I have to confess that I hadn't considered it either -
you took that extra step! (I probably suggested it
when I mistakenly wrote 'admittance' rather than
'conductance' but I was really thinking of the
latter!) I expect it would work fine, and actually I
am not sure that there would be any particular
constraint upon the distance between tuner and
amplifier, provided that cable losses were negligible.
The downside of this technique is that a common
station impedance is not preserved, and to my mind
that is the overwhelming reason for putting the
matching network at the amplifier.
73 Roger
--- Bill Fuqua <wlfuqu00@uky.edu> wrote:
> Using an automatic antenna tuner and shunt
> capacitance to ground at
> the cathode is an interesting idea. I have not
> considered it. However, if
> the capacitive reactance is a fraction of the tubes
> input impedance it
> should work. If it is 1/5 of the tube's cathode
> input impedance the antenna
> tuner, not knowing the difference between an
> amplifier and an antenna,
> should offer the appropriate impedance to the
> amplifier to provide a
> conjugate match. That is the antenna tuners output
> resistance will equal
> the input resistance of the amplifier and the
> tuner's output reactance will
> be equal to that of the amplifier's input but
> inductive. Together they
> become resonant with a Q of about 5. As long as the
> inductance (tuner) is a
> short distance away ,less than 1/4 wavelength,
> should make no difference.
>
> 73
> Bill wa4lav
>
>
> At 03:17 PM 3/30/2005 +0100, Roger Parsons wrote:
> >I think that Steve, G8GSQ, probably has the answer:
> >
> >I have now convinced myself that it is nothing to
> do
> >with the matching network per se, or with flywheel
> >effects. Many grounded grid amplifiers like/need to
> >have a capacitor directly across the input. My
> belief
> >is that this is sometimes (and only
> >sometimes)necessary to ensure unconditional
> stability
> >over all of the driving cycle, and only marginally
> to
> >reduce harmonics.
> >
> >As Steve rightly points out, it is not possible to
> >'remote' a capacitor over any length of
> transmission
> >line unless the reactance at the end of that
> >transmission line is optimised for each particular
> >frequency.
> >
> >The use of a pi network at the amplifier input
> allows
> >a capacitor to be placed directly at the input
> whilst
> >preserving (on average) a resistive input.
> >
> >This could be proved with a bit more effort than I
> am
> >prepared to make. A T network could provide exactly
> >the same impedance transformation and Q as its
> >equivalent pi counterpart. But it would not place a
> >capacitor directly across the amplifier input, and
> I
> >wager it would not achieve the desired effect....
> >
> >73 Roger
> >VE3ZI
> >
> >
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>
>
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