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
conductors)?
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.
Thanks,
Grant KZ1W
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