Without using triangulation, how might one broadcast a signal that would only be picked up by one antenna out of the many present? Or, if not one antenna, then perhaps just a few close by. These antenna are not stationary. We can use any frequency that we want, although a higher frequency would be best. Each antenna has a slightly different Q, length, and width.
The simplest answer is to use a very directional antenna. Pay attention to specifications like side lobe levels since side lobes point in much different directions than the main beam.
The next level of sophistication is not the antenna, but using some sort of spread spectrum coding scheme. SS signals are very difficult to detect as they can be well below the noise level. Correlation is used to integrate energy over a long enough period of time to bring the signal above the noise, but you have to know the PN sequence to do it.
In addition, pay attention to antenna tracking, in case of mobility.
You can also code the signal, or if this is not a problem, use a prefix to address the intended antenna.
Using directional antenna for one receiver would make it large antenna to concentrate the radiation in small angle making it very directive. If one could make a pencil like beam this would be a very suitable solution. It will also very power efficient. The ability of the designing antennas with high directivity increases with the increase of the frequency. If the use of optical communication system is allowed, then one can use laser beams. The use of MIMO system can be used to focus the radiation on an intended direction and null it in other specific radiation.
A second solution is to use a coded transmissions. such those used in the code division multiple accessing. The one which is very simple is to encrypt the data transmitted such that it can be only decrypted by the one who has the key.
Best wishes
The frequency you use will help determine your answer. At high frequencies (VHF and UHF) you can use multiple element yagi antennas, including two or four in a phased array.
Beams can be used for high frequencies (HF) but they are not as directional
At medium wave frequencies, you might need to use phased verticals, which would be harder to tune to a highly directional pattern.
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