> Basically, VHF voltage-amplification is Mu*VHF-Rp
No, it isn't.
Let Ra = plate (anode) resistance
VHF-Rp = Rp = the impedance of the parasitic suppressor at the frequency
in question
Rl be the impedance of the tank circuit at the frequency in question
The gain at any frequency is mu*Ra[Rp + Rl]/ [Ra + Rp + Rl]
You cannot ignore the plate resistance, and strictly, you cannot ignore
the non zero value of the tank circuit impedance when considering the
VHF gain, although it may be (should be) very low. Indeed, the fact that
it isn't is what causes the trouble.
What value of shunt resistance is used in the W8JI type suppressor? Why
can't you get the same lowering of impedance by dropping the value of
the shunt resistor? My (possibly faulty) understanding of the way the
suppressor works is that the inductance is added to increase the amount
of inductance in the circuit so that the shunt resistor is then damping
a higher proportion of the parasitic circuit, thus reducing its gain.
The higher the stray inductance in the parasitic circuit ( bad layout
etc), the bigger the suppressor choke has to be. The bigger the
suppressor choke, the more volts of fundamental appear across it, and
the more fundamental power gets dissipated in the resistors. If the
value of the resistors is too small compared to the reactance of the
choke at the parasitic frequency, then the effective impedance that the
suppressor offers drops too far, and the assembly looks like a lossy bit
of wire. So there's an optimum value for the impedance of the assembly
at the parasitic frequency, and too low is just as bad as too high.
After doing a few sums, I'm not convinced that the nichrome approach
doesn't have advantages in some circumstances - at least you aren't
searching for high power (5-10watt) low capacitance, low inductance
resistors.
73
Peter G3RZP
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