[Amps] low Q output matching

[Carel, pc5m]

Hi Peter (and Gerald)

Basically my aim is to get the simplest output match circuit possible
(simplest = lowest value of C's, because high valued, high current, high
voltage variable C's are difficult to get and/or expensive). I don't mind
even 0 dB of harmonic suppression.... because that is easily solved
afterwards with fixed valued C's/L's lowpass filter. My question is
basically are there any other parameters I am overlooking (beside the afore
mentioned harmonic suppression) if I go for this ?. In essence, could I just
a high power 3kW broadband 50 to 1700 Ohm RF transformer and the tube would
be happy ?
Gr, Carel.

-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of peter chadwick
Sent: zondag 11 november 2012 9:32
To: amps at contesting.com
Subject: Re: [Amps] low Q output matching

What is the value of the lowest Q?
What harmonic suppression are you aiming for? It should be 43dB + 10 log P,
without exceeding 50dB. If there's a tuner on the antenna, that may well
mean you don't need the low pass filter.
73
Peter G3RZP





========================================
Message Received: Nov 11 2012, 07:46 AM
From: "Carel, pc5m" 
To: "'AMPS'" 
Cc: 
Subject: [Amps] low Q output matching

Hi, I was wondering what else, besides bad harmonic suppression, will happen
when I design my (single band) output matching network (PI-L) with low Q
(lowest Q needed to get the match done) ?

When using low Q design I will get
- much smaller (variable) capacitors, especially for the bigger load C, so
easier to get components
- broader no-tune range
-smaller RF stress on the components

As an example for a 160mtrs amp, with Rload of 1700Ohm and intermediate
impedance around 330Ohm:
With Q of 10:
CLoad =1200pF
L-l=10uH
Ctune=293pF
Lpi=33uH



With lowest Q:

CLoad=400pF
L-l=10uH
Ctune=132pF
Lpi=33uH



When you plot both matching networks in a Smith diagram you see immediate
the difference. Low Q is much more straightforward.
To get the needed harmonic suppression a lowpass filter with fixed
components is easy to construct (first harmonic at 3,8MHz needs 25dB and
second one 20dB to get the same values as higher Q design). But maybe I have
overlooked something . Thanks for your ideas 73's Carel, pc5m





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