Sponsored Part 15 fm transmission range
I've noticed that the fcc rules on license free fm transmitters are very confusing. I guess the power output and antenna doesn't matter. It's all about the signal strength. But the 1991 part 15 fcc memo states that 0.01 is an allowable amount of power. But than it says microwatts, which doesn't make sense because that would only be enough power to go about 3 feet. It would make more sense if it was 0.01 watts or 10 milliiwatts, but that would be enough power for 7/10 of a mile radius under optimum conditions. So that doesn't make sense either. There's actually several things about the 1991 memo that don't coincide with what the FCC States elsewhere
I'm no professional when it comes to low power transmitters. But I have messed around with them a lot as a hobby. Here's some power and antenna combinations. That are very close to the license-free parameters. (1 Milliwatt with a full wave vertical wire antenna) (10 Milliiwatts with a half wave vertical wire antenna) (25 Milliiwatts with a quarter wave vertical wire antenna) (100 milliiwatts with an 1/8 wave antenna) (500 milliiwatts with a dummy load) all of these combinations are roughly close to the license free limits of 250 microvolts at 3 meters.
It's really not to complicated. You seem to be mixing up FM and AM in the first post.
Simply stated for AM it's power but not power out at the antenna it's power (no modulation) into the final amp not the output at the antenna and no field strength or distance rule at all Just an antenna length rule. Most certified transmitters operate at that power and use a loading coil to make the short antenna work like a longer one. So you don't have to worry about that.
For FM there's no rule on power, microwatts, milliwatts, watts, antenna type, nothing. Ignore that. It's field strength only. Yes you have heard about 200ft distance but there is no rule about that. Depends on a few things...location, receiver quality etc. I don't know what the 0.01 power measurement you are referring to or where you are seeing microwatts but ignore that. It's irrelevant.
Don't go by a memo go by what the FCC says in the actual part 15 rules.
Oh on my first post, when I mentioned 10 Milliiwatts is enough power to reach 7/10 of a mile radius,. That would only be possible with high sensitivity car FM radios. It would obviously cover less distance with regular portable radios. And what I meant by optimum conditions would be a full wave vertical wire antenna at 100 FT elevation over flat terrain with no obstructions.
Range is always a contentious topic with Part 15 FM. There are so many factors that influence range, the most important being the sensitivity of the receiver you're using. Other factors include height (the higher the better), obstructions, even the weather. And what constitutes a listenable signal is very subjective - minimal noise, a lot of noise, fence boarding?
You're right with the fact that the Part 15 rules call for a maximum field strength for legal transmitters, and not output power. However, that field strength (250uv/m at 3 meters) can be translated to a power output under ideal conditions. That calculation has been done here in this Forum in the past, and is approximately 90 nanowatts.
The FCC states in one of their technical bulletins (but not the legal rules) that the maximum range you can expect with a Part 15 legal FM transmitter is 200 feet, but that doesn't take into account the myriad factors that affect range. It's probably true with common receivers and under common conditions, but I (and others) have seen much better range with good car radios (which can have sensitivity of under 1uv) - up to 200 meters or more.
In Canada, the maximum allowed field strength is 100uv/m at 30 meters, or roughly 4 times (or more) that allowed in the U.S. In one of my installations, using a certified Decade MS-100 located on a rooftop overlooking the intended coverage area with few obstructions, I saw ranges to good car radios anywhere between 250 meters up to almost 1km (parked in the right spot in a ferry lineup, very weak, but the signal was still there). With portables, range was limited to under 200 meters and in some cases well under 200 meters - it all depended on the receiver.
The Canadian maximum field strength translates to about a microwatt of power, at least according to Industry Canada or ISED, our regulatory body. So, 1 microwatt can certainly cover more than 3 feet.
Once you get into the multiple milliwatt output, potential ranges increase significantly. Our friends in New Zealand are allowed up to 1 watt on the FM band boundaries, and I was told once that although ranges for a solid signal were typically several km, line of sight could get ranges well into the double digits.
I hope that clears up some of your confusion. Unless you can afford to own a field strength meter, it's wise to purchase an FCC certified transmitter. If you do that, the responsibility for compliance rests with the manufacturer (as per the Part 15 rules); if you use something else, you're on your own.
I was in the middle of responding to the initial post, had to leave for a bit, came back and finished it. In the meantime, I see that there have been several other intervening posts with some duplication. Anyway, I'll leave my original post up without modifications.
Cool thanks for the info on fm transmitter range, I have also been playing around with a part 15 AM transmitter. I'm using a 102 inch cb whip antenna with a magnet base. I have the 100 mw transmitter in the attic just under a 16x16 metal roof directly iunder the middle of the roof. the magnet on the antenna has felt on the bottom and is located on top of the roof in the geometrical center of the metal roof the felt on the bottom of the antenna base makes ian insulator so it's not actually electrically connected to the roof. But it has like a counter pose ground plane effect. The whole roof is working like a ground plane. My coaxial antenna lead wire from the transmitter is only 8 in Long. There's a hole drilled through the center of the roof for the 8-in coaxial lead wire. The center of the lead wire attaches directly to the antenna. Iandthe ground part of the lead wire attaches directly to the mounting magnet on the antenna. So I have exactly 10 ft of antenna and wire. So it's perfectly part 15 compliant. It works really well I'm getting about almost 2 miles radius listenably clear.
@pete2169 What's ground conductivity like where you run the station?
When I ran on Bowen Island (essentially 3 rocks - mountains - sticking out of the ocean), ground conductivity was extremely poor, and I couldn't get more than 1/4-1/2 mile AM range no matter what I did. And yet, in my current location, a flood plain with great conductivity, I just pounded a metal pipe into the ground, attached the ground lead to the top of the pipe (which was a few feet above the earth) and I was getting 1 mile listenable range in all directions, 2 or more in one (parallel to the river). If I had your setup, range probably would have been more.
With AM, other than the receiver, ground conductivity is one of the most important factors in getting good range.
The situation I have where I run my am transmitter is high in elevation. Its on the roof of a 4 level apartment building. It's a 16x16 flat metal roof open gazebo on the roof of the building 45 feet above the ground . The gazebo has metal posts holding it up. There's a swamp cooler system on the roof. So I attached a ground wire between one of the gazebos metal posts and the water pipe that feeds the swamp cooler system that goes down to the basement. I also found a grounding Post in the basement. So I attached a ground jump wire between the water pipe and the grounding Post in the basement. Which gave me about 30 to 40% more coverage area. With the grounding system. The high elevation position of the transmitteri and antenna help a lot also. I'm getting three quarters of a mile radius of clear coverage. And listenable coverage out to about a mile and a quarter. If I take the felt off the bottom of the antenna base I gained about another half mile of listenable coverage.
I'm using one of those help you sell your house transmitters. It makes a big difference frequency I'm using. It works the best at 1680 khz, and works almost as good at 900 khz. Of course if I run the transmitter without the felt on the antenna base, makes it actually not part 15 compliant. But without the felt I'm getting close to 2 mles radias of a listenable signal. And with the felt attached and getting three quarters of a mile to a mile and a quarter listenable depending on the direction.
I also have a great place I'm experimenting with FM transmitters. I rent a small office on the 9th floor of the office building. So my FM transmitter is about a hundred feet off the ground. I'm using a range Master transmitter. It claims to be part 15 FCC compliant when using the low power setting which is 100 Milliwatts. It has an 8 inch rubber antenna. I'm getting a clear coverage area of over a mile to the north and east. And about a half mile to the west and south. The coverage area works better to the north and east because my office is in the Northeast part of the building. And that's with my wife's Toyota which has a great high sensitivity stock receiver. With my Ford van and only get about half the receiving distance the radio isn't as good.
The transmitter mentioned here is the CZH-05B under various brand names and yes it has an FCC certification with number. It can be looked up with testing info. If I was in the USA I would be using it. I purchased one in Canada at Long and Mcquade music store here last January under the Retekess name and tried briefly and audio was fine, no humming with decent 12 volt power supply and measured harmonics and found well with in allowed specs contrary to what is said about these Chinese "splatter boxes" and since it is not certified in Canada except for a licensed category I returned it but it managed to get approval in the USA under part 15. So much for that 200ft guideline. I liked it and the small rubber duck antenna. There was nothing wrong with it. Just too bad I couldn't use it here. The store web site said RSS-210 compliant(the Canadian equivalent to part 15), but found out that that was a misprint.
Yes, the fm transmitter is a CZH-05B. It sounds good. Clear quality sound. I'm running the audio through a graphic equalizer so I can cut the low frequencies a bit and boost the highs a little. I'm able to get a stronger clearer audio without distortion when using The equalizer. The transmitter didn't come with a certification number on the unit. So I called the company and they sent me an FCC ID. (Z9v001) but i wasn't sure if I could just use it? or do I have to contact the FCC first and get permission? I'm not sure? Anyway I did a couple of short tests with it, but I didn't want to use it full-time unless I'm sure it's legal.
I'd stay away from the Rangemaster FM transmitter, and other similar Chinese-sourced transmitters with output in the multiple milliwatts. Most claim to be FCC compliant, and some even have FCC Part 15 certification, but it's difficult to believe. Some have been reviewed here on this Forum, and found to grossly exceed the allowable FM field strength of 250uv/m @ 3 meters (the Rangemaster being one).
I suspect that those manufacturers who claim compliance are just shading the truth. And it's difficult to understand how some got actual certification, unless the transmitters were tested without antennas or in some other, non standard, way.
A few of us were curious about who was actually responsible in the event of using a non compliant transmitter that the FCC had certified. It does state in the FCC rules (I'm too lazy to look up exactly where now) that the manufacturer is responsible if using a certified transmitter. That may get someone off the hook legally if they are caught using one, but it likely won't stop the FCC from shutting you down.
Certification is there for two reasons that I can see. A transmitter has to be certified in order to market it. And certification also supposedly lets the end user know that they are compliant with the regulations; you have to have very expensive test equipment to determine compliance otherwise (in the order of many thousands of dollars).
You can't wholly use range as a measure of compliance, as there are too many factors that influence it. Getting a range of more than 200 feet (which is commonly used) does not automatically mean that you're exceeding limits. Getting huge ranges is likely a good indication.
Some of the true FCC certified transmitters I'd recommend using include the Broadcastvision AXS-FMT, the Whole House 3 or the Decade MS-100. The first 2 have been reviewed here, and found to exceed Part 15 specs, but not nearly as much as those multiple milliwatt ones. They're closer to Canadian BETS/RSS210 limits and are, indeed, certified for unlicensed use here in Canada. It should also be noted that some users have found the Whole House 3 to have a high 1st harmonic - it's unknown whether that is common across all examples.
Canada seems to be far stricter with certification than in the U.S. The Retekess TR-508, for example, advertises that it has 2 power levels, low (< 100 milliwatts) and high (500 milliwatts), similar to the Rangemaster electrically and physically. It carries the Part 15 certification label in the U.S., and yet in Canada, it is only certified for RSS-123 - a licensed use for strictly bounded areas (such as malls) or time limited events. Canada's Radio Equipment List (REL) data correctly lists the power output as 400 milliwatts. Even though our field strength limits are greater than Part 15, you cannot use the TR-508 unlicensed in Canada.
You have to be fast here with your posts. Once again I started to post, stopped a bit, then continued, and find several since the first. Oh well, I'll once again leave mine.
Like I said, I'd stay away from the CZH or the Rangemaster, whatever name they stuck on it. Some may be certified, but they're not Part 15 compliant. You'd probably be shut down if the FCC drops by. You shouldn't be getting around a mile range with a Part 15 transmitter.
And a true, Part 15 certified transmitter also has to have a label permanently fixed to the body, with certain wording and the ID number. Just having a number I don't think would be enough.
If I were in the U.S. and just experimenting, I'd go with the Broadcastvision (the AXS or the earlier 3001, both are certified). I've owned both, they sound good, and they'll keep you out of trouble. The Decade is the Rolls Royce of unlicensed FM transmitters, but you pay a lot for that quality (for a lot less $, you'll get just a little bit less quality with the Broadcastvision).