[ARDF] Attenuators for ARDF...

[Dale Hunt WB6BYU]

> 
> On Sun, 17 Oct 2004, Matthew Robbins wrote:
...
> > Does anyone know how much attenuation is needed for ARDF?

Jay Hennigan replied:

> 90 dB should be plenty, but without an offset or modifying the receiver
> you aren't going to get that much with a step attenuator in the real
> world...

   It also depends on how well the transmitters are hidden.
   The ones at the WC that could be seen 100m away are
   not typical.  Kuon has twice had her Ron Graham receiver
   swamp out during a competition and still not been able
   to find the transmitter.  (Each time she was within 10
   meters - but now that she knows how to interpret the
   symptoms, she will know to start beating the bushes
   when that happens!)

   Jay's comments about pickup through the case are the real
   limiting factor for using an HT or scanner.  Even with a
   well shielded attenuator and double braided coax jumpers,
   60dB is a practical limit without shielding the radio
   itself.

   A further problem (which affects the usable step size)
   is the limiting action of the FM receiver.  Most of
   the mobile rigs I've tested have about a 10dB range
   on the S-meter from no indication to full scale.
   (My old TR-2600 HT with analog meter approaches 20dB,
   but that is the best I've ever seen.)  You can't use
   a 30dB step, because there will be times when one
   step is not enough and the next is too much.  If you
   are using the bargraph meter on the HT, I'd suggest
   10dB steps at the most.  (However, on 80m I get by
   with a 4-position slide switch for my gain control.
   With an AM receiver you can use larger steps on 2m.)

> > As I see it, here are some of the trade-offs:
> > 
> > offset: 
> > 
> > 1. More difficult for beginners (requires tuning off/ knowledge of 
> > the radio controls)

     I usually have a new person run the equipment while I drive
     on mobile hunts.  Trying to get a non-technical person to 
     understand a set of binary stepped switches is not always
     easy (especially in the dark when they pick up the attenuator
     backwards).

     For ease of use, I recommend making the offset attenuator
     sensitive enough that you can use it from the beginning
     of the hunt.  Then you never have to switch modes while
     hunting (or, especially, between transmitters!)  This is
     one reason why I use a double balanced mixer instead of
     a single diode in my attenuators.


> > 2. Batteries (which is the main reason I want to try a step atten for
> > loaners---no maintenance, and no batteries to run down during periods
> > of non-use.)

     Same problem with receivers of any type, which is why many
     are designed to apply power only when the headphones are
     plugged in.  If your HT has a DC connection across the
     antenna jack (or if you modify it to apply a bit of
     voltage there), you can arrange the circuit to switch
     on only when attached to the radio.  Or run the earphone
     cord back to the attenuator and have an earphone jack
     on that unit that turns it on only when the 'phones are
     in use.

     (I also heard a recommendation that carbon-zinc batteries
     cause less damage when they leak than alkaline, so might
     be a better choice in this case.)


> > 3. Better for very strong signals.
> > 4. Once the radio is off-tuned, the controls are very simple.  Twist
> >  the knob.
> > > > 
> > step:
> > 
> > 1. Nominally easier for beginners.

      Not necessarily - see above.


> > 2. No batteries
> > 3. Set up with just enough attenuation for ARDF, might not be usuable
> > for mobile hunts.  (Not really a concern for me.)
> > 4. Might be simpler if a small enough number of stages can be used.
> > 5. The resister values seem to be "reasonable" up to about 20 db

   And #6, NOT usable on strong signals, as Jay pointed out.  

   But "offset" and "step" are not mutually exclusive, and there
   may be some good alternatives.  The pot in the offset attenuator
   simply adjusts the level of the HF signal applied to the mixer.
   You can do this by switching discrete resistors.  To get maximum
   attenuation you need to pay attention to reducing the stray RF
   floating around inside the case, but I suspect that a 8- or
   10-position rotary switch could work reasonably well.  This
   would allow 10dB steps (though the actual attenuation with an
   offset attenuator is not constant:  due to threashold effects
   in the mixer the actual attenuator depends on the applied
   signal level.)  So you will have to do some experimenting with
   resistor values, but a single tapped voltage divider will
   probably do the job.  (The attenuator I use for "serious"
   mobile hunts has a "numb" setting, where I'm using the
   voltage drop across a short circuit applied to the pot.
   At maximum attenuation, the actual oscillator signal 
   applied to the mixer is only about S6 when applied to
   my HF receiver.  You may find that a resistor (or a dead
   short) to ground from one of the switch positions makes
   it easier to get the desired levels.)

   A similar consideration to the limited meter range is the
   available tone range in whoopee mode (although this doesn't
   apply to using an HT for ARDF.)  The wider the dB range 
   in each step, the less resolution available.  With the
   VK3YNG sniffer I can hear less than 0.2dB difference in
   signal strength, which allows me to take a sharp bearing.
   If he used the same audio range for a 30dB step, the
   resolution wouldn't be as good, so the pointing accuracy
   would drop.



> > One group I am thinking of is Civil Air Patrol.  I figure they'll 
> > need two
> > antennas, one for ARDF and one for ELTs.  I was thinking the
> > attenuator-built-in-the-lid might be a way to reduce the cost.  And I
> > don't think CAP people need extreme levels of attenuation to find
> > extra powerful foxes.


   Actually, CAP probably has to hunt a wider range of signal
   strengths than we would encounter in ARDF, even though the
   ELT transmitters are not as strong.  (And then there are
   the 50 watt transmitters left on by mistake...)

   First of all, ELT hunting requires all the sensitivity
   you can muster, especially in the early part of the hunt.
   Signals can be quite weak, especially if the antenna is
   damaged, it's in a canyon, and the batteries are almost
   dead.  (And ham HTs can be rather deaf on the aircraft
   band - some are 10 to 20dB less sensitive than on the
   ham bands.)  So the attenuator must be capable of low
   loss - or, better yet, be swapable with a pre-amp.

   On the other hand, most ELT hunts are non-distress, at
   times ending up on a shelf or in a trash can.  (We found
   one in a bedroom closet, and in a box of junk in the
   garage.)  This may require sniffing within inches of
   the transmitter.  The Little L'Per has at least 120dB
   of attenuation range for this reason.

   One tool I built for ELT sniffing in the last 50m would 
   meet many of your critieria - a simple crystal set. 
   Cheap, and can be built into the antenna boom, and doesn't
   require any batteries. The only user interface is the 
   headphones.


   Although I like the idea of building the attenuator into the
   antenna boom, I've had good results using broom clamps to 
   secure a box to the boom.  Easy to remove and move to another
   antenna.  There are a couple types of these, designed to hold
   a broom handle to the wall in the garage.  Basically a
   U-shaped clamp - bolt it to the box and it will hold on
   either 1/2 or 3/4" PVC pipe.

   Or, instead of building the attenuator in the lid, why not just
   remove that section of the boom and put it in the other
   antenna?  I never glue the joints in my beams anyway so I can
   take them apart and put them in my hand luggage.


   Hope this gives you some ideas,

       - Dale WB6BYU
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