Hi,
I would like to build an amateur radiotelescope for SETI. I know I need at least a 3meter antenna, but they seem to be too expensive.
Would I get the same results by using three 1meter antennas? How would I connect them to make them work as a single one?
Thank you! Regards.
Three 1meter antennas = One 3meter antenna?
Re: Three 1meter antennas = One 3meter antenna?
Solved. 9 1meter antennas
Re: Three 1meter antennas = One 3meter antenna?
In theory it works, but you in order to achieve the same gain you have to align all dishes. Alignement is imho the main obstacle to achieving the desired gain.
Re: Three 1meter antennas = One 3meter antenna?
How have you got on with this project

 Posts: 67
 Joined: Thu Jan 25, 2018 10:48 pm
Re: Three 1meter antennas = One 3meter antenna?
It's not so clear which frequency and which type of antenna did you means.
I googled a little about SETI and probably you're talking about 14201720 MHz.
https://www.seti.org/setiinstitute/pro ... servations
Parabolic antenna gain is:
Gain = 10 * log10( k * ((pi * D/lambda)^2 )
Source: https://www.electronicsnotes.com/artic ... tivity.php
where
k  is the efficiency factor which is generally around 50% to 60%, i.e. 0.5 to 0.6, let's assume it's 0.5
D  is the diameter of the parabolic reflector in meters
For 1420 MHz we have:
3meters antenna gain is 29.98 dBi = 31.5501 times gain for amplitude.
1meter antenna gain is 20.44 dBi = 10.5196 times gain for amplitude
31.5501 / 10.5196 = 2.9992 antennas
Let's check to make sure.
When we have coherent antennas, it's power sum will be:
For coherent sources P1 + P2 = 20 * log10( 10^(P1/20) + 10^(P2/20) )
Lets calculate result gain of three 1meters diameter antennas:
20.44 dBi + 20.44 dBi + 20.44 dBi = 20 * log10( 3 * 10^(20.44/20) ) = 20 * log10( 31.5589 ) = 29.98 dBi
So, we have correct result  one 3meter antenna is equals to three coherent 1meters antennas.
But these antennas needs to be properly directed to the same direction and their phase should be adjusted (to be inphase) before sum of their output. Otherwise your antennas will be incoherent (noncorrelated/not in phase sources). The sum of incoherent signals is different:
For incoherent sources: P1 + P2 = 10 * log10( 10^(P1/10) + 10^(P2/10) )
In your case 20.44 dBi x 3 = 10 * log10( 3 * 10^(20.44/10) ) = 10 * log10( 331.9871 ) = 25.21 dBi
I googled a little about SETI and probably you're talking about 14201720 MHz.
https://www.seti.org/setiinstitute/pro ... servations
Parabolic antenna gain is:
Gain = 10 * log10( k * ((pi * D/lambda)^2 )
Source: https://www.electronicsnotes.com/artic ... tivity.php
where
k  is the efficiency factor which is generally around 50% to 60%, i.e. 0.5 to 0.6, let's assume it's 0.5
D  is the diameter of the parabolic reflector in meters
For 1420 MHz we have:
3meters antenna gain is 29.98 dBi = 31.5501 times gain for amplitude.
1meter antenna gain is 20.44 dBi = 10.5196 times gain for amplitude
31.5501 / 10.5196 = 2.9992 antennas
Let's check to make sure.
When we have coherent antennas, it's power sum will be:
For coherent sources P1 + P2 = 20 * log10( 10^(P1/20) + 10^(P2/20) )
Lets calculate result gain of three 1meters diameter antennas:
20.44 dBi + 20.44 dBi + 20.44 dBi = 20 * log10( 3 * 10^(20.44/20) ) = 20 * log10( 31.5589 ) = 29.98 dBi
So, we have correct result  one 3meter antenna is equals to three coherent 1meters antennas.
But these antennas needs to be properly directed to the same direction and their phase should be adjusted (to be inphase) before sum of their output. Otherwise your antennas will be incoherent (noncorrelated/not in phase sources). The sum of incoherent signals is different:
For incoherent sources: P1 + P2 = 10 * log10( 10^(P1/10) + 10^(P2/10) )
In your case 20.44 dBi x 3 = 10 * log10( 3 * 10^(20.44/10) ) = 10 * log10( 331.9871 ) = 25.21 dBi