w8ji.tom wrote:
>The stuff about high SWR making a tube type PA dissipate extra power or
>produce excessive voltage at the tube is the worse kind of nonsense, unless
>that tank is not reloaded or able to reload to the new load impedance.
>
>2.) Another reason to set an SWR is matching range. Above a certain SWR,
>the tank might run out of capacitance and won't tune properly.
Trying to expand and clarify this...
It makes practical sense for amplifier manufacturers to specify a
maximum SWR, because customers can measure SWR but not true R-X
impedance. However, in an engineering discussion we need to recognise
that SWR is not the true limitation. What the amplifier is sensitive to
is the actual load impedance at its output terminals.
Going around a constant-SWR circle on a Smith chart, there are an
infinite variety of impedances that all give the same SWR. Depending on
the length of transmisison line involved, these range from high-
resistive through resistive-inductive to low-resistive, then resistive-
capacitive and back again to high-resistive a half-wavelength further
along the transmission line.
The manufacturer's limiting SWR is based on the fact that the amplifier
can't handle load impedances on (or outside) *some* parts of the
limiting SWR circle. With a different length of transmission line, there
might be load impedances on other parts of the same circle that it
could handle - but it isn't practical for an amp manufacturer to get
into that subject. A limiting SWR is a nice, simple thing to specify.
>
>Reflected POWER has little to do with anything except as it affects
>voltages, currents, and impedances by presenting a different impedance to
>the PA.
That makes complete sense. From the amplifier's point of view, it can't
tell the difference between a load consisting of an antenna->line->
transmatch->another_line, and a lumped R-X load that presents exactly
the same impedance.
There are two separate problems here:
1. "How does the amplifier react to a range of different load
impedances?" (Legitimate answers MUST NOT mention transmission lines or
SWR!)
2. "What happens to the reflected power in the transmission line?"
IMO the only workable approach is to answer question 1 first, and then
look at question 2... if you still think it exists.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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