[FoxHunt] Attenuator

[Kuon & Dale Hunt]

>Dale, can you explain the advantage of the double balanced mixer...


   Sorry to take so long to get back to the topic, but I had to
head off to Georgia for the ARDF Championships.  A great time
was had by all...


   The basic circuit is the same - an oscillator passing through
a pot and feeding a mixer.  We've used the SBL-1 for kits and
club projects because it is simple, but I actually get better
attenuation characteristics building my own with 1N4148 diodes
(though the insertion loss is a bit higher.)  With the SBL-1
and other commercial mixers, apply the oscillator to the
IF port, which should be rated down to DC.

    The primary advantage of the balanced mixer is a reduction
in interference and stray images from other services.  Sure, you
can still hear signals off by a multiple of the clock oscillator,
but the number of possible mixing products due to multiple strong
signals is reduced.  And, unlike some commercial attenuators, I
have no problems hunting signals on harmonics of the offset
oscillator frequency.

    Usually I add a second switch position that feeds DC to the
pot instead of RF.  This gives me a variable on-frequency
attenuator.  The DC should be limited to 10ma or less (if you
burn out the diodes in the mixer, you can reverse the polarity
and have a nice DC controlled attenuator - but a bad mixer!)
This allows me to get reasonably close to the transmitter
before needing to switch in the offset - and that also helps
keep me from chasing spurs, because by the time I need the
offset mode, the signal I'm chasing is one of the strongest
ones around.  The attenuation in "THRU" mode is limited by
the forward voltage drop on the diodes, so you could use a
couple diodes in series in each leg of the mixer.  Because
of the "threshold" effect (effective signal strength is
the amount that the signal exceeds the forward diode
voltage and the bias voltage) this attenuation is not
linear - the attenuation will appear to be greater on
weak signals than on strong ones at the same setting.

   Having the THRU position means I can pad down the
oscillator output before applying it to the pot.  I try
to make the minimum attenuation on OFFSET mode close
to the maximum in THRU - this gives me two steps with a
bit of an overlap.  For kits, we usually add red and
green LEDs so you can tell which range you are on in
the dark.

   For really close work, I have added a "NUMB" position
to some units.  This connects a resistor (20K?) load on
the oscillator output, and taps off the voltage developed
across a pair of switch contacts grounding one end of
it.  You can adjust the inductance of the shorting wire
to get just a whiff of RF going to the pot - on my
usual mobile hunt attenuator the oscillator and mixer
are in separate shielded boxes.  When I couple the
output of the oscillator box to my HF rig, with the pot
at minimum in NUMB position, the signal delivered to
the mixer is about S3.  My HT starts to melt at about 10'
from a 25W transmitter, but with a mobile rig and
a switched passive attenuator in line ahead of the
offset attenuator I have parked beside the fox and
identified the driven element on his yagi.

   I put a BNC "T" connector in one of the cables where
it plugs in to the offset attenuator.  This allows me
to bypass the attenuator easily when signals are weak.
When they get strong, I just unplug one of the cables
on the "T" and plug it into the other port on the
attenuator.

   I don't have any problem transmitting through the mixer
because I remove the microphone before putting it in line.
(I routed the "S" meter voltage to an unused pin on the
mic connector and use this to add an external audio
"S" meter, which also makes sure the mic is unplugged.)
But even the SBL-1 mixers ARE repairable if you
transmit through them - though probably not easy to do
during the hunt.

   If we had more hunts - and more difficult ones - I
would consider improving the packaging and convenience.
But we've gotten a lot of folks started hunting with
a simple 2-element PVC/wire quad and the attenuator.
A friend of mine (now SK) designed a board a few years
ago, the local club had 200 of them made.  I think they
are just about sold out, so there must be a bunch of
them out there around the Pacific NorthWest.

      - Dale WB6BYU