[FoxHunt] Attenuator

[Larry Benko]

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."