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Re: [TowerTalk] Johnson Matchbox

Subject: Re: [TowerTalk] Johnson Matchbox
From: jim Jarvis <>
Date: Tue, 25 Mar 2008 11:15:57 -0400
List-post: <">>
TT,  Some added thoughts on this:

The Johnson matchboxes were rated at a time when the FCC power rules  
were based on
INPUT power, not output.   So, the 275W version matched the specs for  
the Valiant, and
the KW version matched the specs for the Johnson KW.    Our present  
rules limit OUTPUT.

It was common to run open wire line at high vswr,  and sometimes coax  
at 5:1 vswr in those
days, and the matchboxes were significantly overbuilt to handle the  
high voltages involved.
This gives us headroom.

These days, we tend to operate more in 50 ohm systems, and the peak  
voltages are much
lower.   Still, the capacitor spacing should withstand several  
thousand volts easily.   And the
coil conductor size and spacing were appropriate to much higher peak  
voltages, and higher
average power than present day SSB or CW would present.

I remember a discussion about the peak power present in a DSB AM  
signal, vs SSB suppressed
carrier, long time ago.   It said something like there was 4x the  
peak in the AM as compared with
SSB.   Intuitively, this doesn't seem right.   For equal power  
output, the PEAK voltages should be
the same... but the average power in the signal envelope would be 4x  
higher in the AM signal.
(or something like that.)    Anybody care to enlighten me?

OH... and the practical solution of ... 'crank it up til you smell a  
problem'  still sounds about right.
I wouldn't expect the 275W matchbox to handle 1500w, however.



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