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:
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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.
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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.
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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.
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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.
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I started with the BLT years ago.
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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.
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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.
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Resonant antennas are not required.
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A separate counterpoise is not required,
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Many impedances are highly reactive, and a tuner with several different variable options is required to get a perfect match.
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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