Re: [amsat-bb] focal lenght and helix

[Jerry Pixton <jpixton@xxxxxxxxxxx>]

Ed, Al, et al

My comments between. Thanks

Jerry, W6IHG


At 06:41 AM 7/5/01 -0800, Edward R. Cole wrote:
>Jerry,
>
> >From the picture of the SPG-36, it looks like the square feed support can
>be unbolted or the feed unbolted.


Yes, it does unbolt nicely. It will be easy to exchange different antennas 
to test.


>f you can find similar square tubing it
>looks like this would work out for a helix support if it is under 1-inch
>square.


The tubing is 1 inch square OD


>Make the helix from 1/8 inch refrigerator tubing or No. 6 solid
>copper wire and place a short piece of 1-inch ID pvc tubing over the square
>boom to insulate the helix form the boom and solder it to an N-type square
>flange connector mounted on a 3 inch square of aluminum plate.
>Alternately, you could place heat shrink over the square tubing instead of
>using pvc tubing.

  I like the heat shrink idea. The PVC seems a little thick as it would 
take 1.5 ID tubing to fit the 1 inch square - making the inside diameter of 
the helix too large even with thin wall tubing


>The helix is nearly self-supporting.  if you drill a
>hole in the aluminum plate so the square tube can slide thru, you could
>farily easily slide the helix in-out to find the optimum focal point using
>sun noise.  Secure it after that.

My idea was to set up half inch points to fasten to with angle brackets on 
the back side to keep the reflector aligned. It looks like one of these 
male N panel connectors that have two screws and are flat on the sides will 
fit tight up to the tubing and not move the end of the helix too far out of 
line. That way I could have a 3-turn, 4-turn and 5-turn helix to test.

Have you found a way to turn the AGC off in the FT-847 when making sun 
measurements?  Right now I am trying to turn the RF gain down as far as I 
can with the AF gain full to get a measureable signal on cold sky.


>One note, since the reflector is a grill it will only support linear
>polarization.  A dipole feed would work with no lining of the dish.  You
>will need to line the dish for a helix to work right.


It would seem that I could line the dish (as an interim step towards the 
helix) with out destroying the linear feed. Is this true???



>   For AO-40, remember
>that the feed has to be opposite sense to what is wanted [i.e. LHCP feed
>for RHCP from AO-40].
>
>You know the stock feed may work fine if you can get add a preamp.  You
>might want to try that before modifying it.


The problem is the stock feed has 44 inches of RG213 cable. I could problem 
cut that in half and put a new connector on it for the downconverter. But 
that is still a lot of loss before the downconverter. would rather have the 
downconverter out on the strut behind the helix reflector.



>   The log-periodic antenna is
>wide-band and from the description is designed to work from 2100-2700 MHz.
>It also exhibits moderate gain in the direction of the short dish
>dimension.  This may actually give slightly more efficiency and gain than
>using a helix.  This looks very similar [except for the feed itself] to the
>Myers dish my freind, WL7BQM, has.  Using a 1 dB NF Conifer preamp and a
>144 Mhz modified Drake he hears very well.

------------------------------------------
Dr. Jerry R. Pixton, PIXOS Designs LLC
http://www.pixos.com/designs/
jpixton@shentel.net
------------------------------------------

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