How far can you run power over coax wire?

Question

I am needing to run DC over coax wire. How many feet can I run power over it? I need to run it 1000 to 2500 feet or more. I will need to run two radios at 7 watts each and each with 18 volts at the other end.

Answer 1

The short answer is...you don't want to do it that way if at all possible.  Is there AC power service near the radios?  If so, just get a AC to DC power supply.  If that is not possible, you will need a "local" source of power as the other answerer said.  Small solar power units for just this purpose are available.  

Getting AC service to the transmitter site is the best option though.

Answer 2

You didn't tell us which coax cable you are going to use.  Will it be RG6 with a 20 ga center conductor or RG11 with a 12 ga center conductor?  Gotta know this to determine the loop resistance and make the voltage drop calculation over the distance.  My gut says that you want to use a small cable and that the voltage drop will be too great with RG6 or RG59.  If you have a known length of the cable you want to use short one end and measure the loop resistance.  Use this value adjusted appropriately for the operating length to make the voltage drop calculation.  You can run the calculation and up the drive voltage to compensate.  E.g. if the voltage drop is 4 volts you could run the input at 22V to have 18V at the other end.

6/12/08

Guy:

When you measure the open circuit voltage it will be the same at the far end as at the near end if no current is drawn to experience the voltage drop.  I.e. if you measure the near end voltage at 18V and then you go to the far end of the cable and measure the open circuit (no load) voltage it will be 18V too.  You must put a load at the far end then measure the voltage across the load.  It will be reduced by the resistance of the cable.  The voltage drop on 1000 or 2500 feet of RG6 will be substantial.  You could run an interstitial wire or even a pair along with the coax to lower the voltage drop. Seems to me that it would be cheaper to run RG11 than to run parallel RG6s.  Are you running the coax to deliver an RF or AF signal to the far end plus DC or are you using the coax to deliver just the DC to the far end?

Answer 3

7W / 18V = about 400 mA.  The coax has 26 ohms per 1000 ft resistance in the center conductor and about 2.6 ohms per 1000 ft in the shield, or about 28.6 ohms total resistance per 1000 feet.

The voltage drop across the 1000 ft cable is 400 mA x 28.6 = 11.44 V, so you'd have to start with about 30 V if you wanted to end up with 18 V @ 1000 ft.  For 2500 ft, you'd have to start with about 47 V; choose an appropriate power supply.

Note that the cables will be highly inductive -- when you need additional power (like when transmitting), it won't be available instantaneously.  You should count on connecting large bypass capacitors at the radio end as well as clamping diodes and/or a voltage regulator to eliminate overvoltages when power is turned off.

You should seriously consider a local rechargeable system, using either solar or wind power (or even a water microturbine) to recharge a battery large enough for your power needs, especially if this is to stay in place for any length of time.  Cables above the surface are easily damaged; repair costs and down time will soon pay for the local systems.