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Generally, an antenna tuner is nothing more than an impedance matcher. Badly termed, as it does not tunes any antenna.

For loop antennas though this is not the case. The antenna tuner does not do any impedance matching but it instead really tunes the antenna to resonance for a given frequency.

is the above correct ? Please explain.

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  • $\begingroup$ The antenna, all by itself, will have some resonance. If the rest of the chain between the antenna and your receiver is not impedance matched then the entire system will not as effective as it could be. Getting the tuner right maximizes the energy transfer from the antenna to the receiver. If the antenna is not close to resonance with the incoming signal, that is yet another way to not maximize your signal. $\endgroup$
    – Jon Custer
    Mar 29 at 16:00

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Whether for an inductive antenna, like a loop, or a capacitive type antenna, an antenna tuner is correctly named, because it does tune an antenna, when used at the antenna feed-point. The problem is most people who use an antenna tuner, use the tuner in their shack, to tune-out the reactance/impedance mismatch that is carried back to their shack via the transmission line; so they're not solely tuning the antenna; and depending on the mismatch, this can lead to large losses since you might get the transmitter to see 50 Ohms going into the coax, but the coax is still dealing with a mismatch at the antenna feed-point that is causing reflections that cause loss on the coax. And if someone wants to call it an impedance matching device, then so be it, because that is exactly what it does.

There is a way to run an antenna tuner in the shack, with less transmission line loss, but it involves using a balanced tuner and balanced transmission line, and a BalUn at the antenna feed-point if the antenna is unbalanced.

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  • $\begingroup$ Alright. So then for best operation of a mag loop antenna first tune it to resonance then match resulting impedance to the rest of the radio ? $\endgroup$
    – kellogs
    Mar 30 at 23:43
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    $\begingroup$ I think you may be missing what Louis is pointing out. With your measuring device at the feed point, bring the antenna to resonance. Now you need to impedance match the resonant antenna system (which may include loading coils for example) to the feedline. A “tuner” may accomplish both of those steps for you depending on your antenna system. Then you can measure the radio end of the feedline to see what if any additional matching is needed. $\endgroup$
    – webmarc
    Mar 31 at 2:46
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is the above correct ? Please explain.

No.

In both cases you're adding one or more reactive elements to the radiating element in order to get a better impedance match and improve power transfer to the radiating element. In both cases part of this process is generally to bring the system as a whole into resonance (zero net reactance as measured at the transmitter).

The only difference is that we tend to think of the adjustable capacitor in a small loop as "part of the antenna", and the adjustable elements of some other tuner as an "add-on" — perhaps because there's some feedline between the tuner and the radiator. But there's no difference in principle.

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  • $\begingroup$ I think "no difference in principle" is wrong. The capacitor in the middle of the small loop has a dramatic effect on the antenna resonance, to the point that you end up sliding a narrow window across the band outside of which there is not even any reception. I don't think other impedance matches change antenna working frequency quite so dramatically, and in some cases not at all. $\endgroup$
    – user10489
    Apr 2 at 4:52
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    $\begingroup$ @user10489 the tuning is so sharp because the antenna is so small (and because, at least if you're lucky, the losses are low), so the result is a high-Q filter. The same applies with any sort of antenna. The smaller you make it, the lower Rrad goes, the higher the ideal Q, and the sharper the tuning. $\endgroup$ Apr 2 at 16:46
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The small loop is an extremely lossy antenna, so it doesn't work at all unless it is tuned, and the tuning device is usually in the middle of the antenna.

A conventional "antenna tuner" is actually tuning the antenna system including the feed line. It reduces the SWR of the entire system and improves power transfer from the radio to the system, but it doesn't reduce loss in the feed line caused by the mismatch. This loss is typically much worse in coax than in balanced line.

Moving the antenna tuner closer to the antenna would shorten that lossy coax. Putting the tuner at the feed point reduces it more. Putting the tuner inside the antenna changes the antenna design. However, in all of these cases, it is changing the antenna impedance.

However, changing the antenna impedance with a tuner doesn't necessarily make it resonant. In some cases, it just lowers the SWR, but the antenna may not radiate better. So if the antenna doesn't want to resonate at a particular frequency, the antenna tuner might give you good SWR but leave you with basically a dummy load and possibly a lot of heat generated inside the antenna tuner. This is not the general case, but it can happen with antennas with a very narrow bandwidth.

Alternately, some antennas need the antenna tuner for a different reason. The antenna may resonate just fine without the tuner, but its impedance might be 25 ohm or 75 ohm instead of 50 ohm, so the antenna tuner (or a good balun) can fix this. But transmitting without the tuner might work just fine if you can stand the loss in the feed line and the high SWR at the radio.

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