[ARDF] Attenuators for ARDF...

[Matthew Robbins]

I've been planning to make several tape-measure beams both as gifts
and to have some available for people who show up at an event with no
gear of their own.

I have several of the offset attenuators off Joe's website, but I've
been trying to decide if there is a better option.  I've been
considering building a step attenuator.  I am attaching a .jpg (that
might not make it) of an 1/2 inch aluminum conduit box that I've been
trying to use in the aft boom of the TMB (between the driver and
reflector), sort of as a handle and a convenient place to hold the
attenuator and controls.  The radio will go aft of the reflector to
balance the CG.

attachment: (The image is one of the larger sizes.  The 1/2 inch ones
fit nicely in my hand.

I like this conduit box because it has a good feel.  Some of the
plastic ones are sharp-edged and boxy.  Even some of the aluminum ones
are like that.  I figure the aluminum provides some shielding as well.
 The only downsides are cost ($3-$4 per box), weight (more than
plastic, but not too heavy), and the need to use either (1) threaded
schedule 80 plastic pipe, or (2) 1/2 NPT threaded to slip adapters. 
The threaded pipe is often not straight, but the adapters seem weak. 
I'm going to try both.

Does anyone know how much attenuation is needed for ARDF?  Is the
two-button arrangement on some imported gear two steps of attenuation
or something else? What are the two levels of attenuation?  I know my
Bryan Ackerly radio has 15db steps, and I usually use 5 or 6.  Closer
than 24m it switches to more, but I don't usually need it at that
point.  So 5 is 75db, and 6 is 90db.

Here's a little tutorial on step attenuators:
http://www.electronics-tutorials.com/basics/attenuators.htm

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)
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.)
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.
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
attenuation per stage.  Above 20db, the calculations I did show the
resister values "exploding", which tells me the attenuation is getting
very sensitive to small tolerance differences in the resisters.  I'm
not using this for lab use, so I can handle it if it's not exactly a
certain db attenuation, but I personally am uncomfortable trying to
use stages greater than 20db per step.  I'd probably just try to built
a few to see if it gets weird.

Things I see as even:

1. Can't transmit through either, unless you make special design
compromises for the step (i.e., big components).
2. Both sort of difficult to use (that is, harder than the (brainless)
Ackerly wonder).
3. Cost, unless the step gets more than about three or four stages.

I was thinking of a BCD switch with a numerical indication of
attenuation, but I'm not sure exactly how to do that.  My first
thought was a binary-stepped set of three or four stages (say, 15, 30,
and 60 db steps).  (that would leave the 8, 9, and 0 BCD setting
unused), but again, the resister values for 30 and 60 db seemed a
little crazy to my unexperienced brain.)  (Do BCD switches ever have
NC switches? I can see how to do it with NC, but not NO, and I can't
find any to look at.  I'm also assuming each "stage" has a separate
ground.) If zero NC is straight-through, then 8 and 9 could have just
a gap.  I wonder if that could have some interesting level of extreme
attenuation?  (I know Bryan Ackerly's 80m receiver has a resister
network mod that uses a BCD wheel.  I'd like see how that works.  This
schematic appears to switch between potentimeters, one per step:
http://www.users.bigpond.net.au/vk3yng/foxhunt/80m_sniffer/80m_range_control/Figure%201%20-%20Range%20Control%20Schematic.pdf
 Photo here:  http://www.users.bigpond.net.au/vk3yng/foxhunt/80m_sniffer/80m_range_control/range_control.html
 But I don't see how he did the BCD switch version.  )

(Bryan: How did you do that?)

I've been trying to build the attenuator onto the lid of the conduit
box so I can move the lid and attenuator to different antennas.
(Although if it's cheap enough, I'd just leave it in the antenna, and
not have the additional coax connector.) One reason I'm looking at
this is I'd like to have a design available for group builds.  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.

If you've got any ideas or experience with any of this, please let me
know.  I'm trying to think of a very simple user interface.  (On the
two-step radios, are the buttons momentary or continuous?  Or do you
switch in one step say 700m away and then have a momentary for 150m or
so?)  Does anyone use the peak to get close and the null to sniff?

Matthew
AA9YH
Cincinnati, Ohio USA