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Re: WTB: AO-40 BBQ Dish

Jon Ogden asked:
> What a great way you mounted it!  I was trying to come up with a way
> to do mine and since I have all the mounting hardware, I think I'll
> follow your lead!  A couple of questions if I may:

> 1.) How does have the dish to slightly one side of the rotor assembly
> "balance" things?  Did you pick a spot where it and the UHF antenna
> would balance out the 2m antenna?

It's as close to center as I could make it, and the S-band dish is on the
same side as the 430 antenna which is much smaller than the 2M, so it's at
least close.

> 2.) I noticed no counter-weights arms.  Did you not need these?

There aren't any, but there really should be.  The 2M and 430 antennas are
a little back end heavy which is over compensated by the forward heavy
S-band dish.  One of these days, I will properly balance it.

> 3.) Your helix is about 2.5 turns.  I've seen references to 5.  What's
> the best to use?

I antenna range tested the dish and helix starting at about 7 turns (if I
remember right) and cut it down 1/4 turn at a time all the way to only 1/4
turn, and the 2 3/4 turns came up with max gain as mounted on my dish.  The
testing was posted here on AMSAT-BB a year or so ago.

> 4.) Does mounting the ground place for the helix place the phase
> center of the helix at the focal point?  I've been trying to figure
> out where exactly to mount the helix (mind you, I've not spent a lot
> of time on this yet).

I tested this when I range tested it.  I was quite surprized how
non-critical it was.  Where I ended up with the helix was as much for
physical convenience as anything else.  It's not far enough off that it was
worth the effort to change the design.

Jim Walls - K6CCC
Mobile Radio Operations
Southern California Edison Co.
Ofc:   626-302-8515   -   PAX   28-515
FAX:   626-302-7501   -   PAX   27-501

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