On Mon, 22 Oct 2007 10:02:57 -0700, Jim Lux
wrote:

Dave Typinski wrote:
On Sun, 21 Oct 2007 01:01:19 -0400, "Jimmie D"
wrote:

"Dave Typinski" wrote in message
...

Anyone have any ideas about what one might do with a television
receive-only (TVRO) antenna? The thing is 10' diameter and it's mine
for the price of dismantling it and hauling it out of the present
owner's back yard.

I'm thinking radio astronomy. Might be nice to make my own radio map
of the galaxy. I'm guessing that this would work okay somewhere
between 1 and 10 GHz... which means making a feed horn... which is
easy enough to do.

What else could I do with this antenna? Other than covering it in
polyethylene sheet to make a really big bird bath...
--
Dave Typinski
AJ4CO

I know of a couple of guys that took two of them, one for rx and one for tx,
tx was a microwave oven mgnetron. He and his frined talked to each other via
troposcatter. I do know if a maggie operates within a ham band or not but it
didnt matter too much for these guys because they didnt have a ticket
anyway.

Jimmie


I thought about magnetrons... but not for communication. My idea is
to set it up as a radar station to measure the distance to the Moon. A
microwave oven magnetron operates at 2450 MHz, which is right at the
upper edge of the 13cm ham band. Unfortunately, magnetrons produce a
really dirty, wide output. Worse, being right at the edge of the
band, half the RF energy would be out of band.

Just how precise a measurement do you want to make? You could do quite
well using a 100W 2m SSB rig and a decent yagi antenna and some signal
processing software.

See, e.g. http://home.earthlink.net/~n5bf/eme2.html

I don't know if one can re-tune a magnetron by the addition of
external components; I'd like to get the thing down to 2.42 GHz to put
the emission in the center of the 13cm band. There are such things as
tunable magnetrons; but, they aren't found in microwave ovens, which
means they won't likely be had for free.

That said, I'd sure like to look at the output from an oven magnetron
with a spectrum analyzer. Maybe the output isn't as dirty and wide as
I think it is.

it is quite dirty, even more so in an oven because:
1) it's fed by a half wave voltage doubler, so it's pulsed at line frequency
2) the voltage waveform is hardly nice and regulated
3) the temperature of the tube changes during operation


Why not build/buy some small no-tune transverters for 3.4 or 5.7 GHz and
use those. At least then, you can do coherent processing over some
reasonable time span, and what you lose in raw pulse power, you pick up
in processing gain.

A nice idea, thank you.

The beamwidth of your 10ft dish at 2.4 GHz is going to be pretty wide,
so you're illuminating a lot more than the moon (which is wasted power).

In absolute terms, on receive, the fraction of the signal power received
from that power which hits the moon will be the same (same physical
aperture.. gain in dBi is larger, but that's just because an isotrope
has smaller aperture at higher frequencies).

So, your goal should be to illuminate JUST the moon (and a bit more to
cover pointing errors). Beamwidth is approximately = 70*wavelength/d.

So
GHz m deg dBi
2.4 0.125 2.9 35
3.4 0.088 2.0 38
5.7 0.053 1.2 43

Given that you want to light up the moon with the same power, at 2.4 GHz
it will take 8 dB more than 5.7 GHz.

Since the moon is about 1/2 degree wide, even 5.7 GHz is a bit low for a
10 foot reflector.

Thanks a bunch for pointing that out. I'd wondered about the
relationship between beam width, reflector diameter, and frequency.

Now that I think about it, doesn't focal length or focal ratio play a
part as well?

I don't suppose you could point me at a decent reference for this
stuff, could you? I've found that Google (i.e., the web) is somewhat
lacking when it comes to explanations - especially accurate ones - of
where those nifty approximation equations come from.

There's a little more than this to the whole problem, but, in general,
if what you want to do is measure the distance to the moon, a radar
built with an oven magnetron probably isn't the best way to go about
doing it.

So noted. The goal here is not so much to do it the best way
possible, but rather to see what kind of results I can obtain using
the scrap (i.e., free) materials I have available.

If I were after accuracy, I'd point a laser at the corner reflectors
left by the Apollo missions. 8^)
--
Dave Typinski
AJ4CO