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Fixed Wireless Technology

Trouble With AM and FM Towers

It's not easy to handle the power of the commercial radios on these towers, but some are able to provide strategies.

[August 21, 2008]

On the ISP-Wireless list in August, GP asked

Any one have experience (good or bad or attempting) to co-locate wisp equipment on AM or FM towers???

Several members reported problems. AV warned:

We are experiencing Ethernet problems on all three of the FM towers we are on. On two of the towers we have no problem since the Ethernet line coming down is only for POE and management when we're on-site, but the third one is having all kinds of troubles with Ethernet link and we need to pass traffic on it.

This tower site has two FM transmitters one at 97.3MHz at 4100 Watts and the other is 98.3 at 3900 Watts. This site has been up and running for 6 years and, just recently, we've run into problems with our Ethernet links. They have never worked at 100 Mbps so we've always dropped them to 10 Mbps and have been OK. Well, now we have more and more traffic and it's causing us to drop pings across the Ethernet cable up to the AP.

We originally had regular outdoor cat5 cable and thought we could fix the problem with shielded cable. After replacing the cable, we still can't link up at 100 Mbps and are still getting dropped pings at 10 Mbps.

So we figure we have three possible problems.

1. The transmitter inside the structure is just pounding all of ourequipment with RF

OR

2. Leakage off the FM transmission cable is interfering with ourEthernet signal.

OR

3. Something else.

We've heard people saying they are putting conduit up the tower or run fiber up the tower, but both will cost unneeded $$ and time if it's the problem inside the transmitter structure.

[KA replied] "Get a media converter and run the data on fiber, ootherwise the RF is going to eat you alive."

Asked why this would work, M7 explained:

An old ham trick. If the cat5 is inductively picking up stray voltages, or the RF energy is inducing a current on the 4 lines used to carry power to the radios, a simple capacitor in the POW leads will bleed them off. A capacitor will not pass DC voltage (POE) but will pass an AC voltage. You would have to make sure the negative side of the POE was also bonded to your common ground for this to work. It wouldn't be wise to have a floating ground or negative side of the POE anyway.

[MC objected] "If you are running equipment that requires GPS sync, a cap across the power might not be your friend. You can put filters on both ends of the run, as well as, try different grounding approaches (both ends, bottom only, etc)."

[M7 replied] "My advice DID NOT include putting a capacitor on the data lines, but on the POE lines. Since POW used 1, 2, 7, 8, Id put a small capacitor across the power leads. The data to fiber converters, with a seperate power feed thus protected, is, however, a good idea."

[KA suggested] "The ideal solution that works really well for me is to run one line for power and then run fiber. On the power line at the radio end place a common noise suppressor/filter (like automotive sterieo type) this will filter any RF coming in on the power line and makes the DC power nice and clean. Then run your data through a media converter to fiber down to the bottom. also make sure your radio is in a Metal NEMA style box so that the actual radio circuitry is RF shielded."

KA added that if there's an alternative to the tower, especially if it's an AM tower, you should use it:

AM towers have so many problems that it isnt even funny. First off an AM tower (unlike FM) is driven, the whole tower is hot and is the radiating element. One problem you will have is that the AM tower has undergone what is called proofing, so once that is done it is highly unlikely that he broadcaster will want you to add anything on it which will change its radiation pattern, or will ask that you pay for the engineers to re-proof the tower again. (Expensive)

Second problem is since the entire tower is driven, it becomes a very expensive feat to decouple the tower from your equipment at the bottom and at that its only going to be for power leads. Depending on the transmitter power level these tuned inductors can cost between $10K and $50K. There is no direct Ethernet isolation other than Fiber.

It can be done, but it's a mess and every time you need to work on it, the transmitter will have to go low power so you dont get RF burns/cook while your climbing the thing. something else the broadcaster does not want to do.

JS agreed, writing succinctly: "Avoid if possible."

—End