one gps antenna two cmm's

has anyonetried to use one GPS antenna to feed morethan one cmm, I will be adding a third cmm and dont want to add another GPS antenna if possible. Curious if anyone has tried this?

3 CMMs at one location? dang!

Do you seriously have 16-24 Canopy units at this location? and why?

Not sure, but I believe the antenna that comes with the CMM is powered. You could use a cap to block the DC from the second CMM and just pass the RF back into it. But the CMM comes with an antenna. Do you have a damaged one?


I have the same question as ahull. Are you deploying three AP clusters on different frequency bands?

I’m considering building a CMM replacement to either drop the cost for timing only or to add features like remote power cycling and defaulting individual APs and BHs. I could also explore support for more than 8 units.

7 - 5.7 AP’s
6 - 5.2 AP’s
3 - 5.2 BH’s
5 - 5.7 BH’s

All on the same building

just really didnt want to mount another antenna, but whatthe heck, Ill do it anyways.

I’m amazed you don’t have problems with channel overlap between APs and BHs, especially with the 5.2s. Kudos.

I’d guess you’re better off mounting another GPS antenna. The signal strength would drop to each receiver on a parallel connection.

No need to re invent the wheel. I installed 1 of these this week and was able to use a real switch instead of the integrated one that comes with the CMM.

I disagree. Reinventing the wheel is the sworn duty of all red-blooded engineers.

Adding Ethernet ports is the trivial part; adding timing ports may or may not be easy. The chip providing the 1pps output has a limit to the load it can drive; a single CMM may or may not be able to reliably drive the timing inputs on more than 8 APs and BHs; FDDI has done this for up to 16 units. A simple solution would be to add another layer of drivers to increase the load capacity. This, however, may add enough delay to the timing signal to make it incompatible in a network with Motorola CMMs.

As for lowering the cost, that’s a goal that drives a lot of people who insist on reinventing the wheel; they call it building a better mousetrap. Why rodent-elimination suggests a more positive image than transportation-refinement is beyond me.

As for adding features, maybe I’m off base, but I’d like a CMM that can do more. Like an NTP server that converts UTC to local time, with automatic adjustments for Daylight Savings. Or distributed BAM-like capabilities. Or a cellular modem for remote access to help locate faults.

If you’re happy with what Motorola – or any manufacturer – gives you, and you aren’t driven to find better solutions, you’re lucky. This kind of stuff keeps me up at night.

I did some experimenting with adding a modified 8 port switch in the CMM micro. This used the PoE as well as timing from one port of the CMM and gave me 14 possible radio connections. Powering 7 AP’s from one CMM port became a problem.

I ended up splitting the new switch in two, powering each half from a single port in the CMM. Giving me 12 possible radio connections. After running that for a while I was worried about reliability and reutrned the CMM to original configuration.

That mini GPS thing could use two receivers and a receive multicoupler (two will not drop the level to much) and use them to drive the 16 ports. I wonder if they have a schematic for it. One could also install a Managed PoE switch near the CMM in a different enclosure (I’ve thought about this)…