Based on your power values, 200 watts forward and 80 watts reflected the SWR
should be 4.44:1. I would agree that a 1:2 balun with a load of 25 ohm on
one side should produce about 50 ohms on the other side. However, your SWR
value of 1.3:1 is likely due to circulating currents on the feedline which
otherwise make the SWR reading being non valid.
With a 4.44: 1 SWR this would project the impedance being either 222 ohms or
11.26 ohms reference 50 ohms. Are you sure the 1:2 balun is connected
correctly? If it is reversed with a 25 ohm load on the "2" side then the
"1" side would show 12.5 ohms. Might be worth looking a bit closer.
There are several SWR calculators available on the web. The require one to
input forward power and input reflected power and the SWR value is shown.
73
Bob, K4TAX
----- Original Message -----
From: "Jim GM" <jim.gmforum@gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec@contesting.com>
Sent: Wednesday, October 17, 2012 11:27 AM
Subject: Re: [TenTec] Model 238C Tuner Questions
Hi Jimmy B.
I have been doing some measurements with My bird Watt meter. 50 ohm match
through a 1:2 balun. Input impedance to inverted L is 25 ohms. SWR is
about 1.3:1 however Forward power 200 watts with Reflected power of 80
watts. Feed line from balun to shack is 25 feet. Reflected power just
gets
worse on other bands. Lots and lots of common mode currents. Too much
for
just some balun to absorb that much RF.
So from what I have seen is that SWR is one animal and forward and
reflected power is another animal that needs to be dealt with.
This type of flow is just a waste, I read up on FCP single redial for a
single band seams to work just fine. Multi band an antenna just creates
all sorts of common mode currents. Not Good. Causes all sorts of problems.
What I am trying to shoot for is some antenna with very low return RF.
That
would mean that ether ground or the antenna is radiating all or most of
the
energy put into it. a 50 50 chance on getting it right. A small florescent
tube, or NEON light taped on the wires will tell the story.
DX Engineering has those clamp on cores for common mode currents seam to
work well on the residual RF coming back but only after the Hi Reflected
power coming back is taken care of.
--
Jim B K9TF
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