I just wanted to get the forumla listed. Too much speculation about field strength on the 'net, and way too many myths. I like facts. This way, other people have a proper starting point.
As we know, numbers are one thing, real-world tests are another.
The key to determining the range for a given field strength transmitter is the sensitivity of the receiver at the other end. That, plus any obstructions that lie between the transmitting antenna and the receiving antenna.
I've started some research on the sensitivity of receivers to get that key number.
There are basically three classes of receivers. Home receivers, car receivers and portables.
It's been relatively easy to find specifications on high end home receivers, both vintage and modern, which is what you might expect. Audiophiles, after all, are into those specifications in a big way. It's not been so easy to get that information on mid range home receivers. And where it was once quite common for manufacturers to list the sensitivity of car receivers, that's not so much the case these days. As for portables, good luck. There's a lot of work still to do.
But to move forward, there are basically three sensitivity measurements for any receiver. The first is useable sensitivity for a mono signal, which means that you can hear it, it's very noisy and you probably wouldn't want to listen for any length of time. The second is the sensitivity for a mono signal with some quieting - the absolute best receivers generally list a high quieting figure of 50 or 60db. 30db quieting, as an example, would still be noisy (but listenable, at least to most people). And finally, the sensitivity with quieting for a stereo signal (with stereo separation at a certain level).
Here is an example, for a high end McIntosh tuner. The useable mono sensitivity was claimed at 0.9uv. The mono sensitivity at 50db quieting was 1.5uv. The stereo sensitivity at 50db quieting was 15uv. So as you can see, if you want to listen in stereo, you need a much stronger field strength than mono. And this is for the best of the best. It should be noted that most manufacturers attempt to meet that 15uv sensitivity for stereo in their specs, but what they do is they lower the quieting levels, so you might get 15uv with 30db or 40db quieting.
Now, this has been stated many times before but it's worth repeating again. A field strength of 15uv/m at your receiver's antenna does not translate into a 15uv signal at the antenna terminals. There is a factor of somewhere between 2 and 2.5 (the math can be found elsewhere on this site). So to get that 15uv stereo signal into the McIntosh, you need a field strength of 40uv/m or so. For mono, the numbers are better - for the McIntosh, you would need a field strength of 4uv/m or so to get 50db quieting. And around 2.5uv to hear a very noisy signal.
Car receiver FM specs can approach the best home receivers. An Alpine that I used to own claimed a mono sensitivity of 1.9uv at 40db quieting. A Pioneer - 1.6uv at 30db quieting. But note the quieting numbers; both are probably around the same sensitivity if you compare apples to apples.
And portables are worse yet again. It's difficult to find sensitivity numbers, and selectivity on these radios tends to be poor as well. Strong signals from adjacent channels bleed over into yours, increasing interference, effectively making the sensitivity even lower.
Obstructions will reduce the field strength of any signal. If your antenna is elevated significantly, that will reduce the effect of obstructions close to the antenna. But these definitely have to be taken into account when calculating desired field strength.
The ultimate aim of the receiver research is to obtain not only the best and worst sensitivity for each class of receiver, but to attempt to define a 'typical' receiver in each class. It should then be possible to calculate typical ranges to each receiver class, as well as best and worst case scenarios.
If we look at the best case scenario for a home receiver. Assume that our ideal listener owns that McIntosh receiver, and it is 1000 meters from the transmitting antenna, line of sight (ignoring obstructions for the moment). We need to get a 40uv field strength to the receiving antenna for stereo sound. That works out to a field strength of 13330uv/m at 3 meters. To get the 4uv mono signal to the receiving antenna with full quieting would be 1333uv/m at 3 meters. And to get the 2.5uv mono signal to the receiving antenna would be 1000uv/m at 3 meters.
Again, that's the best case scenario.
For more typical receivers, I expect to find that car receiver specs will approach the best home receiver numbers, and be better than the typical, mid-range home receiver. I also expect that the best portable will still have worse specs than the typical car receiver. We'll have to see.
So there a number of decisions that ultimately have to be made. Do we use stereo sensitivity or mono? Do we target the best or typical receiver? Car receivers or home stereos, or even portables? And how do we take into account obstructions - a fudge factor? Or attempt to build some obstruction assumptions into the calculations? The complicating part about the effects obstructions have on a signal is that they are really not linear - the farther you get from the transmitting antenna, the more obstructions will be between you and the transmitting antenna, so the more they attenuate the field strength.
My own personal opinion is that, if the decision is made to petition the FCC for increased field strength, the target range should be moderate distance (say 500 meters or so) for a stereo signal to a typical home stereo. That would translate to a field strength, line of sight, of 6708uv/m at 3 meters (not taking into account obstructions). Those that own the best home stereos will get better range, as will those listening in cars; those that own portables only will get worse. The mono signal, line of sight, will travel much further, of course - over 6km theoretically, but the reality is that it will likely get noisy and die out a lot quicker due to the increased number of obstructions.
Perhaps that 500 meter range is too large, considering the range the mono signal might get, but then, that's the purpose of throwing something out there. To get discussion.
Any and all comments are welcome.
You're right. There are far too many myths and far too much misinformation being bandied about on the Internet about Part 15 field strength. I just read some more today. You can't argue with mathematics and physics (well, you can and some do, but they just look silly in the end).
I did say and I was wrong at the meeting that if 500mW would equal ¼ mile that 1 Watt would equal ½ mile and I'd need to run 1.5 Watts into a rubber duck to get a mile. I was actually told if I were to run 1.5 watts into a rubber duck I'd probably go 5 miles or more. Something to do with the square root of the RF power would double the range. What isn't clear to me is if it is the square root of 500mW or the square root of 1 Watt and how would I figure that on a rubber duck antenna? Would the db gain also have to be in the formula? I would imagine it would which would really throw me off. Math was not a subject I was good at in fact I'm mathematically challenged. That said someone would have to have the formula for that. And problem was too that the ¼ mile was to a car Radio which had a vertical antenna and it was an SUV type vehicle. So it had some height compared to a regular car. The Radio in it had RDS capability so it was probably even more sensitive than a regular car Radio. So in actually 500mW may go 1,300 feet as I've looked up some 500mW transmitter's specs for range from a rubber duck. I'd assume they mean to a boom box. Someone done a test of a 5 and 7 watt transmitter and put it on youtube and it went 20 miles on a car Radio with some drop outs but came back in booming across town. He said he had a dipole antenna in the video. So what I want to see is 1 Watt. I can't find a demo yet for that I'll keep looking.
I'd hope they would give the ALPB an experimental FM permit (I'm sure they would tell us no more than 1-5 watts probably more like 2 to the final of the transmitter). Surely the FCC is gonna keep you under 5 watts because they deny 10 Watts for new LPFM permits. That in mind their not gonna let us go stupid wild with the power. Its why I say for the experiment we keep the power close to a watt and see what it takes to go that 1 mile range in Stereo to a home receiver (probably digital) on a T antenna or telescoping antenna on the receiver. One should have to record the sound quality so we could hear when the receiver faded to Mono and hissed and finally faded to the abyss. There is another technical issue and that is that you can only run so much power into a rubber duck till it gets HOT. Plus even if it were legal I'd not want to run a 7 watt FM Transmitter because it runs hot without a good fan. Even 5 Watts gets hot too so that is a bad thing for me don't want that. So its another reason I don't want to run that kind of power even if it were legal. Worst came to worst I'd put the transmitter in the attic and either run a A/V transmitter to it or a Bluetooth transmitter/receiver system to it and try the 500mW at an elevated level and see what happened or at least a 2 story house or building. I'm willing to bet if I got up 10-20 feet that I'd get out 1 mile just fine @ 500mW and it could be the great magic power to a rubber duck. If it was than we'd have to construct a logic circuit and a chip that could not be easily duplicated to be molded into the antenna system and test that and make it hack proof as much as possible. One could enforce that the transmitter be sealed sort of like those brick power supply's where it can't be easily opened as a safeguard for certification for the FCC. After the rubber duck test we need then test a dipole and a telescoping antenna. However you'd have to come up with a sort of test so someone can't add a roach clip to the antenna and wire to extend the range. Also the blank frequency scan should be put into this experimental FM Transmitter. If we show that we mean to do everything to block spurs and harmonics we'll get further with the FCC. We may have to add band pass filters to our transmitters to block anything above and below the FM band from even passing to the antenna. Lots of Pirates use 7 pole low pass filters. Yes I look around their sites to see how they are blocking harmonics. One suggestion on a dark Radio forum was to use a 7 pole filter so No harmonics can get through. Someone said it makes the range of the transmitter less, but we'd figure that out in the 1 mile range experiment. How many filters are in the Whole House FM Transmitter? Has anyone taken one apart to find out? Does it even have a low pass or band pass filter? The SainSonic AX-05B claims to have one, but the transmitter is so small where is the filter? I think filters need to be made so one could screw in the filter like a cable TV trap used to block the beep beep beep so you could pull in HBO positive trap or notch filter back on those days. If they made filters like that now I'd surely use one just to be extra safe on my FM transmitter. I don't know how to make one. Yes I hope we get the permit so we can show real world results to the FCC.
@ Artisan Radio: I must have been writing as you were posting. 6708 uV/M at 3 meters does sound good. And 500 meters which is 0.310686 of a mile and 1640.42 feet and to a home Stereo is awesome. Actually its way more than the SainSonic AX-05B into a rubber duck which was what I was trying to accomplish in the range boost petition. This would make the high power (500mW) setting into the stock antenna of this transmitter 100% legit and therefore a hobbyest could use that transmitter and axtually add a slightly better antenna (if they wanted to) to get the max field strength of 6708 uV/M @ 3 meters. I'd love to get my hands on a field strength meter that would be fun for FM. Sorry I was writing the same time you were and didn't see the feild strength number you came up with for 500 meters. I guess I'm happy with that one.
Conventionally the so-called "filter" which keeps a transmitter from emitting harmonics is part of the output circuit between the final RF amplifier and the antenna.
It is a form of "low-pass" filter which allows the fundamental frequency and the sub-carriers up to 200 kHz pass unimpeded into the antenna, but sharply attenuates (reduces) all RF energy above that point.
Since the filter is built around the frequency of the transmitter, there is no such thing as "one size fits all."
Properly made transmitters should already include an output filter, if it does not, it is not a professional transmitter.
When a committee of members designed the 13.560 MHz "Big Talker" Part 15 shortwave transmitter we got hung up for quite awhile on the output filter, but the design we ended up with does the job very well.
We have never talked about designing a Part 15 FM transmitter but that could be interesting.
The more I think about what we want vs. what the FCC wants, extending the range of Part 15 is just not going to happen. That means that every part 15 FM transmitter manufacturer would want to extend their range, making the FM band noisy.
So I think, as stated before, we should push for a new classification.
Very low power (numbers to be determined by math, experimentation based on topography)
Intent is to not go over a 1000 meters (5uV at 1000m?)
Broadcaster has to apply for a license. Don't want 12 people in the same area.
Annual fee (this keeps the number of broadcasters down)
Possibly opening up 87.9 (should this be a separate issue?)
Signal to be confined, as well as possible, to a geographic location.
Apply for an experimental waiver to test numbers for 3-6 months?
This brings me to another question - is there an FM transmitter with variable power output? I'm not talking about something like the Whole House with the secret switch, I mean a device that can vary output wattage with very low numbers. Or, is this something that's going to have to be built?
A very excellent FM transmitter is the Ramsey FM25-B Kit, which has a variable power control and a choice of two antennas - 1.) built in telescopic extendable to about 2-feet, and an output F-connector to attach transmission line to antenna of choice.
I have built two of them so far and both have been "borrowed" by friends who seem unable to return them, so I'm going to get a 3rd one.
Yes, there are many transmitters that can vary output. From commercially certified ones down to what is currently considered hobby or Part 15 (you just have to know how to do it for the latter - that's how the Decade MS-100, for example, can be manufacturer-tuned to either U.S. or Canadian maximums, and it can also be tuned for the Canadian special event class, with significantly greater field strength and range).
However, I would recommend a continuously variable commercial transmitter, to which you can add an external antenna, such as a quarter wave vertical.
Power Out: 500 mW
Rubber Duck Antenna: -5dB loss
Transmitter Antenna height inside: 1 meter
Car Radio Antenna height 1 meter
Frequency 103.1
GREEN 44 dBuV/M, 3 Homes Covered
YELLOW 34 dBuV/M, 17 Homes Covered
Cow Pasture to the north
https://wa4jm.shutterfly.com/pictures/9
The Green Coverage Spot shown on John WDCX's map looks like something one could live with.
I would guess that the Green Spot based on the present FM rules would not quite cover one house.
ArtisanRadio in post 32 has some great points. One thing to consider though is car antennas are disappearing. There used to be a vertical ¼ wave antenna for the center of the FM band on every car, which was great. Now some antennas are either nubs that have XM antennas built in, or horizontal metal trace elements on the windshield. The nub antennas have lots of loss. The horizontal metal trace antennas will not be the correct polarization for most consumer transmitters.
Has anyone looked at how the FCC works? If someone at the FCC were to get excited about 500mw community broadcasters, that would be the first step on a journey that would be fraught with peril every step of the ay. The idea would eventually need to go out for comment, and I think it is safe to assume that at a minimum NAB, NPR, and EMF would comment negatively on it. Those three groups would put the lawyers/engineers on it and show how 500mw radio stations across the country would blanket the FM band with noise and hurt the big players in radio; it happened to LPFM. Even if the 500mw community broadcaster idea survived the comment phase, when it came time to go on the air, full power broadcasters could fight to keep the new service off the air. Right now if a translator tries to come on the air but a station on an adjacent or the same frequency has listeners who receive interference from the translator, the translator has to fix the interference or it has to go off the air. If a station knows a translator is going to come on the air some stations will ask listeners who may be affected to contact the station so the station has documented bona fide listeners in the translator’s service area to mount a case against the listeners. In any of the big markets or adjacent to any of the big markets, any frequency clear enough to put a 500mw signal on probably has an adjacent or co channel occupant who can demonstrate that it is possible to receive their signal where the 500mw would want to operate. Assuming it was possible to get past all that, then there is 1 problem left.
100 watt LPFM’s are asking for a power increase to 250 watts because they do not get building penetration with 100 watts. A 500mw signal would admit from the start the target audience was either people with outside FM antennas or people listening in cars. The people who have outside FM antennas probably are trying to listen to a distant market station, so if the 500mw signal interferes with their ability to listen to that station, the 500mw could be forced off the air because likely the distant signal is a primary user of the channel.
Getting word out about a new station receivable in a car when people drive in a specific are could be a challenge. I would guess the 500mw service would be noncommercial. Since a license was issued, there would be no way around royalties for any copyrighted material broadcasted.
There is the real challenge. If the coverage area were 1 mile wide, then at 30MPH, someone would be in the coverage area for 2 minutes. From that alone, it would be easy for someone against the new service to argue the new service does not do the public good since it is only of fleeting temporal use to most passersby.
Since we are talking about the complications of reaching three households and one car, an alternate solution would be to purchase four complete record libraries and deliver them to the four listeners so they could enjoy the fine music.
Save us all the trouble of broadcasting.
This service could be used to Promote an Internet Station. The main point in such a service covering just under a mile would be for the sole purpose of getting listeners to tune in via the Internet if they want to receive the station further than 1 mile or 1/4 mile. Now I already go off air @ 6PM to avoid temperature inversion interference. If a listener wanted to listen to the distant station when the inversion is happening they would not get interference from my station. I think this would probably be from Noon-6PM for these stations that way there is even fewer issues when the DX at night comes in and people want to listen.
I've not seen data about anyone receiving a 500mW transmitter from inside a building. I'm sure our folks in New Zealand could give us that data. Now as far as yearly fee's they should not be more than a Live365 broadcaster would pay for say 20-30 listeners per month. I'm sure royalties would be included in the license if that was gonna be an issue but I don't think it will (Maybe in Huge Cities). Plus I already said that NO Car FM Transmitters should have the new field strength only stationary transmitters should. That would help to stop that fear from the nay sayers. Maybe like I said before one would have to be sure or apply for the area they want to transmit to make sure no other micro powered station is wanting or using the same frequency.
We have already expressed disinterest in being required to maintain an EAS system as an obligation of a new CLPFM category.
Another responsibility no one needs is the keeping of a "public file."
Licensed stations are bogged down with so many extraneous responsibilities that there's no time to enjoy the art of broadcasting, which probably has a lot to do with why program quality gets left out.
If radio is to be fun it must have room to happen and not be wrecked by constant compliance inspections.
If radio can't be fun for the broadcaster it certainly won't be fun for the listener.