Troubleshooting Reversible Beverages

When I closed off the article on my latest reversible Beverage antenna it was working in one direction and not the other. It is now working. The problem was nothing profound and I had every confidence that I would find the fault and fix it. Beverages are really very simple antennas. Reversible Beverages, although they may seem magical, are uncomplicated as well.

The two challenges I face fixing Beverages are the tiny transformers and the trekking. Working with fine gauge coated wire and tiny (and abrasive) ferrite cores is a test of patience. Although I have more patience now than when I younger it is never enough for these confounding creatures. I get less frustrated soldering and de-soldering SMD components!

Following best practices on building Beverage transformers never seems to be enough. I get intermittent shorts, broken wires, confused windings and when I go to test them with an analyzer or VNA I am never quite sure that I understand what I'm seeing. The latter is especially true with tapped (balanced) transformers that are used on 2-wire Beverages. It is perhaps no surprise that most hams who build Beverages prefer to buy rather than build these transformers.

The trekking is due to the location and length of these antennas. From my house, the far end of this Beverage is perhaps 400 meters away. It is by no stretch a technical hike. However the fields and bush are bug infested, full of hazards that will catch the unwary, and are either wet or snow covered. Tools have to be carried in. Multiple trips to diagnose and repair the antennas are tedious. 

Offense is the best defense

Diligence during the design and construction of a Beverage will avoid many problems down the road, and many treks into the bush to correct mistakes. I test each component, each circuit, the antenna itself and I double check. Problems still arise despite that effort, though perhaps less than otherwise. Trusting to luck or believing that ignorance is bliss are poor strategies.

Diligence includes but is not limited to the following:

• Clear all brush and trees that could fall onto or grow into the antenna
• Follow best practices when winding transformers, then test and test them again
• Measure impedance transformation and balance of transformers
  
• Check RF and DC paths through the switching electronics
• Test head end switching and transformers with a dummy load, and do it again at the antenna
• Test continuity of the antenna wire(s), ground connections and coax

Problems can and still do occur, though hopefully fewer than the "hope and pray" style of construction practice.

Understand what you're building

It is very worthwhile to understand how Beverages of all types work. Without that understanding it can be quite difficult to interpret what you hear and what you measure. For that I will refer to the article where I described how reversible Beverages work, in a manner that I hoped would be helpful to those unfamiliar with these antennas. For reading convenience, the diagram I used and that I will refer to, is replicated below.

There are characteristics of reversible Beverages to keep in mind. Each direction must have a load. That load is either a dummy load (resistor) in the head end electronics or the receiver. Without the load the Beverage is bidirectional. The Beverage wires operate in both common mode -- as an antenna -- and in differential mode -- as a transmission line. The only difference in this respect between a coax and open wire Beverage is that in common mode there is just one wire: the outside of the coax shield. The inside of the shield is part of the transmission line.

Centre tapped transformers are used to combine and separate common mode and differential mode signals on open wire reversible Beverages. Read the above linked article to see how. Understanding is key to testing and repairing these antennas. 

I am also reproducing from that article the annotated diagram copied from ON4UN's Low-Band DXing book. It will come in handy for the following discussion about transformers and testing.

Transformers

I don't enjoy winding Beverage transformers. The BN73-202 binocular ferrite cores are tiny (½" square), the holes are tinier (⅛") and fine enamel wire is fragile and hard to work with. 

After they're built they need to be tested, and they must be securely mounted in boxes or on PCBs to avoid stress and breakage. Clumsy handling can damage them. It is helpful to use long leads so that they can survive trimming when damage does occur. At low frequencies an extra centimeter (or inch) is inconsequential.

Mistakes are difficult to correct, and usually involve building a new one from scratch. The ferrite cores are robust and usually can be reused. In any case, they're inexpensive

On the left are a pair of Beverage transformers. The one with the enamel wire is the reflection transformer (T3) responsible for the malfunctioning east-west reversible Beverage. The secondary winding is tapped to pull off the common mode signal travelling from left to right (in the diagram above). 

This is not the first time I've had a faulty reflection transformer. Despite following good construction practices failure is not uncommon in my personal experience. Assuming I'm not a worse builder than others it is a task everyone should approach with care.

Too much tension when pulling the winding tight can cut through the teflon liners and abrade the enamel coating. Conductor contact with the core can result in poor performance or a short. But if you don't make the windings tight it will be difficult to fit the many turns in the holes despite the fine wire being employed. 

I did discover a short that was intermittent, which only occurred when one of the leads was under tension. I redid that winding and testing went well. Yet in the field it didn't work. The symptoms were different, as measured by an antenna analyzer, but still faulty.

I rebuilt the transformer from scratch. Plastic insulated wire is far less prone to shorts, and that is my preference. The problem is that it is thicker and few turns can be accommodated. The transformer on the left has 8 turns (2 + 6, for a 9:1 impedance transformation) of Cat5 wire (AWG 25). The one in the picture comes from my first Beverage.

Those 8 turns of insulated wire are about the best I can fit into these cores. During construction I have to tamp down the wires with a narrow jeweller's screwdriver to get that many turns. I doubt I could get 10 turns with my best efforts. Unfortunately the reflection transformer requires 11 turns (6 + 5).

I went with a hybrid design: the 6 turn winding with a centre tap made from Cat5 insulated wire and the 5 turn winding with the same small gauge enamel wire. Rather than teflon inserts I used the insulated wire winding as the protective bed for the enamel wire. I carefully tamped the insulated wire down, making sure the turns covered the inner hemispheres of the holes. The enamel wire was carefully placed on top of the other wires so that it never touched the abrasive ferrite -- ferrite is a hard ceramic.

This construction method worked out well. After testing it was reinstalled and the Beverage worked as it should in both directions.

Test before deployment

The head end with T1, T2 and the reversible electronics is quite simple to build. Testing can be tricky. You need the following:

• A way to inject 12 VDC into the connector and block the DC from reaching the RF analyzer
• Ohmmeter
• Several resistors
• Antenna analyzer or VNA -- the VNWA3 from SDR-Kits is great for plotting and measuring insertion loss

In the picture I am testing the failed reflection transformer. A small breadboard from an Arduino kit comes in handy. You must be careful not to bend or break the fine wire when inserting the winding tails. I use needle nose pliers close to the wire ends to push them in. You want no more than ⅛" of wire between the pliers and the bread board to avoid wire damage. Do it a bit at a time until the wire makes firm contact.

For transformers with a low turns ratio, use resistors not too distant from 50 Ω for best accuracy. Analyzers and VNAs become increasingly inaccurate as the impedance deviates far from 50 Ω, whether lower or higher. Since the reflection transformer has a near unity 5:6 ratio I used a 75 Ω resistor on the 6 turn winding, which is near to 50 Ω on the 5 turn winding.

For high ratio transformers such as 9:1 connect the VNA to the low impedance winding and a suitable resistor on the high impedance winding. A 470 Ω works well in this instance, or 680 Ω if you are aiming for 75 Ω on the low impedance winding.

Insertion loss measurements with a VNA (S21) are more accurate with low ratio transformers. For high transformation ratio one VNA port will have to measure either a very high or very low impedance. Insertion loss is not critical for receive antennas and most of the time I don't bother measuring it. A single port antenna analyzer is therefore suitable.

For balanced transformers it is a good idea to do two further tests. Test each half of the tapped winding to check that the impedance transformations are identical. They won't be exactly identical but they should be very close. It is also a good idea to test for balance by connecting the VNA to the non-tapped winding and tying the centre tap on the second winding to one side of the first winding. When balance is good there will be almost no difference in the measured impedance.

The test works because the centre tap is "neutral". This is similar to a power transformer where the centre tap of the secondary can be grounded. Had I been more careful I would have noticed that this test failed: the impedance dropped to nearly 1 Ω. But I was in a hurry and thought that I'd accidentally shorted the wires during the test. Instead it was an internal short in the transformer, one that did not appear when measuring the transformation ratio, other than a somewhat higher X value than I expected.

I always sweep transformers from 1 to 10 MHz to uncover any anomalies. If you use long leads expect an increasing X value at the higher frequencies.

The head end is tested in a similar fashion, except that we must be careful to understand the common and differential modes that coexist on the two wires of the Beverage. We will use resistors to represent the antenna modes.

The diagram from ON4UN's book shows coax to both directional ports. The switched head ends I built have one coax port, for the selected direction, and the other port is connected to a 75 Ω resistor. Both directions need a 75 Ω load (resistor or coax + receiver) or the Beverage will be bidirectional, just as happens in a simple unidirectional Beverage.

I test the head end switching before T1 and T2 are installed. After the transformers are installed both ports are shorted to ground at DC and the switching cannot be tested with an ohmmeter. After the transformers are installed they are tested separately for continuity, and only then are T1 and T2 connected (see the earlier diagram).

You'll have to inject 12 VDC, as mentioned earlier, to test the non-default mode. In this antenna the west direction is more likely to be used so I made that the default mode (reverse, or differential mode) and east is the powered mode (normal, or common mode). The DPDT reed relays allow either direction to be wired as the default. I stuck temporary labels to the relays so I wouldn't make mistakes

Testing the differential mode (reverse direction) is quite easy. Place a resistor (red in the diagram) across the wire terminals of approximately 670 Ω (I used 470 Ω and 220 Ω resistors in series) and measure the impedance at the coax port across the frequency range of interest. The SWR should be a flat line close to 1.5, up to at least 10 MHz. At higher frequencies the sloppy internal wiring will exhibit an increasing inductive reactance and the SWR will rise.

Testing the common mode (normal direction) is less easy. I've never bothered. In principle you need to tie together the Beverage wire terminals -- there is no easy access to the centre tap of T2 -- put the resistor (as above, but green) between the wire terminals and the ground terminal. Since there is no effective antenna ground in the workshop setting it is most expedient to tie together coax and earth grounds. Well, that should work, I think. 

I prefer to do the common mode test in the field on the actual antenna. So let's do that.

In the field with an antenna analyzer

A VNA is inconvenient in the field so I rely on my Rig Expert AA54. It requires no computer, it has large keys you can punch wearing gloves and the plastic body withstands abuse. A single port analyzer is perfectly adequate for the following tests.

All tests are done from the head end. Since Beverage antennas are so long that can require walking back and forth a few times to diagnose and resolve problems. That trekking is inadvisable for my situation because the bush grows thick and the ticks and flies are everywhere and the uneven ground is a hazard because you can't see it through the high vegetation. I assigned a high priority to repairing the misbehaving antenna so that it would be complete before the work became hazardous. It was that or wait for September or October.

We need to test both common mode (normal) and differential mode (reverse). These are east and west, respectively, for this antenna. In common mode both wires are in phase with the primary antenna current. The wires are tied together and terminated in the normal fashion with a resistor to ground on the other side of the transformer primary winding. The resistor for the test is the analyzer, on the transformer secondary winding.

Since the nominal impedance is over 600 Ω, it is a little high for accurate measurement with the antenna analyzer. I am using a 9:1 transformer to bring it down to about 75 Ω, which is an SWR of 1.5 on a 50 Ω analyzer. The container with the transformer and UHF connector was saved from an earlier Beverage project and conveniently saved for future use.

If you look closely at the left panel you can see the SWR plot oscillating around the 1.5 line. The oscillations are large because the far end is not terminated (wires shorted in this case, and no reflection transformer) making the Beverage bidirectional. Impedance oscillations are greater in bidirectional mode than in unidirectional mode for any Beverage. The test is successful.

For the differential mode test the open wire is a transmission line bringing the west common mode signal back to the head end via the reflection transformer. The SWR plot should look similar to that for the previous test since the common mode signal is not terminated at the analyzer, which again makes the antenna bidirectional during the test. For the two failed reflection transformer trials, in one case the impedance was very low and in the other it was too high.

It's amusing that this latest Beverage was completed right before the close of the 160 meter season. Although there is activity through our summer, it is a poor time for DX. For me the deadline is more concrete since the growing hay requires that the radials be rolled up in the next week or two. The short vertical will suffice until fall.

The growing hay is contrasted by the above picture taken the morning of May 1. Yes, snow in May. This is normal and the vegetation is well adapted to our climate. The flowers shake it off and continue their spring growth spurt. So does the rhubarb patch at the tower base. Rhubarb muffins were baking in the over soon after the May snowfall departed.

Soon the bush will soon be too wet and overgrown to traverse except for an emergency repair. I don't anticipate the need. With the new Beverage I have 6 receive directions, and it is very nice to have. They continue to be useful on 80 and 40 meters during the summer months when I am largely absent from top band.

With this article I am officially transitioning to summer mode. That means towers and high band antennas. Since summer is also a time to plan it is quite likely there will be an article or two about low band antennas.

Sporadic E Season Begins

Seven weeks before the 2021 peak of sporadic E propagation in the northern hemisphere the 6 meter band is hopping. There is lots of activity and DX contacts are in the log. Although nothing exotic what I have worked and heard wets the appetite. I see many others hungering for the DX as well. I know that I'm ready.

The DX I've worked so far is in the Caribbean, Central America and one in Europe. I have heard more, including several of the European big guns, the west coast (California), and north and central South America. Stations not far to the south have had more luck linking sporadic E to TEP to work South America. I'm a little too far north for those openings. Our time will come.

What is clear this early in the season is that the FT8 activity level on 6 meters is greater than I've seen before. There are many signals even when there is no sporadic E in evidence. These are paths of less than 1000 km, typically tropospheric and perhaps some E-layer scatter. There are lots of CQ'ers hoping to get lucky.

With so much excited anticipation of DX propagation there is little excuse to not work DX on 6 meters this year. As many old hands on the magic band have noted, FT8 has uncovered the existence of sporadic E propagation and TEP linkages that had previously gone unnoticed. The ability to monitor all activity without tedious dial spinning or monitoring beacons, data collection by PSK Reporter and the continuous CQ'ing by enthusiasts have highlighted numerous instances of unexpected propagation. This includes single decodes of distant stations, propagation at unusual times of day, polar paths and extremely long path DX. The latter includes VK/ZL and central Asia from here in eastern North America.

Despite my initial reticence to abandon CW and SSB on 6, I am now a believer. The reason is a simple one: FT8 delivers the goods. It isn't just "youngsters" like me who've gone digital. Octogenarians of my acquaintance have learned the digital modes and are having a blast on HF and on 6 meters as well. Old dogs can learn new tricks, and so can you.

Objectives for 2021

At the end of sporadic E season in 2020 I stated that my DX goal this year is to surpass 100 DXCC countries on 6 meters FT8 this year, and I've since made the promised station improvements to make it possible. With 90 countries worked and 81 confirmed it would seem that this is low-hanging fruit. It isn't. 

I am at the stage where additional countries are increasingly difficult to work. They are farther away, rely on rare multi-hop opening, and many operators in those distant locales have small stations or deal with pervasive man-made QRN. The station improvements will help, especially the kilowatt amplifier. Barring disasters I fully expect to reach this milestone in the next few months.

My other objectives are closely related. I want to explore more polar path opportunities to work west, central and east Asia. Some of these can occur at odd hours so I will more often leave the station monitoring overnight for hints of polar propagation (I already monitor activity during the day when I'm busy elsewhere). The north polar region is in full daylight at the sporadic E season peak and the terminator is enticingly just out of reach from my QTH. This is one of those times I wish that I lived further north!

OH, LA and SM stations occasionally appear in our late evening, and central Asians such as UN can appear after sunrise in the very early morning (5:00 to 7:00 AM local time). If I see these signals in my overnight monitoring I will make a point of being in the shack at likely hours during the weeks bracketing the summer solstice.

Other objectives include KL7, ZL, HL, LU, CE, Middle East, Russia, TZ among other Africans, and other known active countries that I've heard but have not been able to work. It's a matter of luck and, yes, power. All are possible. There are also numerous European countries that I've been unlucky with. You would think with so much within reach that another 10 countries would be easy, but it really isn't. My stretch objective is 100 countries confirmed on LOTW, which will likely require working 20 to 25 new countries. Not everyone uses LOTW and I no longer use paper QSL cards.

Using a kilowatt

FT8 is not a low power mode. For digital, as it is for traditional modes, many times low power is sufficient and other times no amount of power is enough. Sporadic E gives few gifts and you do what you must to meet your operating objectives. My objectives are challenging enough without a power handicap.

The ACOM A1500 works very well on 6 meters. Tuning and high power behaviour are not noticably different than when used on HF. For the time being I am being cautious about the power level in case of splatter/distortion and overheating. At approximately 900 to 1000 watts of average SSB power -- our legal limit is 2250 watts PEP -- there are no problems. 

A nearby friend confirms that my signal is clean. Temperature for continuous FT8 operation is fairly high but within the amplifier's comfort zone. After several minutes the temperature plateaus during transmissions. It drops during receive periods. For FT8 and similar modes the duty cycle is a little below 50%. While similar to CW and SSB during transmission, it is closer to 25% during a QSO since half the time is spent listening. So do watch your amplifier closely if you are new to digital modes.

Not surprisingly my QRO CQs get more replies. When I call a big gun with a weak signal there is a good probability that I will be copied. Several DX stations have answered my CQs, and that's a promising indicator. Of course when conditions are good the same happens when I run 10 db less power.

Despite the QRO there are stations that give a signal report lower than the one I give them. This may be due to QRN or other problems at the other end in comparison to my typically very quiet rural QTH. The SNR calculation by WSJT-X provides relative rather than absolute signal reports so puzzling disparities are common.

I don't use the amp at all times. I will turn it on when DX is likely, spotted or heard and I need it to get through. PSK Reporter is used to monitor whether my high power CQs can open the band. Once the amp is on I tend to leave it on since tube life is negatively impacted by power cycling. 

Focussed on DX

When there is a DX opening or an opening is likely I focus almost exclusively on that. Single hop signals (within a radius of 1000 to 1500 km) are ignored other than to see what they're working so that I can predict the progression of the opening. 

This creates a dilemma when close stations call me when I am focussed on DX. At those times I almost never reply. FT8 is slow enough that I will not take 1 or 2 minutes of precious time to work them. This may seem impolite to some. It mostly seems to be stations that do not or cannot hear the DX and are eager for what contacts they can make. Some may want my grid, and there are Americans who consider Canada to be DX.

When the rate is slow I may take the time to reply, but not otherwise. However, one rule I live by is that when I make a directed CQ -- CQ DX, CQ EU, CQ SA, etc. -- and a nearby station calls my on my transmit frequency I almost never work them. It is very impolite for them to do that since if a weak DX station replies on my transmit frequency I might not be able to decode them. Most operators know that a transmit frequency is chosen because it is quiet and will reply there for the best chance of a QSO.

I wish that automatic reply on the called station frequency was not a feature of WSJT-X. It is and we must deal with those who use that feature, appropriately or otherwise. So if I call CQ EU or CQ DX and you are not in Europe or DX, respectively, and you want a QSO with me do not call on my transmit frequency. Transmit elsewhere and I may reply.

A few go so far as to stalk me. To escape their unwanted calls on my transmit frequency I QSY. They follow. I continue to ignore them. You would think after 5, 10 or more attempts to work me they'd give up. I have to wonder whether it's a deliberate tactic to punish me for not working them. My memory is good and I usually remember their calls. I am therefore more likely to ignore them in future when I am not focussed on DX.

Those using JTDX software have another way of dealing with non-DX callers. There is a feature to filter callers that are not in the target region, and there are other helpful feature. The price is that you may miss some stations that you might otherwise want to work. Since I use WSJT-X my approach is to disable the "Call 1st" feature and choose who I reply to, or no one at all. Many big guns on 6 use JTDX and I may eventually do the same.

Perpetual CQ'ers

There are some hams who seem to CQ on FT8 for hours on end. I think that's odd behaviour but they seem to like it. For my style of operating it is a nuisance since they fill the monitoring pane with their transmissions and during busy openings it adds to the QRM. Those within ground or tropo range are inescapable unless, by chance, the DX opening is in a direction that puts them in a pattern null.

The perpetual CQ'ers from last year are already active. I don't understand them and I don't think I want to! As a friend of mine is wont to say: "it takes all types!" They're present on HF and 160 as well.

Dealing with more activity

It is wonderful to see increasing activity on 6 meters. My subjective impression from activity so far is that activity will be higher than last year, and almost all of it will be on FT8. With 3 kHz of spectrum used by the vast majority there will be QRM. Due to the nature of the mode there is a strong incentive for everyone to congregate on one channel.

Alternatives include the intercontinental window at 50.323 MHz and FT4 at 50.318 MHz. I wish more DXers used the intercontinental window since that would benefit those of us focussed on long haul contacts. This has not worked too well since most hams are glued to 50.313, and since this is as true of those far afield as those in NA, the majority of DX can only be worked there.

I did work a few new ones on 50.323 MHz in 2020. This year I hope that more DXers will QSY when the QRM grows intense. My observation is that use of the intercontinental window declined in 2019 and rose in 2020. Many don't experience as much QRM on 50.313 MHz as those of us with quiet locations and good antennas and so may see the need to QSY. However, I am merely speculating.

QRM gets so bad at times that I have failed to complete DX QSOs due to a North American stations jumping on the frequency and obliterating the weaker signal that they probably aren't seeing. This was common for Pacific openings since I must contend with QRM from across North America. Local QRM is less of a problem when pointing northeast to Europe. WSJT-X is very good at decoding more than one signal at the same frequency (audio offset) but there are limits. The software can't perform magic.

I am also hoping for more FT4 activity this year. QSOs are twice as fast as FT8 and thus can put more DX in the log during the usually brief openings. I tried 50.318 MHz a few times last year during DX openings and only heard North American stations. The majority of the time FT4 works as well as FT8 since its SNR performance is nearly as good. 

Monitoring...

I am monitoring 50.313 MHz as I type these words, and all I see is a few nearby stations. DX is uncommon this early in the season but you never know. There are hams who want to tell them when DX openings are expected. It comes from an HF mindset with its more predictable daily and annual cycles. Sporadic E propagation patterns are suggestions and probabilities, and not dependable predictions.

Regular monitoring of 6 meters is required. Alert tools are available that can help but it's really requires making the effort to monitor. The unpredictability is both aggravating and a challenge.

I hope to see you on 6 meters this season. If you're not DX and I don't reply please don't be offended. It's nothing personal. There are many DX-free days when I am happy to spend an hour or more working single hop openings. Although not alone in FN24 I appear to be the best equipped, which may explain the attention while I focussed on DX.

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.