[FoxHunt] antennas for harmonic sniffing

Sun, 25 Jan 2004 16:55:20 -0500

Hi, Ive been using this technique for several yrs and have found it to
be VERY effective in close-in hunting for ELTs and EPIRBs; I mostly use
486.000(4th) and 850.500(7th) as they are freqs most HTs and scanners
cover. I keep 243.00 and 486.00 in memory CHs of my IC-R100, right above
121.500, and can easily switch between Att or Preamp settings. I use 486
as my "drive by" freq as I usually get a weak signal on it within a half
mi or so for a beacon, and a quick "spike" when Im within 100 ft or so.
This is handy unless you enjoy taking DF readings in the rain or when
theres a 20 below wind chill,,. Then I stop and get out my "secret
weapon": A 5 el yagi meant for Cell band use. Tie this into my FT50 or
AR8000 and I can literally "point" to the exact location of the beacon.
At close range those using the standard issue ELPer are often left
running around in circles, partly due to simply not turning the rec gain
down enough. Plus in most "urban" enviroments, reflections on 121.5 can
drive you batty!!

One word of caution though, is many of the "new generation" "406" beaons
seem to have a very "clean" signal, thus you will not get a 486.000 or
even a 243.000 harmonic until you are within a hundred feet or less.
Plus the 121.500 signal is usually  somewhat weaker as its only about 25
mw instead of about 100 mw. This difference from the typical "signature"
of an older higher pwr, harmonic laden beacon can throw you the first
time you encounter one.

Ques: Is anyone else on the list from CAP or other DF groups,
encountering problems from false triggered "new gen"(406) EPIRBs.
Typically  Alden Electronics "SatFind 406" series? Ive tracked  several
of these over the past couple of yrs which have been triggered for
unkown reasons. These were all installed on boats and were set off while
the boat was docked(as far as I know). The only common thread with
couple of the earlier ones, was they may have been exposed to some
degree of rain or dampness. However w/ one last wk the boat was stored
for the season and covered w/ shrinkwrap. The only variable was the
overnite temperature had rissen from below zero to about plus 10.
Although the batteries were technically expired, by about 6 mo, the
beacon was putting out a healthy 121.5 signal w/ sweep tones, and the
strobe was flashing too. Although I could not detect a 406.025  dat sig.
On atleast one other one from last yr, the batteries were good til '08.
This one was just a dead carrier(no sweep tones), no 406 data burst, no
light. On yet another one, there was a sweep tone, strobe, and even a
nice data signal on 406.025, but AFRCC had NO indication of the
signal(even on 406). Any idea as to what these use as a trigger switch
when they are in the Automatic mode? The older "float free" beacons used
a simple mercury switch, which required the beacon to be stored, laying
horizontal in a special cradle. The new 406 beacons are stored in the
upright position.

Capt Steve Donnell
NHWG CAP
Abenaki 102

Kuon & Dale Hunt wrote:

> 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
>
> ---
> [This E-mail Scanned for viruses by Onlinemac.com]
>
> _______________________________________________
> FoxHunt mailing list
> [email protected]
> http://mailman.qth.net/mailman/listinfo/foxhunt