Re: [amsat-bb] WTB: AO-40 BBQ Dish

[Jim.Walls@xxxxxxx]

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.


73
-----------------------
Jim Walls - K6CCC
k6ccc@amsat.org
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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