> - change of impedance over drive cycle
The best way to look at this is with harmonic content required to
make a wave some shape other than a sine wave. The waveform
MUST have harmonics of the drive frequency to change waveshape
during a fraction of an RF cycle, because the required frequencies
to modify the sine wave in such a small time interval are at least
two times the drive frequency.
If there is significant attenuation of signals at two times the drive
frequency, you can be absolutely sure anything happening at the
cathode is unimportant
What confuses people is they look at the change in drive
impedance with drive level and assume this affects the fractional
RF-cycle impedance. It does not, if the input has almost any Q at
all and is a low pass network.
> Its precisely these sort of things which cause me to prefer Tetrode
> amps which are grid driven (esp. at VHF) - even then I like a 6dB pad
> on the input of the amp.
Because of the lack of inherent negative feedback, many practical
tetrode amplifiers are not nearly as clean as a cathode driven
system.
The high amount of feedback also moderates the SWR. For
example the input SWR on my AL1200 is 1.15 : 1 at 1 watt and
only changes to 1.00 at 200 watts of RF drive.
What may confuse people is if you vary the loading control or plate
tuning, the input SWR changes a noticeable amount. This is
especially true in higher gain cathode driven stages.
It was a bit of advertising fluff by a tetrode marketing department
that mostly touted the "tetrode" advantage. That was ironic, since
at the same time the same marketing people thought grids could
be largely unregulated based on over-the-air "am I splattering much
Herbie?" measurements.
IMD measurements show how dirty most tetrode amps commonly
used actually are.
> For your amp you might like to try a 1/2-wave length of coax between
> rig and radio on the band of interest [test] (taking velocity factor
> into account) to eliminate the "transform effect" of the coax from the
> equation.
Having a certain length of cable in fractions of a wavelength
guarantees nothing, except how far away the PA can be physically
placed.
Just make sure the amplifier is matched to the cable impedance.
The normal range of 50 ohm cables I have measured is 45-55 ohms.
73, Tom W8JI
W8JI@contesting.com
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