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 Post subject: Power question
PostPosted: Sat Sep 02, 2017 11:36 am 
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Joined: Sat Jul 22, 2017 2:26 pm
Posts: 33
When I drive by a cell tower, I often notice that it's baracaded inside a fence with a power sub-station and generator. I suppose it's not really a "sub-station", as it isn't really redistributing any power outside the facility. Typically there are very substantial insulators mounted on the top of the large enclosures (in one case I recall they're being about 10 or 12 inches in length). So - 12k? 22k? My question is: why?

Your cell phone typically uses one half watt of power to communicate with the tower. It should take no more than that to communicate in the other direction. So, in our area there are enuf cell towers such that each of them would have to service no more than maybe 10,000 phones. That's 5000 watts = three hair dryers worth.

What gives?


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 Post subject: Re: Power question
PostPosted: Sat Sep 02, 2017 3:03 pm 
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Joined: Fri Feb 01, 2002 1:01 am
Posts: 1772
Location: Mideast USA
From what I know, if a cell tower is handling at a given moment, 100 cell phones; the power transmitted is still 0.5 Watt.
There is multiplexing. One cell tower transceiver slices time into as many portions as cell phones are 'logged in'.
Of course, each transceiver is limited to the number of cell phones it can multiplex. If there is 150 cell phones logged-in, a second transceiver helps handling the excess. But the figure of how many channels are handled by each transceiver (64, 128 ?), I do not know.

When a certain number of phones are logged-in, extra channels of the radio spectrum come into operation. How many channels are available, I do not know.

In a cell tower there has to be other equipment, computers, microwave links, backup battery chargers, climatic control, fiber optic interfacing to landline central offices, whatever else using more Watts. And multiplied by the number of service providers hosted.

You should be able to find details in a search.
---->https://www.extremetech.com/extreme/158342-a-rare-look-inside-an-lte-cell-site-operated-by-sprint-in-san-francisco

A little extra ----> http://blog.taitradio.com/2012/10/09/ch ... -and-cdma/



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 Post subject: Re: Power question
PostPosted: Sat Sep 02, 2017 8:33 pm 
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Joined: Sat Jul 22, 2017 2:26 pm
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Hi Externet,

Thanks for the links. In that first link, it mentions the RF amplifiers on the towers max out around 100 watts. That seems awfully high, IMO. The time-slot description was interesting reading, and the part about the heavy involvment of SDR (which I already knew). But it was a pretty high altitude flyover description. The articles didn't get into very much detail, and the author did mention that it's a very tight-lipped industry.

I'm still not sure I'm believing those stations can eat up as much power as what they seem to be equipped to use :-)

According to this site, it's probably 23kv feeding the cell tower:

https://electronics.stackexchange.com/questions/71614/guessing-power-line-voltage-from-the-number-of-disc-insulator

Seems like a lot of power capability for a few SDR transmitters/receivers, some other switching, and a 100 watt 900/1900/2500 MHz xmit setup. Half inch ASCR is good for 250 amps. So, that's a max potential power of 5.75 megawatts. And - the ASCR is probaby bigger than that. I can't get close enough to measure, of course.

- Ron


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 Post subject: Re: Power question
PostPosted: Sun Sep 03, 2017 5:24 pm 
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Location: Maryland
At the site, there may also be redundant power feeds, with automatic switching in the event of an outage.
Batteries don't last forever!



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 Post subject: Re: Power question
PostPosted: Thu Sep 07, 2017 12:39 pm 
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Joined: Thu Aug 01, 2002 12:01 am
Posts: 2822
Location: San Jose CA
even if there were no power on the tower at all, some daredevil will break the lock, climb to the top and hang from one arm while they take a selfie or smoke a doobie. It happens all the time. A guy got fried on a high V tower just a couple weeks ago because he wanted to climb it. Fortunately his flaming body fell next to a playground with fire extinguishers handy, I think he lived but not very well. When they do it on a water tank, they often drain the tank out of caution just in case they took a leak in it.


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