The answer to this problem is unrealistically simple...
Their is reactance on the line. When changing the length of the coax
changes the SWR on the line you have proven this point.
Either the input to your amp has a reactive input, or the output of your
transceivers is not matching correctly to 50 ohms...
I would suspect the input of the amplifier being the real culprit,
however, you can check this by taking the output of the exciters and putting
them into a real (Non-reactive) 50 ohm dummy load and measuring forward and
reflected power.... If they both measure 0 watts reflected, the
transmitters are fine.
Also, you should be measuring power input to the amp at the input SO239
connector... Using a double PL / PL connector. If you are not, then their
is the chance that you are not seeing the "true" input power to the amp,
especially if the input to the amp is reactive.
Finally, your exciters are showing different power output levels for input
power levels because of the inability to deal with the reactive loads
presented.
Remember that a piece of coax WILL act as a transformer, and the inability
of your solid state exciters to deal with a reactive load is what is causing
this phenomenon.
Just a note from a lowly CBer...
--Toll Free--
______________________________________________________
Hi Mike,
Both auto tuners are off, but in playing around I put a 10' RG-8X jumper
between the radio and amp instead of the 6' 9913 jumper an interesing enough
the SWR dropped to flat, but now the Icom needs 75W to get 1500 out and now
the Yaesu needs 60. OK, I am going to give up now - 3cx800's arent cheap
and I think the time will be more well spent on the air instead of the dummy
load, hi hi. I guess I should be satisfied it has full output and a low
input SWR....
73,
Kevin
----- Original Message -----
From: Mike Blier <w6ffc@pacbell.net>
To: Kevin Polchan <kpevin@mediaone.net>; <amps@contesting.com>
Sent: Tuesday, January 25, 2000 10:17 AM
Subject: Re: [AMPS] Input power question
>Do you have the auto tuners switched out in both rigs? The Yaesu has a
>higher SWR than the Icom so perhaps you have the tuner still in line.
>
> Mike B.
>
>-----Original Message-----
>From: Kevin Polchan <kpevin@mediaone.net>
>To: amps@contesting.com <amps@contesting.com>
To: <amps@contesting.com>
>Date: Tuesday, January 25, 2000 7:58 AM
>Subject: [AMPS] Input power question
>
>
> >
> >Hi All,
> >
> >Just a quick question after experimenting with a couple radios here the
>last
> >couple days. I have an AL-800H amp (two 3cx800a7) and 2 different
>exciters,
> >an FT-990 and an IC-756. I am curious why it takes 80watts from the
Yaesu
> >and only 55 watts from the Icom to drive to a 1500 watt carrier. I have
>put
> >a wattmeter between the radio and amp and the amp and antenna. I assume
>its
> >some kind of impedence thing, perhaps from the finals of the radio. Both
> >radios at 100watts out will drive the amp to the same output.
> >
> >I am not that concerned about it, even though the amp is rated at legal
> >limit for 55W of drive, 25 watts seems like quite a large difference to
> >achieve the same output. The input SWR is < 1.5:1, but the Icom shows
> >1.19:1 and the Yaesu shows 1.46:1.
> >
> >Any comments?
> >
> >Thanks and 73,
> >Kevin
> >KB9UPV
> >
> >
> >
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