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[TowerTalk] Measuring a 70cm MOXON turnstile antenna

Subject: [TowerTalk] Measuring a 70cm MOXON turnstile antenna
From: Grant Saviers <>
Reply-to: "Tower and HF antenna construction topics." <>
Date: Wed, 21 Apr 2010 17:03:06 -0700
List-post: <">>
For some 425 Mhz high power amateur rocket telemetry (APRS packets) I've 
built a MOXON turnstile (Cebik, QST August 2001) from brass brazing rod 
0.093" dia.  Now the challenge is to try and measure what I've built. 
This is my first foray into UHF measurements so advice is appreciated.  
I've looked in various handbooks and note the lack of "how to's" for 
measuring UHF antennas.

Some of my questions:

Where is the end of a PL259 terminated cable at UHF? 

I've been unable to measure a 1/4 wave 70cm stub with my MFJ-269, there 
doesn't seem to be any way to get the leads out of the measurement, so 
I've measured a 46" length of RG316 into a PL259 (measured to the end of 
the barrel).  Here the MFJ and AIM4170 disagree on the electrical 1/4 
wavelength, MFJ measures the 46" (to end of PL259 barrel) as 139Mhz and 
the AIM measures the same piece as 133.66Mhz.  (I checked the MFJ 
counter accuracy at 133 Mhz and it is right on.)  The AIM measured a Vf 
of 0.689 vs the MIL spec 0.695 so it seems to be the one to trust.  Is 
the Vf of RG316 at 425Mhz the same as 133Mhz (TFE insulation, Ag plated 

The MFJ at UHF can only measure SWR, but with the foregoing I wonder if 
it is of any value at all.  It measures one of the crossed antennas 
having decreasing SWR to the MFJ lower UHF frequency limit of 417Mhz as 
1.7:1.  The antenna is exactly per the 425Mhz dimensions of the MOXON 
rectangle generators on the web.  Again, a question about where is the 
conductor exactly, I'm assuming it is the center of the rod.

Might a brass element implementation vs a copper model be a problem?  
I've also spaced the ends of the elements with PVC spacers made from 
0.250" rod to make the tip to tip the exact 0.50" per the MOXON 
generators.  Did I make the right plastic choice at UHF?  Any better way 
to hold the tip spacing?  (I tried heat shrink but it doesn't grip well 
enough and is too flexible).

Is there a 75 ohm subminature coax like RG316?  I need to make a 25 to 
50 ohm 1/4 wave transformer of two parallel lines.

I do have a signal gen that goes to 1Ghz so am puzzling of how to use it 
to measure the antenna properties.  Any how to's appreciated.

I've read EZNEC has trouble with crossed wires, per Cebik I soldered the 
centers of the reflectors together,  so wonder if this antenna can be 
modeled with any accuracy.


Grant  KZ1W


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