[FoxHunt] antennas for harmonic sniffing

[Kuon & Dale Hunt]

With the "DC to daylight" coverage of the newer
hand-held receivers and transceivers, sniffing
transmitter harmonics for close-in work can
easily extend beyond the common 146 -> 438
combination.

K7FM was interested in hunting ELTs, and sat
down with a calculator to determine the options.
It turns out that 730 MHz is a good choice:  it
is 6 * 121.5 (actually 729 MHz) and 5 * 146 MHz.
You can hunt both bands with the same antenna.

This sounded promising so I scaled the W4RNL "OWA"
6-element yagi design to 730 MHz and built one
using PVC pipe and #8 aluminum wire.  The antenna
is less than 11" / 28cm long.  Set out my
ELT practice beacon and it works great!  Using
the frequency knob as an attenuator (the ELT has
a VERY WIDE signal - only 40dB down at 100kHz off,
which is why the old 121.6 MHz practice frequency
could be heard on the emergency channel) I could
get clear bearings at 3' / 1m away.  The pattern
is very clean, with just one peak above the noise.
I could hear the beacon at 250 m / yards away, and
could probably stretch it to a quarter mile ( 400m )
line of sight.  (Maximum distance was about 100m
when the signal passed through a building.)

Haven't tried it on 2m yet:  although odd harmonics are
generally stronger than even ones, most 2m rigs should
have much lower harmonic output than the ELT...

...OK, I'm back.  The rain stopped and the temperature
warmed up to 36F / 3C so I set out one of my 2m
transmitters:  a Hamtronics synthesized kit at about
1 watt output.  I could copy the signal on 732.825 MHz
from 400m away.  Probably could go further, but the road
went over a hill at that point.

Receiver in both cases was a VR-500.  The minimum
discernable signal is at least 6dB better using CW
than AM or FM.

If there is interest maybe I can post the dimensions
and a photo somewhere.

     - Dale WB6BYU


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