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focal lenght and helix


My questions for the Holiday -

Jerry wrote -
The big question is where to place the focus--where is the phase center? If
you are using the stock dipole feed, I would recommend locating the focus at
half the distance between the dipole and the reflector (as a guess).
Jerry, K5OE

The spec sheet on my Philips Tech Parabolic Grid Antenna say -

Focal length 15 inches
Efficiency 50% Typical
Beamwidth 10 - 13 degrees
Dimensions 32 x 27 inches
F/D Ratio 0.46 to 0.56

The current dipole asssemby sits out about 15 inches but is several inches 
long (have not looked inside yet, don't want to distroy an operational 
station before I make my first AO-40 contact, hi). The feed is described as 
"Quasi Log Periodic End Fire Array Feedhorn", Humidity Leak tested?????

So I think that the "focus" and the "focal length" are the same 
thing.  Does this mean that I should design a strut to place the helix some 
30 inches away from the dish. This seems wrong to me. Can some one explain? 
I was going to put the reflector behind the helix at 16 inches and was 
going to start with a 5 turn helix.

Jerry, W6IHG
Dr. Jerry R. Pixton, PIXOS Designs LLC

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