[aprssig] 8 hour Backup Power Rule

Rick Green rtg at aapsc.com
Tue Mar 18 00:52:59 EDT 2008

On Mon, 17 Mar 2008, Steve Noskowicz wrote:

>    Transmitter power is quite low in many cell sites - many under 10 watts
> into 17 db gain antennas.    That 300+ amps at 24 (or 48) volts (7kW++) doesn't
> all go out the antenna and, therefore, heats the site.  100 channels at 10 W
> ea. is only 1kW.
   That implies ~6KW of heat to be pumped out of the building.  In a study 
of a data center I did a few years ago, I came up with a ratio of 70:30, 
that is 70% of your power bill can go to running your computers, and 30% 
will have to go to the A/C unit.
   It seems to me that the bits you're spitting into a fibre line or three, 
no matter how fast, don't contain a whole lot of energy, so the 14% you're 
accounting for going up the feedline makes cooling a radio room that much 
easier than a computer room!
   Anyway, my simple arithmetic tells me that you'd need 56KWh of battery 
capacity to run the gear for 8 hours, or 80KWh if you include the A/C, 
plus some to allow for inverter inefficiency.  The deep-cycle RV battery 
in my shack is rated at 100AH, which would be 1.2KWH, so it would take 67 
of them to supply your needs.  Make that 68 so you can build a 24V bank, 
and at ~60lbs each, you've just added two tons to your weight loading!
...Then build a battery box, charger, and ventilation fans, etc.

I don't blame the cellphone companies for balking.  It's one thing for 
some politicians to set a uniform standard with the stroke of a pen, and 
quite another to implement it.  Even my over-generalized 
back-of-the-napkin calculations just show the tip of the iceberg when you 
consider some 300,000 cellsites, each with their own unique assortment of 
zoning, environmental, lease, local legislative, and physical 
restrictions.  The mind boggles...

Just in my own neighborhood, I've spotted cellsites hidden within the 
large crucifix on a mega-church's front lawn, behind the fiberglass 
medallions that make up the freeway sign for a local college, and 
disguised as a large blue lightning-bolt sculpture on the lawn of a large 
corporation's national headquarters campus.  All custom, one-off 
engineering jobs.  ANd I imagine that there are equally creative, and 
difficult to modify, installations like this all over the country.

Rick Green, N8BJX

"Those who would give up essential Liberty, to purchase a little
temporary Safety, deserve neither Liberty nor Safety."
                                   -Benjamin Franklin

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