[FoxHunt] antennas for harmonic sniffing

[WolfBob]

Great idea..

The 5th (6th) harmonic getting out is a very transmitter specific issue. 
A good pi network in an ammo can transmitter has 3rd harmonic at the 
connector down about 45 dB and the 5th is down another 15 or so. The 
usual 1/2 wave at the fundamental antenna down't help much either. I 
wish you'd check a lot of transmitters before getting too excited. Of 
course -60 dB below a watt is a microwatt, still something that can be 
heard for a ways. I did a hunt at 1200 mHz once and it bounced 
everywhere off of everything, so 400 yds is probably all you use anyway 
and get anything in the correct direction.

Bob, WB6JPI

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
> FoxHunt@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/foxhunt
> 
> 
>