[ARDF] Does anyone have a good design for a Halo Antenna?
jay at west.net
Mon May 19 14:33:23 PDT 2008
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 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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