PtP 550 and Radio Station Transmitters

Hi all, I have a PTP 550 on top of a radio station and every time their backup transmitters fire up the ethernet port on the 550 and the mikrotik router settle on 100mbps. Once the transmitters shut off the negotiation remains at 100mbps but after I reboot the 550 or the router, it begins to negotiated at 1000mbps. Shielded cable and ends being used. Wondering if I can no longer coexist in this locaton?

I’ve had a bit of experience with co-location at TV and FM broadcast sites. You have 2 choices to remain on site:

  1. Lower your 550 as low as you can go on the tower - ideally 30m or more below the lowest broadcast antenna element) to make the ethernet cable as short as possible.
  2. Run solid conduit up the tower and run the ethernet inside of the conduit. Ground the condiut at the top, every 30m on the way down, and at the bottom of the tower before you turn off the tower into your cabinet or building.

You may also have to disable auto-negotiation on both your 'tik and your 550. The power influence of the broadcast transmitters on the ethernet is significant, as you have discovered. I’ve worked on sites in the past where I could not provide enough isolation for the twisted pair cable, and if converting to fiber is not an option, you may have to move off this site.

Dave Clelland

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Is this the same PTP550 that you were talking about in this other post?

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As DaveClelland has pointed out a double shield may be required depending on how the broadcaster has their equipment configured. Usually if there is a backup transmitter it is coupled to the main antenna and is configured to their frequency. Some are same power but most are usually 1/10th power (the main TX radios are expensive! so backups are usually a lower power unit that is just there incase the main goes down). Very rarely will you see a twin system with separate feeders and antennas, but they do sow up where things have been added or taken over.

This aside, it is best practice to keep your cables as far way from the feeders as possible and enclosing your cable in a conduit for passing the broadcast antennas and baluns. The PTP series radios use standard thread sizes and can have conduit all the way to the port if needed. I have used a LB pull box right off a PTP600 before and a flex conduit to go from the LB to the tower into an LPU (a version that has a removable case lid). This was on a tower that was shared with a 10KW FM station. just make sure you use pipe dope or silicone seal to seal the threads as water tends to get in without it.

There are some others out there that have used ferrite toroids to mitigate this issue with some success but I have found them to be limited use as you need to calculate the ferrite needed from your signal (125Mbaud for 100M/1G), the FM rejection frequency and the amount of power to be captured. This can lead to some very hard to find ferrites and each needs to be hand wrapped and to ensure a tight and even spacing around the toroid. Personally, its easier and usually more cost effective to run a metal conduit from your control box to the radio up the farthest leg from the feeders and to make sure you use a ice shield and place one 1.3m below your radio if your above the FM antenna or 2m above if your below the FM antenna there is a good reason for the discrepancy in placements. If its below your radio, you can stand on it if you get one that can handle it else place it 2m away. The shield needs to be 2.5 times wider and 1.75 times as deep as the FM TX frequency period and properly bonded to the tower and the ground cable. This will effectively block the FM TX power from being injected into your equipment and cables as they pass from the tower to the radio.

The long and short is to look for areas that could be points of injection and either eliminate or shield. just make sure you bond everything together and avoid grounding loops.

Really appreciate the reply! I will weigh all my options

Thank you for your reply! I think that running fiber is probably my best bet.