[FoxHunt] Attenuator

[Jim Forsyth]

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" <wolfbob@csnsys.com>
To: <foxhunt@mailman.qth.net>
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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