Figure 1: The trifilar wound “unun” in a weatherproof minibox.
I had an assortment of antennas, but on 160 and 80 meters I had nothing of consequence. I have an 80m horizontal loop, but stopped using it due to its proximity to my building structure and attendant problems. I had been looking at VK6YSF’s longwire web page, and the little 23 meter (75.5 foot) end-fed “long wire” antenna that he had formulated was quite interesting to me. He had taken the time to do a complete analysis of the impedance of a variety of not-quite random wires, assessing the impedance on each ham band. He settled on 23 meters as a good fit for almost all the bands (except 17 meters). I already have a nice loop for 17m, so didn’t feel hobbled by the antenna’s high impedance on that band.
He uses a 9:1 unun to feed the end-fed longwire (it’s really only a longwire at 20m and above). Probably one should slap a 1:1 current balun in behind the unun to keep coaxial radiation at bay, in my non-expert opinion. Balun Designs sells a nice hybrid unit that is both a 4:1 unun and a 1:1 current balun in one box. I ordered one for a 4:1 loop connection, but I don’t think they have a 9:1 combo version, which would simplify my longwire antenna arrangement. I procured a nice 9:1 unun with a 2 inch core (1.5 KW capable) – and tried a few of the bands. I never bother with the little cores because the big ones will work with QRP as well as QRO. For end fed wires, it’s always only a QRP (max of a couple watts) power level that I use. I could easily load power from the Omni A into the antenna using, in addition to the outside transformer, a tuner in the shack. The impedance transforming unun is outside in a minibox, and a run of RG213 coax feeds the tuner inside the shack.
Some hams think that they can slap a 9:1 voltage balun onto the wire, and all will be good. But, such a device does nothing to isolate the antenna to prevent mantle radiation or the feeding of RF into your shack. A good quality 1:1 current balun seems to be in order for that task. I am a big fan of using transformer baluns instead of chokes. Some baluns are built with short coax sections that are covered with concentric ferrite beads, but IMO these may not isolate the antenna in all cases. The “Balun Designs” page gives info on transformer balun versus choke considerations (balundesigns.com: “Striking a Balance.pdf”). You can search for the pdf on their site. It seems to me that they are expert in this arena. They successfully pushed me in the direction of their own product, LOL.
Note I’m not giving advice for how to build your antenna here, but am just writing about what I did with mine. Incorrect (or for that matter even correct) usage of end fed wires, baluns, ununs, and counterpoises can create high voltage nodes on your wires. BEWARE of end fed wires and other such things, and don’t say I told you to do it any particular way! I’m not a qualified balun/unun/counterpoise design expert. For my own preferences, I would never run much power to an end-fed radiator such as what is described here. QRP power levels of one or two or five watts are in the domain of what I run.
There! Disclaimer entered.
I found the performance on 40, 30, 20, and 15 meters to be mediocre, which probably is the expectation beforehand. On twenty meters it may be a bit better than my dipole or my vertical in its favored direction. The graphs on the VK6YSF page show wildly varying radiation patterns, depending on the band in use. On 17m, my tuner could not do the job. I didn’t try 10 or 12 meters. Just for grins I decided to load the antenna on 80 meters. On the low end of the 80m band, the wire is about a quarter wavelength. On the other end of the band it is approximately a 1/3 wavelength radiator. I had not harbored great expectations for the use of it on 80m, but after a few days I’ve decided it ain’t all that bad!
Granted, I currently have nothing else on the band, so cannot make any valid comparisons, but in the first few days many CQs seemed to garner a 579 or 589 or 599 report, so long as the station was within maybe five or six hundred miles. Conditions may have been above normal, but maybe it is (at least for now) my 80 meter antenna, and I’ll let the vertical and the loops handle the other bands. When it’s all ya got, of course it’s viable, if it works at all, depending upon your patience level. It seems to do pretty well on the 20m band. On that band, it’s a true longwire (equal to or greater than one wavelength). Due to the up and down complications of band conditions, this is all very much a horse shoes swag of course.
The VK6YSF page is at https://vk6ysf.com/longwire_antenna.htm
Figure 9 on his page is the important one – and shows the tune-able impedance on 80 meters and the other bands. This antenna should eventually be upgraded to something more traditional, like a high dipole on 80m, but for now it’s easy to run out a length of 75 feet of wire, to get a signal of sorts onto that band. Due to my lot, the full length 80m antenna will be much more labor intensive.
I went so far as to try this wire on 160 meters, and made a QSO there with difficulty, three states away (Hi there W8BJO!). On 160m it is only a 1/8 wave horizontal wire, so expectations are considerably lowered. The efficiency may be a bit better due to the counterpoise I use in preference over an earth ground. I DO have an earth ground on the coax feed for the equipment!
Addendum 02/25/23:
I played around with the counterpoise length quite a bit. Its length seems to make a substantial difference, depending upon on the frequency of operation. Looking at the info at aa5tb.com/efha.html – you can get an idea about counterpoise lengths. For a half wave antenna, end fed, he shows that the impedance starts to climb quickly as the counterpoise reaches about 80 percent of the antenna length, ultimately going to about 5,000 ohms.
Figure 3 on the aa5tb page shows the impedance chart vs counterpoise length. I still haven’t settled on a good length for the counterpoise, because it is a multiband antenna and the counterpoise is subject to a percentage of the vagaries associated with the antenna wire itself. So, I’ll continue to experiment, and maybe eventually I’ll settle on the perfect counterpoise length for a 23m longwire LOL.
Again, none of this is design advice or a recommendation! 73, – Ron
WB8LZR 