[FoxHunt] Attenuator

[Bob]

WOW what a bunch of naysayers.  Wonder if anything new ever
gets thru..

I have a ten element beam on the top of my 4Runner (5 el in
each pol) and can find 1 mW at 90 miles into a 1/4 inch whip
(proven performance) and can also locate my 5/8 vertical on
the same vehicle running 150 watts. It also finds my
computer and one of the two GPSs in the vehicle.  That is
with my 250Hz bandwidth SSB soupedup Kenwood with a 0.5dB NF
pole-mounted preamp and my own 70 dB attenuator. I
understand these things.

Tony understands sniffers. He has built at least 100 of them
in the last 10 years. They include offset attenuators,
preamplifiers, handheld radios, fixed attenuators and all
possible combinations of these. Now he is getting into
logamps as they offer the fastest and least error prone
solution to bearings in rugged terrain with transmitters
that have short transmission windows. His "clicker" has a
audio range from a few clicks/sec to 10 KHz (or as high as
we old farts can hear). This allows a sniffer that can be
swung in a fractions of a second the tone will follow and
the swinger doesnt have to watch meters, lights, turn pots
flip switches or anything or even stop running (or walking
in the case of us old farts). No switches with a device with
70 dB dynamic range or at least not very often do you have
to turn a pot or flip a switch, just keep on truckin'. 
For transmitters that are hidden in places where they are
not the biggest thing around simply use something else in
your bag. I know of very few places in Los Angeles area
where I can drive and not get closer with the 4Runnerto the
hidden transmitter (in power) than other transmitters. Mt.
Wilson in one such place and I have hunted there a lot. I
doubt if the logamp sniffer would help much there. I found
that very little helps much there. I have four helical
resonators, a cavity and a passive attenuator into a
Radioshack handheld tuned off freq as a usable sniffer.
Other handhelds seem to have a lot more intermod and
desense. Anything with a diode is a disaster. Offset
attenuators do nothing useful as the idea is to keep the
stuff away from anything non-linear until it is smaller than
the hidden transmitter. 

Now what about "jamming". Hidden transmitter that are very
near or co-located with puposeful transmitters set up to
prevent certain systems from getting a bearing. The logamp
sniffer is just another tool in the bag. If the jammer is
set up to defeat the logamp, then it must be suseptable to
some other system. I even have an Infrared heat sensor that
has found active transmitters from passive decoys in my bag.
BTW, my bag is a lot of radios. The 4Runner has six two
meter radios and I carry another 6 handhelds one with no
antenna, one that is on the hunt frequency with a very small
antenna (you are there radio) and two backups. The doppler
radio and one of the handhelds are RS HTXs (available for
$40.00 on eBay), two Yeasu VX-1Rs (great "spinning" radios)
and two Yeasu 211s. I have a LPER, two diode sniffers, a bug
detector, an assortment of offset attenuators, fixed HP,
Telonic and other switched passive attenuators and a lot of
stuff I have forgotten about over the last 35 years of
hunting. But I wander off the subject...

Bob, WB6JPI

Bruce wrote:
> 
> On Monday 22 April 2002 13:55, 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.
> 
> I'd have to agree with Larry here. Early sniffer designs here used to be
> basically nothing more than amplified diodes. Yes they had 1 or two tuned
> circuits, but that really wasn't enough to knock down Pagers, Power Lines,
> Petrol Bowsers, Street Lighting etc etc.
> Almost no teams use this form of sniffer anymore. Yes they work, and in say
> 70% of cases where the fox is the "loudest" thing around work fine. It's just
> that peasky 30% that made the difference.
> 
> Pretty much all of the recent more sucessful designs have been dual conversion
> receivers. Of course it's more complicated, but these things are done for a
> reason.
> The MZ Super, TJN/XAJ Ultra and the new YNG designs became "standard"
> equipment here.  They work well for ARDF too where the signals can get quite
> weak. Often normal foxhunt signals here are weak since the fox is down a hole
> somewhere, or is a mini-fox only emitting a couple of mW.
> You can spend your time chasing the street lights if you like, but we gave up
> on that long ago.
> 
> BTW: Most FM limiter chips have a dynamic range of 60->90dB so this is
> nothing out of the ordinary. The thing is you can't use this whole range into
> a Whoopee (tone VCO) since it'll be a bit unexiting (a 3el beam only has a
> useable F->B of 15dB or so). You need to have ranges.
> Our Polar pattern system in the car uses such an FM chip, but I only ever
> display about 20dB on the CRO screen at any 1 time. The rest is held as
> internal ranges the micro can use to "fill in" between external dB switching.
> 
> > 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.
> 
> Agreed. It'll work more than 50% of the time, especially if you all use Mega
> Watt foxes.
> 
> > Just for FYI purposes these are the possible signal strengths that
> > can be expected:
> 
> The Ultra sniffer has a useable dynamic range from -120dBm ro +15dBm.
> Not bad, but *still* not really good enough for the high powered foxes (>10W).
> It does this in 9 overlapping ranges. It really needs a couple more ranges
> on top of that !
> 
> > 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)?
> 
> I can _just_ do this by effectively causing a PAD by unplugging the BNC a
> little bit from the Ultra sniffer box. This only works because the Ultra is
> in in a very well shielded metal box. Forget it with a handheld !
> When you're dealing with levels like this a good pad would probably need to
> be in it's own "compartment".
> 
> > > My friend Tony has built a sniffer around the Analog Devices
> > > Logrithmic amplifier chip. Here is his report...
> 
> --
> Cheers,
> Bruce                                          ICQ: 32015991
> _______________________________________________
> FoxHunt mailing list
> FoxHunt@mailman.qth.net
> http://mailman.qth.net/mailman/listinfo/foxhunt