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My IC-7300 has an internal antenna tuner that can handle a SWR of up to 3:1.

The other day I noticed that a frequency I was transmitting on (the 40m FT8 frequency FWIW) has an SWR of 1.5 (was my first time running FT8 on 40 meters). I expected that because the antenna itself is tuned for best 40m SWR in the phone region, which I use more often.

If I turn on the IC-7300's antenna tuner it can, unsurprisingly, knock that back to 1:1. But should I bother doing that? From what I've read, a SWR of 1.5 isn't that bad. What are the cons of using the internal tuner when I "don't have to"? And is 1.5 a "don't have to" or is that far enough from 1:1 that it's a good idea?

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    $\begingroup$ These related Q&As may contain helpful info. $\endgroup$
    – Mike Waters
    Sep 5 at 23:07
  • $\begingroup$ See Phil's answer on his separate question about the answers here. :) $\endgroup$
    – Mike Waters
    Sep 9 at 20:49
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For transmit, compared to a perfect SWR of 1:1, an SWR of 1.5:1 will give about 4 % loss of power radiated by the antenna due to reflections at the antenna / feed line junction.

All other things equal, this will result in about 0.2 dB drop in signal strength for a remote station receiving your transmissions.

Similarly, for your receive , compared to a perfect SWR of 1:1, an SWR of 1.5:1 will give about 0.2 dB loss of received signal, noting that 1 S point is equal to 6 dB of change in signal.

These losses for an SWR of 1.5:1 would normally be considered negligible and would not be noticeable.

The radio may heat up a bit more during transmit with a 1.5:1 SWR, however i imagine that all Icom radios are designed to easily handle the slight increase in temperature without problems.

In general a SWR of 1.5:1 is perfectly acceptable, and considering as already mentioned in another answer that you have to wait for the radio to re-tune every time you change frequency with the antenna tuner enabled, i would leave it off because the negligible difference it would make in this case is not worth the effort.

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    $\begingroup$ The 4% calculation doesn't account for loss in the feed line, which the tuner can't fix unless it is between the antenna and the feed line. Also, loss during receive will be different than loss during transmit. I agree that the difference will be negligible for transmit and receive, but the radio may still overheat, more so at higher power, although also probably not a big deal below 100w. $\endgroup$
    – user10489
    Sep 5 at 23:27
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    $\begingroup$ @user10489 Thanks for that, i updated the answer a bit to suit. $\endgroup$
    – Andrew
    Sep 6 at 0:44
  • $\begingroup$ Seems to me hams like to put the tuner between the transmitter and transmission line even though the mismatch is between the transmission line and antenna. I would think the entire transmission line gets mismatched even though the transmitter sees a good match, maybe I'm wrong. $\endgroup$
    – pgibbons
    Sep 7 at 22:13
  • $\begingroup$ @pgibbons The impedance seen at any point on a transmission line depends on the boundary conditions, ie: what's connected at both ends, so regardless of where a tuner is located, when a match is obtained the result is that the entire transmission line has no reactance at any point, and the impedance is the characteristic impedance everywhere on the transmission line. So even though the mismatch is at the antenna, a tuner at the radio can in fact be used to get rid of reactance in the entire system. $\endgroup$
    – Andrew
    Sep 7 at 22:41
  • $\begingroup$ @Andrew so basically, the antenna can be crap and the transmission line can be crap but a tuner at the beginning fixes it? Let's assume 3 completely different characteristic impedances for the transmitter, the transmission line, and the antenna. the tuner here would get rid of the reflection from transmitter to transmission line but there would still be a reflection from transmission line to antenna as well as from the antenna itself. For example, cut the transmission line in half and tape it back together. the tuner can still make this look great, but is it... $\endgroup$
    – pgibbons
    Sep 10 at 17:35
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If it is convenient to reduce the swr by fixing the antenna, that would be a more effective solution. But, as you say, 1.5 is not that bad, and likely you would like to continue using the antenna in the phone region anyway.

So is it better to run it at 1.5 or engage the tuner? It is probably easier on the radio to use the tuner. The disadvantage of the tuner it that it decreases efficiency slightly. But with the swr fixed by the tuner, the radio may be able to put out a higher power without overheating, which should more than make up for the efficiency loss.

The antenna likely will not radiate as well with the tuner as it would without the tuner if it had 1:1 swr, but it is better to have the tuner heat up than the radio's finals, so I would run with the tuner. The only real disadvantage is that you have to wait for the tuner to match to the antenna (a one time thing each time you tune), and you are less certain how much of your power is going into the antenna. (If you know you have a 1.5 swr, you know how much is not going into the antenna. The tuner covers up the loss without removing it.)

It's really a toss up if the swr is less than 1.5.

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If you do not use the tuner, you are getting some power (another answer here says around 4%) reflected back into the final stage of the power amplifier (PA) of the radio. This will cause the final transistor(s) to heat up, and the cooling fan will be used to remove the heat.

If you do use the tuner, you move the mismatch from the PA to the tuner circuitry, and instead of heating up the PA transistor(s) you will be heating up the tuner components. The cooling fan will be used to remove the heat.

There is a small insertion loss when using the tuner, but its components are much less sensitive to heat. If you operate in a hot environment, you want to keep as much heat away from the PA as possible, and the insertion loss is a small price to pay for that. If you operate in a cool environment, if makes little difference, as the cooling fan can remove the heat very easily.

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    $\begingroup$ Why does the reflected power necessarily cause the finals to heat up? Couldn't the phase of the reflected wave be such that the finals see a higher impedance, and thus can make the same power at a lower current, thus reducing heat? $\endgroup$ Sep 7 at 13:57
  • $\begingroup$ That's a good point that the radio's immediate environment should be considered. $\endgroup$
    – rclocher3
    Sep 7 at 17:47
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    $\begingroup$ Also why would the tuner be heated? An ideal tuner consists of only reactive components which can't be heated. Of course a real tuner will have some small loss, and so there will be some small heating, but your answer seems to imply something more? $\endgroup$ Sep 8 at 18:20
  • $\begingroup$ Phil: impedance mismatch doesn't work that way. The only way it doesn't cause heating is if the transmitter reduces power output. $\endgroup$
    – user10489
    Sep 10 at 2:15
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    $\begingroup$ The tuner may heat up less than the transmitter would. $\endgroup$
    – user10489
    Sep 10 at 2:17

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