Ethernet in an RF Environment

so….   an interesting development.  The regional command vehicle that I help to support has been chasing down some network gremlins ever since it went to a heavily networked configuration.  We’ve got 3 large switches – one in the front, one in the middle, and one in the back of the rig.  These switches all connect (ultimately) to the main router, and all equipment (computers, clocks, printers, radio equipment, display equipment…) connects to the switches.  Miles of ethernet cables to have interesting things happen.

At one point, a number of upgrades were done, and new switches were installed – these new switches were Fiber capable.  I had asked previously about using fiber between the switches, mostly because of network load, but the throughput between the gigabit switches was not limited enough to cause problems.  That being said, a more recent upgrade DID switch to fiber – for a number of reasons.  Amazingly, the network gremlins completely disappeared.

One of the other comms guys who is more or less guiding the upgrades brought something to mind that he thought might be related:  baudrate.  While it is true that gigabit ethernet can move 1 Gb/s, and similarly, older network configurations move 100 Mb/s, this does not mean that gigabit ethernet has higher frequency content on the lines!  Both use 125 Mb/s symbol rates – meaning the data on both is around 125 MHz.  The difference is that Gigabit uses multiple pairs of the ethernet cable, while 100Mb/s ethernet only uses a single pair.

Since our rig runs both VHF and 800 MHz trunked radio systems, there is a high possibility that we are seeing either RF energy near the 125 MHz operating frequency (the sytem has frequencies throughout the 136-174 MHz band), or harmonics from the 7/800 MHz system affecting the 125 MHz system.  When the fiber got introduced, the long cables between routers that may have been acting as antennas disappeared, thus eliminating the problem.  

just one more thing to consider when planning a large scale installation.

Leave a Reply