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My 3.2m EME dish
project
Par/by:
Bertrand
Zauhar,
VE2ZAZ |
last updated :
16/11/2009
For this project, I
rely a lot on the experience of others with dish antennas. What they
share on the web is very valuable. So I thought I would do the same to
help others the same way.
Owning a large parabolic dish
antenna
has always been the ultimate goal for me. EME (moonbounce) has given
me the excuse to proceed! Besides, two years of EME on 432 MHz has made
me realize that my QRP setup would not allow me to work tons of
stations. I work more or less the same stations in rotation. And I am
not willing to grow my 4 x 4.3 wavelength yagi array for several
practical reasons. The other consideration is that there are many more
stations on 1.2GHz EME than on 432MHz. And I need a new project to work
on!
A 10.5-foot (3.2 m) dish will provide a better performance on
1296 MHz (10+dB sun noise vs. 6dB wih my 432 array) and an even better
performance on 2304 MHz.
I will spare you the details about negociation with my wife, the
swimming pool, neighbors and so on. The bottom line is that thing are
progressing faster than expected!
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Fall 2008
My friend Robert - VE2ASL calls me up to
inform me that he spotted a 3.2 m all-aluminum TVRO mesh dish. It lies
on
the ground in good shape and is available for 50$. It is the dish only,
no hardware, no mount, no feed; just the dish. I say yes! This marks
the
beginning of my adventure...
So my friend brought it
back to his place on a boat trailer during the winter season.
My first contact with the dish was through this picture to the right.
It would have to do for now...
This
project was supposed to be a long term one. I had agreed with my wife
to wait until our old above-ground swimming pool wears out beyond
repair before
installing the dish. This could be a few years...
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Summer
2009
I take posession of the dish and bring it home on our camping pop-up
trailer. The dish mesh is in pretty good shape. The
screws that hold the mesh in place are rusted though. I contemplate
replacing them with stainless steel hardware.
On another
front, I have the opportunity to purchase two more TVRO
dishes! They are a 2.7 m dish and a 3 m dish, both complete with all
the hardware, the actuators, the feeds and mounts. Even the receivers
and the cabling come with it... 75$ take them home. I don't really need
the dishes,
but I can certainly use the mounts and the actuators! Now I have what I
need to get going, at least I have a polar mount I can adapt for EME
use and a linear actuator to steer the dish.
Upon inspection, I notice that one of the linear actuators took water.
I completely disassemble it dry it out and lubricate the gears and
motor. In the end, it was a good learning exercise to understand how it
is built and controlled.
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September
2009
The multi year wait is turning into an active
project. It occurs that there is a housing development just getting
going right behing our backyard fence. So with my wife's approval, I
decide to quickly erect one of the dishes (non-functional for now) as a
way to "mark my territory". The chosen location for the dish is not
ideal for west-pointing EME, but this is a temporary location (for a
few years, until the swimming pool disappears).
One critical decision before I start is to select the type of moon
tracking to implement. I elected to use polar tracking for my EME
operation. Implementing
a
Azimuth/Elevation mount requires heavy duty sprokets, chains, motors,
bearings and lots of welding. Since I am an illiterate in welding, this choice will save
me a lot of pain and money in the future. Besides, the ice and snow
here in Canada complicate everything even more if 12-month operation is
desired. Polar tracking will be much easier to implement, but will have
limitations and will require manual interventions, like changing the
actuator position to cover more of the moonrise or the moonset. I am
willing to deal with this for now.
The first step is to get a pipe to mount the dish on. A quick visit to
a recycling yard yields a 10 foot (3 m), 2 3/8 inch (6 cm) OD, thick
wall steel pipe. This is not as big of a pipe as I would have liked,
but it will work in my environment (houses that break the wind, not
living in a windy area). So I dig a 42 inch (1.1 m) deep, 15 x 15 inch (38 x 38
cm) square hole, a long and painful job since we live in on rocky land.
I then pour 8 cubic feet of concrete and also fill the pipe with the
mix.
After a week to
let the concrete set, I install one of the 10 foot (3 m) dishes with
the un-modified mount and without feedpoint or actuator. This is not
the dish that I will be using for EME; it is merely a placeholder. I
will work on the real dish this upcoming winter.
After
installation, I noticed that the pipe bends a little bit from vertical
due to the dish weight. It is not much for now, but once a feed is
installed, it will become more significant. I may decide to add a
counter-weight or guy wires. I will adapt when I get there.
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November
2009
My
decision
to install a temporary dish to "mark my territory" was a good
one. Two houses are currently under construction behind my backyard
fence! Close call...
I
have completed the
replacement of the 500 rusted screws that hold the dish mesh in place.
I
have stainless steel screws in place now. Oh what a job! Regular steel
and aluminum do not get along well... It took me about 5 hours of work
divided down into ten half-hour sessions to complete the job.
I
have also started to put
together the VE4MA feed. For the cylinder cavity, I used a Tx-Rx
Systems VHF duplexer cavity. Cheap and rugged (0.100 inch wall!). I cut
off the welded top end cap; that yielded a nice 6.4-inch diameter,
24-inch
long cylinder. I kept the removable bottom end cap, but added more
screws to fasten it.
I drilled the N connectors and polarization screw holes as per VE4MA's drawings.
Note that, as
recommended by VE4MA himself, I
did not implement the nulling post. It apparently provides little
improvement in TX-RX port isolation. The result is what you see on the
right hand side pictures.
The initial testing of the feed without scalar ring and pointing to the
zenith gives promising results, with better than -20dB of return loss
on both port at 1336MHz. Port isolation is better than 28 dB. The peak
performance should normally show up at 1296MHz though, so there will be
a need for some tweaking. Axial
ratio (circularity) will be measured later.
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The
list of things still to do is long:
- Build a PHEMT LNA
- Build a 200-300W Power Amplifier
- Design changes to the polar mount
- Design and construct the feedpoint arms
- Design the Actuator controller
- ...
Stay
tuned!
Bert, VE2ZAZ
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