Field strength is inversely proportional to distance, and it is linear (at least in a vacuum), not exponential. The relationship of output power output to field strength is exponential - to get twice the field strength, you need to square the output power.
Of course, ground reflections and other environmental factors can significantly affect field strength, so generally there is a constant involved in the mathematics as well. Ground reflections and nulling are the reason why height is so important for FM - signals in that frequency range tend to bounce and scatter, whereas in the lower frequencies they are absorbed.
So that 100uv/m at 30 meters allowed in Canada is a lot more than 4 times 250uv/m at 3 meters. You're still in the microwatts levels, but compare that to the nanowatts allowed in the U.S.
As for range, that's a highly contentious issue. There are many factors that affect FM range, even Part 15 FM range. The biggest factor is the sensitivity of your receiver, combined with the receiving antenna. In the FCC bulletin that is routinely referred to with regards to legal range, no mention is made of the type of receiver being used. Or, in fact, any of those other significant factors that can affect range, such as obstructions, height of the transmitter/antenna, even weather.
To give you an idea of how bogus that 200 foot number can be, Industry Canada states that the expected range for a BETS signal (with our increased field strength) is 100 feet. They both can't be correct. In fact, neither is, given the right conditions.
Line of sight, and with significant height on the transmitting end, you can certainly get more than 200 feet range with an excellent car receiver (i.e., around 1uv or better) in the U.S. Enough to cover a reasonably-sized parking lot, for example.
I can receive my FM signal, generated by a Decade MS-100, out 400-500 meters here in Canada, line of sight, and with my car receiver. I'm also on the 8th floor of an apartment, which helps. But that's in one direction. In front of the building (I'm in the back), where the signal has to pass through multiple rebar-reinforced concrete walls, it can't be received on the roadway below.
It just goes to show you that the simplistic 200 foot rule in the U.S., or whatever rule you want to apply in Canada, just isn't very accurate.
Further to my previous post, that's why I say that you should always use a certified FM transmitter. In Canada it's in the rules, I thought it was in the rules in the U.S., but obviously I was wrong there.
Certification is obtained under controlled conditions in a laboratory environment, with no modulation. The tests don't deal with obstructions, reflections or any of the other significant factors that affect field strength and range in the real world.
It's pretty much guaranteed that a transmitter that meets FCC Part 15 requirements in the lab will perform much differently outside that lab, even though the FM rule thumpers hate to admit it. Very few people have the necessary test equipment to actually determine field strength; that's one of the reasons that certification exists (along with the permission to market and sell the transmitter). Adding an audio cable, for example, in field tests (conducted by a member here) of the Whole House 3 increased the field strength. Moving the audio cable around, the antenna orientation, pretty much blowing on the transmitter changed the field strength. But most people wouldn't know that, or by how much.
The FCC (and Industry Canada) are obviously aware of this. It's probably one of the reasons the allowable field strength is set so low - to take into account all these other factors, so that if you do go over, you won't interfere with other signals.
We already know that range can be useless as an estimator of field strength (except at the extremes, of course - you're not going to get a mile of range from a Part 15 transmitter no matter how high you install it, or how long an audio cable you have, or how you hang it out a window, despite what some have claimed). So the only way you have of knowing that you are even close to being legal using FM is to use a certified transmitter.
And, in fact, it is my opinion that if you do use a certified transmitter, you are legal. Period. Regardless of your real world field strength. Because the FCC and Industry Canada have taken that variability into consideration when forming the rules.
If you're not using a certified transmitter, or you've modified a certified transmitter, then you're on your own. Even getting 200 feet of range may mean you're over the FCC limits, particularly if you're using a cheapo receiver and are going through obstructions.
We already know that range can be useless as an estimator of field strength (except at the extremes, of course - you're not going to get a mile of range from a Part 15 transmitter no matter how high you install it, or how long an audio cable you have, or how you hang it out a window, despite what some have claimed).
I know your saying a part 15 FM cant acheive a mile (since that's the topic), but it still sounds off when you say a part 15 transmitter cant acheive a mile, because several part 15 transmitters do (not just AM).
Just saying.
Now I got to go google linear and exponential feild strength to see what your talking about
@mark - in May 2023 I noticed that, although their website was still up, there were no links for ordering any of the transmitters. I had originally ordered my transmitter from them with a mono chip, for better coverage, and since decided that I also wanted a stereo chip to have on hand "just in case". I emailed them and luckily, they were able to sell me a stereo chip. I mentioned feeling sad that they were no longer selling transmitters, and received the following reply,
"Since COVID our Chinese supplier of the metal enclosures basically shut down. After COVID, they have very long waiting times to catch up with orders. Last year we could not get any production, and none so far this year. They also produce for motor companies, so that gets priority over our stuff naturally.
We can still produce the all modules complete, but no enclosures."
Now, I notice that the website has disappeared, so it looks as if they are indeed completely out of business.
Yeah, I left the FM out of that statement - it should have read "you're not going to get a mile range from a Part 15 FM transmitter..."
It's easy to get the relationships between field strength, distance and power mixed up. Distance and field strength are linear, field strength and power are exponential.
So, if your field strength, is, say, 250uv/m at 3 meters, doubling the distance will half the field strength (in a vacuum). So at 6 meters, your field strength would be 125uv/m. And so on. Nulling from ground reflections, even weather (such as humidity, which is really just water particles in the air) will all change that field strength number.
If it takes 10 nanowatts output (I can't remember what the 'best' theoretical number is) to get you that 250uv/m field strength at 3 meters, to double it to 500uv/m at 3 meters would require you to square the power output, or 10x10=100 nanowatts. Doubling the field strength again to 1000uv/m at 3 meters would require a power output of 100x100=10 microwatts.
Of course, those calculations assume a resonant antenna; you'll require more power if you don't have one.
@rugster I also have an EDM FM Transmitter and think it's a wonderful device. For now, however, it is out-of-service with a blown final RF amp transistor caused by static discharge from lightning.
I just checked, and the final in the EDM transmitter looks to be an MPSH17. It's the only transistor in the whole transmitter (in a TO-92 package), and it's next to the output filter inductors, so I think it must be the final. If that's the case, it's easily replaceable. Not sure if you're handy with a soldering iron but if not, I can do it for you.
Are you sure it's just the final, and not any other devices too? If you're picking up a weak FM signal from the transmitter on the signal frequency, then it probably is just the final. If my transmitter had been hit by static, I'd be concerned about some of the other semiconductor devices in the circuit too, such as the pic processor.
@rugster It is super generous of you (Rugster) to offer courtesy soldering! Many thanks. Luckily, I am an old hand at that sort of thing and just need to face up to the job and get it done.
I have a clear memory of the day it happened. By sheer chance I happened to be walking toward a rear window when an explosive sound was heard and what looked like fireworks appeared up in the trees. I didn't believe I had seen what I saw. Up until then the storm was far away. It was not a direct hit, I surmise that static-electricity over-taxed the antenna.
When tested a weak signal was detected.
@carl-blare - a weak on-frequency signal is a good sign. It sounds as if the only device affected was that one transistor in the final. The MPSH17 is now an obsolete part, but they are still available. I imagine there are many NPN bipolar transistors that would work in that position (2N3904, PN2222A etc) but seeing that the MPSH17 is still around, that's the one I'd use.
Yes, I know what you mean about facing up to the job. Often, with these sorts of things, I just need to wait until, one day, the inspiration hits and I suddenly feel the urge to attend to a particular fixit task. Then afterwards, I wonder why I didn't do it sooner! 😀
One of these days, I'd like to acquire a Part 15 certified FM transmitter, to see the range I can get with an approved device. I don't have a functional 'scope with which to measure the power from my EDM transmitter. If it is close to the stated 1mW at the minimum power setting, then 30.5dB of extra attenuation is needed to bring the power down to 90 nanowatts which, from @artisan-radio, I understand is the power level that will, under ideal conditions, achieve close to the allowed field strength under FCC 15.239. I built an inline RF step attenuator recently that can give 30dB of attenuation, so that should work nicely.
@rugster The way I fine-tuned the power output of my EDM was to make its reading on a spectrum analyzer match that of a C.Crane certified transmitter.
@rugster I agree with you, with no extended range liberties it tends to extinguish any hopes of getting out..
But keep in mind Radio Sausalito was originally a network of part 15 FM transmitters spread around the town.
Also remember what Commissioner Pie said to the church's a few years ago when they wanted a power increase for part 15 FM? He told them to just "use more part 15 FM transmitters"..
So, though I agree with your "why bother" with part 15 FM" attitude, a part 15 FM station network is certainly feasible and has been accomplished before.
But with that said, the above transmitter would not be a good option
Churches are able to apply for a license more in less no need to pull strings to use cheap transmitters
I didnt notice your reply untill just now @khitsradio. I dont know if it's actually any easier for a church to acquire an FM licence than anyone else (it seems ludicrous to suggest churches have any advantage in being granted it, but I dont know). But it is clear that the churches did want more power for their part 15 FM broadcast during covid, based on a 2020 Radio World article:
“I understand that this makes it more challenging for churches during this difficult time, but I want to emphasize that there are options available,” Pai continued... Churches also could explore using multiple Part 15 devices to cover a larger area if they continue to provide ‘drive-in’ style services."
But what really prompted me to post is to find out who you are? I see this is the only post you had made here so far..
Welcome, and introduce yourself!