[FoxHunt] Attenuator

[Bruce]

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=20
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 s=
ay=20
70% of cases where the fox is the "loudest" thing around work fine. It's =
just=20
that peasky 30% that made the difference.

Pretty much all of the recent more sucessful designs have been dual conve=
rsion
receivers. Of course it's more complicated, but these things are done for=
 a=20
reason.=20
The MZ Super, TJN/XAJ Ultra and the new YNG designs became "standard"=20
equipment here.  They work well for ARDF too where the signals can get qu=
ite=20
weak. Often normal foxhunt signals here are weak since the fox is down a =
hole=20
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 gav=
e up=20
on that long ago.

BTW: Most FM limiter chips have a dynamic range of 60->90dB so this is=20
nothing out of the ordinary. The thing is you can't use this whole range =
into=20
a Whoopee (tone VCO) since it'll be a bit unexiting (a 3el beam only has =
a=20
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=
=20
display about 20dB on the CRO screen at any 1 time. The rest is held as=20
internal ranges the micro can use to "fill in" between external dB switch=
ing.

> 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 M=
ega=20
Watt foxes.=20

> 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 range=
s
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=
=20
little bit from the Ultra sniffer box. This only works because the Ultra =
is=20
in in a very well shielded metal box. Forget it with a handheld !=20
When you're dealing with levels like this a good pad would probably need =
to=20
be in it's own "compartment".=20

> > My friend Tony has built a sniffer around the Analog Devices
> > Logrithmic amplifier chip. Here is his report...

--=20
Cheers,
Bruce                                          ICQ: 32015991