On Sun, 28 Sep 2008 22:21:49 -0700, "Sal M. Onella"
wrote:

"Jeff Liebermann" wrote in message
.. .
My guess(tm) is that most modern HF radios can also safely operate
into high VSWR loads, but suspect that manufacturers are also hesitant
to guarantee such operation. Most manufacturers don't even specify a
maximum VSWR and simply reduce power if excessive reverse power
(usually 2:1 at full power) is detected.

I have two HF radios, (Kenwood TS-120, TS-870) and I often observe the
forward power climbing as I manually tune and reduce the reflected power.
At first glance, this would seem to indicate the rig had previously
throttled back its output in response to the consequences of a mismatch.

That's normal. You're not measuring forward power. You're measuring
RF voltage near the radio antenna connector. Once the load (coax +
antenna) moves away from a 50 ohm resistive load, all bets are off as
to what the voltage means. The same for a VSWR meter or watt meter
installed between the radio and the coax cable to the antenna. Once
the load gets away from 50 ohms, the voltage readings are not an
indication of power.

I never intentionally tune at anything more than a few watts, so perhaps the
protection circuits operate at less than full power, too. The autotuner in
the TS-870 uses about 10 watts.

Yep. That's the way it should be done. The auto-tuners intentionally
reduce power to prevent damage if they cross through some odd
resonance that causes the voltage across some internal component to go
sky high. Half the power, 1/4th the voltage. It could zap the tuner,
but also zap the power amplfier if tuned at full power.

That begs the question "What happens to a transmitter if one
intentionally runs it with a high VSWR?" I don't know the full
answer. Conventional wisdom is that the xmitter finals overheat. I've
never seen much of that except with a tube power amplifier. With the
power amps I worked on, the voltage across the transistor would either
increase dramatically, or the current through the transistor would
increase dramatically. Excess voltage would zap the junction. Excess
current might cause overheating, but more commonly, would simply turn
the xsistor wire bonds into fuses or melt the junction even before the
heat sink could extract the heat.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558