[ARDF] Attenuators for ARDF...

[Dale Hunt WB6BYU]

Matthew Robbins wrote:
> 
> The vibe I'm getting is that offset is cheap enough and easy enough to use.
> Furthermore, it's just more capable than a step attenuator.  It sounds
> like offset is the way to go when you have to use an off-the-shelf
> radio.

     Again, you can make an offset attenuator with discrete steps...


> 
> Dale: Tell me about this double balanced mixer...


     Nothing special that anyone else couldn't come up with.
     I just used a stock SBL-1 DBM (or homemade with 4 diodes
     and 2 transformers) in place of the single diode.  The
     transformer-coupled ports ("LO" and "RF") go to the
     rig and the antenna (interchangably).  The oscillator
     signal (and/or a DC voltage for straight-through use)
     are applied to the "IF" port (which must be rated down
     to DC.)

     There are two modes:  "THRU" applies DC to the pot, and
     "FREQ" enables the oscillator to drive the pot.  Make
     sure that the mixer can't be driven by more than 10mA
     in either condition: use series resistors between the
     two sources and the pot to adjust the ranges.  Then I
     added green and red LEDs to indicate the mode.  Don't
     have a good electronic copy, but I'm sure I have copies
     of the article around here.

     About 10 years ago we had some boards made, and the local
     radio club just found a stash of them.  Let me know if 
     anyone is interested and I'll pass along the info.

     The big advantage with the DBM is fewer images when hunting
     in an urban area.  Also, having the "THRU" mode means that
     the offset mode isn't emabled until the signal is fairly
     strong, further reducing problems with spurious responses.
     But I'm not sure really how much practical difference it makes.



> (And the crystal set, too.)


     Hmm...  I thought all hams started in Radio by building
     a crystal set!  Guess those went the way of the Trimm
     Lightweight high-impedance headphones, though.

     Basically you build a resonant LC circuit for the desired
     frequency.  The antenna taps about 20% of the way up the
     coil, the diode taps 60 - 80% and feeds a high impedance
     earphone with an RF bypass capacitor to ground.

     Look at the front-end of Joe's "Sniff Amp" for the basic
     circuit.

> 
> One last question (no promises, though): I've been looking for a
> schematic of an older offset attenuator that uses one AA battery...


     The original QST article used a single penlight cell.  Basically
     it used a 2N2222 LC oscillator on 500kHz.  I've done the same
     thing with a 1 MHz crystal.  This replaces the canned oscillator.

     The canned oscillator is easier to build because it has far
     fewer parts.  Commercial crystals (4 MHz) are around 70 cents,
     so you don't save a lot of money unless you can find some
     surplus.  The voltage regulator is about 50 cents.  The penlight
     cell holder will be about 50 cents vs. a 9V snap for 25 cents.


The SBL-1 mixers are $5 or more: you can save a lot
     there if you are willing to wind your own transformers.


Marvin Johnston wrote:
> 
> One of the problems I've run into is saturation when close to the
> transmitter...

     The biggest problem I had was minimum resistance (and stray
     inductance/common ground paths) in the pot.  First, add enough
     series resistance between the oscillator and the top of the
     pot so you just have adequate minimum attenuation with the pot
     at maximum.  This gives a better control range.  Then check 
     the wiring in the ground leads: there should be one ground
     lead coming from the oscillator to the bottom of the pot, and
     a separate lead from there to the mixer.  Any common wiring
     will increase the minimum output from the pot and therefore
     decrease the maximum attenuation.  (And the "NUMB" option
     that reduces the RF applied to the pot helps alot, too.)

     One other thing that the DBM allows is to have one of the
     coax connectors totally isolated from ground.  This prevents
     any common-mode currents flowing on the cable from swamping
     the receiver when you are up close.  The only signals that
     can get through are transformer coupled.


> My junk box could handle a
> couple, but certainly not 20 without buying parts :)!


     Well, if you aren't picky about the frequency... no, I don't
     want to even think about how many I could build!  I've 
     probably done 6 or 7 so far of various types.  (If anyone
     has problems finding parts for an ARDF project, let me know.)
     Now if only I had more time for building circuits...
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