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Author Topic: Fox Hunting attenuator suggestions  (Read 1701 times)
N9VKC
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Posts: 33




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« on: May 13, 2004, 05:56:40 PM »

Hello All

  I'm looking for an attenuator in complete or kit form that would be usefull for fox hunting in the 2m and 70cm bands.  Ay suggestions for good attenuators would be greatly appreciated.  My local AES has the MFJ attenuator but I thought I would search about first.

73's

Chris / n9vkc
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WB2WIK
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Posts: 20543




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« Reply #1 on: May 13, 2004, 06:45:45 PM »

The MFJ's actually not a bad deal and costs less "new" than a good "used" precison step attenuator.

I use the HP (Agilent) model 355D for this purpose, it's 0-120 dB in 10dB steps and accurate to a fraction of a dB up through about 1 GHz; however, for fox hunting purposes, you don't need precision, you just need something that adds loss and is preferably adjustable so you can add more or less loss.

If you can find a 355D or equivalent surplus for $50 or so (they're definitely out there!), it's a good deal.  But if you can't find something like this, the MFJ is a good deal and very reasonably priced.

Now, the trick will be to shield your fox hunting receiver well enough to actually take advantage of an attenuator!  Most HT's and many mobile rigs don't have sufficient shielding to work in strong field, due to leakage right through the case; as such, you can add 120dB of attenuation at their input and often absolutely nothing changes...but that's the challenge of the fox hunting game!

WB2WIK/6
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AC5E
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Posts: 3585




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« Reply #2 on: May 13, 2004, 06:46:05 PM »

The MFJ switchable attenuator seems to work quite well - although I have a commercial attenuator I bought some years ago. If you can find a recently calibrated commercial jobbie (so no one has tried to transmit through it and cooked the resistors) I would suggest that.

Before that I used either the "shield the HT with your body" method, which works pretty well - or a sleeve attenuator made up of two sections of coax sliding inside one closely fitting brass tube.

The closer the ends of the coax are the more coupling and the less attenuation. That worked about as well as the current switch type but the exact coupling was pretty critical so I usually spent some time readjusting the coupling. Not something you really want to do when you are one of a dozen hunters closing in on the fox.

73  Pete Allen  AC5E
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KT8K
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Posts: 1490




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« Reply #3 on: May 14, 2004, 12:41:15 PM »

For 2M fox hunting I have long used an old TR9000 all mode rig, which has an RF gain control.  I have a pair of commercial step attenuators (got 2 for $25 at a swap) but the RF gain control has done the job by itself.  I've never seen an RF gain control on an FM rig, though.  It has to be a rig with SSB/CW.
Happy hunting es 73 de kt8k - Tim
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WB6BYU
Member

Posts: 13026




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« Reply #4 on: May 14, 2004, 04:16:43 PM »

Chris -

Start with a visit to K0OV's web site:

www.HomingIn.com

His book on transmitter hunting contains plans for
building both switched and sliding-tube attenuators.

One big problem with using standard attenuators for
fox hunting is that there is an effective maximum of
about 60dB or so, regardless of how well the unit is
built.  At this point, the signal leaking in through
the case of the radio will overpower anything coming
in through the antenna jack.  When the S-meter is
pinned without any antenna, no amount of attenuation
in the antenna cable will give you a usable signal.

The currently popular solution to this is called the
"Offset Attenuator" (or "Active Attenuator".)  It
consists of an oscillator and a mixer:  the receiver
is tune to the receiver frequency plus or minus the
oscillator frequency, so it has some buffering from the
direct transmitted signal.  The response of the system
is reduced with a pot controlling how much of the
oscillator drive is applied to the mixer.

Not only can these work VERY WELL, they are relatively
simple and inexpensive to build, often with junkbox
parts.  And you can reasonable performance even without
any shielding.  There are many such circuits available
on the net - and most are pretty similar.  K0OV has one
on his site, others have been posted by WB2HOL and
WB8WFK (from memory).  The local ARES group sells some
boards that I designed 10 years ago, and at least one
ham sells ready-made units.

(We may have linked some circuits from the equipment
page of www.ardf-r2.com?)

Most such circuits use a single diode in the mixer.
I use a double balanced mixer, and seem to have far
fewer problems with image mixing of strong local signals
than the single diode circuits.  But not everyone has
 such parts in their junkbox, or is confortable building
their own using coils of wire wound around their finger.

If you follow the links and still have questions, feel
free to email me and I'll do my best to help out.

Happy Hunting!  - Dale WB6BYU (at arrl.net)
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WB6BYU
Member

Posts: 13026




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« Reply #5 on: May 14, 2004, 04:46:24 PM »

Check the Product Reviews section of this site, under
Direction Finding Equipment.  The review of the K0OV
Offset Attenuator shows the kit available from KE6HTS.
Or you can build your own from the instructions at
www.HomingIn.com
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