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Re: [Amps] Fwd: Pi-L In-circuit Adjustment Question

To: r@somis.org
Subject: Re: [Amps] Fwd: Pi-L In-circuit Adjustment Question
From: TexasRF@aol.com
Date: Tue, 22 Feb 2005 16:15:51 EST
List-post: <mailto:amps@contesting.com>
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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