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Author Topic: Johnson Matchbox Problem  (Read 694 times)
N2UM
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Posts: 12




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« on: October 20, 2009, 07:01:19 PM »

I've recently acquired a nice 275 watt Matchbox.  I also have a Palstar AT1KM.  I'm running approximately 75 feet of 600 ohm open line feeder to a 132 foot long doublet up about 30 feet. The Matchbox tunes 10, 15, 17, 20, 30 and 80 down to 1:1 SWR.  The best I can get is 3.5:1 on 40.  I've tried shortening and lengthening the antenna, no change.  I've tried adding a 4:1 balun at the matchbox, no change.  Because of my location, I'm unable to lengthen the feeder.

Any ideas why and what would help on 40?

TNX, Gary  N2UM
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WB6BYU
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Posts: 13487




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« Reply #1 on: October 20, 2009, 10:23:06 PM »

Do you have the same problem at both ends of the 40m band?

A quick check with EZNEC suggests the impedance may be
too capacitive:  10 to 20 uH of inductance in series
with each side of the feedline where it attaches to
the matchbox would be a quick check to see if that
is the problem.

When in doubt, open the Matchbox and see which capacitor
is at the extreme limit of its range.  That should give
a good idea of what the problem is.
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N2UM
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Posts: 12




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« Reply #2 on: October 21, 2009, 08:37:48 AM »

Do you have the same problem at both ends of the 40m band?

Yes

A quick check with EZNEC suggests the impedance may be
too capacitive: 10 to 20 uH of inductance in series
with each side of the feedline where it attaches to
the matchbox would be a quick check to see if that
is the problem.

When adding approximately 12uf of inductance to the feeder, there was no change at 7005kHz (3:1) but at 7295kHz there was a significant improvement. (1.75:1) But the match is now off on all the other bands.

When in doubt, open the Matchbox and see which capacitor
is at the extreme limit of its range. That should give
a good idea of what the problem is.

"Tuning" is half meshed and "Matching" is fully meshed on the bottom.

TNX for you help!
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WB6BYU
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Posts: 13487




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« Reply #3 on: October 21, 2009, 09:14:33 AM »

Not having my Matchbox open in front of me at the moment,
I can't see which is top or bottom, but I'd guess that
the impedance is too low (so the center sections of the
matching capacitor are fully meshed.)  A quick check would
be to add some resistance - say 100 to 200 ohms - in
series with each leg and see if that allows you to match
the load.  Also, adding some capacitance across the output
terminals should also fix this - try 100pf to start.  

These may not allow a match on all bands, but they will
confirm why you can't get a match on 40m, and with that
information we can look for a solution.

(I have heard that reducing the number of turns on the
input link coil will increase the matching range, but I
haven't tried it.  You could try moving the 40m tap by
one turn and see if that makes a difference.)
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N2UM
Member

Posts: 12




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« Reply #4 on: October 21, 2009, 09:55:11 AM »

A quick check would be to add some resistance - say 100 to 200 ohms - in series with each leg and see if that allows you to match the load. Also, adding some capacitance across the output terminals should also fix this - try 100pf to start.

OK now!  I added 100 Ohms in series withe each leg, I now have and SWR of 1.0:1, 52 ohms resistance and X=0 on 7005kHz.  I only have 1/4 watt carbon resistors so other than my analyzer, I didn't put any power to it.  10,15,20 & 80 tune up perfectly.

Obviously, 1/4 watt resistors will not hang in there at 125 watts output from my transceiver.  What would be my next course of action?

TNX again for all the help!

Gary
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WB6BYU
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Posts: 13487




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« Reply #5 on: October 21, 2009, 12:04:55 PM »

That confirms that the impedance is too low for the
Matchbox.  Based on that, there are several possibilities:

(1) adjust the antenna or change the feedline length to
raise the impedance at the Matchbox;

(2) modify the Matchbox to match a lower impedance by
moving the 40m coil taps;

(3) adding capacitance across the antenna terminals.

You'll have to experiment to find what combination gives
the best tuning across all bands.  You may end up having
to switch something in and out - for example, a capacitor
that you plug in for 40m and unplug for the other bands.

There are probably other options as well, but these are
the first ones that come to mind.
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N2UM
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Posts: 12




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« Reply #6 on: October 21, 2009, 12:15:35 PM »

I had 12 feet of 600 ohm feeder left.  There was no way to add it to the end so I spliced it into the middle. My wife hasn't seen it yet! MY SWR on 40 is now 1.5:1 R=47 and X=0.  All the other bands are happy too.

Thanks for all your help!

73, Gary  N2UM
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WB6BYU
Member

Posts: 13487




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« Reply #7 on: October 21, 2009, 05:13:16 PM »

The matchbox doesn't handle low impedance loads, and the
simulation showed you were dropping to 100 ohms or so
(with >800 ohms capacitive reactance) at the bottom of
40m, depending on the exact antenna and feedline lengths.
The fact that the variable capacitors were at maximum
between the antenna and center tap confirms this was
the problem.

Glad the feedline fixed it.

Good luck with the Matchbox!
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