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mode-s helices: Network Analysis

I've finally gotten around to hooking a few 2400 MHz helices
I've built up to the network analyzer here at work.

Anyway, I tried three types:

1. K9EK/G3RUH type 1.5 turn LHCP "dish feed" with a brass strip
match.  (Actually Ed K9EK recommends more turns)

2. 33-Turn RHCP "grounded-feed" (grounded, tapped transmission line

3. 14-Turn LHCP "Resonant Ring Reflector" antenna (AF9Y/K5OE) type.

The Type one showed an okay match initally, 1.6:1 at 2400 MHz.
tweaking (trimming the final turn a little, bending the brass strip closer 
to the ground plane and playing with the shape of the brass strip) got a 
1.19:1 match at best.  The return loss tuned was -21.432 dB.  Assuming the 
dish doesn't de-tune it much, this feed should
now be in good shape.

The second antenna (I'll credit the original author as soon as I
find his page again) didn't start off well at 3.5:1 SWR, but I was
able to tune it to a 1.18:1 SWR by moving the tap point a bit,
lengthing the "shorted transmission line" and increasing the spacing
of the line from a 1/8" to 1/2".  This one would be harder to
reproduce than the first one, but would be good if you have test
equipment available for tuning.

The K5OE/AF9Y design (scaled to 2400 MHz) was the big disappointment
of the bunch.  The initial SWR was 4.2:1.  After tuning (by moving the
ring reflector forward almost a 1/2") I did get a 1.4:1 SWR, until
I put RTV on it to hold it.  Apparently the resonance of the ring
is very critical and the dielectric constant of the RTV threw it
off quite a bit.  I was able to finally get a 2.5:1 SWR with the
RTV in place-- which might be okay for receiving mode-S still.

It seems that for reproducability, the "traditional" brass shimmed
"periphery fed" antenna is the winner of the bunch.  Both the
second and third antenna was built on 1" PVC (thinwall) with
12 AWG THHN insulated wire.  The PVC form seems to have little
effect on matching.  So the most "reproducable" design is the "Mode-S
the Book" design or the G3RUH design on the amsat.org pages...
Not too suprizing...

I'll try to scan in the plots from the network analyzer, take some
picture of the test antennas, and write up a more detailed analysis--
maybe figuring out a better explanation (and solution) for the
"ring reflector" type antenna in the next week or so.

Figured I'd pass along this info to the group however.. hopefully
it won't be long before we will be needing mode-s antennas.


Fred Spinner, W0FMS

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