[Amps] Link coupling

[R.Measures]

On Jan 8, 2005, at 5:22 PM, Rob & Terri Sherwood wrote:

> Can anyone who goes back before me (licensed in 1961) explain the 
> theory behind the link input on a Johnson Matchbox?

Basically, its a link primary, resonant step-up xfmr with a 
dual-differential variable-C V-divider to vary output Z.  However, it 
has a fairly disappointing Z-matching range.  A balanced L-network is 
way mo' betta.   Ironically, even though the balanced-L network Z 
matcher was invented by Bell Labs in the 1930s, and E. F. Johnson Co. 
mfg all of the components needed for a balanced L network antenna tuner 
in the 1950s, they definitely dropped the ball with their 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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Richard L. Measures, AG6K, 805.386.3734.  www.somis.org