(Note: John Hill, KF7SQQ, has written a fine manual entitled “Portable Wire Antennas” that gives a wealth of information on just about every type of wire antenna there is. He addresses the end-fed antenna and various techniques to make them work. The focus is on portable use, but the theory applies to all wire antennas. Well worth the money. You can get it on Amazon.com.)
In its January/February 2015 issue, CQ Amateur Radio, in Cam Hartford’s, N6GA, QRP column, “Some Serious QRPp Work and Some Serious QRP Antennas,” featured a section, “Fine Tuning the EARCHI End Fed Antenna.” (I have requested reprint permission for the article, but haven’t heard anything yet.) EARCHI, Emergency Amateur Radio Club of Hawaii, is probably the top promoter of the concept and used to sell their version based on plans posted at their website <http://www.earchi.org/proj_homebrew.html>. (The club discontinued sales as of 2/25/2015. You can purchase my version of the matchbox on this page: The W4KGH HF Multiband End-fed Antenna).
N6GA does a thorough job of testing and makes a couple of important discoveries in the process. Here are some excerpts:
…most folks who will feed this matchbox with 16 feet of coax will have the coax lying on the ground, and this is not a good thing… you end up warming the earthworms instead of the ether… One way of treating this situation would be to elevate your coax and put a choke between the coax and the rig. A better way would be to cut piece of wire to 16 feet and attach it to the matchbox as a counterpoise, keeping it raised several feet off the ground. Then place a choke between the matchbox and the coax.
. . .
[On testing coax on the ground vs. counterpoise and choke using the Reverse Beacon Network] … On 15 meters I sent a a CQ with the coax connected directly to the matchbox, then changed frequency about 15 KHz and transmitted another CQ with the addition of the counterpoise wire and the choke. The first CQ netted two spots. The second CQ received spots from the same two stations at 5 and 6dB stronger reports, plus an additional four spots. So six spots versus two, and with stronger signal reports.
In the end, what we have is a pretty simple end-fed antenna which is easy to carry, easy to erect, and provides for practically instantaneous band changing. Adding the counterpoise and choke will make it an even better performer [emphasis added].
UPDATE: Cam Hartford, N6GA, was kind enough to answer my email and responded with his experiences doing further tests of the end-fed matchbox antenna. Here is the text of his most recent email:
Ken –Think I’ve run enough A/B tests to go crosseyed permanently.My neighborhood antenna guru (Charlie W6JJZ, aka Charlie Tuner) brought over a couple of his home-brewed antenna current probes and we checked for actual current at several different spots on the system, at different frequencies. Results are condensed as follows:Using a 33 foot vertical wire, roughly an end-fed half wave on 20 meters. The results on 20 meters were pretty much as we expected. With counterpoise and choke, there’s roughly the same amount of current at the bottom of the antenna (top of Earchi) and at the point where the counterpoise leaves the transformer. I view this as an L-shaped off center fed antenna. I used a 16’ piece of wire for the counterpoise which is close to a quarter wave. The current in the counterpoise drops off to almost zero by the outboard end. There’s an insignificant amount of current on the feed line. With no choke or wire, the original Earchi configuration, the current at the bottom of the antenna and on the outside of the coax where it enters the transformer are roughly equal. I used a 16’ piece of coax so the current on the feed line drops off to almost nothing at the radio end, similar to the 16’ wire counterpoise.On 18 MHz the results were different. Without the choke and counterpoise there was a significant amount of current on the feed line including quite a bit at the radio itself. If you were running any power I’d think you’d notice the effects. With the choke and counterpoise there was an insignificant amount of current anywhere on the feed line or at the radio.Pretty much the same was true for 15 meters. The choke and counterpoise reduced the RF on the feed line to an insignificant amount.I also ran multiple spot checks using the Reverse Beacon Network to see if there was a noticeable different between the two configurations. On 20 meters there was probably no statistically significant difference, but that’s expected because the current on the counterpoise/feedline was relatively small. I did notice differences on the other bands, although this type of testing is subject to the variations in propagation. Sometimes there was no difference and sometimes there was a big difference. Overall I’d have to say that having the choke/counterpoise and lifting the counterpoise up off the ground made a noticeable difference. If I received 10 spots with the choke/counterpoise, I’d receive 6 or 7 without them.Hope this helps. Charlie says that if you are going to try to mount the choke inside the box with the transformer, try to keep them separated. The end of the antenna, being a high voltage space, you need to be careful of capacitive coupling between input and output, so put the antenna and counterpoise connectors at opposite ends of the box. If you need to use a bigger box, make it a long skinny one if you are able to!73,Cam N6GA