[TowerTalk] N4XM's X Match Tuner Info

[Jim Reid]

Hi,

I have had one of these tuners for a few years now.
Have not used it much at all,  as I no longer have
antennas suitable for 160 meters;  now using my
modified kW Matchbox to my doublet for 80 and 40.

The XMatch series capacitor which is switched to be in
series with either the tuner input or output connector is
a "bank" of doorknob type capacitors which total 575 uF,
rated at  5 kV.  The two sections of the LARGE
variable capacitor  (which is switched to the opposite
connector from the bank of fixed capacitors) are
in parallel,  forming a large amount of variable C,
I presume enough C to equal, or come close to
the same 575 uF,  but perhaps not.  The instructions
suggest that if you find the variable C approaching
high numbers,  something is amiss and you should
change the length of your feedline, or change the antenna 
particularly if you are going to be QRO.

The inductor is wound on a ceramic form;  might be
a Caldwell (Sp?) unit.  One end of the coil is grounded,
the other is open.  The roller tap connects between
the two capacitors in the usual T circuit configuration.

Per tuner theory,  one or the other of the two "series"
capacitors of the T circuit should be a maximum value
for best efficiency.  This is the point of the XMatch
design, apparently,  as the fixed capacitor is obviously
at its' maximum value all the time,  hi.

I have found that if the antenna system has reasonable
Z,  that the input capacitor (from the rig) is where
the high value fixed C wants to be,  with the variable
feeding the output connector going to the feedline and
antenna system.  Note that if the antenna system
appears to have a Z below 25 ohms or so,  the
575 uF fixed C almost certainly must be in series at the
output connector terminal.

Incidentally,  the front panel of the unit has the sub-title,
"Classic Lowbander";  this is certainly where it is best.
I found it not that easy to set up on the higher bands,
but is fine for 160, 80, and 40.  I guess the components
selected by N4XM are optimum only on 160,  but not
sure about that either.  I suspect the fixed C of 575 uF
is pretty high for 10 Mhz and higher frequencies.  Other
tuners I have seen usually have no more than 350 uF
or so for the max value of the input terminal C;  some
provide an optional 300 uF or so that can be switched
in at the output for low band,  low Z antenna set ups,
the Palstars, for example.

Not using this tuner at this time,  but not sure I am ready
to sell it yet either,  hi.

73,  Jim  KH7M