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Unable to tune EFHW on 17-15m.

I recently constructed a simple linked EFHW based on the design by VK1NAM (https://vk1nam.wordpress.com/2014/08/08/link-end-fed-half-wave-antenna-and-tuner-for-sota/), however I have cut the lengths for the CW ends of the bands.

Using either my BLT tuner or my qrpguys EFHW Mini-Tuner the antenna tunes and works fine on 40, 30 and 20m. However, I simply cannot get it to tune on either 17 or 15m with either tuner. I have tried many options of lengthening and shortening the 1/2 wire and the counterpoise lengths to no avail. The BLT tuner can tune a normal 1/2 wave doublet or dipole on those frequencies so I know the toroids are properly wound. I have triple checked the toroids on the qrpguys mini-tuner and all is fine in that respect.

I may have access to a MFJ antenna analyzer but not sure how useful that will be with a high impedance EFHW wire??

Any ideas on where to start are greatly appreciated - I am not an antenna expert! Thanks.

Hi Larry,
I have been using the EARC antenna http://www.earchi.org/92011endfedfiles/Endfed6_40.pdf and found the very same problem of not been able to tune it on 17m with my MFJ-941B tuner.
It was like that when the RG58 coax length from the unun to the radio was 5m. One day I changed that RG58 coax length to about 30cm of 75 ohm coaxial and the impossible to tune 17m band became easily tuneable without loosing the ability to tune on all the other bands.
For various reasons, I gave up using that short length of 75 ohm coax and I just use a short 50cm or so length of 50 ohm RG58 and the antenna is back untunable on 17m.
The usual length of the radiating wire is 10m, but sometimes I use a longer wire of 21m. The impossibility to tune on 17m with the 21m long wire remains the same.
I hope this may give you a clue on things to try with your antenna.
73,

Guru

Take out that capacitor and check the lowest possible capacitance. Often you have to adjust the screws on the back to get a good low capacitance.

Larry,

There are lots of ways out of your problem.

I suspect the root cause of your difficulty is that your tuner must have a very low minimum capacitance in order to match your antenna at 18.08 or 21.06 MHz.

I see that VK1NAM already has links shown, and I suggest you use links, if you have not already put them in.

I use a 20.1M wire for activating 60M through 15M. For 40, 20, and 15M, the antenna is an approximate multiple of a half-wave - 1/2, 2/2, and 3/2 respectively. A high impedance tuner is required. The minimum capacitance needs to be very low for 15M.

You mention the BLT - I have a lot of experience with this design, and I have built a whole set of them for low and high frequencies. It’s a good tuner, but it’s not designed for very high impedances or end-fed antennas, although it will often find a match. It’s not always efficient for this service.

Some options:

  1. Modify the QRP guys tuner - or build another one for the higher frequencies. Reduce the number of total turns on the toroid - maybe by 1/3 or even 1/2 of the current number.

  2. Add a switch so you can switch to a lower number of total turns - and put a tap on the toroid inductor, several turns from the high-Z end of the coil (top end on most drawings). Use the switch to select your normal turns or your shortened low-inductance tap.

  3. If you have an MFJ or other analyzer, you can connect a 2K resistor to your tuner, where the end-fed antenna would connect, and then see if your tuner can match it. Connect your analyzer to the 50-ohm jack on your tuner. Set the analyzer for 18.1 MHz and 21.06 MHz - then adjust the tuner and see if you can get a 50-ohm match on the analyzer. This is the basic procedure for testing tuners. If it won’t match a 2K resistor, it might not match your antenna either. From how the analyzer acts, you may be able to tell what you need to do to modify your tuner.

  4. You could move the links in your antenna. For my 20.1M, ~66-foot wire, I have a single link at 52 feet from the feed end. This is roughly a half wave out on 30M, and roughly a full wave out for 17M. This may be the only link you need.

However…Most of the low-cost tuners available have a limited ability to match complex impedances and real-world loads, over many frequencies. This is not a simple problem.

This is why I wrote this article for SOTA operators, and I why I developed my tuner.

  1. I started with the BLT years ago.

  2. I made a small single-ended (unbalanced) tuner for end-fed wires with several switches and two variable capacitors - it worked very well. It was somewhat like the BLT in its topology.

  3. I came up with a different but similar unbalanced design, with a series capacitor connected to the primary winding of the inductor. This design has worked well for me.

With my tuner, I can perfectly match a simple 20.1 meter wire, with no links, and no added counterpoise, end-fed, on 40, 30, 20, 17, and 15M.

The 17M match requires absolute minimum capacitance and a lower-inductance tap on the tuner inductor. Only one 60 pF section of a polyvaricon capacitor, set almost to minimum, can be used. This is a very reactive match, so that’s how it is. It still gets out and makes lots of contacts. Often it’s really nice not to have to open a link on the antenna wire.

With the same wire I can operate on 40, 30, 20, 17, and 15M with the link open at 52 feet. Sometimes I use a 12-foot counterpoise, but many times I am jumping to get an S2S, and I omit the counterpoise, using what I call a “forced” match, letting the tuner force the resonance of the non-resonant 52-foot wire on 40M or 20M. My tuner has many variables and can do this easily! It is a wonder to use this thing - you can jump from band to band almost like magic, once you learn the settings.

  1. Resonant antennas are not required.

  2. A separate counterpoise is not required,

  3. Many impedances are highly reactive, and a tuner with several different variable options is required to get a perfect match.

  4. It is almost always possible to get a perfect match with reasonable wires and reasonable frequencies that we use! The tuner must be flexible to permit this magic to happen.

I do activations every few days, and you wouldn’t believe how many times I just use a forced match with the wrong-length wire, the wrong link or no link, no CP, on the wrong band, adjust the settings and the switches anyway, null the LED bridge until the LED is completely out, call the guy, and he’s in the log!

Here’s the article - please go up to the top:

You can try different lengths of counterpoise too, and you may eventually find lengths that will allow your tuner to get a match.

Some of the ATU’s are also quite good, and they can get a match with many combinations of wire, CP, and links.

73

George
KX0R

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Yes, the capacitor has trimmers built in and adjusting them to a minimum will maximise the tuning range.

Also make sure you are testing in an open environment. Any wire or metal fence or building nearby will detune the antenna.

Can you get a noise peak on receive. If that is at minimum C then see first point.

The antenna analyser will be of most use if it has a live indication when set to band centre or where you want to operate. Connect it to the end of the coax that would go to the rig. As you adjust the tuning of the matcher you can see how close you are able to match.

Using different feedline lengths is an old favourite to get improved matching. I would use 75 ohm coax and start with convenient length.
The closer to a quarter wave length of coax the bigger the impedance transformation. Tune for minimum SWR at the transceiver connection.

Anything less than 2:1 is fine. For me, 1.5:1 or less is bliss. It can be bad for your health if you keep trying for 1.0:1.

Good luck. Let us know how you go.

73
Ron
VK3AFW

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Hi Larry,
I suggest you contact Andrew directly - by the way his callsign now is VK1AD having upgraded from the standard to the advanced class licence in Australia. As others have said if you have followed the design of this linked end-fed antenna and counterpoise - the possible difference may lie in the atu/matching unit that you are using. If you have an antenna analyser yoy can borrow, you could use that to read the antennas values (X, Z etc.) on the bands where you are having issues and then check whether the end-fed ATU/Matching units that you have tried are supposed to be able to cover an antenna with those values.

When I built a similar antenna some years ago I used a Hendricks matching unit but also a “tuner” unit from a “magic antenna” also worked well.

73 Ed.

Hi George, do You use a feed line? If so, which kind and how long?

Thank You for Your detailed description of Your antenna and tuner!
73 Axel DF1ET

Axel,

I plug the 20.1M end-fed wire into my tuner, and it sits on the ground. There is a 2-foot 50-ohm coax from the tuner to the radio. If I use a separate counterpoise, it plugs into the tuner.

The various wires connected to the radio (short coax, headphones, paddles cable), the tuner, and my body form a small counterpoise that works well for the high-Z end-fed antenna. I often use a 12-foot counterpoise for 60M, although I can get a perfect match without it.

73
George
KX0R

Thank you so much to all for your insightful comments. Yes, the problem was that the polyvaricon trimmers were not set to minimum capacitance. Once I did that I was able to tune using a 2-3K ohm resistor terminator and the MFJ-259B analyzer. I have yet to test it on a real antenna but I suspect that was surely the problem.

George: I like your tuner ideas and may keep that for an upcoming winter project!

73’s, Larry

George, thank You for Your explanations!

Axel DF1ET

Hi George,
I’m keen to try your 66 footer tomorrow. Do you use insulated wire or bare wire? Or doesn’t it make any difference because of the need to use the tuner anyway?
73
Geoff ZL3GA

As others have pointed out, the capacitor has too much capacitance for the high fq bands. You can adjust the trimmers for minimum capacitance, but in my experience this won’t be enough either. the Capacitor has two sections. You can disconnect one of the section AND adjust the trimmer to minimum capacitance, then you will be able to tune 17 and 15M. You will adjustment on other bands becomes MUCH easier, BUT you may not have enough capacitance to reach 40M. You could devise a switch to add the other section back in for 40M only OR you could use the switch to switch in a fixed capacitor of say 100pf and retain the advantage of tuning with a smaller capacitor for easier tuning… 73, Fred KT5X (aka WS0TA)

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