[ARDF] Reverse-bezel fast compass?

Jay Hennigan jay at west.net
Wed Aug 20 23:57:01 CDT 2003

I've been experimenting with 2-meter antenna designs, and have mounted
a ball-style automotive compass on the boom of my tape-measure LPDA
units. [1]

These compasses are cheap and read correctly in terms of the view
of the compass reading the direction that the antenna is pointing.
As they're designed to be put on the windshield of a car, this makes
sense.  The compass is designed so that the observer is behind the
compass, thus the displayed bearing is the direction the car (or antenna)
is pointed.

However, these compasses are slow to settle and not very finely graduated
for taking accurate bearings.

I acquired a Moscow 11 baseplate orienteering compass which is much
better in terms of giving an accurate reading and doing so quickly when
running or swinging the antenna.   Cool, just mount the compass on the
boom and you're done, right?  Wrong.

This style of compass is designed so that when the user lines up the
needle against a line on a rotating plate, the compass baseplate will
"point" toward the direction of travel.  Nice for use with maps or a
"sticky" bearing remembering the rotation of the capsule relative to
the baseplate.

But, mounting such a compass capsule on the antenna such that north
on the compass lines up with the business end of the antenna causes the
indication to be swapped east-west.  If the antenna is pointing west,
the compass needle points towards the "East" marking on the dial.
North and South are fine, but everything else is swapped around.

I know this, and could mentally flip it when drawing bearings.  Keeping
track of several transmitters, my position on the map, the clock, running
through the woods all at the same time are enough that the mental math
isn't something I'd like to add to the mix.  :-)  Likewise, using it in
the traditional manner of spinning the capsule and lining up the needle
with the line takes time.

A Google search turns up rowing and kayak compasses that read "right"
for this type of thing, but they're large, heavy, and somewhat costly.
I also don't know how fast-settling such compasses would be, but would
suspect that they're fairly heavily damped compared to a good orienteering
compass.  The Brunton, Silva, etc. sites don't seem to offer a "reverse
bezel" for any of the baseplate style compasses.

Has anyone on the list addressed this problem?  Is there available a
compass with good speed and stability with a bezel that indicates the
needle bearing relative to the base as opposed to the other way around?
Or has anyone done a scalable graphic of a compass ring that is correct
for this use that they're willing to share?  Alternatively, is there a
nice, fast, stable automotive-style compass that is recommended?

[1] Ken WM5R took a picture.  http://www.wm5r.org/photos/2003_usa_ardf/
Yes, I will publish plans, right now tinkering again for the next set
of improvements.  At least I'm documenting things this time.

Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
WestNet:  Connecting you to the planet.  805 884-6323      WB6RDV
NetLojix Communications, Inc.  -  http://www.netlojix.com/

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