Dual Antnenna Installation Configuration

[FrischEng]

I have a telemetry communication application that requires 2 antennas to be mounted on the same mast. The radio modems are low power (1 watt) unlicensed units operating in the 902 ~ 928MHz frequency range. I am looking for some guidelines as to the required vertical and horizontal separation required. I am also concerned about interference by the grounded mast on the lower antenna.

 

[VEBill]

Rigorously calculating the required separation would require a lot of information that is likely not available to you. For example, the detailed performance characteristics of the receivers and transmitters (some are better than others). So you're probably left with doing the best you can and trying it.

I believe that vertical separation tends to provide more isolation than horizontal separation (assuming vertical antennas). Stand by for confirmation from others on this point.

If you offset (in the usual manner using a horizontal bar) the lower antenna from the mast by one wavelength (only about a foot, pretty easy), you probably won't see much impact on the pattern from a practical point of view. Since one foot is so short, maybe try two or three feet instead. A three foot horizontal bar shouldn't be mechanically difficult.

 

[Mike0008]

Are the separate antennas for RX diversity or independent TX/RX paths?

Vertical separation of a vertically polarized antenna gives the best isolation bang for the buck, but you have to balance that against the height required to maintain your desired link budget.

When side mounting an antenna, tower shadowing can be a significant issue for a omni-directional antenna, far less so for directional antennas. Making sure your lower antenna is mounted on the side of the tower facing your coverage area and using a side arm to mount the antenna at least 3X the wavelength from the tower face will help.

If your devices are stationary, use directional antennas if possible. Directional antennas will 1) improve your link budget 2) reduce interfere to you caused by other users (and your interference on them!) and 3) greatly reduce the impacts of tower shadowing.

//m

 

[Higgler]

do you know what isolation is needed? Isolation in a dB value that is.

kch