[Fwd: Re: [TenTec] RE: answers to Omni V/WM-1 observation]

[Dr. Gerald N. Johnson, P.E.]

Message-ID: <36894B04.2315@ames.net>
Date: Tue, 29 Dec 1998 15:35:00 -0600
From: "Dr. Gerald N. Johnson, P.E." <geraldj@ames.net>
Reply-To: geraldj@ames.net
Organization: Gerald N. Johnson, P.E.
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Subject: Re: [TenTec] RE: answers to Omni V/WM-1 observation
References: <36893C7C.726A@vbe.com>
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All amplifiers are sensitive to the phase angle of the load and anytime
the SWR is greater than 1, the phase angle of the load swings around the
smith chart each half or full wave of length of line change. In the old
tube amplifiers that phase angle had to be compensated for by adding or
removing the loading capacitor on the Pi network. When you ran out of
capacitor due to phase angle of the load you ran out of load control,
whether too much or too little. Generally the problem was not enough
loading C on 160 and 80 and too much on 15 and 10m.

But the PI net output radios had a greater tolerance generally than the
fixed tuned solid state radio. What you have now is that the changes in
phase angle, hence resistive and reactive components, of the load get
transformed through the output low pass filter and matching in the solid
state radio to upset the load on the PA devices. Raising the load
impedance on the PA devices means a greater voltage swing, but if
perfectly matched and driven for maximum output, they can't swing any
more so you loose output, but if the voltage rises too much you loose
devices. Lowering the load impedacne reduces the voltage swing and
raises the current swing. Still can swing past zero so you loose output
and the peak currents can fry marginal devices. The ALC in the radio
attempts to pretect the devices. Some do, some don't depending on the
phase changes through the radio's output networks and low pass filters.

While the modern radio tolerates some SWR, it would be better for device
longevity and for distortion products if the load was always matched
with an external antenna tuner, just like we used to do with tubes that
required it because their load resistance was to much further from that
of coax fed antennas that a broad band match wasn't practical.

73, Jerry, K0CQ


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