[altusmetrum] Antennas for rocket tracking

John Sahr jdsahr at gmail.com
Mon Apr 18 19:42:16 MDT 2016

regarding the fluctuation of the signal strength, it can be understood, I
think, given the innards of the rocket and the propagation path.

(*) launch-apogee-main deploy (seconds < 950 on time axis).
      Without knowing much about the rocket flight dynamics, it's
reasonable to suppose that the rocket was spinning pretty rapidly (based
upon rockets that I've seen).  Given the fact that there is also
dual-deploy electronics near the radio, one should not be surprised if the
antenna pattern of the TM transmitter is much more complicated than
quasi-omni-directional.  I know that in the rockets that I'm building with
dual deploy, there are a lot of wires, including excess wire length to
accommodate assembly of the electronics bays.

(*) descent under main (seconds > 950 on time axis).
     Here, notice that the variability of the signal strength is much
larger.  This can be attributed to the fact that the rocket is now
approaching the horizon.  This means that the ground station antenna is now
seeing significant direct-path and ground reflection, producing both
constructive and destructive propagation (phenomenon known as "Lloyd's
Mirror", well known in radar and communications).  In particular notice
that the signal when the rocket is on the ground is relatively weak, which
is what would be expected from Lloyd's Mirror and random orientation of the


On Mon, Apr 18, 2016 at 11:36 AM, W7AMI <terry.w7ami at gmail.com> wrote:

> 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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John D Sahr --- Seattle WA --- jdsahr at gmail.com
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