[TowerTalk] Reflections, Conjugate Matching and Jim Reid's comments

[w8ji.tom]

> Steve Best said:
> 
> >     Ask yourself why transceiver manufacturers, even ones with built in

> >     tuners, specify a maximum VSWR load (like 3.0:1) and why the output

> >     power is lowered when operating into a high VSWR?  If all of the 
> >     antenna reflected power was reflected back to the antenna, why
would 
> >     it make a difference?  And yes, some transceivers and tuners do
burn 
> >     up because of reflections.
> 
> W7NI offers an explanation:     
> 
> You are apparently talking about relatively modern, solid state
> transceivers.  It is well known that solid state devices respond very
poorly
> to over voltages.  They short, permanently, unlike vacuum tubes which may
> actually arc internally but then recover to operate again.  The presence
of
> a high VSWR probably creates an over voltage situation that could
> permanently short the output transistors.  This is a different failure
mode
> than having them simply dissipate too much power and melt down.  So I
think
> modern transceiver protection circuits are really protecting the output
> transistors from too much voltage across the junctions than from too much
> power dissipation.  But I could be wrong since I am not an RF design
> engineer . . .

Since I design PA's and matching systems I can shed some light on this.

Tube type PA's or solid state PA's with adjustable networks. 

1.) One reason I specify a maximum SWR in a tube type PA is the voltage
rating of the loading capacitor or current rating of any wire leads or
components between the tank and the output connector.

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.

Fixed tuned PA's.

1.) The load impedance change causes excessive current or voltage at the
output device or in the circuitry between the output device and the output
connector.

2.) Stability (solid state PA's)

3.) IMD

Reflected POWER has little to do with anything except as it affects
voltages, currents, and impedances by presenting a different impedance to
the PA. 

73 Tom

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