I get a fair amount of email about my earlier posts. Recently, a reader asked:
I was reading your post about end-fed, long-wire antennas, I think it was from 2016. I have a couple questions about end feds. Do you mind if I ask?
Feel free to ask, but I don’t claim to be an expert at all. My blog posts were just reports of some playing around that some of us did with end-feds.
So, he asked:
Thanks and I understand you are not an RF engineer or anything like that. This is my situation. I have a KX3 which I run exclusively 5 watts with CW. It has an internal antenna tuner. I have a 6 foot piece of RG58 that goes through a second story window and the center conductor connects directly to a wire about 135 foot long. At that same point the shield is connected to a wire that is about 35 foot long and it goes to a grounding rod. I don’t have any types of coils etc installed.
Depending on the RF gods, of course, this antenna can do an excellent job. For example I have a confirmed QSO to VK land, 5 watts via CW. Other times I hear nothing.
So this is what I am wondering. Am I missing something not using a unun? Is there an optimal length the wire should be for this situation? All bands except 80 tune up super. Eighty gets about 3.2 – 1. Leave well enough alone?
Any thoughts?
I replied again:
Personally, I don’t think you’re missing anything by not using an unun. All the unun does is bring the impedance down to a value where a lesser tuner can match the impedance to the transmitter. If your antenna tuner can do it without the unun—and the Elecraft tuners can really match just about anything—then why use an unun? It’s just adding some loss into the system.
As for as optimal lengths go, I think that the longer the wire you can string up the better—as long as you stay away from lengths that are exactly halfway multiples. Mike, AB3AP explains this pretty well. If you take a look at his figures (see below), it looks like 132 – 135 ft. is a length to stay away from.
That’s a half wavelength on 80, and I think that’s why you’re having trouble getting a decent SWR there. Try coiling up about 5 feet to make it shorter, or add 5 feet to the end and see how that works.
Thom - W8TAM says
I had a 93′ sloper antenna, with a 9:1 unun and a random length (20’+) counterpoise at my house for quite a while. It performed much worse than my 100′ doublet did. I relegated it to my second station upstairs where I mostly ran digital modes.
I eventually took it down, I didn’t use it often enough for the space it was taking up.
I few months ago, I decided to re-use the 9:1 unun to terminate a 40′ vertical wire. We made about 20 10′ radials spread east and west in a fan pattern. I’m shocked at how good that antenna performs. It’s tunable 6m-80m, and hears things the doublet can’t hear. It’s become a regularly used antenna in our shack.
I think counterpoise/radials matter a lot. I think any antenna is better than no antenna. I think if an antenna doesn’t work exactly the way you have it deployed, try another way before dismissing it.
Rob w4zng says
Dan’s right, add a little more length to the long wire and it’ll probably tune right up. At the AB3AP link Dan gave, scroll down a little to the multi-colored length chart. It shows that 135′ is right at the edge of not working, but 137′ would be centered on a “good” range, so 2′ additional length is probably all you need. I can say from experience, 137′ random wires have always tuned up easily for me – so far
Now, about an unun… It might keep some RF out of the shack, if that’s ever a problem, if you were running more than QRP power levels. But you’re not, so skip it. Also, if you use a 4:1 instead of a bare cable end, it might make your tuner’s job a little easier, but that doesn’t seem to be a problem here either. So unless you just can’t get 80m to tune or are having RF in the shack, it’s probably not worth the bother. Like Dan said, just another source of loss.
Anyway, hats off to you and the good successes you’re having with a long wire. Hope you get 80m working soon too.
Walt - AC8RG says
Thanks for your thoughts. I will play with the length in accordance with the chart and see what happens.
Ed says
A End-fed random length or half wave antenna has a high impedance at the feed point, By connecting 50 ohm coax at that point without any matching device, I would expect that you are losing signals both on receive and transmit – so contrary to what others have said here, I would recommend you to add an UNUN at the point where you want to connect the coax to the antenna – a 9:1 ratio is often used but to find the perfect ratio, you would need to attach an antenna analyser to see what impedance you have at the antenna’s feed-point.
If you were operating portable and bringing the wires all the way back to the rig, then the KX-3 ACU would probably be adequate, it’s the fact that you have a run of 50 ohm transmission line in circuit that nullifies to some effect the ability of the KX3 “ATU” from being able to act as an impedance matching transformer.
If you want to avoid installing an UNUN, you would also get some improvement switching from that 6 foot of coax to 600 ohm ladder line for the run from the antenna feed-point to the ACU.
73 Ed DD5LP.
Dan KB6NU says
You’re right about whether or not you are connecting directly to the radio or not. In our case, we were able to connect directly to the radio. I have a BNC-banana plug adapter for just this purpose.
Walt says
Thank you – I will check into it.
Walt
AC8RG
Bob Truitt says
KISS…For QRP portable operation in the field, connect a random length, end fed wire to the output of your antenna tuner, dial up an operating frequency, let your tuner do the match and operate! Don’t bother with coax or lossy matching transformers. I use LDG817 and LDG817H tuners on my FT817 and FT818 rigs and have found that about any length end-fed wires work fine on 80-10 meters. I even get away without a counterpoise! Try it! I’ve even made successful contacts on 5-watts to wires that were lying on dry ground, shunt feeding grounded guy wires and clipping onto elevated CATV cable messenger wires! Experiment!
73, DE WA4A