What is the max output power of an fm transmitter?

[Vishwesha]

I want to build an fm transmitter that can broadcast signal to the whole city. So i need help. What is the standard maximum output power of transmitter suggested by FCC? Usually at what power the commercial broadcasters do transmit? Please help me. Thanks in advance.

 

[tcmtech]

What you can broadcast at Vs what they hunt you down and toss you in jail for is different.

The bigger FM stations run from KW's too tens of KW's power output depending on distance covered. A few hundred watts could still cover a few miles though.

You would likely need to brush up on your vacuum tube theory to be able to build a transmitter that can work at that power level.

 

[sfink06]

I have always though that you have to have a license to broadcast FM (ham radio?)

 

[smanches]

I think you can (or used to be able to) broadcast 1w on FM stations. Our local uni had a station, but did not have a license and could only transmit 1w of power. Wasn't even enough to cover the university campus itself.

Also, I believe max FM power output is 100kW.

 

[RadioRon]

I want to build an fm transmitter that can broadcast signal to the whole city. So i need help. What is the standard maximum output power of transmitter suggested by FCC? Usually at what power the commercial broadcasters do transmit? Please help me. Thanks in advance.

It depends on the size and topography (hills, mountains, large buildings) of the city. According to Wikipedia, typical FM broadcast stations running 100,000 watts can be heard for about 100 miles. If you want to have less power, and assuming your city is relatively flat, then perhaps it would be ok to scale the power to 12,500 Watts to cover 50 miles, and 1563 watts to cover 25 miles.

These are just based on simplistic assumptions, but you should learn that distance is mostly affected by antenna height and antenna performance, so the best advice I can give you is to build a transmitter of less power but build a very high and very good antenna.

I would be remiss if I did not add that in most countries (not sure where you are) it is against regulations to transmit at these levels without "type approved" equipment and without a license.

 

[Sceadwian]

smachens they probably had some kind of license, or at least there's a waiver for it in FCC law, you can't legally transmit more than a few mw's max on the FM band without a license, the licenses are not cheap. You could buy an FM transmitter and do it anyways but you're technically pirate broadcasting.

 

[smanches]

I'm not sure how they got away with it. It was also 20 years ago, so maybe the laws have changed somewhat since then.

I know the FM station (frequency) had to change when they finally got their broadcast license. They were also broadcasting over cable at the time, so maybe there was some provision in that which allowed them the 1w. Although I doubt it since cable is basically private, unlike airwaves.

EDIT: Decided to look it up. 100mw maximum, or the edge of the campus for school stations. I think it's the latter that got them to 1w. https://en.wikipedia.org/wiki/List_of_North_American_broadcast_station_classes

 

[mbarazeen]

FCC only allows some 50mW i remember, since its on comercial FM band any broadcasting more than thsi limit should have a licence. also building a transmitter as you say is not that easy sonce you may need much experience on how to tune it. also stable frequency would be another challange.

1st try to build a small one of some 100mW and get it stable and tuned, if you succeed in it, then you can add some stage of amplifier to boost your out put. but keep in mind if any one would complain for interfearence then local authorities may take action for violating FCC requirements. in some countires there may be a big panalty too.

a 5-10W transmitter can cover upto 5miles radious in open flat ground depens on the height of the antenna

 

[kc8gpd]

i know this is an old thread but i am posting this so people doing a search in the future will have correct info.

the relevant rules are..

§ 15.239 Operation in the band 88–108 MHz.

(a) Emissions from the intentional radiator shall be confined within a band 200 kHz wide centered on the operating frequency. The 200 kHz band shall lie wholly within the frequency range of 88–108 MHz.

(b) The field strength of any emissions within the permitted 200 kHz band shall not exceed 250 microvolts/meter at 3 meters. The emission limit in this paragraph is based on measurement instrumentation employing an average detector. The provisions in §15.35 for limiting peak emissions apply.

(c) The field strength of any emissions radiated on any frequency outside of the specified 200 kHz band shall not exceed the general radiated emission limits in §15.209.

(d) A custom built telemetry intentional radiator operating in the frequency band 88–108 MHz and used for experimentation by an educational institute need not be certified provided the device complies with the standards in this part and the educational institution notifies the Engineer in Charge of the local FCC office, in writing, in advance of operation, providing the following information:

(1) The dates and places where the device will be operated;

(2) The purpose for which the device will be used;

(3) A description of the device, including the operating frequency, RF power output, and antenna; and,

(4) A statement that the device complies with the technical provisions of this part.

[54 FR 17714, Apr. 25, 1989; 54 FR 32340, Aug. 7, 1989]

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§ 15.219 Operation in the band 510–1705 kHz.

(a) The total input power to the final radio frequency stage (exclusive of filament or heater power) shall not exceed 100 milliwatts.

(b) The total length of the transmission line, antenna and ground lead (if used) shall not exceed 3 meters.

(c) All emissions below 510 kHz or above 1705 kHz shall be attenuated at least 20 dB below the level of the unmodulated carrier. Determination of compliance with the 20 dB attenuation specification may be based on measurements at the intentional radiator's antenna output terminal unless the intentional radiator uses a permanently attached antenna, in which case compliance shall be deomonstrated by measuring the radiated emissions.

also part 15.221 and part 15.209 are other provisions for unlicensed broadcasting.