[altusmetrum] Antennas for rocket tracking

W7AMI terry.w7ami at gmail.com
Mon Apr 18 12:36:52 MDT 2016


Bdale,

    After a lot of reading I built a crossed Moxon dipole antenna for 
just the purpose described below.    It was designed by L. B. Cebik (sk) 
and described in the August 2001 issue of QST starting on page 38.  
Don't miss the addendum in QST for October 2001 starting on page 78 for 
more information on the Moxon.

    The Crossed Moxon antenna has a radiation pattern with about 6 to 7 
dB of gain overhead and a similar loss on the horizon.   I liked the 
fact that the antenna was circularly polarized since the polarity of the 
received signal is unknown and random during the descent phase of the 
launch.

   The hardest part of the construction is building the phasing 
section.   It is tough to get it cut and installed into the antenna with 
as short of lead lengths.   I chose to place mine inside the supporting 
pipe to help keep it out of the elements and to reduce the lead 
lengths.   I also added ferrite sleeves over the coax feed line to 
reduce radiation from the feed line but this probably isn't needed.   
Despite what is shown in the QST article it is important to keep 
material away from the ends of the wire gaps.   I found that placing 
anything near the gap detuned the antenna significantly. This was 
measured using a VNA.

    I am using a TeleGPS in the rocket and two antennas for tracking.   
The Crossed Moxon antenna and a vertical with drooping radials which 
should work better at the horizon.   The Moxon is connected to a TeleBT 
and my cell phone.   The vertical is connected to the vertical and goes 
to the back up PC.   Both antennas are mounted on 5 foot tall PVC pipe 
tripods.

So far, only one flight, the results are mixed.  There is more variation 
in the RSSI of the Moxon than I expected and vertical has a steadier 
signal.   The Moxon does do better at apogee as expected, but not as 
much as expected.   This was a 3,000 foot flight so it will be 
interesting to see what happens on a 10,000 foot high flight next 
month.   Also interesting is the difference of RSSI while the rocket is 
sitting on the pad.






Terry
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