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measuring the output power of an FM transmitter

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darklordchris

New Member
i recently bought a small cd/ipod/car radio style, fm transmitter capable of transmitting about 5 feet. i was wondering if their is anyway i can measure the transmitters output power. can anyone give me an idea of how to do this?:D
 

Sceadwian

Banned
Why would you want to do that? It's be less than a milliwatt.
 

darklordchris

New Member
Why would you want to do that? It's be less than a milliwatt.
curiosity mostly. i'm doing research about power and transmition distance in the fm band. i was wondering if theirs a way to just connect a multi meter to the antena wire and figure it out like that. the transmiters part 15, but i still want to know roughly how much power its putting out.

also are you sure its less then a mw? isen't regulation power like 250 mw in the usa?
 
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Sceadwian

Banned
Wikipedia is your friend =)
I'm quoting this under fair use, link is for the entire article.
Title 47 CFR Part 15 - Wikipedia, the free encyclopedia

Unlicensed broadcasting

Unlicensed broadcasts on the FM broadcast band (88 to 108 MHz) are limited to a field strength of 250 µV/m at a distance of 3 meters from the antenna. This is equivalent to 0.01 microwatts.[1] Emissions must be kept within the 88.0 to 108.0 MHz band under Part 15 rules.
The FM broadcast band is limited but not restricted to 87.9 MHz (but see note below on TV) to 107.9 MHz. There are a few class D FM stations on 87.9 MHz.[2] Any unlicensed 88 to 108 MHz broadcasting with a signal strength greater than 250 µV/m at a distance of 3 m from the antenna is punishable by law and confiscation of all broadcast equipment.
Unlicensed broadcasts on the TV broadcast bands are prohibited, except for certain medical telemetry devices. 87.5 to 88.0 MHz is considered part of the VHF TV band (channel 6 audio is on 87.75), though it shows up on most FM tuners. For TV, 15.241 and 15.242 deal with high VHF (channels 7 to 13), 15.242 also deals with UHF.
On the standard AM broadcast band transmission is limited to 100 milliwatts of power (with restrictions on size, height and type of antenna) or, alternatively, under 15.221, if the AM transmission originates on the campus of an educational institution, the transmission can theoretically be any power so long as it does not exceed the field strength limits stated in 15.209 at the perimeter of the campus, 24000/F(kHz) µV/m.
 

flat5

Member
As a start, you could measure the current leaving the battery.
Also measure the battery voltage while it is doing it's job.
Perhaps consider 50% of the power is what you are after.
I don't know what would be close.
It certainly won't be more than what the battery is providing.
 

audioguru

Well-Known Member
Most Helpful Member
Those MP3 to FM car radio transmitters have an attenuator at the antenna to reduce their range. People have removed the attenuator so that the transmitter inside their home transmits to a radio that is anywhere in their yard.

My FM transmitter draws 53mA from a new 9V battery and goes pretty far. I used it for only one hour because I don't wan't to be caught by the RF cops.
 

darklordchris

New Member
Those MP3 to FM car radio transmitters have an attenuator at the antenna to reduce their range. People have removed the attenuator so that the transmitter inside their home transmits to a radio that is anywhere in their yard.

My FM transmitter draws 53mA from a new 9V battery and goes pretty far. I used it for only one hour because I don't wan't to be caught by the RF cops.


thats impressive. how did you do that?
 

Sceadwian

Banned
Pop yours open and take a real clear well lit picture of it with a decent camera, post the pic here someone might be able to point out the attenuator if one exists. One of the first things I do whenever I buy just about any gadget (excluding the really expensive ones) is open the sucker up and take a peek around.
 

stevez

Active Member
Measuring the power input to the entire transmitter is a good start. To get a better handle on power output you might isolate the supply to the final amplifier then measure the voltage and current input to that stage.

I'd consider this part only partially reliable - it's been a long day. If you are able to measure the current and voltage to the final amplifier then you'd need to add the power input from the prior stage - which is likely to be low enough so that it's negligible. If you know the power input to the final stage then the output will be something less than that.
 

Sceadwian

Banned
stevez, the posters question was answered yesterday morning =)
 

darklordchris

New Member
Those MP3 to FM car radio transmitters have an attenuator at the antenna to reduce their range. People have removed the attenuator so that the transmitter inside their home transmits to a radio that is anywhere in their yard.

My FM transmitter draws 53mA from a new 9V battery and goes pretty far. I used it for only one hour because I don't wan't to be caught by the RF cops.


ok heres a picture http://viewmorepics.myspace.com/ind...iendID=9997320&albumID=24128&imageID=62460958 . any ideas?
 
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audioguru

Well-Known Member
Most Helpful Member
The BA1404 FM transmitter IC is an antique piece of garbage. There were so many complaints about its poor performance that Rohm discontinued it and replaced it with a modern line of FM stereo transmitter ICs that use a crystal-controlled frequency synthesizer with a phase-locked-loop.

If the BA1404 IC does not have an attenuator on its output then its output power is so low that it might transmit across a street. Some very old kits are still sold with it.
 

darklordchris

New Member
The BA1404 FM transmitter IC is an antique piece of garbage. There were so many complaints about its poor performance that Rohm discontinued it and replaced it with a modern line of FM stereo transmitter ICs that use a crystal-controlled frequency synthesizer with a phase-locked-loop.

If the BA1404 IC does not have an attenuator on its output then its output power is so low that it might transmit across a street. Some very old kits are still sold with it.

weak. so what are some good ones?
 

audioguru

Well-Known Member
Most Helpful Member

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