[BC] Metro PCS Cables on tower

[Broadcast List USER]

>From: Mike McCarthy <towers at mre.com>
>
>I also believe the DC cables are of some significant size.  I'm not sure of the function or purpose of the equipment being supplied power, but it would seem to me that it must be a very special purpose or AC could have been send up and a power supply located local to the gear. 

You are correct, there is no reason to run DC, other than the fact  
that running DC is the way it is commonly done on towers which are not  
AM radiators.

Keep in mind that for a very long time, cellular operators not only  
didn't want to be on AM radiator, but wouldn't even consider a tower  
within a mile of an am radiator.

It is common to run DC up the coax to power the receive pre- amplifier.  It is much less common to run DC conductors, but it was occasionally done.  Today, not so much.

Once upon a time, a positive Voltage selected one pre-amp and powered  
it, a negative Voltage selected (via RF switches) a second pre-amp and  
powered it, and no Voltage made all the RF relays drop out and  
connected the transmission lines so no pre-amps were in line.  So, a  
positive Voltage could select one preamp until it failed, someone  
would then switch to a negative Voltage to select the backup preamp,  
and no power sent tup the tower to bypass both preamps.  This is still  
likely done for combiner systems at VHF and UHF, but I'm not sure it  
is still done for cellular as I haven't built one of those systems for  
a few years and technology is marching on pretty fast there.

--chip