Performance of a Combination of Two Different Antennas?

[ferdem]

Hi all.
Suppose we have two different antennas whose frequency bands are next to each other, say first one ise VHF(30-300MHz) and the second is UHF(300-3000MHz). And we have a receiver having the input range of 30 to 3000 MHz.
In this case, how can we utilize these antennas? Do we switch or combine them? Switching is obvious, I wonder what happens if we combine them using an 'ideal' combiner. Actually, I have never seen such a combination but it looks reasonable to me? Two issues arise:

1) Around 300 MHz, the both antennas will receive, we need to watch for array factor, provide constructive superposition for desired direction. This is OK.

2) May one antenna degrade another's performance in its midband where the first antenna does not receive at all? If we have a single antenna covering the whole band, in what metric we can observe its superiority over the combination?


Thank you.

 

[FvM]

I wonder what happens if we combine them using an 'ideal' combiner.

The "ideal combiner" will reduce the received power by 3 dB. A diplexer (high pass/low pass combination) can combine both antenna signals, but can't give optimal performance near the transition frequency. A switch would be the best solution.

If the frequency bands have unused gaps, a diplexer can work well.

 

[Warpspeed]

One potential problem with this is that the phases and amplitudes of the two signals you are combining may not always add. they could just as easily subtract.

So as the positioning and directivity of the individual antennas with respect to the signal source direction may create some dead directions and higher gain directions. Its a bit of pot luck really what is going to happen.

I suppose switching directly between individual antennas, and the output of a combiner will at least provide some choice.

 

[biff44]

I would us singly terminated HP/LP filters to connect the two antennas to the receiver with less than 1 dB loss. At exactly 300 MHz, if you phased the transmission line lengths you could get the two antennas to somewhat combine.

 

[ferdem]

Thank you all for the answers.
Mr.FvM, we do not loss power in combination(not division) direction, I expect less than 1 dB loss there.
The combination works, no doubt. However, I m still curious whether there is a difference in performance (possibly pertaining to noise) between the following two cases.
Case 1: A single 300-3000MHz 3 dBi antenna is used to receive at 2000 MHz.
Case 2: A 30-300MHz 3dBi and 300-3000MHz 3dBi antennas are combined via 'ideal' filters and an 'ideal' combiner. The receiver is tuned to 2000 MHz.

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Edited to add:

"I have noticed my mistake, as you pointed Mr.FvM, even if we have an ideal combiner, there is a 3 dB loss that we cannot avoid"