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B&W Swinging Links

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Author Topic: B&W Swinging Links  (Read 694 times)
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Posts: 176

« on: March 08, 2017, 10:37:15 PM »

A thousand years ago, B&W offered three shielded swinging links for coupling transmitters to antennas.  They were the #3781 1-turn link, the #3782 2-turn link, and the #3783 3-turn link.

I'd like to find whatever tech information might be available that addresses these links, especially info that aids in selecting which link is used under what conditions.

Thank you.

-al hart

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Posts: 1692


« Reply #1 on: March 11, 2017, 02:49:41 PM »


   It has been a long time since I have messed with that stuff.

What I remember is that it is very helpful to have a series variable capacitor with that link to tune out the link reactance. Also, the link should be adjusted such that full loading is obtained with the link about 50% of the way into the plate tank coils. A lot of variables come into play here, and the plate tank Q being one of them. Ordinarily you'd think the extra turns would better support higher Z loads, but when you series L-C resonate the link, then this isn't necessarily the case. That said, a 1 turn link might work out better on the higher frequency bands.

Not much of an answer is it? :-)


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Posts: 2015

« Reply #2 on: March 11, 2017, 07:56:38 PM »

I have a push pull 100TH rig  HDVL coils   It has a 2 turn link and works well  on 40 M 20M and 15 M  .
I also have a 250TH push pull rig with a 3 turn link that works well on 40 M and 75 M..
Both rigs will tune with the link 100 % coupled....
Not really the answer you were looking for....... but  Thats all I got
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« Reply #3 on: March 11, 2017, 07:59:20 PM »

If I remember correctly, the shielded link was to filter out harmonics in the days of terrestrial TV. The MB-40SL grid circuit or plate tank uses a half copper shielded  swinging link for the same reasons. It takes a bit more drive in a grid circuit over a non shielded type which is the only drawback. It may also reduce feedback from harmonics. 

The secrecy of my job prevents me from knowing what I am doing.

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Posts: 65

« Reply #4 on: March 12, 2017, 01:58:40 PM »


Depending on your application, these things are easier to design by cut and try rather than theory.  Here are some pointers that may help.

If you want wide range impedance matching, like for a transmatch, use the tightest coupling you can get.  If you look at the pictures of my tuners, the primary is would directly on the secondary.  A series capacitor then serves as the variable coupling, like a swinging link, but with more range of adjustment.

Try experimenting with the number of turns on the link to see what works best using scrap wire and low power, then build your coils.

If you are building an amplifier, use the coils in the handbooks, but use fixed links and series capacitors.

Good luck.

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