[TenTec] SWR Fold-Back

[Larry, K4WLS]

Yes, the Ten-Tec rigs do not have the normal ALC circuits to reduce
the power output in cases of loads that significantly depart from 50
Ohms. The result, however, is the same. You get less output power
for the same amount of current drawn from the power supply for loads
that significantly depart from 50 Ohms purely resistive. The load (Ant)
Z is is always reflected back to the rig when not using an external
antenna tuner. One of the worst case scenarios for the older Ten-Tec
rigs that use power BJT's in the finals is a reflected load Z that is
predominately inductive. This causes very high RF voltages on the
power BJT's and can wipe them out.  Curcuit breakers in the associated
power supply and the internal fuse will detect no abnormal DC current
to the finals, for the most part, in this siuation and will not help.

Power MOSFETS's, on the other hand, have protection diodes "built-in"
to handle such scenarios. Yes, any antenna tuner does have losses.
I would still prefer to use, per say, the cheapest MFJ to keep the SWR
seen by my Corsair finals 1:1. I always tune the antenna for a 1:1 SWR
on the Corsair and not for zero "reflected power" on the tuner. If you want
to get down to "brass tacks", the term "reflected power" is actually a
mis-nomer. A mis-match between the ant Z and the coax Z results in a
reflected Z mismatch back toward the rig. SWR on the transmission
line and power delivered to the antenna is a concept seldom undertood
by most Hams.

Putting a tuner in the line to your Ten-Tec rig and adjusting the tuner
for a 1:1 SWR on the rig SWR meter will in most cases give you 100W
output (if the rig is working properly) for a key-down current of 18 to
20A. The reflected power meter on the tuner may show some reflected
power, but this is the apparent reflected power at the point in coax line
to your antenna where you have the tuner placed. This is not the true
story however.

If you put another power meter between your tuner and the ant coax,
such as an MFJ set to "Bypass", if the power meters are calibrated
correctly, for an SWR of 3:1 on the coax line to the ant, the forward
power will read 150W, and the reflected power will read 50W. This
of course neglects any losses in the "tuned" antenna tuner in the
line. SWR = (Pfwd + P ref)/Pref

The SWR on the coax from the antenna tuner to the antenna may be
3:1, but with a tuner the rig is going to see a predominately 50 - 52 Ohm
resistive load and this is the best scenario for the finals. For a
coax to the antenna that has low dielectric losses, you are still going to
going to deliver most of that 100W to the antenna (except for some
small losses in the tuner) even with a 3:1 SWR on the coax between the tuner
and antenna.

So "SWR Fold Back" applies to any solid state rig whether it has ALC
ro reduce the power output to the finals or not. The end result without
a tuner is the same, you still get less power output from the rig with a
a SWR that significantly departs from 1:1 at the antenna connector
to the rig.

Well, I am an OF and could not sleep last night. Guess this could be
worded a bit better, but I am tired. Hope it helps someone out a bit.

Regards,   Larry  K4WLS

By the way, I need to put a 500 Hz CW filter in the 1st IF of my Omni 6
Plus. I have major health problems, including poor dexterity. I am
lucky to still be able to send CW. Ten-Tec rigs are great, but the
manuals have always been a bit helter skelter. Could someone be
so kind as to give me a step by step procure. Do I have to remove
both covers to get to the filter board. I am a retired (long ago)
civil service Navy RF Communications engineer. To tell you the
truth, I am tired of digging info out of tech manuals, and my eyes
are bad. You can send me a personal mail - sure would appreciate
the help of some kind soul - God Bless Ten-Tec for the best CW
rigs, and God Bless all you Ten-Tec fans.