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Fixed Wireless Equipment

A Tower Too High?

For wireless broadband, it may be possible to obtain access to a tower that's too tall.

[January 9, 2006]

On the ISP-Wireless list on Christmas Eve, 2005, DR wrote:

I have access now to the top of a 375 foot FM radio (not AM at all, not live) tower. I would be using all DC equipment (12V, Mikrotik). The only power to the top is to the strobe light, which is only on at night time, controlled at the bottom of the tower by a photocell. Here's my questions:

Since POE is out of the question (375 foot), would I have any issues withrunning AC power up a FM radio tower? What about using DC power instead? I know we have a lot of problems at our non-live AM radio tower POP, mainly requiring us to use chokes on the AC power and fiber optic cable to the bottom.

I talked to another WISP who is mounting on street lamps, using a DC battery charging arrangement, such that the system charges when the light is on, and runs battery when it is off. Is that viable for a 375 foot tall tower?

I've read that FM radio causes interference with ethernet, so I was thinking about forgoing the entire ethernet access at the bottom, since if I need to get access, I could just login via the AP anyhow.

What about having the strobe light reconfigured so that the photocell is at the top, and I have constant power..?

What do others do? We've never done anything bigger than 225 feet.

[CH asked] "Since you don't need data to the ground could you run DC up some heavier gauge wire?"

DR explained why the problem would not be so easy to solve:

Well, the only problem I can see is that the owner of the tower is concerned about extraneous power being a conduit for lightning to come inside of the equipment room directly. What gauge of wire are we talking about—if we're talking about using something that's big enough to run AC, what's the real advantage of running DC, other than having a single point of failure (the DC power supply at the bottom), which in reality could be made redundant with the use of high current power relays and dual supplies. I'm leaning towards reconfiguring the strobe light to have a top mounted photocell—is that really complicated? I read the FCC lighting rules, and having current sensing alarms to detect lights that are out.

This is a remote tower for this radio station, so they may have an automatic alarm on it.

All in all, I'd rather not have a top mounted battery setup, that's for sure. I think if price were a top consideration, converting the light's photocell might be the best bet.

Also, if the light at the top is plain AC, and I put an outlet up there, I have the distinct advantage of being able to use powerline networking to connect down the tower—which means I can have ethernet at the bottom with little trouble."

[FM replied] "Forget about the power at the top. You said the lights were strobes. The lighting power supply, at the tower base, Sends up one kilovolt static with a 15 kV kick to fire the strobe. There is no 110 V AC up there. Also, AC does not travel any farther in a circuit than DC. However, with AC, you can keep on increasing your voltage by using transformers, as in power distribution by utilities."

[BF agreed] "The advantage AC has is it can be stepped up or down using transformers. DC can be transformed but the devices are much more complex, and work like a switching power supply."

[JV opined] "If the grounding system on the tower is good, there should be no issue. It is probably worth pointing out the simple fact that the coax to the FM antennas surly contains more copper and goes all the way from the top of the tower to the transmitter. The fact that AC is being carried is irrelevant to lightning."

[JR agreed] "I agree. But the guy who started this thread is going to have to convince the tower owner."

BL thought that 375 feet was too high for the radios, anyway:

I would look closely at the radiation pattern(s) of the antenna(s)—at 375 feet; much of your signal may miss any useful target (fading before reaching ground level or projecting very high above ground). We don't have much that is higher than about 250 feet as APs—Point to Point is a different matter. Take a look at the horizontal radiation pattern. You might be surprised.

—End