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Author Topic: multiple link coupled trans matches  (Read 4661 times)

Posts: 91

« on: October 27, 2017, 02:21:26 PM »

ok guys...project time...
I have 3 working johnson, do not want to sell them...
But I am thinking of making a project out of two of them....
One for 80 thru 30 or 40 meters
 and a second one optimized for 30 or 20 meters thru 10 meters....and perhaps 6 meters.
The third one will leave as is.
Definitely will home-brew one for 160 meters.
Any one else do this before?

Posts: 595

« Reply #1 on: October 27, 2017, 10:37:35 PM »

Not exactly watt your asking, but the following may be of interest to you.

Examining the Johnson Matchbox ATU         Walt W2DU


EXTRALight  1/3 less WPM than a Real EXTRA

Posts: 833

« Reply #2 on: October 28, 2017, 01:06:21 AM »

I think the capacitor values are a bit large for operation on 6.

For 30, I would experiment with the 40 and 20 metre settings - there's a good chance it will give a match on 30.

Similarly, try either the 15 or 20 metre positions for 17 metres and either the 10 or 15 metre positions for 12m. The Matchbox is pretty flexible and may well not need modifications to cover these bands.

Posts: 354

« Reply #3 on: October 31, 2017, 02:28:47 AM »

In my experience the box does not do well with impedances with
a low resistance component. Living on a postage stamp lot my
antennae are frequently short of a half wavelength and the box
needs to handle impedances with a resistive component below
25 ohms plus what I consider some fairly obtuse reactances. To
that end if I was planning to redesign the box I would opt to
tap down on the inductor to obtain the ability to match impedances
with a low resistance component, and not use the capacitive divider.
Then I would adjust the coil inductance to allow working two or
three bands with a variable cap tuning the matching network. Few
ops these days have the luxury of full size loops or folded dipoles so
much of the utility of the Johnson Matchbox is unused.

I have several of the Matchboxes and they do shine when using
horizontal loops and folded dipoles. I have found it is hard to beat
a loop antenna with a Matchbox for operating flexibility. On the
designed frequency it nicely matches the nominal 100 ohm feed
point. It also can match the loop on even and odd order harmonics
without a lot of power loss in the matching network. Dipoles, doublets,
folded etc do not offer the even order ease of match of a loop.




Posts: 18079

« Reply #4 on: October 31, 2017, 07:42:12 AM »

Yes, in spite of its hallowed status, the Matchbox does have a limited matching range.
You can increase Itby changing the number of turns on the link coil:  that's the first
place I would start, as the modifications are relatively minor, and it can still cover
multiple bands.

The next step would be to see how it matches on 80m, for example, if the antenna
is tapped across the 40m coil taps.  You may want to include the output capacitor
across the antenna, rather than leaving it across the main tuning capacitor.

What you really want for low impedances is a series tuned circuit rather than
parallel tuned like the original.  This requires a bit more modification, but in
theory would be a matter of cutting the coil in the exact center and connecting
the feedline across the two ends. 

Perhaps the most versatile circuit arrangement would be to connect a large
variable capacitor across the center of the coil instead, and move the output
terminals to the ends of the main tuning capacitor.  (The added capacitor can
replace the 4-section output cap, and needn't be rated for as high of a voltage.)
Now you haven in effect, a balanced pin network tuner: the added capacitor
is in series, and the original is in parallel.  Depending on the ratio of the two,
you can match a wide range of impedances.

The difficult part likely will be cutting the coil in the center, and you can
experiment with a jury-rigged tuner to get a sense of the matching range
and the optimal number of turns on the input link.
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