Steve Thompson wrote:
>W3LPL wrote:
>>A class B amplifier produces many harmonics as it switches from
>>conducting to non-conducting, and the input tuned circuit must provide
>>an excellent input termination impendance not only at the fundamental
>>frequency, but also at the 3rd harmonic and ideally at the 5th harmonic
>>as well. This would not be a problem if it were a Class A amplifier,
>>but that's certainly not practical for a high power RF amplifier!
>>
>>Even a short piece of RG-58 between the input tuned circuit and the
>>tube socket significantly increases the input termination impedance at
>>the harmonic frequencies.
>
>I'd guess that the phase changes the cable introduces at harmonic
>frequencies has a lot to do with it. The loss in a foot of RG58 even at
>150MHz is small.
It seems that the input network is actually providing a reflection for
harmonic energy, not a matched termination.
So... just speculating... in a multi-band amplifier, would it make sense
to wire the 10m pi-network permanently in place at the tube socket? On
the lower bands, that network would look like capacitance, and it might
be enough to compensate for a short coax link to the switched networks
for all the lower bands.
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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