## Three 1-meter antennas = One 3-meter antenna?

Discuss commercial and home made antennas.
alpha015
Posts: 5
Joined: Tue Mar 20, 2018 7:56 pm

### Three 1-meter antennas = One 3-meter antenna?

Hi,

I would like to build an amateur radiotelescope for SETI. I know I need at least a 3-meter antenna, but they seem to be too expensive.

Would I get the same results by using three 1-meter antennas? How would I connect them to make them work as a single one?

Thank you! Regards.

alpha015
Posts: 5
Joined: Tue Mar 20, 2018 7:56 pm

### Re: Three 1-meter antennas = One 3-meter antenna?

Solved. 9 1-meter antennas

snn47
Posts: 179
Joined: Tue Dec 27, 2016 11:00 pm

### Re: Three 1-meter antennas = One 3-meter 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.

Dotty1
Posts: 4
Joined: Fri Jun 07, 2019 7:25 pm

### Re: Three 1-meter antennas = One 3-meter antenna?

How have you got on with this project

Posts: 48
Joined: Thu Jan 25, 2018 10:48 pm

### Re: Three 1-meter antennas = One 3-meter antenna?

It's not so clear which frequency and which type of antenna did you means.

https://www.seti.org/seti-institute/pro ... servations

Parabolic antenna gain is:
Gain = 10 * log10( k * ((pi * D/lambda)^2 )

Source: https://www.electronics-notes.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:
3-meters antenna gain is 29.98 dBi = 31.5501 times gain for amplitude.
1-meter 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 1-meters 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 3-meter antenna is equals to three coherent 1-meters 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 (non-correlated/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