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An antenna tuner colocated with the transmitter may be required to prevent transmitting into an SWR higher than the allowed specification for the transmitter, or if the transmitter might overheat or "fold back" Tx power if the SWR is too high.

However, if one is using a small QRP or QRPp rig that can transmit into a very high SWR (even a completely open or shorted antenna connection) for extended periods without exceeding the output transistor(s) specifications or thermals, is there a reason for still using a transmitter colocated or built-in antenna tuner to lower the SWR into the feedline to the (perhaps badly mismatched random wire) antenna.

e.g. might some configurations of antenna tuners act as a bandpass filter to improve receive dynamic range, or reduce the potential for wideband receiver overload, etc., when adjusted for lowest SWR?

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    $\begingroup$ The tuner will still improve power transfer into the antenna. The tuner might improve sensitivity both through better power transfer and some filtering, but only in special cases will it filter significantly. More likely, a tuner will cause some insertion loss, and at QRP, you might not be able to afford that. $\endgroup$
    – user10489
    Nov 30, 2021 at 23:42
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    $\begingroup$ I wish you would expand that into a proper answer @user10489. You're certainly right about the receive sensitivity, I have roughly tuned my manual transmatch many times by tuning for loudest static volume. It stands to reason that if a better-tuned transmatch improves coupling efficiency enough to make a noticeable difference on receive, then the same improved coupling efficiency would improve radiated power out: less reflections rattling around until they dissipate as heat. $\endgroup$
    – rclocher3
    Dec 1, 2021 at 1:48

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Just because the finals don't go up in smoke when driving a mismatched load, doesn't mean they're working well. They will probably deliver less power to the antenna than they would in a matched condition, and they might also have more distortion. So from the perspective of getting signal out, and getting clean signal out, you're probably best off having it.

You might get some benefit in terms of receive noise floor, since most tuners act fundamentally like a low-pass or high-pass filter, which might augment the pre-selector filters (or lack thereof) on your portable rig, reducing interference from out-of-band signals while coupling more of the one you want... but that's rather situational and I don't think you could count on a tuner having good rejection in a place where you need it.

As for tuner losses, they tend to be less significant than most people think. A decent tuner, correctly adjusted, operated near the middle of its design frequency range (i.e. 20-40 meters), with <10:1 SWR, probably has less than 1dB loss. You can get a tuner to absorb 50% or more of the power you put into it, but usually it's by doing something foolish.

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    $\begingroup$ I wonder what the efficiency of tiny little QRP transmatches marketed for trail use is... I have a couple of those, but I've never tested them for efficiency. $\endgroup$
    – rclocher3
    Dec 1, 2021 at 23:17
  • $\begingroup$ @rclocher3 I'm very fond of my Xiegu G1M. At only 5W for SSB I'm always summing up the losses. $\endgroup$
    – wbg
    Jan 13, 2022 at 17:37

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