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Author Topic: Twin Lead Fed 6 Meter Moxon?  (Read 2070 times)

Posts: 80

« on: October 02, 2011, 05:47:27 PM »

I have a lot of excellent 300 ohm twin lead feedline. Seeing as how a Moxon is a balanced antenna (and I would be constructing a 6 meter SSB Moxon) it seems to be a match made in heaven considering the low loss characteristics of the twin lead.

So if I use this twin lead instead of coax, do I need to change in my Moxon's dimensions (and if "yes" by how much)?

David, N1ZHE

Posts: 17483

« Reply #1 on: October 02, 2011, 06:49:04 PM »

The standard Moxon equations are designed for a 50 ohm feedpoint.  You can shift the impedance somewhat
by adjusting the dimensions, but not as high as 300 ohms.  Perhaps the best you can do is get it up to 75 ohms
and use a 4 : 1 matching transformer.

Well, I suppose you could try a delta match - I've used that on various yagis to match a 200 ohm impedance
(4 : 1 coaxial balun)...

Here is a possibility:  build a standard Moxon to formula, but without the split in the middle of the driven element.
Make the delta out of two pieces of #12 wire about 18" long that connect to your twinlead.  These angle up
to tap onto the driven element about 14" either side of center.  At that point you will need a capacitor between
the wire and the element of about 10pf.   Using 3/4" material for the antenna and 1/8" delta wires, my model
suggests the SWR is under 1.2 : 1 from 50 to 51 MHz.

I used this method to match aluminum yagi elements, and wanted to avoid any copper to aluminum contact.
Instead of trimmer capacitors I used longer insulated wires for the delta section and wrapped the ends
of the wires around the driven element to form a capacitor.  This was fairly easy to slide along the element
for tuning, and I could unwind turns to adjust the capacitance.  It seemed to work pretty well, but I'd
suggest using wire with low RF dielectric loss - perhaps Teflon, instead of common PVC types.

The spacing between the taps on the driven element sets the feedpoint impedance, and adds some
inductive reactance.  Normally you could shorten the driven element to make it work, but with the Moxon
it is probably easier to use the trimmer capacitors instead.  (The capacitors could also be at the feedpoint
end of the wires - it changes the tap point and the required capacitor slightly.)  The length of the delta
wires makes some difference, but isn't critical:  the best solution is to build it and adjust the various
lengths until you get a good match.

Then, of course, you need a transformer from the 300 ohm twinlead to either 75 or 50 ohms, whatever
your rig is happy with.  A 4 : 1 coax balun using 75 ohm coax might be adequate.

It's something to play with, anyway.
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