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The Kenwood TS-570S comes with a warning against using the internal antenna tuner for antennas with SWR > 3, even though it may tune (e.g. at SWR 3.5 or 4).

warning

It says the transceiver or antenna may be damaged. What specific damage might occur, and under what circumstances?

Second, if the radio can handle an SWR of up to 3 without a tuner (what I've heard), what's the point of having an internal tuner if it shouldn't be used with SWR > 3?

I am using 50 ft of RG-8X coax to feed an EFHW-1080-2k antenna. This antenna typically doesn't require a tuner, but the higher SWR is due to a horizontally bent configuration conforming to the limits of my outdoor space.

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    $\begingroup$ Hello Edward, and welcome to ham.stackexchange.com! $\endgroup$
    – rclocher3
    Jan 8 at 1:10
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First, even if the radio won't be damaged by 3:1 SWR, it will certainly have to cut back the output power, perhaps to 1/10 of full power. So the tuner will be what allows it to function correctly even with a modest SWR antenna.

The problem with operating a tuner with high SWR is that for some antenna impedances, typically the Low R High C of a short whip, it will generate very high Q solutions that result in very high circulating currents. These can overheat the inductors. Even if the tuner is not intended to tune a short whip, and won't have enough series inductance to do it at low frequencies like 3.5 MHz, at a higher frequency like 14 MHz there might be accidental tuning solutions that do seem to work and provide a good match.

For example (without pulling out my calculator) - if the antenna is 3:1, being about 20-j100 Ohms, this can be tuned to resonance by an inductor of about 1+j100 Ohms, and the inductor loss will be a small fraction of the total power.

But if the antenna is 1-j200 Ohms, the tuner might manage to match this using all its inductors, say 3+j300 Ohms, and then some parallel capacitance to bring it back near to 50 Ohms. In this case you can see the tuner will absorb 75% of the transmitter power, which is not ok.

And the examples above could happen to the same tuner, at different frequencies, depending on the antenna. The tuner doesn't measure the antenna impedance, it just searches for low SWR.

A clue that the tuner is overloaded is that the SWR will climb up while you are transmitting, as the inductor heats up and changes shape.

Finally - the antenna you link to (seems too good to be true, for a simple wire antenna) says it shouldn't require a tuner, but the SWR might go up to 2:1 in places. This are exactly what the rig's built-in tuner is designed for.

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  • $\begingroup$ Nice, a very helpful answer. For "why the internal tuner", it sounds like you're saying that it allows more power to be radiated than without, because the transmitter doesn't need to cut back its power to prevent damage. That's really good to know! As for damage, it sounds like you're saying it depends on impedance mismatch more, which SWR doesn't measure directly. Last, it sounds like you're saying that if any damage were to happen to the tuner, it would likely manifest as a transceiver SWR reading > 1:1 while the tuner is enabled. Does that all sound right? $\endgroup$ Jan 7 at 20:49
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    $\begingroup$ Yes, mostly. Impedance mismatch is measured by SWR, what I didn't say was that my second example is an SWR of 60:1. As for damage - the inductors will take several seconds to heat up and be damaged with just 100 Watts, so you can watch and see the SWR climbing. If this happens they might overheat and be damaged. You can't tell if they have been damaged like this, just if they are heating up, and some heat is normal. At some point they'll burn out or damage the PCB. This is not the only way the tuner could be damaged, better to check SWR and don't force the transceiver to tune > 3:1. $\endgroup$
    – tomnexus
    Jan 7 at 20:59

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