FM Transmitter

[k6ccc]

OK, here is the radio guy to explain FCC Part 15.  First of all, this is a set of regulations from the Federal Communication Commission in the United States.  That means that for those of not in the U.S.A., you can ignore this post.  And for the lawyers on the forum, it's really Title 47 of the Code of Federal Regulations, Part 15.  If you really want to read it (you likely don't), here is the entirety from the U.S. Government Printing Office:

http://www.ecfr.gov/cgi-bin/text-idx?SID=d1b38a77f47f0889627bc2fe65c3de06&mc=true&node=pt47.1.15&rgn=div5

Part 15 of the FCC rules in general covers things that radiate a Radio Frequency (RF) signal - whether that radiation is intended or not.  For example a computer radiates some RF noise and has to comply with Part 15 as an unintentional radiator.  A radio transmitter is intended to transmit a signal and is therefore an intentional radiator.  For intentional radiators, Part 15 specifically covers radiators that do not require a specific license.  For the most part that is low power transmitters.  There are a large number of intentional radiators that operate under FCC Part 15.  These include such things as your WiFi, Baby monitor, Wireless video link, LOR's ELLs, Wireless weather station, BlueTooth devices, and a host of other stuff.  Unintentional radiators include things such as your computer, a microwave oven, CFL and LED lights (that are operating from an AC source), almost every electronic device.  What I will call "real" radios such as CB, FRS, Ham, business band, utility, and public safety, and broadcast radio (AM, FM, and TV) all operate under different parts of the FCC rules, as do cellular telephone, microwave and satellite.

Part 15 defines limits for either transmitter power level (how many watts of power) or a field strength level at a specified distance from the antenna.  Some of the frequency bands specify the limits as transmitter (measured either at the transmitter or the antenna depending on the band),and some specify the field strength.  In the case of the Part 15 transmitters operating in the FM broadcast band (what we care about), the limit is specified as a signal level.  That is a little unfortunate because it's harder to measure.  So a lot of the advise given is an approximation.  For example you might be told that you can transmit 0.1 watts (low power on a CZH-05B) and you are OK, or maybe that you have a usable range of 200 feet.  Both these are approximate values.  The effectiveness of the feedline between the transmitter and antenna (if any), and the antenna can radically affect the actual field strength.  And think about this one.  How do you define a useful range of 200 feet?  Remember that radio signals in reality go on forever - they get weaker and weaker.  The telemetry transmitters on the Voyager 1 spacecraft are transmitting only about twice the power of a CB radio, but are being received from about 20.6 Billion Kilometers away, but it takes a massive antenna and very sophisticated receive system to dig that weak signal out of the noise.  For our purposes, the useful range is the point where the signal no longer fully quiets a receiver. In other words, when a receiver will be completely quiet when the transmitter is transmitting a radio signal but no audio.  For most of us, that 200 foot guideline mentioned above is likely fine since someone farther away than that is not likely going to be able to see the display anyway (and yes, I know there are some exceptions).  Getting the antenna closer to the audience helps since the signal has less distance to go.  In my case, I am transmitting about 0.025 watts into a good antenna, but it's only 10 feet from the street. 

For those that want to know what of the massively long FCC Part 15 really applies to us, here it is:

> §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.

There are pages of different frequency segments.  This is the one that we're all operating under.  Note that this covers 88 - 108 MHz ONLY.  That means that those of you operating on 87.9 or 87.7 are not on a legal Part 15 frequency.  In other words, you are operating illegally - period.

As to how you could get busted for excessive power.  It was mentioned that the FCC is not driving around looking for part 15 violators.  That is absolutely correct.  The most likely way you would get busted is that a listener to a station on (or maybe adjacent) to your channel is hearing you instead of the station they want to hear and complains generally to the station.  The station engineer might drive around about find that there is an unlicensed station on their frequency and maybe even find it.  The station then complains to the FCC.  There are priorities in how aggressive the FCC is in finding interference problems.  At the top of the list is interference to aviation and public safety radio systems.  I personally have had an FCC engineer out at 10:00 PM on a Saturday evening because of interference to a Public Safety radio system that I run for a living.  A little below aviation and Public Safety is broadcast.  Pirate broadcast stations will get responded to if it gets to the FCC.  They do take them pretty seriously.  If you actually get a visit from the FCC, my advise is complete co-operation and play dumb.  I.e. "Gee, I didn't know I was running excessive power."  If they tell you to turn it off - do so.  If you can turn it down to a legal power see if they are OK with that.  Hint - don't turn it back up after they leave - they may come back.

OK, if anyone has actually read this far, I commend you!

 

[wmilkie]

I just skipped to the end

[bobschm]

Every year we go through this. If you want a tiny bit more range, just extend the antenna wire that came with your transmitter. In many cases the transmitter is overpowered but complies by shortening the antenna. 

There are NO empty bands in my area, so I just step on a very weak foreign language station. 

So far, no men in black have knocked on my door.