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Re: [TowerTalk] Johnson Matchbox

To: "'jim Jarvis'" <>, <>
Subject: Re: [TowerTalk] Johnson Matchbox
From: "Gary Schafer" <>
Date: Tue, 25 Mar 2008 12:00:46 -0400
List-post: <">>
With AM modulation the plate voltage doubles with 100% modulation so peak
envelope power is increased by a factor of 4.

With the plate modulated AM transmitter having an efficiency around 70% or
so, with 1 kw carrier input that would produce around 700 watts carrier out.
With full modulation the voltage of the signal would double so the peak
envelope power would increase by 4 times. That would put the peak envelope
power at 2800 watts out.

The old 1 kw rated tuner should easily handle 1500 watts PEP.

The 275 watt tuner, used with a 275 watt input rig should put out 192 watts
of carrier (275x.7) and the Peak envelope power out would be 4 times that or
768 watts.

Gary  K4FMX

> -----Original Message-----
> From: [mailto:towertalk-
>] On Behalf Of jim Jarvis
> Sent: Tuesday, March 25, 2008 11:16 AM
> To:
> Subject: Re: [TowerTalk] Johnson Matchbox
> 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.
> N2EA
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