[Amps] Link coupling

[Rob & Terri Sherwood <rob@sherweng.com>]

Can anyone who goes back before me (licensed in 1961) explain the theory 
behind the link input on a Johnson Matchbox?  I started with a 275 watt 
model in the early 60s, and now have a pair of KW versions for my two 40 
meter 165 foot center fed extended double zepps.   I don't think there 
is a finer balanced tuner out there. Of course there aren't very many 
true balanced tuners to choose from period!

The question is, it uses a 2 turn link for the 50 ohm port.  (The 
obsolete 300 ohm adjustable link of 4 to 6 turns has been disabled on 
mine units since 300 ohms receivers are hard to find today.)  It has no 
series C to adjust the coupling.  The secondary is band switched for 80 
- 10 meters, yet the link stays constant.  How can one get away with a 
two turn link from 80 through 10 meters?  Maybe that was common even on 
the transmitters with plugin band coils. This is just before my time. 
The design with a split stator cap for tuning and a dual differential 
for matching was / is ingenious.  No taps like previous tunes or 
amplifier output stages for that matter.

Does the one even think of the turns ratio of the link to the 
secondary?  That ratio obviously varies all over the map on 80 vs. 10 
meters.  While one has to juggle the feedline length to get full band 
coverage on both 80 and 40 on  whatever centerfed antenna one is using, 
the tuner never seems to run out of range on 20, 17, 15, 12 or 10 
meters.  (Depending on the antenna and feedline length, with some 
combinations it will occasionally tune 30 meters.)

The C values are rather low, I am guessing between 100 and 200 pF.  If 
they had been larger I would guess the matching range would have been 
better on 80 and 40, but the tuning rate and minimum C might have been a 
problem on the higher ham bands.  Considering one only has to juggle the 
feedline length get get a good compromise on 80 and 40 while the higher 
bands tend to fall in to range automatically, I guess E. F. Johnson knew 
what they were doing.

73, Rob, NC0B



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