But what about the transceiver that uses tubes for the output - like the
many hf rigs which use a pair of 6146's or similar? Surely if a
tube or, for that matter, a transistor/power module is tuned to match into
the grid circuit for Class C operation the impedance it "sees" is very
close to a VSWR of 1:1?
I think that what you are trying to suggest is that an amplifier which is
normally biased for Class AB1 and is then over-driven into Class C would
cause the impedance to change as the grid current starts to flow. How is
this different to, say, a grounded grid Class B amplifier which runs a fair
amount of grid current under normal operation. Surely in this case the
input Z also changes from one state to the other? But doesn't the input
impedance change gradually as the grid current increases from zero to its
maximum?
There is no doubt that some solid-state rigs do not produce acceptable IMD
products when mis-matched into reactive loads. But from experiments that I
and others have performed the degradation is not as bad as some would have
you believe, provided the ALC circuitry in the transceiver is designed to
gradually back-off the drive under poor VSWR conditions. It may be true
that some solid state transceivers use the VSWR back-off as a "sudden
death" type of protection, which is an exceedingly poor piece of rf
engineering!
Any transceiver worthy of use should have a correctly designed AND adjusted
ALC circuit. If the unit left the factory correctly set up and some "lid"
with itchy fingers then goes and tweaks for maximum smoke (without the
correct test equipment or knowledge of how to use it) they deserve to be
jumped on by his fellow amateurs!
My original comment related to an amplifier specifically set up for Class C
operation for CW. This is how the "old fashioned" amplifiers worked and
nothing was considered unusual about the tuning up or operation in any of
the many ARRL Handbooks or text books on the subject that I have read so
far. Drive was applied until the required amount of grid current flowed
and the anode tank circuit was then tuned for a pronounced dip in the anode
current before the loading was increased to bring the anode current and
output power to the required. Surely nothing has changed as far as that is
concerned? Today we have better tubes available, but the basic principles
haven't changed - or have I missed something along the way? In the days
before VSWR meters, hams where quite happy to have a high VSWR between the
transceiver and the amplifier, one simply adjusted the matching networks in
the transceiver and the amplifier for best power transfer.
It seems to me that too few hams understand the basics of rf engineering.
They should take some time out and read Walter Maxwell's (W2DU) excellent
book published by the ARRL entitled "Reflections". Once the fundamentals
of impedance matching are fully understood it only requires a little
application of common sense to resolve 90% of the problems that hams
encounter.
As a final thought:
Consider the anode impedance of an amplifier. One tunes up with maximum
drive. In this case for the anode voltage/curent in use the tube now
"sees" its correct load impedance and transfers the maximum power to the
load (antenna). Under low drive conditions the tube "sees" an impedance
which is many times higher than what it was set up for. Can some genious
devise an automatic tuning mechanism that will provide the correct anode
impedance for any condition of drive level, to ensure the best efficiency
under all drive conditons?
John ZS5JF
----------
> From: Rich Measures <measures@vc.net>
> To: John Fielding <johnf@futurenet.co.za>; Amps Contesting
<amps@contesting.com>
> Subject: Re: Fw: [AMPS] tuning capacitor & insulator
To: <amps@contesting.com>
> Date: 19 December 1998 09:07
>
>
> >Please explain why varying the VSWR on the driver "causes grief"?
> >
> Because high swr causes the radio to shut down in order to protect its
> output transistor.
>
>
>
> Rich...
>
> R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
>
>
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