Combining UHF antennas

[W6DSR]

I want to combine two identical UHF antennas with a 3 dB combiner, and then feed that signal to a very low noise amplifier for an improvement in SNR. I am also considering using two matched very low noise amplifiers (close phase characteristics, and gain matched), one for each antenna, and then combining the amplifier outputs with the same combiner. I've been told that using two amplifiers and then combining signals will not yield an improvement in SNR vs combining the antennas before a single amplifier. Could someone explain why this is true?
Thanks,
-Doug, W6DSR

 

[G4BCH]

If the combiner is lossless then it makes no difference. The overall noise figure of the system will be limited by the combiner and cable loss in front of the LNA. If the LNA is mounted at the feed point of the antenna then the loss of the feeder and combiner will be eliminated. whether this makes any practical difference or not will depend on the antenna noise temperature. I.e pointing at the horizon it probably is not worth worrying about, pointing into space maybe. You'll have to do the calculation to be sure.

 

[FvM]

Noise power will be doubled with two amplifiers.

 

[W6DSR]

But, isn't the noise of the two amplifiers uncorrelated? I thought I would gain a root-of-2 improvement in the SNR by configuring them with each amplifier having its own antenna. I can see that there is basically no improvement in SNR with each antenna feeding its own separate amplifier vs combining the antennas first, because in the latter case, the noise will remain the same, but the signal to the amplifier input will be 3 dB higher.

What I'm trying to accomplish is to use multiple LNAs to get an improved noise temperature than each of the LNAs independently. Maybe that idea is not possible...

 

[G4BCH]

But, isn't the noise of the two amplifiers uncorrelated?

Yes, it should be assuming there is no common HF noise introduced into the amplifiers. well designed amplifier will be OK in this respect.
Using separate amplifiers is common practice in any array that needs beam steering RADAR, SOANR, and cellular phone base stations. The combiner loss is after the amplifier and phase adjustment can be added if required.
You also get a boost in dynamic range as each amplifier is only handling the signal power from one element of the array rather than array total. for a high gain array this can be significant.
Don't under estimate the potential difficulty of getting a good phase match in the system, that is why phase shifters are often used after the amplifiers.

 

[FvM]

But, isn't the noise of the two amplifiers uncorrelated?

Right, I see my error of reasoning. Power combiner, e.g. Wilkinson works as lossless combiner only for correlated signals, uncorrelated signals are attenuated by 3 dB. Thus output noise power is ideally the same for both configurations, as stated in post #2.

 

[dick_freebird]

Phase matters. At UHF that can be a significant thing in
not much distance-skew (a full cycle being between 4"
and 3' roughly, for the 3GHz - 300MHz band). Think about
the high end, antennae 2" apart might be antiphase and
sum to zero (our other contestants get to keep the noise
as a lovely parting gift).

Multipath can grossly affect incident phase too, and variably-
in-time and -orientation. You might be better off switching to
whichever antenna gives best, than summing "best" and
"worst" without thinking.

 

[c74236228]

when two amplifiers are combined,the amplifier phase is always the case,the gain of the combined amplifier is unchanged,the output power of the amplifier is changed,the output power of the amplifier is inceased by 3dB,and the noise of the amplifier is superimposed.You now have circuit amplifiers that receive signals and improve SNR,andSNR is related to noise, not output power.This type of circuit is usually used in transmitting signal circuits.

--- Updated Jul 21, 2023 ---

For the antenna,as long as the antenna phase is in the same phase.the gain of the antenna generally is increased by 3dB to change the SNR,I think the circuit to impore the SNR is to combine the two antennas and then add a low noise amplifier,which can improve the SNR

 

[W6DSR]

Much useful information - thanks for all the replies.