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Mode-S Antennas



Message text written by "Mark D. Fossum"
>I'm interested to know how everyone plans to
get a signal to it. What is the consensus for a good 2.4ghz antenna?
Helix, dish, yagi? <
-------------

Yes, yes, yes. Since I use all of the above (and variants), here are a few
observations:

- helixes (helices?) never seem to live up their promise of gain. Anything
longer than a dozen turns or  so doesn't get much better. But they are
inherently circular, which counts for something. They are broadband, which
makes them somewhat harder to screw up when building. Make good dish feeds.
Serious downside: The helix is insulated from ground and makes a dandy
static electricity receptor. You should always use a shorted 1/4 wave stub
right at the connector to save the following electronics from normal static
buildup damage. Of course, with a direct lightning strike, all bets are off
no matter what.
Speaking of static, and you from the north country, you wouldn't believe
how much static is generated by dry falling snow!

- small dishes: probably the best bet as far as performance and ease of
feeding. Remember that the dish reverses the hand of polarity, so your feed
must be LHCP so the overall antenna will be RHCP. Myers, et.al. barbeque
grill with linear feeds work fine and are ready to go.

- yagi: long and narrow; easy to mount when there is all kinds of other
stuff on the tower. Note that they are fairly narrow band; a 2304 yagi will
be quite weak on 2401. Available from Down East Microwave
(www.downeastmicrowave.com), who handles antennas from Directive Systems,
Inc.

All this and more in the reference: "Mode S: The Book" available from
AMSAT; $15 for 120 pages of useful stuff.

73,
Ed Krome K9EK
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