[amsat-bb] 5.8ghz Dish, G5500, and receivers...
ko6th.greg at gmail.com
Wed Sep 13 17:23:41 UTC 2017
I think you will have a couple of challenges. But, this could be fun.
The issue with weight on your rotisserie is partly a matter of mass, but
mostly of balance. Find a way to mount the dish so that it doesn't
apply a big torque to the motor. Back when AO-40 was still
transponding, I had a 3' x 2' BBQ grill dish. When I mounted it, I used
the LNB as a counter weight on a shaft behind the dish to offset some of
the torque it applied, being mounted on the side of the cross-boom.
http://home.wavecable.com/~ko6th/Dsc00201.jpg Even my little Alliance
TV rotor (mounted sideways as an Elevation rotor) could move it just
fine. One of the things I did which might have helped was that the
mechanical connection between the rotors and the antennas had some
"give" to them. The idea was that doing so prevented the motor from
stalling when it tried to start moving. No idea if it helped, but that
was the theory. All that said, your dish (one of them) is even
heavier. I would be concerned if you tried this with two unless you can
really balance things.
The other issue could be the size of the dish, compared to the
wavelength you are capable of receiving. From what I remember when
building the AO-40 station, 2.4 ghz needed a dish at least a few feet in
diameter for good performance, and your dish was designed for 5.8 ghz.
If you're only looking below 2 ghz, it might be disappointing. Are you
committed to using the SDRPlay? Something like the HackRF can go to 6
ghz, for example. Lacking that, you're going to need a down-converter.
For AO-40, there were several models of HBO receiver setups that we
re-purposed, for example, my Drake 2880. They naturally receive in the
2.5 ghz -2.7 ghz range, if there's anything interesting left in that
band. They were easy to move to nearby bands by re-crystaling the LO.
Last thought is regarding sensitivity. These SDR devices aren't noted
for being really sensitive, nor very selective (poor front-end
filtering), though some are significantly better than others. If you
can focus on a single RF segment, a narrow band, low noise preamp at the
antenna will go a long ways towards actually hearing something.
Receiving AO-40, with its 10 watt transmitter only 30,000 miles away,
required me to invest in a good preamp at the feed point in order for my
Drake down converter to produce anything audible, and even then it was
right above the noise. Modern SDRs are probably a good bit better than
the Drake / ICOM IC-R7000 receiver lash-up I used, but you're looking
for something potentially a lot weaker.
Then again, this hobby is all about experimenting and seeing what
works. Good luck, and let us know what you find!
Lou Michaels wrote:
> A pair of 25'' 5.8ghz 29dBi Teletronics International dishes fell into my
> lap this week and I'm trying to decide on some projects.
> Here's a pic: https://i.imgur.com/W9oeVSv.jpg
> I have a G5500 rotor from my previous ground station, and I'm thinking of
> doing some solar radio astronomy or transponder chasing.
> Without the covers, the dishes weigh 13.2lbs each. Could the G5500 safely
> handle the load of one or two of these dishes? I'm not considering roof
> mounting, and prefer to do things closer to the ground for the time being.
> Via a SDRPlay, my highest receive capability is 2ghz. Short of changing the
> feed, what might I do or be able to use to bring the frequency down to
> something more manageable or will the dish still feed signals at lower
> I'm not beyond changing the feed, or building transverters, but some
> searching isn't producing more than commercial wifi applications.
> Anyone have any plans, ideas, advice or direction to offer?
> Thanks in advance!
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