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[AMPS] More L-net/Series to Parallel questions

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Subject: [AMPS] More L-net/Series to Parallel questions
From: (Jon Ogden)
Date: Wed, 17 Feb 99 09:40:27 -0600
>I did a bit of playing on the Smith Chart. While adding the inductance in
>series between the plate and C increases the value of C1, it does push up
>the circuit Q as a whole, as evidenced by the narrower bandwidth when a
>frequency sweep is done.  That's working back from the SWR looking back into
>the network when loaded with the correct laod impedance at the plate.

I don't understand.

The extra inductor acts as an impedance transformer.  It's reactance 
changes with frequency and so the resulting output impedance (prior to 
the tank circuit) changes over frequency.  In my case using a 1.5 uH 
inductor the impedance at the pi-net at 28 MHz would be about 1800 Ohms, 
the impedance at 14 MHz would be around 3 KOhms and at 3.5 MHz the 
impedance is just under the "normal" impedance of 4600 Ohms.

So the fact that you have a lower impedance allows one to design a pi-net 
with a lower Q that has "reasonable" real world values.

I don't understand how the bandwidth is narrower as you say.  On 80M, the 
inductor should be barely noticeable in the circuit.



Jon Ogden

"A life lived in fear is a life half lived."

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