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It’s not supposed to be like this!
7 May 2016 | by Not much to report from me lately. The weather was not suitable for portable operations, otherwise too busy or too tired to do anything “radio”. We decided to spend our last money on a trip to Europe this summer (necessary family visits, long overdue) and I am one to prepare way in advance.
“What radio shall I bring along?” was of course the first question I asked myself. But being wise I quickly changed it into: “What antenna shall I use?” Since I never worked with end-fed antennas I thought it would be a good opportunity to learn about them and try them out.
Let’s start with buying stuff: a seven meter fishing rod that fit in my suitcase cost US$20. A length of 30 meter strong light Teflon coated copper wire was about the same. Cut in two lengths of 20 and 10 meter it would cover 20 and 40 meters, hence End Fed Half Wave. For the impedance coupler I turned to M0UKD’s excellent website . End fed antennas are no rocket science: the high impedance of a half wave length of wire has to be turned into a low impedance that matches the radio. A tank circuit is used to optimize the power transfer of the transformer used for this. So I build a tank circuit with a 1:8 air core transformer and a 9-50 pF variable capacitor.
After another hard days work at school I went to the parking lot to do some initial tests before going home. I shot this impromptu video with my phone to show you the result.
So I built myself a Full Wave End Fed! It’s not supposed to be like this! With the 10 meter wire I got the same result: on 20 meters I got a dip, but the SWR wouldn’t get below 2.2:1. I didn’t check the performance on 10 meters, but I guess I will get a near perfect SWR there, too. Of course, on my workbench, using a 3300 ohm resistor as dummy load, I had no problem matching 40 and 20 meters.
I’m a bit baffled and unsure what to do next. I already tried some transformers using ferrite cores from my junk box, but non were giving me a tuned tank circuit (inductance too high). I want coverage from 40-20 meters and my capacitor has a range of 9-50 pF. The inductor should be 11 uH then and with a 1:8 ratio the transformer should make 3300 ohm into 51 ohm. The only thing I can think of right now is that my wire is not suitable, but why? As I said in the video, any suggestions are welcome. In the mean time….I’ll keep studying about EFHW antennas.
3 Responses to “It’s not supposed to be like this!”
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You can make one of these http://www.earchi.org/92011endfedfiles/Endfed6_40.pdf
It works great with my KX3. It is a homebrew 9:1 Unun and 53 feet of wire.
A half wavelength antenna at frequency X is two half wavelengths at frequency 2X, so it’ll have similar impedance characteristics at double the design frequency.
My rough estimate of wavelength at 7.009 MHz says it should be 42.8 meters (neglecting end effects and assuming the speed of light is 299.78 million meters/second), so maybe your 20 meter long wire is too short for a half wavelength? I’d check to see if a higher frequency (like near the top of the CW band) has a lower SWR. Your matching network won’t change the resonant frequency of the wire much, just the impedance seen by the feedline.
A half wave antenna resonant at about 7 MHz will look resonant at 14 MHz (give or take a few kHz.). I suspect that’s why you’re seeing a good SWR at 14 MHz.
One other thing to consider: if your feedline and matching network aren’t too lossy, high SWR isn’t that big a problem unless your transmitter automatically reduces power in its presence, or you’re running it near its design limits and a high SWR will cause an overvoltage condition on the final RF amplifier.
High SWR at the antenna means some power is being reflected from the antenna back down the feedline. It’ll be reflected from the transmitter back up the feedline to the antenna again, but the feedline introduces loss on each trip. On a long feedline with, say, 3 db of loss, you lose half your power between the transmitter and the antenna on the first trip, then half of anything reflected back to the transmitter, then half again of anything reflected back up to the antenna. If you’re using a short feedline (which is a nice characteristic of an end-fed halfwave antenna at HF – you can have a short feedline), the additional losses incurred on the reflected power aren’t too bad. If your transmitter doesn’t mind a 3:1 SWR and you’re using a short feedline (i.e. a few meters) I wouldn’t worry too much about it.
Hello Dave, thanks for your reply.
You are quite right. SWR is not too important if you use a short feed line and if the transmitter can handle the mismatch. As you can see in the video the KX3 does throttle back a bit with a high SWR, but I’m not going to take that rig with me to Europe, just a single band QRP rig. And even with a high SWR I still managed to make a QSO with Japan without much effort, so that proves what you wrote.
For antenna length I always take a velocity factor of 7% in consideration, so that’s why my half wave is 20 meters (plus a little bit). And if it is resonant on the full wave length frequency , then it must be resonant on the half wave frequency, right (14.002 /2 = 7001).
Since there are dips at full- and half wave I think that proves resonance. What I want to try next is using different transformers. I am preparing some with different ratios (1:8, 1:9 and 1:10 and wound in different sizes. The transformer I used so far has one inch in diameter and 24 windings primary, 3 secondary. I wonder if using a smaller diameter and more windings will make a difference.