[FoxHunt] Attenuator

[Bob]

Don't knock it until you've tried it. I think Tony has a
preamp (alinco or maybe a Smith) with four or so tuned
circuits , but he didn't mention using it on this sniffer.
Tony's been in the sniffer business longer than most of us
have been alive. He lives in San Diego and hunts in some
pretty rugged hunts. Realize these are sniffers and not used
for things 10 miles away.

BTW, I hunt transmitters and have had no problems with
multiple simultaneous transmitters located close to one
another. I have used very simple radios including handheld
FM receivers, LPERS and simple diode sniffers. There is
nothing like the dB you get when you are 10 ft from one and
100 ft from another. Makes up for a lot of confusion. 

Bob WB6JPI


Larry Benko wrote:
> 
> The recent discussions concerning the use of the Analog Devices
> Log amplifiers for fox hunting is in my opinion generally a big
> mistake.  The very strength of these devices is their DC to
> 500+MHz bandwidth and ease of use.  However this bandwidth
> definitely is not good for many fox hunts.
> 
> The original message referred to dBW which I assume is dB relative
> to 1 watt at a 50 ohm impedance.  However most specifications are
> usually done in dBm which is dB relative to 1 milliwatt at a 50
> ohm impedance.  To convert from dBW to dBm add 30 (e.g. 0dBW =
> +30dBm).
> 
> Connecting a 2m 3 element yagi to a spectrum analyzer and driving
> around the city (yes I know that I am crazy) shows signals in the
> 88-108 MHz FM radio band, in the 152 MHz paging band, and some
> local TV stations as high as -20dBm.  I made no attempt to actually
> get close to the actual transmitting antenna.  These signal levels
> include any attenuation that the 2m yagi would provide.  "Your"
> log amplifier sniffer will see these undesired signals as well as
> the desired signal.  Attempts to put some simple tuned circuit in
> front of the log amplifier may help somewhat in the FM band but not
> in the nearby paging band.  Even putting a helical filter in front
> of the log amplifier will still allow all signals in the 2m band
> to be included in the signal strength measurement.  We have had
> hunts in the Denver, CO area where I hid 5 transmitters each running
> in excess of 1 watt in a 500 foot circle.  The transmitters were
> totally camouflaged and at any time several may have been trans-
> mitting.  A broadband sniffer would be totally unable to sniff these
> transmitters.
> 
> However given the proper circumstances where the 2m signal to be
> hunted is "guaranteed" to be louder than all other signals in the
> DC to 500 MHz band (after the 3 element yagi selectivity) a sniffer
> such as this is very useful.  Just don't expect to use it for weak
> or even moderate strength signals reliably.
> 
> Just for FYI purposes these are the possible signal strengths that
> can be expected:
> 
> Weak signals:
>  Basic HT sensitivity -130 to -136dBm (S1 on meter -105 to -115 dBm)
>  3 element yagi = +7dB
>  Result with HT and yagi = -137dBm to -143dBm
> 
> Very Strong signals:
>  Fox TX = 10W into dipole = +42dBm (+40dBm = 10W & +2dB for dipole)
>  RX + 3 el yagi at a distance of 8' from fox = +20dBm to HT (100 mW)
> 
> Even a broadband sniffer out in the middle of no where is still
> subject to your competition or an unknowing ham transmit on some
> other frequency in the 2m band.  By the way a +20dBm input signal
> to an HT will probably cause severe intermod and will cause problems
> to an active attenuator.  For extremely strong signals a 10 to 20dB
> pad should precede an active attenuator unless you build it with a
> +23dBm rated double balanced balanced modulator.
> 
> Try this experiment.  Can you hunt a keyed down 2m mobile radio
> running 50W into whip antenna at 20 feet?  What if there were several
> whips on the car?  Could you tell which whip was the primary radiator
> (no touching the antenna)?
> 
> Just my $.02 worth.
> 
> 73,
> Larry Benko, W0QE
> 
> 
> 
> Bob wrote:
> >
> > My friend Tony has built a sniffer around the Analog Devices
> > Logrithmic amplifier chip. Here is his report...
> >
> >    "This week I finished my new sniffer with the AD 8307. It
> > is
> > the best I have ever used. I combined the 8307 and a ticker
> > that goes up
> > in tone with more RF. It starts to tick at -107 dbw and has
> > 70 db dynamic range to the high tone. I then can switch in
> > 30 and 35 db
> > attenuators to numb it  60 db. The lost 5 db is some kind of
> > leakage. (marginal shielding)   I think I did the
> > measurements correctly.    I can stand in my yard and key a
> > repeater 10 miles away and it
> > will start to tick when pointed in that direction. With the
> > switches in, -60db, I can DF my repeater, 80 watts at 25
> > feet."
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