On Wed, 2006-08-09 at 10:45 -0400, ARDUJENSKI@aol.com wrote:
> I noticed that not in all cases will the meter (set to SWR) in the Paragon
> II read exactly the same as that is the 238 ATU. By the way the 238 and the
> Paragon are connected by an 18 inch quality jumper (Belden 8214).
>
> I found a discussion on the topic in the ARCHIVES at
> _http://lists.contesting.com/archives//html/TenTec/2003-01/msg00663.html_
> (http://lists.contesting.com/archives//html/TenTec/2003-01/msg00663.html)
>
> Basically this section in short says to believe the meter in the rig.
>
> SITUATION: I did notice that if I turn the AGC button to OFF the meter will
> match the ATU meter. (By the way I use a MFJ 249 Analyzer to find the
> settings for the ATU but find in some cases it still needs to be tweeked when
> I
> push TUNE.) Is this normal? I could not find anything in the manual that
> even
> mentions the AGC in conjunction with operation of the SWR/TUNE set up.
>
> Is this normal or do I have a problem I need to look into?
>
> Thank you
>
> Alan KB7MBI
> Woodinville, WA
>
>
Cascading SWR meters is a lot like carrying two watches. With one you
could be sure of the time or the reading, but two are never going to
agree.
Each SWR meter shows what it sees at its output. But each is a section
of transmission line with some coupling to the metering circuits that
can change the input Z, more if the SWR isn't purely resistive and the
SWR meter won't say if that 1.5:1 load is purely resistive or reactive
or something in between. So the SWR meter in the transmitter may see a
different load than the one at the tuner. To add complication, and low
pass filter will also act as some sort of impedance transformer,
probably adding its own reactive component.
When tuning a fixed frequency radio through a Bird watt meter, Bird
always recommends adding coax to the watt meter to make the assembly a
half wave long so the transmitter being tuned sees the same Z as the
Bird sees, even if the Bird introduces some impedance changes.
Further, even if the load is resistive and other than 50 ohms, a piece
of 50 ohm transmission line will make it look reactive unless that line
is a multiple of a quarter wave long. And many simple SWR meters don't
handle reactive loads well which shows up most when the SWR changes when
changing the load feedline length though the SWR shouldn't change
amplitude, just phase angle.
--
73, Jerry, K0CQ,
All content copyright Dr. Gerald N. Johnson, electrical engineer
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