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

[Jay Hennigan]

Matthew Robbins wrote:

> Jay---Thanks for the good advice.  I was thinking the gamma match
> might be lopsided, and from what I've read, the T-match can be a huge
> hassle to build.  Would a hairpin be similar in size to a tape-measure
> beam? I'd assume the answer is "Of course", but I've assumed before...

Of course!  :-)  But the simple tape-measure hairpin wasn't designed for 
transmitting, so getting the impedance close may take some tweaking. 
The T-match is essentially a pair of gamma matches with a balun. 
Advantage is that it's balanced to the antenna, disadvantage is that you 
need the balun to make it unbalanced to match a co-ax feed.  Of course 
if you build the transmitter into the middle of it and link-couple to a 
balanced feed, all is good.

The goal is that the antenna, feedline, and match don't distort the 
pattern.  IMHO, perfect SWR is a secondary consideration.  You don't 
want a *horrid* match which would result in a lot of (mostly vertical) 
feedline radiation, but anything better than 2:1 is probably going to be 
OK.  A bead balun close to the feed is probably a good idea in any case, 
and isn't going to hurt anything.

> Another question---I was looking at the ARRL Antenna Handbook last
> night.  There is a page (in my edition) that shows three ways to have
> a certain OSCAR link (Rx or Tx I don't remember).  Two of the options
> were a turnstile and a big loop---not a Halo.  What threw me was that
> the feedlines specified were not what I expected. They didn't always
> use 50-ohm (52-ohm?) feedline back to the radio.  For example, for the
> turnstile, they had the 75-ohm phasing harness, a 52-ohm 1/4 wave
> matching section, and a 75-ohm feedline---If I remember correctly.

The fancy matching harness may have been an attempt to get a better 
match to the native 36-ohm turnstile impedance.  In my opinion not worth 
the bother.

The turnstile is an imperfect match from the get-go.  It is essentially 
a pair of 72-ohm dipoles in parallel, so native impedance is 36 ohms. 
For the power levels we're talking in ARDF, that's close enough to 50 
ohms.  SWR is around 1.35:1.  You can tweak the dipole lengths on a 
turnstile to get a lower SWR, close to 1:1.   However, doing so will 
result in a distorted radiation pattern.

--
Jay Hennigan - CCIE #7880 - Network Engineering - jay at impulse.net
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