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
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