On Mon, 29 Sep 2008 11:47:25 -0500, Cecil Moore
wrote:

Jeff Liebermann wrote:
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.

It's not an indication of *NET* power but it is certainly an
indication of forward power (or reverse power). If it weren't
anyone could sue Bird for false advertising.

Bird is very specific that their 50 ohm wattmeters are only useful in
a 50 ohm system. Here's one of your articles explaining how it works
in a 75 ohm system:
http://groups.google.com/group/rec.radio.amateur.antenna/msg/717e8daabed08fd9
Incidentally, I've been running cheap 75 ohm coax in 50 ohm systems
for years without any problem. Max VSWR= 1.5:1
http://www.qsl.net/n9zia/wireless/75_ohm_hardline.html
It's especially useful at 2.4Ghz, where the rather high coax cable
losses almost totally eliminates any detectable reflected 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.

A high VSWR can cause over-voltage or over-current conditions
depending upon phase. Over-current conditions can indeed cause
the finals to overheat. Over-voltage conditions can cause punch-
through of the finals. The key to understanding the effect of
SWR on the finals is in understanding constructive and destructive
interference.

Agreed. I didn't want to go into the mechanism for where the excess
voltage or current came from, just the risks involved in tuning at
full power. You can run a radio at full power with a high VSWR load,
but it's likely that either the PA voltage or the current are going to
approach the point of no return. Reducing power is much safer than
finding the limit the hard way.

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