[FoxHunt] Moded ELTS

[Jay Hennigan]

On Sat, 23 Mar 2002, sdonnell wrote:

> Also, what have been anyones experience in using more sophisticated  "doppler" Df
> systems w/  ELTs. Does the continuous and varying sweep tone affect the bearing
> indication at all? The doppler that I have is an Agrello jr.   Thanks

We use both doppler and a quad (Cubex yellowjacket) for our 2M hunts,
propagation similar to ELTs.  AM modulation of an ELT shouldn't be an
issue for the doppler.  The FAA "DF fix" is a doppler.  Look near your
local FSS or control tower for a pole with 16 dipoles around it.

As a rule the doppler gives more accurate readings when either the
hunter or the target is in motion, as reflections tend to average out.
The quad or beam will give more accurate bearings when both the hunter
and target are stationary as the level variations from "mobile flutter"
can be misleading.

ELTs will be randomly polarized, and dopplers and the L-Per work better
with vertically polarized signals.  This isn't a big hindrance for the
doppler in our experience on the amateur hunts.  A GaAsFET preamp helps.

Don't forget that a doppler unit requires an FM receiver, and radios for
the aircraft band are usually AM.  You'll want to do some calibration
runs with the actual radio you'll be using as well.  Each radio needs
the doppler's delay pot set for its audio characteristics.

Dopplers absolutely love an unmodulated signal.  The switched-capacitor
filters are quite narrow, so unless the modulation is consistent and
within about three hertz of the doppler's sweep rate it won't really
bother it.  The "Doppler Systems" commercial units are crystal-controlled
at a 300 Hz. sweep.  During a ham hunt we hid and ran a 299 Hz. CTCSS
generator which had one team literally watching their doppler go around
in circles.  A while into the hunt we switched to 301 Hz. and spun them
around in the other direction.  They were not happy campers but eventually
found the fox.

73 de WB6RDV

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