[Moon-net] 2.40m dish

Paul Wade W1GHZ q.w1ghz at verizon.net
Mon Sep 29 03:07:56 CEST 2008


think for a moment - why should loss travelling through space vary with 
frequency?

it doesn't!  it's just that pathloss is defined between isotropic antennas, 
so aperture
decreases with frequency.  for a constant aperture area, pathloss is also 
constant
with frequency.  so if a 10 foot dish works at 1296, it will also work at 
10 GHz, if
you have the same NF and power (which might be a problem).

73
paul

At 02:48 AM 9/27/2008, Edward Cole wrote:
>For what its worth, I wonder if any out there have looked at the eme 
>pathlink as frequency increases?  Pathloss rises by the square of the 
>frequency and so does dish gain by square of diameter.  So a ten foot (3m) 
>dish should work as well at 10-GHz as it does at 1296.  From what I have 
>gleaned from reading, ten foot is the minimum "standard" for CW.

>Opting for an 8-foot dish (2.4m) reduces gain by 20Log (8/10) or ~ 1.9 dB 
>down from a 10-foot dish.  That being said one would expect the same 
>performance if sky noise, preamp NF were the same.  But it seems that the 
>smaller dish does just a bit better with higher frequencies.  Could this 
>be due to slightly lower sky temperature?  Or is it because the Moon is 
>better illuminated by the smaller beamwidth?
>
>Ironically, today my wife tells me that there is a 14-foot mesh TVRO dish 
>available in our neighborhood for the taking.  Since I have two 1.8m, a 
>2.4m, and a 4.9m dish, I certainly need another one ;-)  I will probably 
>retrieve the 14-foot dish if only to hold for some future eme'r or amateur 
>radio astronomer.
>
>Re the 4.9m dish:  We are fabricating and welding the az-el mount 
>tomorrow.  This mean we should have the dish installed in about two 
>weeks.  Give me two more weeks for installing feed, running cables and 60w 
>PA at the dish.
>
>73, Ed - KL7UW




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