You better pour another cup of coffee...
Probably the number one thing asked about a Part 15 transmitter is range. Be it on this forum, or any of hundreds of other forums where radio and electronic geeks gather. From Christmas Light Show builders to would be community broadcasters. It's easy to see the most often asked question is something like "What is the Range of the XYZ Transmitter". Or "which transmitter will give me the best range".
Here's the answer. If it's a LEGAL Part 15 FM transmitter the answer is "they're all the same" and "it depends".
We all know that FM is limited to 250 uV/m at three meters. That amount of field strength is just that. It's 250 uV/m at three meters. Neither the FCC nor the laws of physics give a crap if you reached that limit with a watt and a super crappy antenna and a ton of line loss, or 2 nanowatts and a 10 element Yagi antenna. It doesn't matter if the antenna is at 5 feet, 10 feet or 100 feet in the air. As long as it's not over 250 uV/m at 3 meters it's legal, and that amount of signal will go so far, period.
Then comes the "It depends".
When I do range tests with Part 15 FM transmitters, (and I really don't anymore) if they're legal level or below, I walk from the transmitter with a fairly decent receiver in an open field. The legal ones poop out between 200 and 300 feet. The few range tests I've done with over the limit transmitters that cover several blocks obviously require me to either walk a lot, or drive. Now you've introduced a lot of environmental factors. Buildings, trees, street signs, the list is endless. The reception you get here, could be better or worse 10 feet to the side. Or under that tree. Or away from that tree. So it's really an exercise in futility. Unless you happen to have access to an open area a mile square. But then it's not a "real world" range test.
Then it completely depends on the receiver being used. I've casually tested both AM and FM range using a wide variety of radio receivers, from those in diffeent cars and trucks to different portables from communication receivers to $5 handheld transistor radios, to boom boxes to high end stereo receivers. "Hey, I get get 450 feet"! Well, with THAT radio anyway. I only get 66 feet with this one". So again, range tests are folly. People always try to quantify with something like "600 feet to a good car radio". How do you know you have a good car radio? Have you compared sensitivity to all your friends cars? Maybe you have two cars and one is better than the other, but neither is nearly as good (or bad) as your neighbors? And you can't go by specs in the manual, either. That doesn't take into account age, damage, corrosion on the antenna connector or the antenna ground to the cars body, or many other variables. Did you know car radios are somewhat directional? It's true. The radios are designed with AGC and other circuits to smooth out reception. But depending on the car's antenna, you have some amount of directionality that can be most apparent when trying to receive a weak Part 15 signal. Antenna on the fender? The cars body is the ground plane. Obviously there's more body metal in some directions than others, varying the pattern. Antenna inside the windshield across the top? Directional depending on the direction of the antenna. None of the "range tests" people want are using any sort of standard for reception. But again, in an open area, with a legal transmitter, the legal limit can only go so far. It's like having three cars all traveling at the legal limit of 60 MPH and then saying "Car number 2 will get there faster because it's more aerodynamic". No. It won't Because it, like the others, is limited to 60 MPH. Or saying "Car number two will get there faster because it has 50 more horsepower than the others". Doesn't matter. It can still only travel at the maximum allowable speed. The law makes them all equals. In the case of Part 15 FM radio the maximum allowed field strength is the speed limit.
Further, these tests -- even the field strength tests -- are done in a wide open area. So, lets say you have a legal transmitter measuring exactly 250 uV/m at three meters in the wide open. What's that field strength going to be when you put it in your house? Is the signal penetrating a 100 year old wooden structure? Is your house stucco from the 1950's with a wire mesh behind the stucco holding it together? Is your transmitter in a window? Does it have an aluminum frame? Is your house covered in aluminum siding? Have a metal roof? Are you in an apartment building or office building with steel girders? Once you're out of the open area and in your house, in your neighborhood, surrounded by other buildings, trees, power lines, phone lines, home wiring, etc all bets are off. You're going to have a wild squiggly RF pattern, and it's going to be dramatically attenuated in some directions and multiplied in others. This makes any sort of range test basically useless. And if the FCC should come by to check you out, they're NOT going to take your transmitter out into some field to see if it's legal. They're going to check and see if you're over the limit, and they may check in more than one spot! If you're over ANYWHERE, you're illegal.
For the most part, all transmitters of maximum limit output (and trust me, you'll never find two transmitters with the exact same output, even among the same brand and model) will have the same range when used in the exact same circumstances.
Lets think about antennas. Lets say you have a transmitter that gives you 250 uV/m at three meters with it's little whip antenna. You decide to put up a dipole for it. You've now put up an antenna that's going to have nulls in some directions, and gain in others. Which means in some directions you'll have increased range making your transmissions illegal, and reduced field strength in other directions greatly limiting range. Lets' review the statement above: If the FCC should come by to check you out, they're NOT going to take your transmitter out into some field to see if it's legal. They're going to check and see if you're over the limit, and they may check in more than one spot! If you're over ANYWHERE, you're illegal. So if you decide to put a dipole on your otherwise legal transmitter you WILL have areas of antenna gain that will make you illegal in some directions. If you think your dipole increased your range, you're right! And it did it by providing signal gain in some directions, while creating nulls in others. It increased range to some listeners, while decreasing it to others. Where that gain is, you're over the limit. Think of a radio signal like a round balloon. You have a nice circular pattern. Then you squish it in on the sides. The balloon bulges out in a couple places, while it's squished in at other places. Just like a radio signal that's forced into a pattern with different antenna designs. You still have x amount of signal, you're just changing it's shape, or pattern. Just as you do when squishing a balloon. This is an easy way to visualize how a typical commercial radio gain antenna works. A typical 100,000 watt FM radio station has a transmitter that actually generates about 20,000 watts. The antenna provides the gain to get 100,000 watts. (this is a bit simplified but the concept holds) The gain antenna is squishing that signal. Take your round balloon, set it on a table. The table is Earth. Squish that balloon as flat as you can and notice that there's no round top anymore, but the sides have expanded and it covers more of the Earth. That's what a gain antenna does. It focuses the RF that's shooting off into space where no one can listen to it and adds that power to the signal going out to the sides where the people are. Again, a bit simplified but gives you the general idea.
OK, so that got off topic a bit, eh?
So, there's some thought on FM range, and why doing range tests of Part 15 transmitters is really not relevant.
TIB