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Re: Mode-S Antennas



 K5OE@aol.com writes:

> >  I'm interested to know how everyone plans to
> >  get a signal to it. 

I made a mesh dish a few years ago. Was going to use a DEM converter but
have the Drake now. Feed is a 3-turn helix (G3UH design). G3RUH found several 
items in the shops in the UK which could be used as the dish: woks, 
lampshades, etc. 

For those interested in crafting their own dish heres how I made mine; I had
machining facilities at work at that time.

Its not well publicised but if you make something like a bicycle wheel 
(without the outer piece to hold the tire) and run some strong cable around 
the periphery, then pull the cable tight so the spokes bend upwards, the
bending of the spokes forms a natural parabola (so no math is needed to 
calculate the arc). Thats what I did.

Starting with a fairly thick six inch aluminium disk in the middle, I bought 
the longest bicycle spokes the shop had. Then I drilled and tapped the edge 
of the disk every ten degrees around it and screwed the spokes in (spokes
come with a screw thread on one end). I formed a loop in the outer end of
each spoke and fed strong cable around the periphery (the cable came from
the handbrake of a car). Then I pulled the cable tighter until the inside
depth of the dish was about three inches and secured the cable with the
metallic part of some electrical connection block (we call it chocolate strip 
over here).

To fit the fibreglass pole I have I first cut a clearance hole in the middle 
of the disk then fabricated an extra piece to screw to it. An exhaust clamp
around the outside holds the pole in place.

The Drake goes at the end of the pole; then several right-angle N-type 
connectors (for rigidity) hold the helix in place. The pole is narrower than
the helix and passes through the middle of it. Feedline (I feed power along
this too) goes inside the pole. Final adjustment is to find a good text then
position the helix at the right distance from the dish (I haven't done this
yet) before tightening the exhaust clamp. Done.

The helix is made from the solid centre conductor of some thick coaxial
feedline, rigidised with fibreglass sheet (PC Board material with the
copper removed), and an aluminium disk for its reflector. N connector is
mounted on the disk.

I now have this 70cm (27 inch) dish in the shack waiting for good weather.
Maybe I'm lazy but I haven't used it yet because there weren't good signals
to test it with at the time I built it.

73
Richard W L Limebear G3RWL
g3rwl@amsat.org
FOC # 1188

          So many beautiful girls ..... (sob) so little time
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