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Re: dish gain for AO-40

Hello James.

jsharp@psychoses.org writes:
> My downlink dish is one of K5GNA's 2 foot dishes with a modified AIDC3733
>  down converter...unfortunately, as it stands, its just not enough to pull
>  anything but an S3 beacon level and only the strongest QSOs out of the
>  mud.  My rig is an FT-736 at the end of 20 feet of RG-6 quad shield.

I suspect your results are typical.  When conditions are optimal (try a 
Western pass when the bird is under 50,000 km and signals are LOUD), you 
should be able to do a little better, and can probably work CW anytime 
without a problem if you can hear the beacon 3 S-units out of the noise.  SSB 
is a "whole nutter' story."  Had the S1 transponder stayed working, your 
setup would have been perfectly fine, but the reality of AO-40 today is you 
need a little more gain than that 2' system provides.
>  Any recommendations on what I can do to kick things up a notch, so to
>  speak?

Yes!  After reading your query this morning, I decided to go ahead and try 
something I have been thinking about for a while.  I have one of those 2' 
antennas sitting in my garage so I cobbled together a "quickie" test of my 
idea.  If I can get together with Bob over the Christmas break, maybe I can 
do one up right and document it all for the all the others out there with 
these 2' systems.

Here's the "short" version (picture link at bottom) to get an honest 5 dB 
gain with about $20 investment and 2 or 3 hours of work:

1.  Using some #10 AWG copper or alum wire, make a circle 83-1/2" long and 
connect it at each corner of the dish using some small gauge wire to "tie" it 
to the corners.  You now have a 26-1/2" (67 cm) round dish outline.  Place 
the dish upside down on the floor and make sure the wire loop is flat, then 
add a few "ribs" to keep it straight.

2.  Using aluminum window screen, cover the "inside" surface of the dish.  
Starting in the middle and working your way out, use small gauge wire to 
weave the screen onto the ribs of the antenna.  Using several smaller pieces 
of screen is easier than doing it with one large piece.  This will make a 
nice, neat, and lightweight package when done.

3.  If you don't have the N-connector adapter for the 3733, order one from 
Bob, K5GNA.  In fact, get two.  Install one on the 3733 in lieu of the dipole 

4.  Take an 11 oz coffee can and cut it at 1-1/2" from the bottom with a hack 
saw.  You now have a "cup" 4" in diameter and 1-1/2" deep.  Pound the bottom 
as flat as you can with a hammer and a flat piece of wood.  Paint it.  Paint 
it again.  

5.  Make a helix from #10 Cu or Al wire 4-1/4 turns long, per the instruction 
in the MMDS article on my webpage:  http://members.aol.com/k5oe
and solder it to the second N-connector bolted thru the back of the cup.  
Center the helix in the cup and wind it for Left Hand Circular Polarization 
(look at my pictures). 

6.  Get a 2' piece of 1" angle steel at the hardware store.  This is often 
available with a bunch of holes already in it, making it lightweight and 
strong.  Bend the end 2" so it can be bolted through the back of the dish and 
can support both the cup+helix and the 3733.  Using a double-male N-coupling, 
connect the 3733 to the feed and cut off the excessive length.

7.  Measure 11" to the back of the cup and bolt the cup to the angle.  If you 
got lucky, there were already holes in the angle at the right location :-)

8.  Put it on the air.  The original antenna had 19.9 dBi gain.  This new 
antenna has 21.5 dBi gain, plus count an additional 2.5 dB for being 
circularly polarized.  A gain of 5 dB!  The new antenna has a focus of 
10.33", giving it a nice 0.4 f/D ratio.

Here is a picture of my quickly-cobbled-together setup.  I used a DEM preamp 
behind the cup, but you can still get the idea.  Just bolt the 3733 where I 
have the preamp.  You could probably run the 75 Ohm cable under the cup and 
through the center hole of the dish and avoid the unsightly cable mess I have.


I had this antenna on AO-40 this afternoon and made several contacts with it. 
 Using AO-40Rcv, I confirm an increase in signal strength of the beacon, but 
cannot really quantify the gain from that observation.
Jerry, K5OE
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