[FoxHunt] Attenuator

Tue, 16 Apr 2002 16:05:43 -0700

Looks like you left out a few details in your original post!

----- Original Message -----
From: "Bob" <[email protected]>
To: <[email protected]>
Sent: Tuesday, April 16, 2002 12:07 PM
Subject: Re: [FoxHunt] Attenuator

> Yes it does. And the attenuator is relay controlled and sits
> right on the radio connector with no cable. The relay box is
> partitioned and double shielded and has four compartments,
> 10,20,30,40dB. I use an external rotary switch and a mess of
> diodes to provide 10dB steps from 0 to 100 dB. The coax
> inside the radio is about 1/2 inch and goes to the steering
> diodes and off the the internal preamp. That portion of the
> circuit is in a can inside the metal case of the radio. The
> attenuator poops out at about 80dB, with 90 being closer to
> 82dB. From then on its a matter of turning down the internal
> preamp. I can get good bearings at over 100 dB, or about 25
> ft from 50 watts into a whip.
>
> Bob, WB6JPI
>
> Jim Forsyth wrote:
> >
> > Bob,
> >
> > I have never been able to use more than 60dB of external attenuation
with my
> > IC275 or my FT736, both are all mode radios. It sounds like the 9000 has
> > exceptionally good shielding.
> >
> > Jim, AF6O
> >
> > ----- Original Message -----
> > From: "Bob" <[email protected]>
> > To: <[email protected]>
> > Sent: Tuesday, April 16, 2002 8:37 AM
> > Subject: Re: [FoxHunt] Attenuator
> >
> > > There are radios and then there are radios. In general, the
> > > All-mode radios have better shielding than the mobile that
> > > are better than the hand held. I can easily do over 100 dB
> > > with my Kenwood 9000 (including the 25 dB of the internal
> > > attenuator) without tuning off freq.
> > >
> > > Bob, WB6JPI
> > >
> > > Kuon & Dale Hunt wrote:
> > > >
> > > > >What are the thoughts on a good attenuator...
> > > >
> > > >   Clete -
> > > >
> > > >       That Radio Shack attenuator probably won't help much.
> > > >   You will generally need more attenuation than it will provide.
> > > >
> > > >   I find 60dB of attenuation is about the limit I can use with
> > > >   most receivers before the signal picked up through the case
> > > >   exceeds that from the antenna connector.  After that point,
> > > >   any additional attenuation won't help.  60dB may get you
> > > >   there if the transmitter is running low power and it isn't
> > > >   well hidden, but I've seen receivers wimp out at a quarter
> > > >   mile from a 25 watt transmitter - the signal was full scale
> > > >   on the "S" meter even with the antenna disconnected.
> > > >
> > > >   The solution is to use a "active attenuator" (or "offset
attenuator".)
> > > >   This is an oscillator and a mixer circuit.  The oscillator is on
> > > >   some frequency in the MF or low HF range - say, 2 MHz.  The
receiver
> > > >   is tuned 2 MHz off from the desired signal, and the level of the
> > > >   oscillator signal applied to the mixer is varied with a pot.
> > > >   Because the receiver is not tuned to the transmit frequency, it is
> > > >   much more immune to overload and pickup through the case.
> > > >   Attenuation levels of 100dB are possible - which is usually enough
> > > >   to sniff out a transmitter hidden in a clump of bushes
> > > >
> > > >   The design on the HomingIN web site is a good start, and there are
> > > >   others around on the net.  Personally I like to use double
balanced
> > > >   mixers in mine, but the single-diode ones are simpler and still
> > > >   should do the job for you.
> > > >
> > > >   Good luck and happy hunting!
> > > >
> > > >        - Dale WB6BYU
> > > >
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