One minor glitch in the process is that plate current may drop due to
the impedance at the tube's input changing. This is noticed at times with
either grounded grid or neutralized grid driven amplifiers. In fact one
common test for neutralization in the older tube type transmitters is to
see that influence on the grid current is symmetrical about the dip in the
plate circuit.
I guess what I am saying if the test does not turn out perfect it is
not due to the plate circuit being off resonance but the input impedance
changing. But other than that glitch, it seems as a reasonable test.
A the primary resonant frequency of a resonant system is the frequency
at which the stored oscillating energy divided by the applied energy per
cycle is maximized after the system has reached equilibrium. In the case of
most impedance matching networks it is where Pout/Pin of the network is
maximized. But there are cases were there is no RF output from the
resonant network and all of the RF power goes into heat. Or in the case of
an antenna most of Pin is radiated as electromagnetic waves at the applied
frequency and a little in heat (electromagnetic waves of much shorter
wavelengths).
73
Bill wa4lav
At 04:15 PM 2/22/2005 -0500, TexasRF@aol.com wrote:
>In a message dated 2/21/2005 4:47:23 A.M. Central Standard Time, r@somis.org
>writes:
>
>
>On Feb 20, 2005, at 6:35 PM, TexasRF@aol.com wrote:
>
> >
> > Hi Rich, no, I said "C1 resonates the network" but no matter, we both
> > know what the intent was.
>
>No capacitor in a L-network or a Pi-network (double L-network)
>resonates the network.
>
>
>
>Hi Rich, here is the plan for the bullet proof dip meter and test:
>
>The PA has an 8877 tube in it with a 1000 ma plate current meter installed
>and connected. We can use this meter to observe the resonance "dip". The Pi
>network is adjusted for maximum output power with 75 watts of drive power
>applied. We have to do this with a dummy load so any antenna related
>influence in
>our test is eliminated.
>
>At resonance, the plate load impedance is all resistive, no shunt reactance.
>Off resonance in the higher frequency direction would entail the presence
>some shunt inductive reactance, which in parallel with the plate load
>resistance would cause the load impedance to be lowered. Off resonance in
>the lower
>frequency direction would entail presence of some shunt
>capacitive reactance,
>also lowering the total load impedance.
>
>Since we know from Ohm's law that current equals voltage divided by
>resistance (or impedance in an ac circuit) we would expect the plate ma
>meter to be
>minimum when the load (network) is at resonance and non minimum when the
>load
>(network) is off resonance.
>
>Now comes the dip check: Expecting the network to be non resonant, as the
>driver frequency is slowly changed, in the direction of expected resonant
>frequency, we would expect the plate current to slowly reduce until we
>reach the
>actual resonant frequency of the load (network). If we go the wrong way then
>the plate current will rise. No problem, we just tune the driver
>frequency in
>the other direction in this case.
>
>Once we find the frequency of minimum plate current (the dip), bingo! We
>have found the resonant frequency of the load (network). If it is
>different than
>the starting frequency then the idea that Pi networks do not operate at
>resonance will be proven. Also, no dip meters have been sacrificed in
>the process
>and any question of what influence is caused by cover removal is avoided.
>
>Anyone out there willing to give this test procedure a try? I am at work
>right now and no access to a big PA to check this out.
>
>Standing by for test results!
>
>Thanks/73,
>Gerald K5GW
>
>_______________________________________________
>Amps mailing list
>Amps@contesting.com
>http://lists.contesting.com/mailman/listinfo/amps
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|