On 23 May 2005 23:56:43 -0700, "
wrote:

Ok. Thanks for all the suggestions. So, now I have to do quite a lot of
work to get back on air with a "decent SWR".

Hi Ramakrishnan,

Well, in fact, you are not in a bad position even when you see 2:1.
There are no brick walls in this game, and the rig will take care of
itself if you try to do something wrong. It has what is called an
automatic "fold-back" circuit that simply refuses to put too much
power into a mismatched load.

Most fold-back circuits start to kick in -gradually- at 2:1. They get
more aggressive as the mismatch climbs.

The point of this protection is to reduce the heat burden to the final
transistors when the power it is trying to deliver just won't go there
(this is called reflection from the load). The fold-back circuit
allows the transmitter to run in a reduced capacity that idles along
at that tolerable heat level.

For example:

You are trying to drive 100W into a matched load. The finals don't
want to live under any more than their existing heating of 60 to 80 or
more Watts (this is power that will NEVER get to the antenna anyway as
efficiency is not remarkably high for HF voice). In fact, after some
minutes your fan will engage to keep the temperature at a safe level.
The net result is that you radiate 100W and waste that 60+ Watts of
heat. This CAN become a problem if your environment is already hot -
if you review the specifications for rated power out, I am sure they
describe the environment as being 20 to 25 degrees C. Hotter
environments demand "de-rating" the allowable limits.

So, in a sense, what is being maintained is an equilibrium of the
maximum tolerable heat burden at the final transistors (there are also
side issues of frequency stability that are heat related, but these
issues are not destructive). So let's press the envelope with two
strained examples:

1. You are trying to drive 100W into a 2:1 mismatch. The finals are
still under the same burden of 60+ Watts of heat to generate that
drive, but the load refuses to accept all 100W and puts an additional
heat burden (the reflection coefficient) back upon the finals to the
tune of about 12%. Hence that original heat burden, plus this
mismatch, boosts temperatures to 72+ Watts. The fold-back senses the
returned power (this circuit is what is driving your SWR meter by the
way) and it drops the excitation level so that the 100W is depressed
to say 80W instead. The heat burden of inefficiency for 80W is
cooler, but with the reflected heat burden now around 8 - 10 Watts the
fold-back has juggled the books to maximize your power out, while
holding roughly the same heat level. In reality, no one notices
(unless you discover the fan is running longer now).

2. You are trying to drive 100W into a 5:1 mismatch. With this
mismatch, roughly half the power you are trying to transmit is being
reflected back to add heat to your finals. The fold-back is likely to
be quite aggressive about how much real power you can expect to make
it to this poor load.

Now, there are two forms of heat. The slow kind, like a clothes iron,
and the quick kind, like a lightning stroke. The fold-back circuit is
incapable of distinguishing the two, and the 5:1 scenario, with the
wrong phase angle (reactive instead of a simple resistive mismatch)
can lead to a voltage breakdown (the lightning heat) or catastrophic
thermal runaway. Hence, you may find that at such levels (5:1) that
the fold-back is complete instead of partial. Hopefully the fold-back
is faster than catastrophe.

Still and all, it doesn't pay to push the limits when the load is
known to be seriously out of whack. There is no indication of this in
your reports of SWR.

So standard advice, conventional wisdom, and law converge with: always
reduce power to a level consistent with reliable communication.

73's
Richard Clark, KB7QHC