If you're building an array of high gain antennas to provide omnidirectional coverage, then you need "receiver voting" wherein the receiver with the strongest signal is automatically selected. Phasing wouldn't normally enter into it (not really worth the incremental improvement).

An alternate approach is a vertical colinear array that will provide gain with omnidirectional coverage. Unless you plan to use a large number of high gain antennas (huge expense), it might be the better solution.

 

If you are talking about different antennas, one for each frequency band, then it is relatively easy to do.  You buy a multiplexer filter that covers each band.  In other words, you might have the following hypothetical spigots on the multiplexer filter:
cable interface
50 MHz to 300 MHz
300 MHz to 700 MHz
700 MHz to 1 GHz
1 Ghz to 2 Ghz
2 Ghz to 2.5 Ghz

You would connect each antenna to its individual connnector, and the signals would all magically appear at the cable interface connector.  The only problem is that at each bandedge (say 700 MHz in the above example), you will have higher than normal insertion loss (maybe 6 dB).

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50 mHz and high gain don't often go together in the same sentence. Are you sure about that?

If each antenna is already high gain, just a 1xN pin diode switch is needed for comparison.

$120 minicircuits 1x4 switch, dc to 3 ghz, http://www.minicircuits.com/pdfs/ZSWA-4-30DR.pdf



 

Usually it's me that misses the OP's concept.

"The array is omnidirectional and will use a series of high-gain unidirectional antennae mounted around an axis facing outwards..."

So I think he's looking for the electronic equivalent of an mechanical antenna rotor (a switch).

But his post raises a brazzilion question.

For example, measuring signal strengths? To what accuracy?

 

To what accuracy? As high as possible. Azimuth (H-plane) radiation pattern should be as omnidirectional as possible.

I was investigating using a discone antenna to obtain uniform  omnidirectionality, however its impedance matching characteristics is poor at lower frequencies (S11 is large).
 
In addition, the vertical E-plane radiation pattern has the tendancy to slope downwards towards the ground (away from 90/270 degrees and towards 180 degress) as the frequency is raised, rendering it nonideal for my objective of measuring signal strengths with minimal reflection off the ground and maximum reception from the horizontal plane (its going to be mounted close to the ground).

My idea was to have several UWB planar spiral antennas mounted around a hexagonal/octagonal 'box' of sorts, with each planar antenna facing a different direction (one on each face of the box), as that would mean i'd maintain a good 'radiation pattern' through the entire frequency range i'm interested in. Furthermore the impedance matching characteristics of spiral antennas such as the self-complimentary archemidan spiral and equilinear spiral is better, and their S11 parameter is more consistent over the desired frequency range.

I should've made it clearer originally.
I have one Rohde & Schwartz EB200 receiver and would ideally like to use that to measure the signals at all of the planar antennae.

As you probably know, calibrated antennas used for this sort of purpose have an Antenna Factor that relates field strength to output. But that normally applies to the boresight, although the data often includes off-axis plots.

You can examine the plots for your proposed antenna choice, and compare your view of accuracy, to determine what value of N antennas would be required using simple switching. But you probably know that already. The answer is probably higher than four (depending on what you mean by "high gain"...)..

If you propose to build an active antenna combiner, then that could help to fill-in the inter-pattern gaps, but max combining gain from 2 antennas never exceeds 3.01dB, so you can estimate a limit of the improvement using nothing more than simple MS-Excel (or insert your favorite tool).

But building or procuring a bespoke active antenna combiner is not insignificant. I've some experience with a narrow band 4 channel UHF device and it cost well into the six figure range.

My recommendation, responding to your original concept of using a circular arrangement of high gain antennas, is to take your allowed azimuth pattern ripple (your error allowance for the antenna) and calculate exactly how many antennas you'll need. It's a three way trade-off of "high gain", versus beamwidth and thus N, and ripple. But you almost certainly know that already.

Except 4 sounds low.

Then use an N-port RF switch controlled by a computer to find the peak reading. Easy project.

"...hexagonal/octagonal 'box' of sort..."

Sorry, I missed the 6 or 8 implied in the above. 8 is a reasonable first guess.


Another thought occurred to me. If you switch through the antennas in sequence, you can probably guesstimate the azimuth to within a few degrees by clever interpolation of the relative signal strength from adjacent antennas (assuming the main lobes are smooth at all frequencies of interest). Then, using that info, lookup the pattern correction. You could even cross check using two antennas.


 

All antennas when electrically small have poor S11, i.e. at low frequencies they all go bad, nature of antennas. So your comment about the discone going bad at low frequencies should not preclude its' use. It's squinty though.  

Discone's and Bicone's are two of the best antennas for ultra wide band 40:1 or more bandwidth. They are Vertical polarization only. You haven't mentioned polarization needs.

You'd need a 60" tall bicone to make the 50 Mhz frequency  efficient. Bicone patterns don't squint, they just narrow at higher frequencies. You'll have a 4 degree minimum elevation beamwidth for a 60" bicone. Although 6 degree is more likely when you build it.
 
 
 

This is for Higgler and is just for fun:

http://www.bigskyspaces.com/w7gj/Welcome_to_6m_EME.htm
6m amateur band is 50-54 MHz
Excuse the off-topic interruption.


 

Unfortunately a 50" high antenna is not feasible for my application. My antenna needs to be compact (will be mounted on a cart), and is to measure the signal intensity for a variety if heights above ground. Hence, small size is a priority. That's the main reason why i though a series of planar antennae would provide compactness, narrow radiation pattern, as well as good broadband responsiveness.

 

Sorry, i meant a 60" high antenna.

 

As per polarisation requirements, the received signal is likely to be vertically polarised, however a circularly polarised antenna like a spiral antenna will allow the provision of horizontally or circularly polarised signals to be recieved as well (in the case of vertical and horizontal polarisation, at a 3dB loss),

If you're really going down to 50 mHz, the wavelength in air is 20 feet, hence you need roughly a 10 to 20 foot antenna to have a narrow beamwidth.
 
-5 dBi gain for a 37" antenna is shown here http://www.ahsystems.com/catalog/SAS-521F-2.php  if you instead use an 18" antenna, you'd have -20 dBi gain at 50 mHz roughly. 9" antenna would be around -40 dBi gain.