low power broadcasting

Broadcasting license free under Part 15 rules in the FM band is challenging.  The FCC restricts the allowable signal by measured field strength.  Yes, they state the legal range is about 200 feet to a normal receiver which leaves it open to much speculation.

Measuring the micro-field strength allowed is difficult if not out of reach of the average user.  Equipment to accureately measure at these low levels is expensive.

License free FM broadcast transmitters marketed in the US must be certified as FCC compliant meaning when used as supplied the field strength should be within allowable limits.  Many of the manufacturers offer "hints" as to how to extend your range but these generally violate the rules.

At 1/4 watt your transmitter could easily exceed the field strength limits if the antenna is modified.  6 blocks to a car radio might be acceptable but seems to be pushing the envelope.  Using a boom-box style radio, if you hear the signal reasonably well at more than 200 to 300 feet your signal is most likely in violation.

You can use the 1 watt amplifier by reducing the efficiency of the antenna so there's no point as the end result must still be within the allowable field strength limit which results in the same distance.

In short, the rule DOES NOT set a limit of 200 feet, rather that's about what you should expect.  What the rule does limit is the field strength which generally we can't accurately measure so we go by the 200 foot rule of thumb.

I got to wondering what the actual obligation of a law abiding person should be when the rules specify adherence that the average person has no means of measuring?

As a comparison, imagine automobiles with no odometers... no indication on the dashboard of the actual speed of one's moving vehicle. Speed limit signs would wisk by as the driver guessed about his speed. What would be the "200 foot rule" in that case?

Using "certified" transmitters as the actual measure of comparison, what I've found is that if the strength of an FM transmitter is just barely present... one step above being "off," just below getting farther than 1-room of the house, it's legal.

That table is somewhat misleading.

It's true, say, that your field strength at 630 feet is 3.8uv into a 1 meter antenna.  However, that doesn't mean that a 3.8 uv signal is injected into the receiver.  There was a series of posts here a while back which calculated that it takes 2 - 2.5 times a given field strength to inject a signal of the same strength into the receiver.  That means that that 3.8uv/m field strength translates to a signal strength of 1.5-1.9 uv at the receiver's antenna terminals, about the lower end of the sensitivity range of an extremely sensitive car receiver that gives you some quieting.  You might be able to hear the signal further than 630 feet with noise, or if you have a better antenna.

Of course, that would also have to be line of sight, no obstructions between the receiving and transmitting antenna (i.e., ideal conditions).

Multiply field strength by 4 and you would get the maximum range in Canada, which works out to be about 1/2 mile with some quieting, a bit more if you can live with a noisy signal.

Considering that most modern 'ordinary' home receivers have sensitivities that are in the 15-25uv range (the exceptions, of course, being high end modern receivers or vintage types, which can rival good car radios), in fact the 200 feet rule of thumb is being generous.  With most, 100 feet range is doing well.

But some digital boom boxes have better sensitivity than others.  Now the Sony CFD-S50 does seem to have improved sensitivity in the FM band compared to some other boxes I've heard.  It could receive a certified FM Transmitter far better than 150 to 200 feet.  Some older FM tuners do have signal meters.  What about a modified 2 meter ham Radio SWR/Field strenth meter re tuned for the FM band?  you could measure how many uV/M your transmitter puts out 3 meters away (10 feet) and make any adjustments.  I'm hoping the FCC will increase the part 15 FM limit to be the same as Canada's after all there is little issues there and one can buy a transmiter while in Canada and bring it into the USA which many people already do so why not simply follow the same suit here.  A petition to the FCC should take place for part 15 FM.  Maybe bring back the 100 mW rule as it seems to be fair and with common sense should not interfere with licensed broadcasts.  Now that 87.9 is FREE it should be considered as a part 15 FM channel for people to use.

There has been some discussion that the FCC begins enforcement at the 1,000 uV/M level already on FM. I will dig around and see if I can find it.

I think the Canikkit transmitters are 1000 uV/m @ 3 meters if I'm spelling the brand right.  It was a USB FM Transmitter for $89 and they say it boes about 900 feet.  Now this is about the same level as the SainSonic AX-05B transmitter at the H power setting.  Funny thing is that the FCC ID# cirtification is on back of the transmitter.  Could this already mean that behind the scene the FCC is gonna change to be the same as Canada's rule?  Another rule I think the FCC should make is that the transmitter scans for FREE frequencies and only allows transmition to function on a quiet frequency and if one tries to broadcast on a used station the transmitter will not operate and display ERROR or USED.  Or at least flash notifying the operator to change the frequency.  Common since manufacturing should be the rule for these devices so Joe Smith can't interfere with a used station.  It should have a sensor to tell if your moving and if you are it drops to a 10 foot range only and refuses to operate at any higher power than 10 foot.  This will kill any issues.  Hell WiFi devices has this feature how hard is it for an FM Transmitter?  Not hard I say.

No I doubt the FCC will change anything, at least in writing. There is a lot of gray area in measuring emissions especially at some distance from the source. The technology now employed for enforcement uses a fudge factor in favor of the suspected culprit. While they certainly can and do use analog FIM's the newer spectrum analyzer/computer utilize such things as Google Earth to measure the distance between the source and the FCC receiving antenna.  Also, they can and often do, take measurements from multiple locations. I remember reading one such evaluation and the fudge factor if I remember correctly, extrapolated to 1,000 uV/m at 3 meters, hence the Canadian correlation.  Makes perfect sense.

Yes it does.  The USB transmiter in question when I read the specs was 1,000 uV/M and I thought of the fact it must be running 500 millawatts into that telescopic antenna.  This is the same specs as SainSonic's AX-05B at the high power level.  The low power level is 61 meters but I'm guessing this would be closer to 250 uV/M because its certified.

At 1,000 uV/M your signal can go 900 feet which is 3 footall fields of distance.  This is a nice neighboorhood transmitter and could cover say 6 city blocks.  Thanks for the info I'll have to do some more reading into this fudge factor.  Also I've read about the Hole House FM Transmitter 3.0 and at the standard power level some have reported a distance of 1/4 to 1/2 mile.  I'm sure with the secret power level it would go well over a mile.  This is another case where I believe behind he scene things are indeed changing for the better and in favor of the hobby broadcaster.  Just want to see the scan feature enforced on the newer FM Transmitters and I may post on whole house FM Transmitter's facebook page suggesting this feature for the 4th version if they ever make a V4.  SainSonic too to force users to use unused frequencies.  Part of having power is to be responsible with it and this is a common sence employment all future transmitter's should have.

Happy Broadcasting!

To add to the above posts, 1000uV/M is the Canadian legal limit and that is in a classification called "anouncement" transmitters(BETS-1) and the whole idea is to get to a neighbourhood. As the other post stated that can have a range of 3 football fields in distance, allbeit with a very good receiver. The CRTC regulates prgramamming and the only thing you have to do is follow the same guidelines as the commercial stations....no foul language, racizim, other things that would be offensive, plus all programming must originate with you...no rebroadcasts and nothing political.

Mark

wdcx, if you get that info. me and others I'm sure would be interested.

Mark

Hmmm That is what I thought the AX-05B and Fail-Safe CZH-05B was sold as an announcement transmitter.  They talk about the American Cancer Soceity using it to announce during Relay For Life.  And No obscenities should be a common sence rule as well as no recest, indeacency of any kind over the air.  This is why folks broadcasting Howerd Stern is an issue.  I'm sure we could petition the FCC so we really didn't have to fudge and have announcement transmitters with 1,000 uV/M legal.  Look at say the Talking House transmitters they are announcement transmitters.  So if you wanted a low power neighboorhood station that plays music and say announces the albums it came from and didn't play more than 2-3 songs in a row from the same artists you'd be following the same guidelines as commercial Radio and yet allowing your neighboorhood to enjoy some tunes you don't hear everyday.  There is many uses for an Announcement Transmitter.  Thanks for the FYI on that part of the legality of it in Canada.  And looks like its starting to trickle down to the USA as well just not in writing yet but I bet will be in the long run.

Even though they are called anouncement the CRTC doesn't say you can't play music or have regular programming....just the rules I said. As I understand in the USA the FCC regulates everything .....the technical things and programming and there's one classification: 250uV/M@3 meters for no licence.

For AM here(Canada), it's 2500uV/M@3 meters in that same classification"anouncement".

The 3 meter antenna, ground and feedline, 100mW etc. doesn't matter here. But when you consider that AM can't get into places that FM can, all the noise interference from hydro, and people not wanting to listen to the bad sound quality FM is the way to go.

Mark

Yes Marc that was my assumption of an announcement transmitter that I could play Progressive Rock (Album rock), Classic Rock not use foul language or do indeacent stuff when I use my FM Transmitter.  This all is good because 900 feet really is nice if I can get full quieting Stereo sound all the way to the 900 foot level on a Sony boom box.  Car Radio's of course would eceive it about 1/2 to 1 mile with some drop outs here and there till the signal is gone.  I'm sure the FCC would really consider this in actual writing if we can petition especially with the hands off pirates unless they cause interference rule but to ease our minds we can try and get this rule in place with the stipulations I've posted for the new transmitters.

Marc I think you and I are on the same page and its nice talking with another hobbyest who has some of the same thinking as I do.  Keep up the faith man and I'll be on air soon still scanning the blank freq's every night during DX conditions to be sure my chosen frequency is always blank.

Best Regards.