Pair of antenna at 90 degrees? (on same access point)

[blapcb]

Hi. I have seen an application working in the 2.4GHz spectrum where the access point had two antennas (I am not sure if it was actually two separate transceivers or just two antennas - I suspect the former) who were mounted at a 90 degree angle to one another (again, this is on the same access point). I was wondering what is the logic behind this. I assume it has to do with something where the orientations of the nodes (i.e. antennas on the nodes) is not known. Can anyone offer an explanation? Thanks.

 

[RF-OM]

Often this is done to mitigate the multipath phenomenon. It is space diversity technique.

Best regards,
RF-OM

 

[blapcb]

Thanks RF-OM. Can you elaborate a little on this? Perhaps provide some terms and keywords that can be Googled? (other than these two).

Basically, are these two distinct transceivers, each with their own antenna? (meaning you actually have to handle two radio interfaces, for example). Or, does this technique imply a single transceiver with two antennas?

Thanks for your explanations

 

[RF-OM]

It may be receiver with two antennas for space diversity or transmitter with two antennas, or transceiver. The main idea is to improve the radio link. Mulipath is phenomenon when signals arrive to the same antenna with different phases due to different path lengths. It may be due to reflections or sometimes other phenomenons. If two antenna used and then signals combined with special way it is possible to mitigate the negative effect and get more stable radio link. It is not easy and requires channel modeling and other techniques. It also can be used to solve the coexistent problems. There are a lot of practical applications for antenna diversity techniques and it is hard to list all of them here. There is probably no one theory that describe all the situations and each practical case requires own analysis and solution. This problem become more and more serious today because a lot of radio devices deployed in close proximity and in adjacent spectra: wireless networking and Bluetooth, cell phones, wireless USB, UWB high speed home networking, satellite TV and so on. Another example is piconet transceiver and moving objects (people, animals, doors, etc) between antennas. This is when we use two antennas for both receiver and transmitter devices.

best regards,
RF-OM

 

[Azulykit]

If the antennas are in an "x" configuration it may be that the designer wanted circular polarization. Orthogonal antennas are one way to create circular polarized signals.

It could also be that they are intending to use orthogonal polarizations to separate two signals. Vertical and horizontal for example.

Of course, it may be a diversity design as discussed above.

Lot's of options (and guesses). Or, did someone just bump one of the antennas out of alignment? I would be inclined to think that might be the case before I looked for something more exotic.