatch loss to -0.6 db. The total system loss is -1.9 db. That is more than enough to hurt when conditions are marginal. The loss needs to be lower.

With the help of a friend the antenna was dropped to the ground for service. I discovered more problems than just the gamma match. Several of the hose clamps holding the boom sections were loose and allowed rotation of the tubes by hand. These were tightened. Weatherproofing of the coax connector was not the best so I upgraded that as well. Fasteners on my original A50-6 are not stainless so those required cleaning and other maintenance.

The gamma capacitor showed weathering effects. It is similar to RG213 with the shield removed but with a plastic that does not age so well. It is a known problem with the Cushcraft "Reddi-Match". After the antenna was adjusted I covered and sealed the exposed section of the capacitor with two layers of Scotch 33+ tape.

Adjusting the match was a puzzle at first. Dimensions of the elements and the gamma match are not at all according to the original A50-6 or the revised and optimized A50-6S. It's a good thing I keep this blog since I documented my redesign of the antenna. Finding those notes in my paper records is not easy.

Tuning was done at λ/2, which on 6 meters is a comfortable 3 meters (10 feet) above ground. This is more than high enough for reliable adjustment. When I last adjusted the antenna I had it closer to the ground but pointing straight up. It is easier to set it up horizontally than vertically, although both methods work well. A step ladder was all I needed to reach the elements and gamma match. I had to move away from the antenna to make each measurement since our bodies alter antenna behaviour as these shorter wavelengths.

In light of the dominance of FT8 on 6 meters I centred the SWR at 50.3 MHz. It is below 1.2 from 50.0 to 50.5 MHz, rising to 1.45 at 51 MHz. This is perfect for my operating preferences of CW, SSB and digital modes. The SWR is slightly better in the shack due the small loss of the improved transmission line (see below).

The antenna was raised with a tag line this time so that it didn't bump into the tower. The gamma match did not fall out of adjustment. With a step on the mast I placed it almost a little higher that it was before, which should further reduce the already minimal interaction with the XM240 below it.

Before winter moved in last year the main run of LMR400 was replaced by LDF5-50 Heliax. The matched line loss is improved by 0.5 db over LMR400. With negligible mismatched line loss the total loss is now calculated as -0.8 db. Therefor the total loss improvement -- matched loss, mismatched loss and ATU loss -- is 0.8 db. That's total system improvement of 1.1 db compared to before. I consider this to have been well worth the effort.

Actually, coax loss was not only calculated. I measured the new sections with a VNA to confirm insertion loss. The VNWA plot is of the 9' RG213 rotation loop. Although the coax is old it was properly stored and has the loss predicted for new RG213 (S21 calibration is off by -0.01 db). 

The impedance variation is small and not unusual. I get similar results for the LMR400 sections of the coax. In my experience, Heliax is more consistently close to 50 ohms, probably because it is hardline and less likely to exhibit small deformations. Then again, maybe I really should do that recalibration!

The final area of improvement is power. I now have an amplifier that will do 1 kilowatt on 6 meter FT8. It is the ACOM A1500. I will have more to say about this amplifier and why I chose it in a future article. Sporadic E openings on long DX paths are brief and weak. I need every advantage to improve my results. 

Now that I have over 90 DXCC countries worked on 6 meter FT8 (81 confirmed) new ones don't come easy. Over the past few seasons I have missed opportunities to work many countries because they ran QRO (I could copy them) and I did not (they could not copy me). Despite having the amplifier available, I will not operate QRO full time, but I will not hesitate to turn it on when the DX beckons. My objective is to pass 100 countries in 2021.

The new coax for the 6 meter yagi is not switched: it runs directly into the shack to one of the antenna ports on the A1500. The A1500 conveniently allows port switching when the amplifier is off. This is handy because sporadic E openings are sporadic (of course!) and I will be able to operate on HF via the second rig and amplifier while monitoring 6 meters with WSJT-X and even working stations on HF and 6 meters concurrently. No longer will I be restricted to one or the other. 

This flexibility is nice to have although I don't know if I'll actually do it. Actually, the more important reason is to avoid consuming a port on the 2×8 antenna switch. Until now I used one of the ports for 160 meters in the winter and 6 meters in the summer. That is no longer necessary and it gives me full time access to the 6 meter yagi year round. Perhaps its availability will encourage me to take advantage of winter sporadic E propagation.

All I need to do right now is wait and monitor for the start of the sporadic E DX season. Openings will become increasingly frequent during May. I hope to see you on the Magic Band.

East-West Reversible Beverage

One slow step at a time my Beverage receive antenna plan is steadily progressing. As of this week I have 3 of the planned 4 reversible Beverages in operation. The latest is the east-west Beverage, and the existing ones built in 2020 are the northeast-southwest Beverage and the north-south Beverage. 

Like the NE-SW Beverage the new antenna is a 2-wire electrically reversible design. As I watch the weather batter the N-S coax (RG6) Beverage I am increasingly of the opinion that an open-wire Beverage is more reliable and easier to maintain. Coax requires substantially more mechanical support since it cannot take much tension. It droops and is more susceptible to wind, ice and falling tree limbs. Although it continues to work well I can see that a day will come when it will have to be replaced.

Regular readers may recall that this most recent Beverage project began months ago. First the area had to be surveyed to find the best location. Next came the clearing of the bush. That turned into a frustrating exercise because I didn't mark one of the termination points (big tree) and I cut the wrong path. Correcting that error wasn't difficult because the chosen route has relatively few trees and bushes that had to be removed.

At right is some of the cut brush alongside the (still frozen) swamp at the east end of the antenna. There are smaller piles of cuttings all along the route. These several acres of bush have no use so I can leave it there to decay naturally.

Antenna specifications

The Beverage is 156 meters long. I measured it to be certain rather than estimating from the dimensions of the property. The west end is in the tree line about midway along the north-south Beverage. The east end is just inside the swamp that I share with several neighbours. 

I could have made the Beverage longer by venturing into the swamp. The first meters of the swamp are frozen in winter but range from damp to standing water at other times. Maintenance would be so unpleasant that I ventured no further east. The performance benefit of another 20 meters (to make it 175 meters) isn't worth wading through the muck.

Since the NE-SW Beverage works so well I made this one the same. The wire is AWG 17 aluminum electric fence wire vertically spaced 4" (10 cm). Height is a pretty consistent 2.5 meter, and slopes downward starting 10 meters from the eastern termination. With snowmobiles, hunters and deer becoming more common I don't dare place the Beverage any lower. I warn my neighbours what's hiding in the bush when I allow them access. The aluminum wire can be difficult to spot if you don't pay close attention.

Supports

Most of the selected path is open ground or with trees too small or not in positions to act as supports. This makes the antenna route easier to use and maintain but requires construction of supports. I used standard ¾" PVC pipes spaced no more than 20 meters apart. Rebar and other scrap length of ~½" metal rods peg the pipes to the ground. Since they tend to sink into the soft ground I later added small preserved wood platforms, pierced to pass the peg.

The picture below shows the variety of supports used for the Beverage.

The first picture (left) show the westernmost 40 meters of the antenna with the tree anchor and two of the PVC pipes. PVC pipe is not strong, but it doesn't need to be. The load is primarily carried by wire tension, with the pipe experiencing very little vertical and lateral force, even in a strong wind. In a way it's the wire that's holding up the pipes! Wood or metal posts with insulators are alternatives. 

Slits sawed into the pipe hold the antenna wire and wire ties hold them there. An advantage of PVC pipes is that they can be easily extended by inserting a short length of the same pipe in the coupling flare at one end (third from left).

Yes, Beverages can cross without coupling. The resistance loss (inefficiency) lowers the mutual coupling to a very low value. In the crossing picture (second from left) the NE-SW and E-W 2-wire Beverages are only 6" apart. The E-W Beverage crosses the N-S Beverage at the west anchor point, as we'll see later.

For the several trees used as supports I used PVC and electric fence screw-in insulators, shown fourth and fifth from the left, respectively. Between supports PVC spreaders (on the right) help to maintain a consistent wire separation of 4" (10 cm). They are made by quartering a 5" length of PVC pipe, cutting ½" slots for the wires and drilling holes for wire ties.

The crossing of the N-S coax Beverage is at the west termination. This is perfectly fine. What you must avoid is terminations at the same location because the proximity of the grounds will degrade the patterns of both antennas. The left picture shows plastic egg insulators, dacron rope and nails on the tree as cleats to support and tension the wires.

A spreader was added at the insulators to keep the wires in position as they are pulled downward to the head end electronics. Two screw insulators on the tree and another spreader ensure the wires touch nothing along the way, including each other. A cable tie holds the box to a length of pressure treated lumber at a height that will keep it out of the snow most of the time. The board is nailed to the tree.

After several trials a gap in the subsurface stone, rock and tree roots was found to pound in the 4' copper plated ground rod. All the Beverage grounds have now been upgraded to that shown on the right. The black AWG 18 wire has a spade lug to connect to the box. On the ground rod end a thin copper plate is drilled and soldered to the wire. A stainless hose clamp bends and holds the plate to for a high surface area copper on copper bond.

Reversing electronics

The head end electronics is identical to that of the other 2-wire Beverage. The only difference is its cleaner layout using a larger prototype PCB. Compare the two below.

I labelled the SPDT reed relays and whether each is normally on (NC) and off (NO). I made west the default direction that, because of my contest activity, west will be used more often than east. The relays are powered by +12 VDC on the coax -- RF and DC are separated by capacitors and RF choke -- to reverse the direction. The unused direction is terminated to a 75 Ω load to prevent reflections that would destroy the unidirectional pattern.

A 7:3 transformer uses the 2 wires in common mode for the east direction. It is fed via the centre tap of the primary winding of the 12:4 balanced transformer. The 12:4 transformer receives the differential mode of the antenna wires for the west direction. A reflection transformer (not shown) at the east termination converts the common mode coming from the west into a differential mode signal, using the antenna wires as an open wire transmission line. More detail can be found in the article on the NE-SW Beverage.

Feed line

I purchased a 152 meter (500') roll of inexpensive RG6 plenum coax, about half of which was used to connect the new Beverage to the remote antenna switch. It is simply lying on the ground. There are several issues I had to deal with for such a long run of coax:

• Animal damage, primarily deer, rodents and (yes) humans
  
• Interactions with the N-S Beverage
• Ease of installation and maintenance
  

I have enough unused bush that there are route choices. A friend help me cut a narrow path through the vegetation. On bare ground the coax was placed alongside rocks and trees. Trampling by deer, snowmobiles and other other vehicles is thus avoided. There is no good protection against rodents other than gel-filled cable which is expensive and usually unnecessary. I have had no trouble with rodent damage with cables on the ground, and that is the experience of many hams. Since it can happen I prefer cheap coax that I don't mind replacing should damage occur. Black jacketed cable is recommended since its tends to be more resistant to the elements regardless of its specifications.

I routing the coax out from under the N-S Beverage as soon as practical (less than 10 meters along) and kept it 5 to 10 meters distant, running roughly parallel on the east (bush) side. At the south end the coax turns west, where it lies close to the coax from the N-S antenna for 10 meters on the way to the remote switch. Initial listening tests indicate little or no discernible pattern degradation of the N-S Beverage in both directions. I am prepared to insert chokes along the coax should interaction become apparent.

There are a couple of places where the coax crosses openings in the tree line where I need to walk and vehicles may cross. I will bury those several inches below grade as I did for the other Beverages.

There is no picture to show the coax in the bush since it is really difficult to get one that isn't fuzzy or confusing. Bush photography is difficult and I am no good at it.

Debugging: direction selection and one dead direction

The new Beverage plugs into a port on my remote Beverage switch. No other work needed to be done to make it available for use at the operating desk. Or so I thought.

When I turned the rotary switch to the reserved position for the new Beverage all I heard was silence. As I've learned over the years the best approach to a difficulty of this type is to sit and think for a few minutes to review all the steps of the construction and testing. 

Suspecting something simple I opened my home brew antenna selector and traced the wires. I discovered that I had not connected the third or fourth ports to the rotary switch. A few seconds with a soldering gun and atmospheric noise was heard from the receiver. 

In both directions the signal level was comparable to the other Beverages. That was reassuring. A quick check on 30 meters (the lowest band open at the time) confirmed that it was reversing correctly and that it had directivity in both directions.

A test with an antenna analyzer showed that it was switching and that the SWR was reasonable though a little high in the west direction. I won't reproduce the SWR scans since they are very similar to those of the other 2-wire reversible Beverage, linked to earlier in the article.

After sunset I gave it a try on 160 meters. The first station copied was C92RU in Mozambique. Their signal was copied well with the new east Beverage and not with the adjacent south and northeast Beverage directions. C9 is due east so this was a promising result. Two evenings later I was pleased to work them.

Unfortunately the west direction didn't work so well. Unlike earlier in the day there was silence. A check with the antenna analyzer confirmed that it was not working in its default direction but all was good in the reverse (east) direction.

The next day I made the 250 meter trek to the head end to see what was amiss. I suspected a loose wire at the head end or the reflection transformer at the east end of the Beverage. There was no obvious fault so I removed the reflection transformer for testing. It took some time until I discovered an intermittent short between the windings. 

I replaced the outer winding, successfully tested it and put it back in the box. We have a few days of rain forecast so there may be a delay taking it back out to the swamp. I saw no reason to delay this article, so I am ending on this hopeful note.

Next steps

The new Beverage direction selector is a work in progress that has been slowed by too many other projects. I continue to use a rotary switch to select the Beverage and a toggle switch to reverse direction. It's awkward but I won't be using it very much over the summer months. Work on the new selector will have to be squeezed in on rainy summer evenings so that it's ready in the fall.

It's a shame that the new Beverage is ready just as the winter top band season is ending. It would have come in handy to more reliably copy the recent A25RU DXpedition. They copied me better than I copied them since I had no receive antenna pointed their way. 

Next month is the annual rolling up the radials ritual for my big vertical. Until early fall I must use a relatively poor base loaded 160 meter vertical. If all goes well my 160 meter station will be significantly improved for the 2021-2022 top band season. In addition to the extra two Beverage directions I have a plan to improve my transmit antenna efficiency.

The last Beverage in my 8-direction receive array is for NW-SE. It is last because from here these are the least useful directions. Because of that and the problem of finding a good route for it in the bush I am contemplating alternatives. One possibility among several candidates is a reversible end fire array using small verticals.

This is the last of my winter antenna projects. Over the warmer months my attention turns to the towers to build 10 and 40 meter antennas and to relocate a few antennas to more permanent locations. I've been moving antennas quite a lot since moving here in 2016 as towers were raised and mediocre antennas were supplanted by better ones. Work on the antenna farm never ends, and that's how I like it.

The Challenge of Wire Yagis is...Wire

I receive many inquiries about wire yagis. This corresponds well with the blog statistics since the most popular article over the 8 year run of this blog is the 40 meter 3-element wire yagi. There are several other related articles that are almost as popular. This isn't surprising because for most hams a yagi for 20 meters and higher bands is not difficult to buy and install, but once you go down to 7 MHz the size of a rotatable yagi, even one with shortened elements, is beyond possibility for most hams. 

Yet there is a desire for better performance on 40 meters than a simple antenna like an inverted vee, vertical or loop. A wire yagi is enticingly within reach. Few bother to contemplate multi-element, higher performance antennas on 80 and 160 meters and, again no surprise, those articles on the blog are far less popular. Many are willing to try on 40 meter wire yagis because they are easily supported and inexpensive, and with a little ingenuity can be electronically switched between its two broadside directions.

In reality it is never quite so simple, as many have discovered. Many hams abandon the project midway through. One of the difficulties is that many hams do not really understand yagis and their subtle complexities. They do not have the technical knowledge, the required test equipment, don't know how to use the equipment or don't know how to interpret what they see. A 2-element yagi is not twice as difficult as a dipole or inverted vee: it is closer to 10× more difficult.

That said, I don't mean to discourage anyone from embarking on a project like this! It can be a very satisfying experience, from the knowledge gained to the on air results. I built my first 40 meter wire yagi over 30 years ago and I got it working despite my (then) limited knowledge of antenna theory, nothing more sophisticated than an SWR bridge for test equipment, ELNEC software (precursor to EZNEC) and a lot of motivation. 

The antenna worked and I was hooked on the potential of these deceptively complex antennas. Readers may be surprised to next hear that I've only ever had two of these antennas. Many designs I did for other hams or they were modelled and never built. I've learned a lot on this journey.

In this article I want to focus on one particularly fundamental challenge of wire antennas: the wire itself. This is the one thing almost everyone takes for granted and should not. A lot of the trouble people run into is that they don't understand the surprisingly large impact of wire specifications and usage on the project. You can easily ignore or remain blissfully unaware of these issues with a single element antenna -- test, prune and you're done. Yagis are not so forgiving.

Before we jump in, a word about the common rotatable yagi with aluminum tube elements. In contrast to wire yagis these antennas are can be modelled to an absurd degree of accuracy and predictability. I recently gave an example of this phenomenon. NEC2 supplemented with an algorithm to correct for stepped diameter (telescoping) tubes works extremely well. 

Whether you can model it in free space or a modest height above ground the result are reliable, and indeed almost exactly the same. Commercial vendors love this since individual hams don't have to adjust their products as long as they're installed at a modest height of at least, say, λ/2: approximately 30' (10 meters) for a tri-band yagi.

The following is a brief list of wire difficulties builder of wire yagis are sure to encounter. All will be discussed in this article. After giving you lots to worry about I'll make proposals on how to deal with the uncertainties when building wire yagis.

• Insulation: material; thickness; stability; or bare wire
  
• Material: copper soft drawn vs hard drawn; copper plated wire; other conductors
• Conductor diameter
• Stranding: solid; number of strands
• Sag and tension
• Wire termination method
  

Let's begin with must seem to be a elementary description of what a wire is. The following diagram is of a THHN electrical wire often used in Canada and the US since it is ubiquitously available in bulk and usually at good prices. It is from a randomly selected web site. There are of course countless varieties of wire in the market, and this is no way is a recommendation to use THHN.

Wire material and construction

For almost all electrical wire the metal is soft drawn copper. Unless alloyed or hardened copper is a soft metal. Put it under tension and it will stretch, and in the process it will also harden. Do it under controlled conditions and you will have made less stretchable hard drawn copper. Some hams do that. It's a good idea although I've never done it. If you do there will be a slight diameter reduction of (usually) no more than one AWG number. Hard drawn copper wire for antennas is commercially available at a higher price than electrical wire.

As soft drawn wire stretches the resonant frequency will be lower due to its longer length. Under moderate tension and the load put on the wire by wind and ice you will find that the antenna will have to be adjusted because it stretches and sags. Expect to do it more than once.

Stranded wire will stretch more because the individual strands do not lie quite flat against each other. Under tension the strand spirals will draw inward and lengthen. This is independent of the each strand stretching as it hardens. Guy strand and rope compress in a similar fashion. The impact of this behaviour on wire yagis is quite and can almost always be ignored.

Another problem with stranded wire is that for the same gauge the rate of corrosion is higher. Chemical reaction rate is in proportion to the surface area, and that is always higher for stranded wire. 

On the positive side, stranded wire is more flexible. Some believe their wire antennas survive better for that reason, although I haven't noticed any difference from solid copper. Of more immediate concern is that careless handling of solid wire is more likely to result in kinks that will weaken the wire. Copper will break from the fatigue of repeated bending. Repairing just one kink will weaken the wire.

I used to believe stranded wire was better and that's what I used. Like many other hams I now build wire antennas almost exclusively with solid wire.

One of the worst choices in my experience is bare stranded copper plated steel. Copperweld and similar products corrode faster and are prone to rust as the copper plating develops fissures. Sanding the corroded wire can remove the copper plating. On the other hand it takes tension well, and that allows construction of predictably stable wire antennas. I know hams who are happy using plated wire for their low band antennas that have survived many years.

Non-copper wire such as aluminum and galvanized steel are rarely used for HF antennas. The lower conductance of aluminum and zinc for low radiation resistance wire yagis is not desirable. There are other concerns I will pass over, such as skin effect (for any plated conductor) and robustness. Copper wire copper plated wire are almost always the correct choice for wire yagis.

Wire diameter

Wire diameter affects reactance and therefore affects the resonant frequency. As the wire thins the resonant frequency rises, and on 40 meters every wire we use is thin with respect to wavelength. As an example, a tube element of 25 mm (1") diameter that is resonant at 7.1 MHz will resonate at 7.2 MHz when made from 2 mm diameter wire (AWG 12). We cannot ignore diameter for our wire yagis.

The ratio of wavelength to wire diameter is the K factor. At 7 MHz the K of the 25 mm and 2 mm elements are 0.0006 and 0.00005, respectively. These are small numbers. On 10 meters they would be only 4 times larger. There's no escaping the very small K factors of wire antennas on all HF bands. Be sure to specify the correct wire diameter in your NEC models to avoid mistakes.

Insulation

In the THHN link above you will find a table containing specifications for the insulation layers. One of the words used should give you pause: nominal. Insulation dimensions will vary for many reasons: manufacturer; forming equipment; business decisions; among others. Provided the wire meets critical requirements like voltage rating, weather and environment, failure rate, temperature/loss at rated current, etc., the product will almost always be accepted. 

Electrical wire is not intended for antennas so expect surprises with the insulation. I've tried using calipers to measure insulation thickness with little success. Insulation is compressible and difficult to make straight enough for the calipers to seat properly. I have done better using the published tables for my EZNEC models despite the variability.

Insulation is a critical factor for antenna design, so we must deal with the details. Since we can't rely on the insulation specs we must adapt to them. The biggest concern is velocity factor (VF). Insulated wire typically has a VF of 0.97 to 0.995. Hence the rule of thumb to shorten a wire antenna by 0.5% to 2%. This is not enough accuracy for yagis. A 1% change is 70 kHz at 7 MHz, and that is unacceptable for those aiming for performance. I assume that anyone building a 40 meter wire yagi has performance as an objective, since otherwise why bother investing the time and effort.

A nice thing about insulation is that corrosion is slowed. Moisture will still get in there if the ends are not sealed, and as the antenna wears under environmental pressure the nylon shell will break off (increasing the VF) and the base insulation will develop micro-fissures. Avoid the more colourful wire and always choose black since the (typical) carbon black pigment resists UV deterioration better than other pigments.

The VF of bare wire is far more predictable, at the price of corrosion due to complete exposure to the weather. If you live in an area with high levels of air pollution or near a sea or ocean the rate of corrosion will accelerate.

NEC2 supports specification of insulation for wires. Use the specifications of the wire if you can find the needed data. The important ones are the insulation's dielectric constant and thickness. The dielectric constant is determined by the insulation material. When I am not sure I use 3.5, and it is usually close enough.

Termination

The method used to secure the ends of a wire affects its electrical length. It is easier to get accurate results with bare wire than insulated. 

Consider the termination at right. Since it is insulated, the entire wire length contributes to its electrical length. That loop is next to impossible to model with NEC2. The wire loop is too small relative to wavelength to break into segments.

The actual end of the wire is the true end and so the small loop has a measurable effect. Since the current magnitude is close to equal in the loop and the phase is opposite on opposite sides of the loop, there is field cancellation that partially negates the wire length comprising the loop. But, as already said, it's difficult to model or predict.

Predictability requires stripping the insulation at the termination and wrapping the bare wire loop onto itself. Now the electrical length is to the outer edge of the loop. It is also easier to make a wrap from bare copper wire that will hold under tension. Plastic insulation is springy and can unwrap -- it has happened to me -- and it's worse with stranded wire. If you leave the loop insulated it is important for consistent results (after calibration, discussed below) that the insulator type, loop size, and length or the wrapped wire be the same on all elements. Follow the same procedure at the centre insulator.

Tension

Sag matters in a wire yagi since it lowers the interior angle at the apex. That increases the resonant frequency and lowers the radiation resistance due to increased field cancellation. 

Under tension wire will stretch (see above) and the resonant frequency will be lower. Unfortunately the frequency lowering action of wire stretch does not equal the increase due to sag. You'll have to adjust the yagi or its elements when either or both occur.

Some numbers will help. Making soft drawn wire hard drawn with tension can increase its length from a few percent to as high as 10% before it breaks. Soft drawn copper under ordinary use will stretch less than that. This is not a minor consideration especially as you go down in frequency and a wire antenna is long and heavy. Wire has to support its own weight in addition to support tension. Consider that 1% at 7 MHz is 70 kHz. That's a lot for a restricted bandwidth antenna like a yagi.

Tension is often more than you might imagine. A few years ago the ground anchor of my T-top vertical for 160 meters pulled out of the ground when the wind speed was 70 to 80 kph. It was just a wood stake pounded into the ground. To prevent further damage I went out in the wind storm to pound the stake back into the ground. I couldn't. The wind load on ~47 meters of 14 AWG insulated wire and the rope catenary was too high. I temporarily tied off the stake instead. After the storm I put it all back together and noticed only a small shift in the resonant frequency. For a single element antenna it was not a problem, but for a yagi it might be.

Straight lines are easy to draw and in modelling software. The ideal angle we get is not the reality. The interior angle of the antenna will be smaller due to sag. Modelling the curve in the wire is quite difficult and nigh impossible to get it right, and this is another problem setting the resonant frequency accurately.

We have a few options available to deal with sag:

• Increase tension: That may stretch the wire and you'll soon be repeating the procedure. Choose stronger wire or avoid excess tension.
• Pull the ends of the vee farther apart: With the inevitable sag you can get closer to the desired interior angle.
• Include a centre tube: Sag is moved outward from the high current center to where its impact is less.
  

Radiation intensity is in proportion to current and that is highest at the centre of a λ/2 antenna like an inverted vee. That is why centre sag is significant. Removing sag improves performance by way of a higher radiation resistance and higher height for the average current. The centre tube option nicely deals with the problem even if it isn't very long. As a bonus, as tension changes the tube will rotate from its horizontal position. It's a highly visible indicator that you have work to do. 

On the negative side, the tube must be split for the driven element and for the switching of a reflector coil. Split wire elements are much easier to build.

If there are no convenient supports for the ends of all elements in a wire yagi with 3 or more elements it is possible to use a catenary rope to support the ends of all the elements. Equalizing tension to avoid excess sag can be difficult. Here is a picture of such a catenary in action that someone brought to my attention. I've contemplated one of these to avoid ground anchors in farm field surrounding my towers for any wire yagis I might build at this QTH (none so far).

A further consideration for setting and maintaining wire tension is selection of the rope. It should not expand or contract in any weather and it should be UV resistant. Black dacron is a good choice that is available from many amateur radio retailers and suppliers. I have a large spool on hand for wire antenna project. 

Avoid polypropylene, nylon and natural fibre ropes that will not fare well outdoors and that will stretch with the weather and with age.

Since tension adjustment is unavoidable you should use knots and devices that allow easy release and adjustment. I am not very knowledgable about knots and mechanical tensioners so I will make no recommendations. I use a combination of knots and rope cleats.

How to get it right

It may sound counter-intuitive that wire yagis with 3 or more elements are easier to design, build and adjust than those with just 2 elements. One reason is that 2-element yagis have narrow bandwidths for gain, F/B and SWR. A second reason is that reversing a 2-element yagi requires a more complex switching system for symmetrical performance.

Yagis with 3 or more elements have simpler switching and the bandwidth is larger. A tuning error of 50 kHz is far less detrimental, to the extent that you might not notice it. You will notice it with a 2-element yagi. The Moxon rectangle variety of 2-element yagi does better with its larger F/B and SWR bandwidth. Its gain bandwidth is the same as a conventional 2-element yagi. If we want the best gain at our favourite frequency with a 2-element wire yagi or Moxon the tuning must be accurate.

How can we adjust a wire yagi to get the promised performance? As we've seen above, the variability due to wire choice and usage is surprisingly large. I follow one of two approaches: calibrate the wire or calibrate yagi performance markers. Let's look at both.

To calibrate the wire we first construct an inverted vee (assuming the yagi will use inverted vee elements) using the wire of your choice. Make the centre and terminations as identical as possible. Details matter. Attach coax of known length and type to the centre feed point. The coax should run straight down or orthogonal to the inverted vee to minimize coupling. Use a common mode choke to prevent the coax from becoming part of the antenna. Put tension of the ropes and measure or calculate the interior angle. Note the heights of the apex and the lower ends.

Measure the impedance of the inverted vee. Use a application like TLW (packaged with the ARRL Antenna Book) to transform the impedance to what it is at the feed point. Shift frequency up and down as required until you locate the frequency at which the feed point reactance (X) is 0 Ω. That's the resonant frequency. Do not adjust the element length at this time.

Construct a NEC2 model of the inverted vee. Take care that the dimensions exactly match the built antenna, for the layout and for the wire diameter and insulation. Set the real ground to what you believe you have in actuality -- medium is a good choice for most horizontally polarized antennas even though many urban/suburban locales have worse ground than that. 

The resonant frequency of the model should equal that of the antenna measurement, but almost certainly won't. If they are far apart, stop to ask yourself why. Investigate and redo as needed. If the frequencies are close adjust ground constants or wire insulation properties until the resonant frequencies agree. You will have to use your judgment. Don't just enter numbers at random!

Move the model to free space and locate the resonant frequency. It will have moved because the ground coupling is absent. Note the difference and adjust the real antenna so that its resonant frequency differs from your desired centre frequency by the same amount (the frequency could be higher or lower). This will get your real inverted vee close to what its resonant frequency would be in free space. This is the procedure I am using to calibrate the elements for my planned 40 meter 3-element rotatable yagi.

As noted many times in this blog and elsewhere, a yagi at a modest height behaves as if it were in free space since there is significant field cancellation in the vertical direction. You could say it doesn't "feel" the ground very strongly, or at least its effect becomes small in comparison to the mutual impedance between the yagi elements. 

Unfortunately it is very difficult to measure each element when it is in a yagi due to that mutual impedance, so we do it indirectly using the method described. All the elements can now be scaled from the calibration data. You must be careful to use the exact same wire and construction technique for the calibration to be effective. An extra inch of insulated wire on the end egg insulator is an inch too much.

The second calibration method is done after the fact. First, build the complete yagi. By whatever means get it close to the desired frequency range as possible. Err on the long side because it's easier to prune elements than to lengthen them.

Work with a friendly ham located broadside to the antenna (in the direction of the main lobe) to find the frequency of maximum F/B. Should that be inconvenient, an alternative is to find the frequency where the feed point resistance component of the impedance (Z = R + jX) is minimum. On the computer model find that frequency. The difference is the tuning error. I used this method to adjust my long boom 15 meter yagis.

To adjust the antenna, scale all elements by the calculated frequency ratio. For example, if the measured frequency of minimum feed point resistance is 7.350 MHz and the desired frequency is 7.275 MHz the ratio is 1.01 and the elements need to be 1% longer.

Dealing with uncertainty

I have been asked several times for the wire dimensions of the wire yagi designs found in this blog. My answer typically resembles what you've read in this article, and I can tell you that it satisfies few of them. The point is that I should not and cannot specify the lengths for the elements of a wire yagi! I don't know your wire, your environment or your construction techniques, and those are critical parameters for the reasons explained in this article. 

Sometimes I have shown lengths, and when I do I also specify the wire and mention that you will very likely have to adjust the lengths. I sometimes wonder if the message gets through to many readers. All I can do is provide guidance and show what can be achieved by those who put in the effort.

That, really, is the point of this article: wire yagis require work! If you must have an exact construction guide you should look at alternatives. Despite the challenge, building a wire yagi can be very rewarding, and educational. I enjoy the learning experience. Maybe you will, too.