[TenTec] Model 238C Tuner Questions

[Bob McGraw - K4TAX]

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 at gmail.com>
To: "Discussion of Ten-Tec Equipment" <tentec at 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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