Re: Dubus..arrggghh


Hi John

I was one of the stations on 3 cm EME when they put VE3ONT on the air using that big dish.
And yes, they were not the strongest signal on the band.  Cleanest perhaps, but not the strongest.

Imagine the Radar Range Equation, but instead to the Radar beam hitting the aircraft, the beam is so narrow it it only hitting part of the aircraft.    Now instead of 100 sq Meters Radar cross section, you only have 10.    So signals are 10 dB less than you might think.

The other way to look at it.   For 3 cm, a dish in the 5 to 6 Meter class now has all the TX energy hitting the moon.  Having the same energy hitting just a small area of the moon does not increase the amount of energy reflected back to the earth.   (Yes, using just the very center of the moon is a tiny bit closer to the earth, and you have reduced Doppler smear, but signal improvements are no longer linear as antenna gain increases when communicating with a smaller dish.)

On the receive side,the smaller dish is illuminating the entire surface of the moon.  The field strength per area is fairly constant.   Looking at only pieces of that field strength always sees the same signal level.    Again the improvement with a big dish is slight.      Now big dish to big dish is a different matter.    VE3ONT was only using about 1% of the surface of the moon.   Signals from a 2nd very large dish would have been very strong, but only if both dishes were pointing at the same 1% of the moon.    Radar studies of the moon suggest the crater Tyco would make a good common aim point.    Eleven US and Soviet Radar studies of the moon can be downloaded at:     73 Kent WA5VJB

On Thursday, June 3, 2021, 04:09:00 AM CDT, John Fell <john.g0api@...> wrote:

There are advantages on the Moon circuit from the use of narrow beamwidth .

I remember listening to some tests from a group who had access to the Canadian equivalent of Jodrell Bank , back in 1994 era ,
Prior to the event much was made of the potential "mega" signal , but for some reason the signal strength during the tests I heard were of average strength for the era - certainly not the levels of WA7CJO or SM4DHN ., who both put in SSB echoes .
However the thing that stood out was the much reduced multipath signal spreading , which was the result of their smaller reflection area , near T9 CW rather than Ar .

Anyone doing EME on the upper Microwave bands should go through the craft learning route of a "small" dish and build/optimise for best performance .Then use the skills with the largest dish form that is practical .

If you get to SSB echoes at any phase of the Moon the fun really starts .....1000Hrs of work per QSO is probably not too far off the mark .


On Wed, 2 Jun 2021 at 23:04, Barry VE4MA <ve4ma@...> wrote:
My philosophy has always been to make a smaller dish work to the you say John there  has always been lots of big underperforming dishes on EME.  Although I think there are fewer of those around now than 30 years ago !

Best 73
Barry VE4MA

Join to automatically receive all group messages.