[ARDF] Does anyone have a good design for a Halo Antenna?

[Jay Hennigan]

Dale Hunt WB6BYU wrote:

> I strongly recommend reading W4RNL's articles on omnidirectional
> horizontally polarized antennas - probably in the VHF/UHF
> section of his website.  

That's the http://www.cebik.com/content/ao/ao16.html link I referenced 
earlier.

> A lot of background information,
> radiation patterns, and notes on tuning adjustments, element
> diameter, etc.  I think he ended up getting best results
> using triangular loops, but there were two different models -
> one with close spacing between the open ends and one with
> wide spacing, and neither had a non-reactive feedpoint
> impedance.

The triangular design with close spacing seems to be the better choice 
of the two according to the notes on the site.  Low impedance but mostly 
resistive.  A toroid matching transformer, T-match, etc. could be used.

> The most critical design criterion, however, is how close
> to omnidirectional you need the pattern to be.  I think the
> IARU Region 1 standard is within 1dB, which is a challenge
> with a standard turnstyle without careful adjustment.  (In
> practice it would surprise me if most competition antennas
> were actually that good.)

1dB is a very tight spec.  An ideal turnstile would do it if well-tuned 
to the exact operating frequency with very good care placed on phasing 
line length including velocity factor, etc.

The scary thing to me on the Cebik site is the vertical patterns shown 
in light of the typical ARDF deployment.  Deep nulls when the antennas 
are tilted even a few degrees from the horizon.  Course setters need to 
be very careful about keeping the antennas in the clear, truly 
horizontal, and secure from buffeting by winds or being knocked around 
by competitors.  In many of our practice sessions the antennas are low 
to the ground entangled in tree branches hanging cock-eyed from a short 
bungee cord.  (Marvin, are you reading?)

> On the other hand if you are just having informal practice
> sessions, I've had good luck with a stiff wire dipole bent
> in somewhat of a circle... or not.  Only rarely do we run
> into problems with being in the null off the ends of the
> wires, and a bit of modeling should come up with something
> that is perhaps only 6 to 10 dB from omnidirectional.

I would say that 6 dB should be more than adequate for training sessions 
and 3 dB would be a lofty goal for serious competition.

> If you need gain from your horizontal omnidirectional
> antenna that would be a different design, possibly
> starting from one of the designs for TV transmission
> like a slotted cylinder, or a spiral conductor wrapped
> around a metal cylinder.  But those are probably too
> large for what you have in mind.

Gain comes at the cost of a more severely compressed vertical pattern. 
   This would make exact horizontal orientation even more critical.  For 
ARDF, adequate field strength in the competition area shouldn't require 
antenna gain when uses with transmitters of reasonable power.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
Impulse Internet Service  -  http://www.impulse.net/
Your local telephone and internet company - 805 884-6323 - WB6RDV