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[TenTec] "A low vswr will kill you ! "

To: <tentec@contesting.com>
Subject: [TenTec] "A low vswr will kill you ! "
From: SEWATKINS@dgs.state.va.us (Sherrill WATKINS)
Date: Tue, 23 Jan 2001 14:42:09 -0500
I agree completely with Dr. Johnson and Mr. Ellingston's comments regarding 
baluns.   Since no one presently builds a well engineered link coupled antenna 
tuner for with a true balanced transformer output for a reasonable price, it 
appears the only way to obtain one is to build one from scratch or purchase a 
used Johnson Matchbox.  If one was to build an antenna tuner with a true 
balanced output, what circuit is the best?  I have had good luck with my E.F. 
Johnson Matchbox but have used it only for 80,40,20,15 and 10 meters to tune a 
132' center fed zep.  I have not tried to modify it to tune 160 or the warc 
bands nor have I tried to tune any other type of antenna.  The only critical 
factor I ever noticed is the length of the feed line seemed to be important for 
operation on 15 and 10 meters.  For operation on 80, 40, and 20 meters the feed 
line length did not seem to matter.  If my memory serves me, 42' was the 
"magic" number to make it tune easily on 15 and 10 meters.  My 1949 and 1956 
ARRL Antenna Books have a chart for recommended feed line lengths for all band 
operation with center fed zep antennas.  The Matchbox is very well built and 
designed and there is nothing today that can equal its construction.   It uses 
a cleverly designed capacitive voltage divider to tune the output or antenna 
coil to resonance with the antenna system.  There are two each, dual section 
air variable capacitors (total of four connected in series) that perform a dual 
purpose of tuning the output coil to resonance and forming a variable voltage 
divider for impedance transformation.  The fact that the Johnson has four 
transmitting air variables connected in series makes for a extremely high 
voltage breakdown rating on the output capacitor.  I think that the Matchbox's 
operating range is limited by the lack of a series tuning capacitor on the 
input coil.  The unit's tuning range could probably be extended by installing 
an air variable tuning capacitor in series with the input coil.  Virtually all 
of the circuits I have seen for the hom
 The Johnson does not and this may by why its range is not as broad as the 
homebuit units?  However, once the proper feed line length is determined, the 
Johnson Matchbox works very well.  It is unfortunate that it is not being made 
today!  Maybe Ten-Tec will start producing an equivalent unit?  I will be the 
first to buy one! - Corn k4own   


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