Output power vs ERP

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brokenshoes

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Massachusetts
I was perusing the FCC ULS database for some public safety frequencies and I came upon a rather curious observation. A UHF frequency license for a Fire Department Fixed Base (rptr) is listed as having 100 watt transmitter output, with a 14.3 watt ERP. How is that accomplished? Are they allowed to transmit with 100 watts of output power into the antenna, or does the transmitter power have to adjusted so the ERP at the antenna is 14.3 watts, which would take into account the feedline loss, antenna gain if any, etc. Anyone? Thanks
 

n5ims

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Transmit power is just the amount of power that the transmitter is able (or configured) to send out (as in your example 100 watts).

ERP is a result of the gains and losses in everything in the line between the transmitter and antenna (the transmitter and antenna are included in the calculation.

For example, you have a system with:
* Transmitter (base signal level)
* diplexer (signal loss)
* feed line (signal loss)
* gain antenna (signal gain)

In the case you mention, the transmitter sends out 100 watts and the gain from the antenna less the loss from the feed line and diplexer would make it appear that the signal was equal to a 14.3 watt transmitter connected directly to a zero gain antenna. This could easily be from a 600' run of heliax where the antenna gain was about equal to the diplexer loss.
 
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