Digest Number 602

Dear William,

Thank you for a careful analysis of my posting.

Message: 8
Date: Thu, 17 Apr 2003 03:11:21 +0100

>
>Radio waves are pesky things. Most people think about them as "line of
>sight",
>but that is not the case at all.
>
>Sidelobes and harmonics are everywhere.
>Radio signals diffract around corners and edges,
>

The equation for the angle at the edge is 1.22*wavelength/diameter
(angle is in radians i.e. 2 pi to a circle, diameter is size of disk);
so you might have a problem below a wavelength of about 10m (about 30
Mhz) for a 100 meter diameter disk.

Is this for the primary (1st order) diffraction angle ?

What about sidelobes ?

>getting rid of sidelobes completely, even at large angles, is impossible.
>The best we can do is employ the inverse square law to reduce the
intensity.
>
Up to a point. If you get too far away from the disk, then you hit the
diffracted radiation cones.

Precisely. How far is too far ? On the Moon, this problem simply does
not exist.
The horizon is so far away that sidelobes can be ignored.

>So we want the edge of the shield to be as far away as possible, thousands
>of kilometres if possible.
>
No. Not correct, see above.

I am confused, you agreed that "diffraction cones" are an issue.

>A few hundred metres would give some attenuation, but not as much as behind
>the Moon.
>
No. At a few hundred meters from the edge, reasonably close to the metal
of the disk, you have no field for all frequencies below about 30 Mhz.

What is "reasonably close" ?

>And there is another problem - metals reflect radio waves,
>so when the receiver tries to focus on a source of interest in the sky,
>it will have to deal with secondary reflections of the source from the
>metal shiled behind the receiver.
>
No, the waves never hit the dish at all.

You missed the point.

Multipaction is when rays of different path lengths hit the same detector.

Ray-1 comes from the source, is reflected off the primary reflector on to
the detector.

Ray-2 comes from the source, is reflected off the shield on to the detector.

The two ray paths conflict and cause multipaction.

>Small shields might be a good low cost interim solution for some
>applications, but they are not the best solution.
>
>Terrestrial radio telescope use two methods:
>
>1) terrain shielding, putting the receiver in a valley or crater, e.g.
>Arecibo.
>
They've got lots and lots of problems you don't have in space. The radio
waves are coming in sideways, and diffraction over the edge of the
crater is more of a problem, since the angle necessary to reach the disk
is low.

Indeed.

>On top of the existing comm-sat output, SPS would increase the noise
>environment.
>
Actually, that's not so bad, it's easy to filter away, it's on a well
defined wavelength, and narrow bandwidth.

Yes, SPS is workable. One issue though. is that they are so high power
that detectable

leakage would occur for harmonics and sidelobes at high angles.

Best regards,

Charles R.