[TenTec] link coupled tuners

[Rob Atkinson]

<<<
Jerry, we all prefer a link coupler tuner.

Unfortunately there are no commercial units available anymore, at least not
in the amateur radio market.
Most of the old Johnson Viking matchboxes on the market are really grubby
these days, and even if you find a good one, they are very limited in their
matching range.>>>

I frequently see nice clean KW MBs for sale on-line.  they are great
tuners; designed to handle 1 KW input AM (i.e. around 2800 w. at 100%
modulation) and they look like it when you see inside the cabinet.
Everyone wants a tuner these days that has this huge range.  But in
reality, that's not a good idea.  There's a point at which you have to
start questioning the antenna and feedline situation if the tuner
cannot achieve a match.
In other words, hams want a tuner that will match a bed spring.  Do
you really want something like that for an antenna?  In the case of
the MB you may have to adjust the length of the ladder line, usually
adding or subtracting 10 feet or so.  It's not that difficult.
Another complaint is that it lacks 160 m. taps.  But few hams can
afford the necessary supports for getting a horizontal balanced fed
wire antenna high enough to make it perform well on 160.  So the lack
of 160 coverage is instructive.  Take it to mean, vertically polarized
unbalanced monopoles are the way to go on 160 m.

<<<Sure you "could" build one but unless you are only interested in one or two
bands, the challenge would be finding an appropriate bandswitch.  These are
really quite complex.>>>

Then don't use a bandswitch, although you just need a ceramic rotary
DT6 or 7 or 9 pole switch.  The two wafers have to be separated 2 or 3
inches so the pairs of taps don't get too close to each other, i.e. a
pair for 80 m. have to be spaced apart so they don't couple.  get your
inductor (you can often find a big johnson edgewound coil for sale
on-line) and wind your link around its center.  You only need 2 or 3
turns for the link.  do some testing to find where the taps should be
for each band.  Solder some short straps at each point and bond them
to banana jacks.  Big banana jacks and plugs are fine for RF.  Now all
you need are two straps with banana plugs you can move around to
change bands.  You can mount everything on a wood board and bolt it to
a wall.
Avoid alligator clips for they do not make good enough contact.  Flat
clips designed for edge wound coils are better.

Actually the MB can tune WARC bands, at least mine does.  My antennas
are two dipoles, one 130 feet long and the other 33 feet long, both
center fed with ladder line.


<<<The MFJ and Palstar designs for symmetrical matchboxes are not as good as
the link coupled, but they get the job done in good fashion.  You need some
pennies for those boxes though, but they'll handle a kilowatt.>>>

But for what you pay for one of those PalStar boxes, you could buy 3
or 4 KW Matchboxes.  BTW, in my opinion the desirable MB is the one
that lacks the SWR bridge option.  You can clean it up in a jiffy.
Just remove the front panel knobs, noting where they are set in
relation to the capacitors, remove all the screws and the cabinet
comes apart.  Take the chassis with the coil, caps and switch and hose
it down.  give it a day to dry.  clean the band switch with some
deoxit and put the whole thing back together and you're ready.  If it
has the TR relay, cover it or remove it so it does not get wet.


<<<For 100w, I have found the MFJ-974B to be a great little box.
Previously, at field day, my club used an old Viking 275w version, link
coupled matchbox to match our openwire fed doublet.
It had great difficulties finding a match across all bands.
We had to play with feedline lengths.>>>

Doing the odd multiple of 1/8 w. at the lowest frequency to get the
right feedline length has worked for me.


73

Rob
K5UJ