[ARDF] TRO-2 ARDF transmitters...

[Jay Hennigan]

Matthew Robbins wrote:

> Here in Cincinnati, we've been building G3ZOI's TRO-2 transmitter. 
> I've put up some photos at http://offcycle.org/ .
> 
> We had some questions about sizing the phasing harness on a turnstile
> antenna.  Perhaps someone here can make it a little more clear.

The phasing harness should be an *ELECTRICAL* 1/4 wavelength at the
operating frequency, 75 ohm.  Note that this is not 19 inches.  It is
19 inches multiplied by the velocity factor of the coax cable that you
use.  This depends on the dielectric of the coax, but is typically 0.66
to 0.78.  solid dielectrics have a lower velocity factor than foam,
teflon is different from polyethylene.  For the teflon coax I use they
wind up at roughly 13 1/2 inches.  Note also that the typical TV coax
commonly sold is a nuisance to work with.  The braid is unsolderable
and the center conductor is usually copper-clad steel, kind of stiff.
RG-179 teflon cable is nice and small, easy to work with, but the center
conductor is a bit fragile.  RG-59 with tinned copper braid is another
choice if size isn't an issue.

Also, expect to see a slight VSWR, about 1.4 to 1.  This is because
there is some mismatch between the feed impedance (about 36 ohms)
and the 50-ohm feedline.  Don't try to tune the antenna for minimum
SWR.  Doing so will distort the pattern.

Good theory and design article here:
     http://www.cebik.com/wire/turns.html

One mod to my design is to use aluminum arrow shafts instead of the
brass rods.  I haven't explored this but it sounds promising and should
offer some advantages.  The larger diameter will widen the bandwidth
somewhat.  Aluminum is lighter and easier to lug through the woods.
Arrow shafts supposedly are threaded 8-32 (I haven't verified this)
which would make construction a lot easier.  No need to attach the
spacers to the rods.

-- 
Jay Hennigan - CCIE #7880 - Network Administration - jay at west.net
NetLojix Communications, Inc.  -  http://www.netlojix.com/
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