Ten-Tec said:
> "...The SWR reading gives no indication of reactive components, nor can it
> separate the resistive from the reactive components. It is calibrated
> with a pure resistive load and therefore has its greatest accuracy with
> pure resistive loads."
Tom said:
"I'm not "anxious to criticize", the statement in the Ten Tec manual
is simply incorrect.
Reactance causes reflected voltage to appear in the directional
coupler just as much as resistance errors do. That's a simple fact
that is often misunderstood."
You simply mis-read what Ten-Tec said. It says that the "SWR reading gives
no indication of reactive components". I understood that to mean that a 2:1
SWR reading simply does not tell you what combination or resistance and
reactance creates that voltage. Ten-Tec's statement does not say that a
reactance will not cause a voltage out of the directional coupler.
Reactance does cause a voltage to appear on the reflected port of the
directional coupler.
Now, with regard to a short of the output shutting the rig down, I know of
at least one case and probable second that it did not. A short in the
output will change the operating conditions of the amplifier driving the
circuit and change the efficiency of the networks. If you had a constant
power source, the current would increase with a short and voltage would be
induced into the reflected pickup of the directional coupler. But, the
source will change drastically with a short. I am going to put an ammeter
in line with a short of the Atlas output and monitor the conditions. The
transistor amplifier certainly will operate differently with a short than
with a open and I will have to put one of the rigs on the bench to
understand the failure mode I understand the intent of the protection
circuit and I understand directional couplers, but I question whether all
protection circuits are foolproof with a short. And, I quoted the manual
from Ten-Tec that addresses the problem. If it was not a problem, Ten-Tec
would likely not have put it in their manual. If the amplifier becomes much
less efficient due to the short, and output drops, then the output from the
directional coupler pickup could be low - not shutting down the alc circuit
in the driver section. The directional coupler output voltage will not
change the severe mis-match to the final transistors.
I am simply going to have to put a rig on the workbench, short the output
and monitor the conditions. I simply reported that in two cases, the SWR
protection circuit did not work to protect finals with a shorted coax
connector. If you want to short your coax, go ahead. I do not plan on
doing so until I fully understand the cause of the failure.
73, Colin K7FM
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