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I have a QRP rig (Mountain Topper) and a variable capacitor QRP end fed half wave antenna with an SWR LED indicator. The LED doesn't go fully out in all configurations but there is always a local minimum on the knob.

  1. Can this problem be fixed by changing the wire length? I would think so if the LED was most dim on one extreme of the knob, but since it is in the middle, I am doubtful the length can make it any better—only positioning. I am not sure though.

  2. Can I tune an antenna wire length with this LED alone? How?

I have an antenna analyzer but it is broken right now so I am trying to do without. 73 73 73

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A tuned antenna should result in a real load impedance for your transceiver. This impedance can deviate from 50 Ohm, resulting in an SWR deviating from 1:1.

The EFHW antenna impedance can be as high as 1500 Ohm and can only be connected to the TX when there is a transformer at the feedpoint of the antenna; the base I assume. Without a transformer (common 1:5 winding ratio should do) the matching can't be obtained.

First check the SWR indicator LED with a dummy load: does the LED dim completely? If not then the SWR indicator fails. Don't worry: a poor indication is better tha no indication.

For QRP equipment: my experience is that a tool with a second LED, connected to an RF detector (telescope antenna) that indicates the local E-field is good addition for tuning the antenna to maximum voltage and maximum radiation. Together with the SWR indication this should give an idea of what happens when tuning the antenna. (For a loop antenna this is not the best tuning indicator.)

Or, best solution, use an impedance matching unit: antenna tuner, with SWR indication.

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    $\begingroup$ The antenna kit that the OP has includes a transmatch and a resistive-bridge-style SWR indicator built into one enclosure. He won't be able to test a 50 Ω dummy load. +1 for the suggestion to use an RF field strength meter. $\endgroup$
    – rclocher3
    Commented Apr 27, 2021 at 16:11
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    $\begingroup$ the tuner circuit has an autotransformer to bring the high EFHW impedance down to something lower. Since the tuner is designed for an EFHW I doubt good results would be obtained with a 50 ohm dummy load. $\endgroup$
    – Phil Frost - W8II
    Commented Apr 27, 2021 at 19:02
  • $\begingroup$ With the schematics (thanks rclocher3 and Phil Frost) it is clear that the output is not designed for 50 Ohm, but already has tuning means for that high impedance from an EFHW or a variation of that. The test with 50 Ohm makes no sense. But a check with a variable resistance dummy load can be done to verify that the SWR indication can dim further. $\endgroup$
    – user16925
    Commented Apr 28, 2021 at 10:24
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The instructions for your antenna kit say to start with a recommended length of wire for the antenna and a different recommended length for the counterpoise, and then decrease the length of the antenna wire by two to three inches (5 to 7.5 cm) until you have the best match, i.e. the LED is the dimmest at the minimum. What the instructions don't say is that you don't need to cut the wire; you can tightly fold the wire back on itself instead. (I'd recommend something non-conductive, like zip-ties or tape, to keep the wire folded.) Electrically, the folded-back part of the wire will be ignored. Folding back the wire is a good idea because the antenna wire length that works best in one location might not be the same length that works best in another location, so you might want to use the folded-back part of the wire again later.

I'd recommend following the instructions in the kit. You might get slightly more accurate results by using an accurate SWR meter or an antenna analyzer to do the tuning and testing (especially if it's bright outside and difficult to see how dim the LED gets). But you're using an inefficient antenna tuner for the convenience of the end-fed antenna, not to mention a low-powered transmitter; so your practical results, meaning the number of stations you can work, will probably be about the same either way.

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    $\begingroup$ Why is this tuner inefficient or not accurate? $\endgroup$
    – Phil Frost - W8II
    Commented Apr 27, 2021 at 16:57
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    $\begingroup$ @PhilFrost-W8II I'd think it would be inefficient because it uses a polyvaricon variable capacitor with a polyethylene dialectric, which I'd think would have a low Q. Also I'd think that an EFHW tuner would be less efficient because it's working on a high-voltage node. But you'd know better than I would; do you think it would be efficient? I didn't say that the SWR meter is inaccurate; I have one like it. But the best match is indicated by the dimmest brightness of the LED, which can be difficult to judge sometimes; reading a meter or a digital readout can be easier. $\endgroup$
    – rclocher3
    Commented Apr 27, 2021 at 17:45

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