[Amps] High SWR

[Fuqua, Bill L]

Yes and No.
   You can. to a limited extent, make an antenna system resonate with an amplifier with a PI network output.
Only if it is slightly off resonance the "Load" capacitor and be adjusted such that you can get an 'conjugate" impedance match.
Making the antenna, transmission line and the PI network all resonate at the operating frequency and also get maximum power transfer.

However, this range is limited and the SWR meter will not indicate 1:1 SWR if you do get a match becuase the SWR meter is still comparing
the antenna system to 50 Ohms and also now the reflections in the transmission line become part of the "resonante system"

 Years ago before hams used SWR meters they tuned their transmitters to the antenna systems by dipping the plate current and
adjusting the Loading capactior of the PI network until they got the proper correct plate current at the dip. But when using
linear amplifiers and particularly GG amplifiers this approach does not always work out right.

 PI-L networks allow for very little range of compensation for antenna missmatch. Still the SWR meter will not indicate a match even
if you achieved one.
73
Bill wa4lav

________________________________________
From: amps-bounces at contesting.com [amps-bounces at contesting.com] On Behalf Of Dr. David Kirkby [david.kirkby at onetel.net]
Sent: Monday, July 04, 2011 5:20 PM
To: 'AMPS'
Subject: Re: [Amps] High SWR

On 04/23/09 04:50 PM, Scott McGrath wrote:
> SWR is an obsession with the CB set and some others CB's could not
> tolerate a high SWR due to the cheap finals used the high RF voltages
> would blow the junction.     This effect created the oral tradition of
> 1:1 SWR == Antenna System Goodness (tm).   It's hard to counteract what
> 'everybody' knows.
>
> In reality and especially with a tube amplifier and a Pi output tank the
> antenna system can be brought into resonance at a wide range of
> impedances and resonance is what we are looking for and it really is
> Antenna System Goodness in MOST cases.

You can't change the resonate frequency of an antenna by adjusting the
amplifier. The frequency of resonance of an antenna is for all practical
purposes independent of how the amp is tuned.

You will get 2nd order effects like the RF heating the elements which makes them
expand so changes the frequency of resonance, but I'm ignoring such things here.

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