[TenTec] SWR readings varying with power out

[Steve Ellington N4LQ]

Perhaps the final amplifier's output impedance isn't 50 ohms at all
power levels. We eliminated the pi-network matching system used in the
old tube type transmitters replacing it with a broad band low pass
filter which doesn't compensate for power power changes.

The swr meter on the rig and an external one never seem to agree
possibly partly due to the above reason. When using a tuner to
resonate antenna/feedline combination keep in mind that the swr
dosen't really improve on the feed line. Rather, we have just created
a spot along the feedline which satisfies the reading on IT'S internal
swr meter (if it has one).

Question: If the swr meter on the tuner reads 1:1 does this mean that
the input to the tuner is now exactly 50 ohms.

Answer: Naw...There's always some reactance left over and the meter
reading is probably not exactly correct in most cases.

Results: That little left over reactance  is seen by the transmitter
and the swr meter in the tx gives us yet another reading.

Which one's right?  Probably neither one.

What to do? Make sure your final amp is not drawing too much current
and carry on. Use the meters for "ball park measurment"

I've got an swr meter in the rig, amp and tuner and they all disagree
for the above reasons.

As for the Omni 6's weird 30 meter problems....I've had two of them
and they both gave bogus final current readings on 30 meters showing
way too much current as compared to the power supply. I trust the
power supply meter.

Somewhere I read where this was caused by a ground loop within the
Omni 6 and there was a cure but I have no idea what it was. Probably
TT can help.
Steve N4LQ
----- Original Message -----
From: "Ken Kinyon - W7TS" <w7ts@uswest.net>
To: <tentec@contesting.com>
Sent: Wednesday, December 29, 1999 10:43 AM
Subject: [TenTec] SWR readings varying with power out


>
> Hi,
> My experience has been that every SWR meter I have ever used showed
a
> slightly higher SWR at higher power levels. That includes the RF
> applications P-3000 that I currently have in line.  I have always
figured
> this was due to fact that it is probably not possible to find
absolutely
> linear diodes.
> If you really want to know what is going on,  use a good power meter
(Bird
> for instance), and measure forward and reflected power.  Then
calculate the
> SWR yourself.
> 73,
> Ken W7TS
> -------------------
> Kenneth E.  Kinyon
> 34 Princeton Circle
> Longmont, CO 80503-2106
> Voice/fax  (303) 684-0037
> E-mail W7TS@uswest.net
> ALPCA #8339
> ARRL-LM
> -----------------------
>
>
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