Output Power vs. ERP (Legal)

[KI4AMD]

I understand the technical difference between ERP and transmitter output power, but I was wondering if someone could help me understand the intent of the difference as seen on FCC licenses. I have several licenses in the Public Safety Pool and I've noticed that on one frequency/location I'm allowed 100 watts output power but only 16 watts ERP. On another frequency/location I'm allowed 40 watts output and 120 watts ERP.

I guess my first question is, why does the FCC even care what I'm pushing off the back of the radio? Shouldn't ERP really be the only figure that matters?

My next question is regarding the 100/16 site. What does the FCC expect me to do here? Use a leaky dummy load for an antenna? Why am I allowed so much power at the radio, but only 16 watts ERP? Is it possible that they are intentionally encouraging me to use a low-gain or lossy antenna to deal with interference on the receive side?

 

[ecps92]

Those fields, in my experience, were completed either by the License Holder or the Frequency coordinator, not the FCC

W/O seeing the License, it is hard to read between the lines

I understand the technical difference between ERP and transmitter output power, but I was wondering if someone could help me understand the intent of the difference as seen on FCC licenses. I have several licenses in the Public Safety Pool and I've noticed that on one frequency/location I'm allowed 100 watts output power but only 16 watts ERP. On another frequency/location I'm allowed 40 watts output and 120 watts ERP.

I guess my first question is, why does the FCC even care what I'm pushing off the back of the radio? Shouldn't ERP really be the only figure that matters?

My next question is regarding the 100/16 site. What does the FCC expect me to do here? Use a leaky dummy load for an antenna? Why am I allowed so much power at the radio, but only 16 watts ERP? Is it possible that they are intentionally encouraging me to use a low-gain or lossy antenna to deal with interference on the receive side?

 

[WA0CBW]

This information is required on Schedule H item 35 (power) and 36 (ERP) and are filled out by the person preparing the license. The instructions for item 35 says "The power entered should be the minimum required for satisfactory operations". The coordinator or FCC may modify the applicants item 36 request for ERP based on the coverage area requested, the adjacent and co-channel users and the height of the antenna.

 

[radioman2001]

ERP is usually dictated by the co-ordinator to prevent interference from the licensee to the surrounding same channel users. A lot of times the power out and ERP differences are a result of line losses and other factors such as duplexers or combiners. For example you transmitter is licensed for 100 watt out, now the loss of your cable up a 400 ft tower is 50 watts, you now have 50 watts out of the cable, which into a unity antenna would be 50 watts ERP. Adjustments can be made anywhere to ensure that you don't exceed the ERP that the co-ordinator has given your license. Another example would be to use a 9db high gain antenna to get best reception from portables, so your transmitting power out would have to be adjusted to match the ERP which includes the antenna gain.
We have a license with 110 watts out, but an ERP of only 1/10 of a watt. This is for our radiating coax in a tunnel which has sizable losses purposely. We would have no effective power out at the other end of the cable if not for the 110 watts going into it.

 

[prcguy]

Generally a high transmitter power and low ERP listing on a license is to accommodate transmitter combining, which can eat up lots of power. A four way hybrid combiner with a 100w transmitter, some feedline loss and a low or no gain antenna could easily result in 16W ERP.
prcguy

 

[freddaniel]

In response to your specific questions.
1. You are correct, generally across all FCC bureaus, the FCC does not care what your actual Tx power out is, but in all cases wants to know the ERP. However, under Part 90 applications there is a requirement to state the Tx power out. Maybe this is a carryover from the old days when little engineering took place in establishing a land mobile system. In all other services, ERP is what is specified.
2. According to 47CFR Part 90.205(H), your ERP is a function of the operating radius and antenna height above the average terrain, and is limited. See the table and formula in the rules that reduces the ERP with antenna height and limited service area. This rule establishes the maximum ERP to be licensed "without a rule waiver". Also, the ERP may have been reduced by the coordinator to "fit your service contour" among other users interference contours on that particular frequency. Also, the ERP may have been entered incorrectly by the person filing the original FCC application with the coordinator.
Keep in mind old licenses were not subject to the limitation on ERP. In the "old days" you could get a license for 100 watts at the top of a 1,000 ft tower or a 5,000 ft mountain, without regard to the actual ERP. So your other licenses may be grandfathered.

 

[12dbsinad]

ERP is typically the magic number. However, many people think that if their license says 100 watts ERP, then they can buy a 100 watt repeater and run it at full power, and are good to go. This is not always the case. Your ERP is figured by all the losses, and gains in your system. Everything has to be caulculated, filtering loss, coax loss, and antenna gain. To put this into an example, Let's say you have a 450Mhz UHF license and the ERP for the repeater is 100 watts. You buy a 100 watt repeater, duplexer, coax cabling, and a 5 db gain omini fiberglass antenna. By the time you subtract the losses of the filtering, and coax, you have roughly 50 watts at the actual antenna, and a 100watts out of the actual repeater PA. So, with a 5 db gain antenna (system gain) the actual ERP of your repeater is 250 watts, ERP. Well over the 100 watts on your license. If you installed a unity gain antenna, you'd be at 50 watts ERP, 2 db gain antenna, you'd be at your correct ERP level of 100 watts. That's pretty much how ERP is configured. Many many many people just look at it and say, yep it says 100 watts.

 

[Jayce]

FCC 14-172

74. We believe ERP limits are more appropriate than TPO limits because ERP more
accurately defines the actual operating power of the radio system. TPO simply describes the transmitter
power without considering other components of the overall radio system, whereas ERP describes the
power of the entire radio system by considering the TPO plus the antenna gain minus any loss factors

At some time in the future , we m actually see the changes to licenses made.