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

[Jim Reid]

On link coupled,  balanced tuners:


> or purchase a used Johnson Matchbox.  If one was to build 
> an antenna tuner with a true balanced output, what circuit is 
> the best?.....

Perhaps the "best"  circuit is presented by L B Cebik in his 
discussion of link coupled antenna tuners at:

http://www.cebik.com/link0.html

This tutorial material is in five parts.

>  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.  

I have modified my kW Matchbox exactly per the circuit presented
by Cebik.  In series with the primary coil is now a 0 - 500 pF
variable,  and each of the five turns of the primary link coil is
now tapped;  thus 1 turn through all five may be switch selected
to tune the primary circuit by switching the L and setting the 
variable series C.  On 80 meters,  the setting is 4 primary coil
turns and about 200 pF series capacity.

The original variable capacitors in the output circuit,  or secondary
of the tuner are very much easier to "tune"  once the primary
circuit has been easily brought to correct resonance at the
operating frequency.  This can be seen using an antenna analyzer
or a good vswr meter even without a good secondary "match"
yet.  I found the original Matchbox  a bit tough to tune
at times until the mods were made in the primary circuit.

I used the panel space occupied  by the (not too useful)  original
Johnson meter.  In that "hole"  is now a small mounting plate to
which the new primary variable is mounted along with the switch
assembly for selecting the desired primary coil tap position.

My doublet antenna is about 137 feet long,  center fed using
4" PVC spacers between the "tuned"  feed wires.

So far seems to perform fine on any of the bands from 80 on
up,  including the WARC frequencies.  However,  I mostly use
this set up only on 80,  40,  and sometimes 30 meters.
As I have a C4 beam,  I have found I can use the 40 meter
element of that on 30 meters,  and always use the C3 part
of the antenna for the bands on up from there to 10 meters
along with a regular T-antenna tuner when necessary.
That tuner is by-passed over most of 20,  15,  and center of 
10;  but the tuner is needed for the WARC bands and low
end of 10,  and upper portions of 20.

73,  Jim,  KH7M


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