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.

Depends on the level of detail and backup you want. There's a EW
Handbook at Point Mugu Naval Air Station, you might be able to find it
with google.

W1GHZs online book is at: http://www.w1ghz.org/antbook/preface.htm





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.

Sure, I understand..

It's just that MO Magnetrons are such ragged edge cost sensitive items
that they spend absolutely nothing on making them good transmitter
components. After all, all they have to do is heat stuff, cheaply.

Even if you have a design from, say, 20-30 years ago, I suspect that
modern tubes aren't going to be as suitable.

I did see a clever design where they took a whole array of MO magnetrons
and injection locked them to build a microwave weed killer. Microwave
Journal, October 2005.

http://www.mwjournal.com/journal/art...p?HH_ID=AR_113

Enjoy...



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

Hmmm.. That's a nontrivial thing.. BUT, you could do (maybe the first)
optical EME contact on the 470 THz band.

Here are two sites that might be of interest to you...

http://www.wa1mba.org/

http://www.w1ghz.org/ and the link here to the online microwave antenna
book provides some good reading.

Scott


Dave Typinski wrote:



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.

"Dave Typinski" wrote in message
...

snip

125 km at 2.4 GHz? How's that work? What were they bouncing the
signal off of? That's definitely further than line of sight.
--
Dave Typinski
AJ4CO

Our local microwave hams like mountaintops. Kansas microwave hams, not so
much.

"Sal"
(California)

And up North, we like rain scatter...

Scott
N0EDV


Sal M. Onella wrote:
"Dave Typinski" wrote in message
...

snip

125 km at 2.4 GHz? How's that work? What were they bouncing the
signal off of? That's definitely further than line of sight.
--
Dave Typinski
AJ4CO


Our local microwave hams like mountaintops. Kansas microwave hams, not so
much.

"Sal"
(California)



--
Scott
http://corbenflyer.tripod.com/
Gotta Fly or Gonna Die
Building RV-4 (Super Slow Build Version)

Just think of the salads that you could make in that giant salad bowl!!! G


Ed, NM2K

What are you expecting for birds? Condors?? The old TVRO dishes were for
2-4 ghz I think. If you used it for 10 ghz work it would have
incredible gain. The surface would have to be extremely smooth as even
minor imperfections woulg be bad for 10 ghz use. I saw an article that a
group used a 10' 1nd a 12' dish to go 125 miles over the desert with
802.11b wireless (wireless computer network if you're not a computer
person like myself). I have a smaller 4'x6' dish I want to mount on my
tower someday so I can screw around on the microwave bands. I live on a
mountaintop in Vermont which makes it easier to get out on the upper
bands that way. Have fun with it and don't stop dreaming of the
possibilities. 73's de KA1LHZ

Dave Typinski wrote:
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...

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.

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.


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.
It's probably cheaper than buying a tape measure long enough to do the
trick... KA1LHZ

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.


Nobody asked me, but fooling around with oven magnetrons is potentially life
threatening, since they are properly shielded only when installed as
designed. What's cookin' might be _you_!

Most in this group know that, so I'm throwing out the caution for the
possible newbie. (There was a rumor years ago about a guy who damaged his
internal organs trying to use a magnetron as the LO for a homebrew TVRO
satellite downconverter. I looked hard and could not find any
sustantiation.)