H. Adam Stevens, NQ5H wrote:

Just a quick back-of-the-envelope calculation.

SWR' = 1.7

or SWR" = 1.0

Let's assume worst case; all the reflected power is absorbed in the source.
This is not necesssarily the case, but gives us the least signal strength in
the high SWR case, SWR'.

So then, comparing the two cases, the change in power to the load in db is
10*log(SWR'/SWR").

SWR'/SWR" = 1.7

2.3 db, barely detectable, worst case.

So it's a question of how much reflected power can the rig tolerate as well.

That calculation completely baffles me.

The ALC in my Icom rig keeps the forward power constant up to the point
where it reduces power, at around 3:1 SWR. This is typical for
commercial rigs. The rig delivers 100 watts to a 1:1 SWR load.
(Techically, this really means a load which, if terminating a 50 ohm
line, will produce 1:1 SWR on that line.) 1.7:1 SWR is a reverse/forward
power ratio of 0.067. The ALC keeps the forward power at 100 watts, so
with 1.7:1 SWR, the reverse power is 6.7 watts. The net power delivered
to the load is 100 - 6.7 = 93.3 watts, or 0.3 dB, not 2.3 dB, less than
the power delivered to a 1:1 load.

Oh, and the "reverse power" isn't "absorbed in the source". Anyone
interested in learning more about this might take a look at "Food for
thought - Forward and Reverse Power.txt" at
http://eznec.com/misc/food_for_thought/.

Roy Lewallen, W7EL