So Gerry from Procaster and I today went out to the park next to his house and did some testing for field strength at 3 meters and 30 meters distances using the Procaster FM. The transmitter was on a tripod 6 ft off the ground and the FIM receiving antenna the same height. 30 meters is where the Canadian BETS measurements are taken, 3 meters is where FCC part 15 is measured from.
First we did measurements with 250 uV/M at 3 meters. I took a Sangean radio that has better than average reception. We had a tape measure to measure 30 meters from the transmitter and 3 meters away and marked the two distances with a stick in the ground.
I have to say that 200 ft that is talked about as the max distance that can be heard on a radio at 250 uv at 3 meters(FCC) is false information. At 15 meters(50 ft) the carrier was gone and just the inter-station noise. We then moved the FIM 30 meters from the transmitter which is just less than 100ft. The FIM reading was 2 or 3 uV/M, if that. That is not enough field strength to create enough signal for even the best of receivers to hear without some elaborate antenna system. No way a 250uV signal at 3 meters is receivable at 200ft(61 meters). That is misinformation. It takes the better of 10uV/M for a high quality receiver to hear a decent station and get out of the inter-station hiss.
Now for the BETS readings. 30 meters is quite a distance. Farther then I thought it would be.
We found that to get 100uV/M at 30 meters(Canadian measurement) You need 3000 uV/m at 3 meters to get that field strength at 30 meters. Still just slightly under. Real world measuring with a calibrated FIM meter. ISED says BETS-1 signals should die out at 30 meters? Not accurate. At 30 meters from the transmitter with a 100uV signal on the FIM the Sangean receiver still had a strong full quieting carrier. Like a local. I started walking back farther and could walk considerably farther than the 30 meters before the reception got spotty and tilting the radio/antenna one way or another still brought in a clean signal. So Canada is much improved over the FCC 250 uV at 3 meters.
Also as mentioned for a decent signal on a high quality receiver you need about 9 or 10 uV/M.
Then we tried a couple of commercial stations to see what the field strength was as the FIM is measures that also. Interesting. Strong signals have a field strength of give or take 1500 uV/M
So that's our findings.
It is really surprising to see how puny the 250 uV at 3 meters is.
I had to read through that twice before I got what you were saying, which if I interpret right is that under the US FCC Part 15 FM rules you really can't get anywhere near 200ft , whereas with Canadian BETS you're allowed like 10 times the amount of field strength.
In other words you demonstrated by comparison, that it sucks for FM hobbyist in the states, but FM is great for those under Canadian rules.
Certainly true.
Although we here in the states got you beat on AM!
Anyway, very cool your were able to mess about with a FIM, it's got to be fun, and thanks for posting about it.. Very interesting observations, and for the most part in agreement with what the former FCC Chief Engineer Reed had said in correspondence with R Fry, however it's not always the case with Part 15 FM, as had pointed out in this excerpt.
"...The expected range for unlicensed FM is considerably less than that of unlicensed AM - .... .., you're lucky to even get 30 feet in the FM band. I did see one experiment achieve 400 feet of range when operated in a remote area with a low background noise level and no other FM stations any where near the selected frequency, using a very sensitive FM receiver with a good antenna, and transmitting in a mono mode with 75 kHz of deviation applied to the modulation. This is why our rules do not specify a range - it's a relative term that is completely dependent on the environment. ...." https://radiodiscussions.com/threads/fccs-john-reed-comments-on-part-15-am-rules-and-interpretations.540838/
...
Oh... Were the broadcast in stereo or mono?
@richpowers You said:I interpret right is that under the US FCC Part 15 FM rules you really can't get anywhere near 200ft , whereas with Canadian BETS you're allowed like 10 times the amount of field strength.
Yes, in the real world, actually what is measured, not on paper, yes going by a 3 meter measurement, to get the 100uV/M at a 30 meter distance where Canada measures it, you need 10 times the FCC max at 3 meters. But if you go by the on paper calculations of halving the field strength for every doubling it's 4 times. All we were interested in is....when you have 100uV at 30 meters from the transmitter and you now bring the FIM up to 3 meters away what do you get, not what is in math on paper. Power and field strength degrade fast as you move away from the transmitter. This is similar to what Timinbovey's tests showed also.
And yes, at a 3 meter reading on the FIM 250 uV/M no radio will hear it at 200ft! As I said it starts to die out on a good receiver at 15 meters and at 30 meters(100ft approx) only a couple of microvolts was measured. No signal on the radio at that.
We were both very surprised at how little that really is when seeing how the reception was lost at just less than 50ft, let alone 200ft!
Stereo wouldn't affect field strength. Listenability perhaps, but not field strength.
Oh to add. This was done with no audio cables/audio, just the transmitter. FIM powered with fresh batteries and transmitter powered with my power station from the 12 volt DC port.
I have to emphasize that this assumption of 200ft range for FCC part 15 FM and the ISED(Canada) assumption that your BETS-1 signal dies out at 30 meters(100 ft roughly) is false.
We didn't take any pictures of our test set up.
Here is some food for thought, if you go by what the part 15 field strength would be at 30 meters, Canada allows just under 100 times that of the USA.
First, I have to say that this was great work by you and Gerry.
2-3 microvolts IS receivable with good car receivers and whip antennas. I owned a Kenwood that had a sensitivity of 0.5 uv 20db over the noise. Even allowing for needing more field strength to induce voltage at the antenna terminals, you could receive a station with that. At 100 feet, mind you, based on your experiment.
At 200 feet, your field strength would be half that times some constant for reflections. Maybe at the 1-1.5uv mark. That might be barely receivable with that Kenwood car receiver. Maybe not. Depends on the value of that reflection constant.
It does seem that you have demonstrated that 200 feet is just about the maximum range for a compliant Part 15 transmitter (at least mounted at a height of 6 feet - I wonder what would happen with greater height for both the receiver and transmitter).
The problem is that the BVE, Whole House 3, even the Part 15 Decade, all get better range than that, and all are certified. I guess that indicates that there are no real, compliant, certified transmitters available. Maybe the C Crane, which has noticeably shorter range.
I said....Here is some food for thought, if you go by what the part 15 field strength would be at 30 meters, Canada allows just under 100 times that of the USA.
To correct my math it would be 33 times better.
@artisan-radio One thing that wasn't done here. Doing this at different frequencies. This was done at 90.5 and it may be that different frequencies may perform differently.
AM sensitivities are expressed in uV/M most of the time which directly relates it to field strength but FM expressed in uV is a bit different than field strength. It means a voltage at the antenna input of the receiver not field strength in space(microvolts per meter)
Assuming you have an antenna( 1 meter long) on a receiver that can, with a 1uV sensitivity get that voltage induced into the antenna by a 3 uV/m field strength, theoretically it should be possible. Yes 3 uV/M or less is receivable on a receiver with that sensitivity provided that tiny field strength can induce a 1,2,or 3 uV into the antenna. Since the space field strength is expressed in per meter it means if you have a length of wire 1 meter long for example a field strength of 250 uV/M with induce a voltage of 250 uV in the 1 meter length of wire.
But the 250 uV/M read at 3 meters and the corresponding 2 or 3 uV/M read at 30 meters that would take the best receivers made to be able to hear something out of that and if you went back 100ft more to 200ft I will stake my life(well that's an expression) on it that the FIM reading would be 0 and no receiver would get a signal out of that. There is no way any typical even good quality radio that most would have would receive a station at 200ft with part 15 field strength.
If it does, it is considerably over. That's why all the certified part 15 transmitters tested out at much over the limit. In fact the BVE, certified, Gerry has one, I gave it to him, with the 20" telescoping antenna it measured, at 3 meters, 17,000 uV/M!! No exaggeration! Even if that was BETS I would be afraid to use that! Way over! That's why I gave it to him. You can also check Timinbovey's tests also with this. That's like the speed limit is 50km/hr and you are going 100km/hr. Why? because to make it at all saleable it has to be considerably over. 250 uV at 3 meters is of little use and was intended to get a signal from your mp3 player across a living room to your stereo wirelessly, or be DJ around your house. Truly not for broadcasting even in a gym or auditorium.
The Wholehouse 3 advertises 150 ft distance but of course it is over even with the stock 8" rubber duck and no secret power boost. It has to be, to be heard over half the length of a soccer field with a Tecson radio as once demonstrated. Tim did a test with this also. Even Ccrane says not happy with the range, here's what to do! Put it on a metal surface, check youtube for a hack, clip a long wire on the antenna.
I think we're both saying the same thing. 200 feet is the upper limit, based on your experiments in the real world, and you'd need a very sensitive radio to even hear it then (if you could at all).
It would be interesting to try different heights (but realistically not possible, at least in a field), and different frequencies.
Stereo wouldn't affect field strength. Listenability perhaps, but not field strength.
I would phrase it as "Stereo wouldn't affect field strength. Range perhaps, but not field strength."
Oh to add. This was done with no audio cables/audio, just the transmitter. FIM powered with fresh batteries and transmitter powered with my power station from the 12 volt DC port. ....
Why is that? That's something I never understand about HBs "AM Transmitter Challenge", why are the test performed without a modulated signal?
@artisan-radio One thing that wasn't done here. Doing this at different frequencies. This was done at 90.5 and it may be that different frequencies may perform differently.
AM sensitivities are expressed in uV/M most of the time which directly relates it to field strength but FM expressed in uV is a bit different than field strength. It means a voltage at the antenna input of the receiver not field strength in space(microvolts per meter) ...
That's interesting, I never actually realized that, but I've never claimed to know much about radio theory, but I find it interesting that you often need to actually shorten your part 15 FM transmitter antenna to maximize your output in relation to whatever frequency you broadcast on. Either by adjusting your retractable whip, or by actually making it shorter by snipping it off if it's just a wire. -- I never knew that either until today. I find it curious that none of the part 15 user manuals even mention this fact.
I don't think Mark's statement about FM being measured in uv is quite correct.
Field strength is always measured, whether AM or FM, in uv/m. Sensitivity of a radio is measured usually as uv, again whether AM or FM, which is the minimum voltage induced at the antenna terminals of the radio by the field (so many db above the noise - usually 20db) that can be decoded. The math to calculate exactly what field strength you need to induce a 1uv voltage has been done on this Forum in the past; I just can't find it. I remember that the factor was about 2.5 - a 1uv voltage is induced at the antenna terminals of an FM radio (I can't remember whether it as a resonant antenna or a 1 meter one - they're about the same for FM) with a 2.5uv/m field strength.
Some car radios can detect a 0.5uv FM signal (generated from a 1.5 or less uv/m field strength (20db above noise).
Dedicated communications receivers have about that sensitivity for FM. About 10uv for AM. Some are even a lot better (particularly the older ones on AM, I've seen some as low as 1uv).
If you have a resonant antenna for AM (so, about 45 meters), you'll get the best results of course. I can't remember if any calculations were done here for the antenna factor for the AM broadcast band.
But given the horrible sensitivity of most receivers today on AM (some well above 100uv), and the noise levels on the band, never mind the short antennas, it's a wonder that AM can be heard at all.
@richpowers You adjust the length of the antenna for the frequency as in the case with the decade MS-100 as it is certified that way to keep the 1/4 wave length for the frequency you are on.
As for why the tests were done in the case of the transmitter challenge without modulation is because the rule states 100mW into the final amp unmodulated. Why? Because if the input to the final is modulated the power is fluctuating and a DC input is needed to get a stable value for the measurement.
@artisan: For example, a GE super radio has the AM sensitivity in uV/M, 65 or better so reception of signals or sensitivity is directly related to the field strength. But FM sensitivity as hi fi is, is stated as in uV.....not per meter, so it's not referring to field strength but a voltage in the antenna. What field strength you need to get that voltage to the input of the antenna with the stock telescoping antenna to the RF amp is really not known. The FM sensitivity of the GE super on FM is 8 UV, still not near as good as hi fi tuners or car radios.
AM in portables and home hi fi uses the coil around a ferrite rod so it is different in operation than a linear length of wire. Like a loop it is the inductance that resonates and couples the signal to the receiver input. Older tube radios had an air coil on the back. In cars with outside whips you can't really have a 45 meter antenna so the receivers are made super sensitive to work with smaller signals from the short wire.
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/%23ham_radio/Receiver/RCA_Superadio_MK2_serv.pdf
Just a thought, while I'm thinking about it.. I don't know why you posted this here where it'll get lost in the midst of the "Everything Else" category. This is a legitimate part 15 hobby related topic, and a good one, why post it here? It's not just another random look at whatever.. It really belongs under one of the part 15 specific threads so it can be found again.