I'll preamble this post by saying this is not a "cut into it and make it work better" type tinker, but using a commercially available (at least a couple years ago) transmitter of a type that passed FCC testing to get an idea what untampered "Mr Microphone" type range actually is and how it compares with the transmitters and ranges often discussed here. Basically an "on the cheap side" bit of practical testing towards understanding legal part15 FM range.}
I was going through some assorted "junk" in the basement recently and found an FCC type approved transmitter I had forgotten. Recalling the remark the one FCC agent made in one case where he told the person to set his gear to about the same range as a "Mr Microphone", I got to wondering what the range of such toys actually is (considering I had one in my hand).
I originally bought the device a few years back for adding voice to a skeleton dummy out on the porch in a chair on halloween. Put a radio under the chair and my teenage daughter and I had great fun making people jump with eerie comments as they came past it to come to the door for trick or treat. Then it was put away with other halloween decor and I ran across it when going through some assorted junk in the basement. It still works, so I got to thinking a bit.
The unit is a "Kawasaki FM Mike" bought at a local store (toy dept) for about 5$ if I recall correctly. FCC II650003, made in China. I looked it up on the FCC site, and lo and behold, there were the test results and schematics and etc. Checking through the test results, if I am reading the report correctly, it tested out at 151.4 uV, which was within the 250 uV limit and so it passed the test for it's ERP being correct to produce a field within the legal limits. (Hey, it's a 5$ toy for cheap mass production and random use by children, so one wouldn't expect it to be right at the actual legal limits).
But I got to thinking that since it's an FCC tested type device (or at least the prototype was), that it might be interesting to see how far such "toys" actually do get out..
First trip was to the park with my daughter and the toy and a portable reciever with battery power. The test reciever was a Phillips AZ1030/17. I don't know the specs on it for sensitivity, but don't really need that for comparative measurement and figure it falls under "typical inexpensive boombox".
The FCCID II650003 transmitter (I'll just call it the 003 for short) did surprisingly well. In open terrain, the signal was good and strong (full quieting of background noise) up to about 90 ft! Speech was still quite legible at 120 ft but some background noise. At about 200 ft the signal was barely audible through the background hiss, and distant stations. Since the unit tested out at it's FCC compliance testing at about 60% of full legal power (151.4/250 * 100%), I didn't expect it'd make the full 200 ft and it obviously didn't. But still, far better than I expected from 5$ worth of toy. I'd never tried using it for a greater distance than perhaps 15 ft on halloween.
That was with the little headset microphone being worn by one of us as they paced away from the reciever while counting aloud while the other "tester" listened to the reciever with headphones.
Next test was trying it with some elevation. I taped it to one end of a 2X4 and stuck it out the attic window. The attic window is approx 30 ft up, so call it maybe 3 wavelengths of elevation on the FM broadcast frequencies. To provide audio, a battery operated CD player was also taped to the 2X4 and the microphone of the 003 taped to one side of the headphone of the cd player. No connection to ground and no electrical connection and all the cables of the devices kept coiled up to avoid the CD player somehow perhaps acting as a significant antenna or ground. Range to the Phillips reciever was quite clear even at 80 paces (with clear line of sight to the antenna) which was about 240-250 ft. Full quieting to the Phillips reciever at that range. By about 300 ft, the signal had some audible static and it dropped out into static and noise at 350 ft. I conclude that most probably the Phillips receiver I use is a bit better than the "cheap radio receiver" referred to in FCC range comments for part15 FM.
So then we tried the range to a "good car radio" which is an Alpine that picks up quite well. Signal was *clearly* audible at 1/4 mile and then became progressively more spotty but would have been "listenable" (if you were direly interested, but most people wouldn't have listened with it being that weak) at about a half mile.
The only *really* cheap portable I have onhand is an "FM multiscan radio" from the local dollar store (like it literally cost 1$ brand new). With the antenna outdoors, it picked up the "003" clearly up to about 150 ft with the transmitter outside the attic window when there was clear line of sight. It is not particularly sensitive at all and only receives the strongest local stations. it's an itty-bitty little piece of junk, but useful when one wants to be out in the yard weeding the flower beds or sitting in the yard with an iced tea and still listening to one's music/news/weather/etc via your station and zara, which is all I'd hoped of it.
And that's in the summer with many trees on the street that are in foll leaf. I was impressed.
Call it "250 ft to a not particularly expensive name brand 'boombox' with enough signal for at least close to full quieting" for a unit that (assuming the manufactured units are reasonably similar to the 3 samples submitted to the FCC) is at only 60% of the allowed power.
The documentation for the "003" can be viewed [HERE], and as you can see from the photos is a very small unit of the def "toy" variety. The only difference I can see between the ones submitted for the test and the one I have is that mine is black and silver instead of pink.
Interestingly, it has *no* external antenna at all. From the photos in the FCC docs, I'm guessing the bit of red(?) wire kind of coiled in a little hank and taped inside the unit is the antenna.
My point on all this is that the "Mr Microphone" range is actually fairly usable if one is interested in "broadcasting" to one's yard and near neighbors. It was not an unrealistic comment/suggestion made by the FCC inspector in the one report where they told the person that his range should be about the same, and it's actually pretty close to the numbers we often toss around on this board for an FM part15 compliant station.
The 003 is a fixed frequency device though, and while it looks like it would be easy enough to modify it to tune to other frequencies, I'm finding it an interesting enough little toy as a "FCC approved part15 FM device" for playing with range comparisons just as it is. It's output frequency is around 100 mhz which is *not* particularly clear in this area, and so I have no plans to see about adapting it for use as the transmitter for a station (though for someone living where there are no licensed stations near 100 mhz, it might be a viable option with a tantalizingly low price for a first transmitter). I know of no way of measuring the actual power output to the bit of wire inside the casing that probably is the antenna without opening it up and getting out a soldering iron. I'm not inclined to do that since I've been finding it interesting seeing what an "FCC approved" type device can actually do, and been finding it rather surprising how well a 5$ toy can do when mounted approx 30 ft up off the ground.
I'd be interested in any comments, ideas for further crude range experiments and etc anyone can come up with, *provided* they don't involve opening up the case of the "003" unit, since I plan on using it to adjust my actual station to closely match the range of the unaltered "003".
Daniel
PS.. Oh, how does my actual station compare to the "003"? I was going to skip that since it's mildly embarassing. Well.. the sound quality is better on my actual station than the way I tested the "003".. But the range of my actual station is a bit less. But that is due to the antenna being inside the house which has aluminum siding and the antenna mounting is at about body height and is near plaster walls and etc etc.. Also the 003 was tested mounted at the front of the house and the antenna for my station is near the back of the house and 20 yrds wasted it well, 20 yrds wasted and not insignificant with part15 FM. I really need to go to an outdoor antenna and attenuate it down to about the same range as the "003" this summer. Which is the reason why I am not inclined to alter the "003" from it's "factory cherry condition". But when I *do* set up my station to match the range of the "003", I'd actually add about a house in both directions up and down the street to my usual listener range. I still don't assume it'd be of any interest to listeners in cars since who in their right mind would tune in a station to listen for maybe 2 whole blocks as they drive past? At 30 mph they'd barely get it tuned in before it faded out.
What about the fact that his range increased when putting his transmitter 30ft up? doesn't this mean he is producing a given f/s at greater distances? say he has a radio that to achieve full quieting requires 2uV of signal. now he has a legal type certified FM that when 1 meter above ground puts out a f/s of 250uV/m over a 3m path. now his receiver goes about 150ft before it starts to get static which would indicate he's at about 2uv of f/s. now he raises his transmitter to 9 meters and all of a sudden the same receiver which received a 2uV signal at 150ft now gets that 2uV signal level at 400ft. would this not indicate that as a fact of power and antenna height he has increased his f/s above 250uV/m @ 3m?
am i over analyzing this? There is a reason they use both antenna height and ERP when figuring out what class an FM station is.
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree
kc8gpd wrote What about the fact that his range increased when putting his transmitter 30ft up? doesn't this mean he is producing a given f/s at greater distances?
Yes, but not because the radiation from his Part 15 FM antenna exceeds the limit then. It is because the losses along the propagation path may be reduced as the antennas at both ends of the path are elevated above the earth, and obstructions.
//
well then, there a good possibility of using part 15 fm in a cellular format using a master Tx and solar powered repeaters scattered about town 🙂 for small rural towns that are maybe 2mi x 2mi, you may get by with a setup like this or along a major thorofare where traffic backs up on a regular basis. a good part 15 fm system seems can serve 300ft radius cells, which means you can place tx's 600ft apart.
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
http://www.ulc.org
Moderator Hunterdonfree
http://groups.yahoo.com/group/hunterdonfree
I'd bet that 30 ft off the ground at 3 meters, the reading would still be the same as on the ground at 3 meters, or very very close. In any case, it wouldn't be anywhere near 600 times more to take an example from the other threads.
Considering actual commercial type approved part15 FM transmitters aren't all that expensive, it's not impractical to consider using several to cover a small neighborhood if they're spaced far enough apart that the overlap definitely wouldn't exceed the 250uV level.. Since FM has "capture effect" and any slight differences in frequency wouldn't result in heterodyne (which you can't hear on FM), for a very small town or a neighborhood, it could be done.
The "approx 200 ft range" is actually quite usable for a lot of things.. Think of inside a church so the hearing impaired could hear the sermon without having a PA system (which would cost considerably more).. Or on the regular community level, say the local stations no longer cover little league games and you like baseball.. get permission to set up in the middle of the stands and 250 feet would allow you to "play old time radio announcer at the ballgame" for a lot of people at the event who might miss that sort of thing. Or for other news coverage events, set up the transmitter where the people are and cover it via something like cell phone or a FRS walkie talkie (if you live in an area where there are usually channels open).. FRS wouldn't have the sound quality for music, but for just voice for a special event it would probably do if one didn't want to risk something like a cell phone.
The antenna for FM is small and easilly made portable. A natural for such things. But even for a fixed station, if you want to cover your yard and near neighbors, it can do it. Depending on how far away the neighbors are.
While there's a limit in the part15 rules to how many homebrew devices a person can have on the air at once, I don't think there are any for approved units? So yeah, it should be possible to cover a very small town or a stretch of road by using a number of legal part15 FM devices.. For a single transmitter station, the answer to better range while staying inside the law would be in getting better receivers to your listeners.
But yeah, I think you might have an idea there, Rev Chrysafis. Solar cells charging a small battery should be able to power part15 FM devices easy enough. I noted that the measurement of RF leakage via the AC line was "not applicable" in the case of the "003" because it's made to use batteries. So something like solar cells might be preferable to power supplies since hooking it up to a wall wart might be using some part15 FM devices designed for battery use outside their test prams. Maybe use something like a zener diode and a current limiting resisitor with a small solar panel and a rechargeable battery to make sure the power supplied isn't above whatever sort of battery the unit was designed for?
But the main tricky bit would be making sure the fringes didn't overlap enough to exceed the allowed levels at the given distance from *any* antenna in the system.
Just "thinking out loud" with all that.
Daniel
Daniel wrote: I'd bet that 30 ft off the ground at 3 meters, the reading would still be the same as on the ground at 3 meters, or very very close.
True, radiation from an FM (but not an AM) transmit antenna system itself is independent of its height above the earth, other things equal.
But even so, the FM field strength present several hundred feet away may change with different heights of both the transmit and receive antennas. This is the result of differences in the propagation path for those different heights, where path obstructions and path geometry have changed.
Also reflections can as much as double the free-space field. They can also cancel it.
For clear paths, and within limits, VHF field strength at a given receive location varies about with the elevation of the receiving antenna above the earth. A receive antenna placed twice as high will "see" a field strength about twice as high as at the lower elevation.
All of this has interesting implications for the distant Part 15 FM fields that the FCC measures and cites.
//
Rattan thank you for a really great thread and Scwis thank you for restoring this thread to the new colo.
I think that we (or at least I) sometimes forget about what happens between our signals and the radios that receive them, especially in some of the thoretical discussion.
Rich's post just above this reply reminded me of that.
Very few of us (yardcasters at least) would bother with placing a P15 FM xmtr and antenna in a clear, unobstructed spot 30 feet up. Obviously pros do it at hundreds of feet or a thousand feet or two when they can to get the better coverage. (That made reading about Rattan's experiment really interesting.)
A long time ago (over a year), I had posted some questions here, and Rich graciously responded to some of the technical details.
QUOTE
RH: There are places inside my house, 3 meters from the transmitter where you can not pick up its signal. Nada, niente. That tells me that there is a lot of attenuation in the signal's path to those spots. (There are also other spots where there is a good signal quite a way's out.)
Rich: Receiver performance depends in part on the radiation patterns of the transmit and receive antennas, obstruction losses, and signal reflections from other conductors and surfaces along the radio path. Together they can produce the results you report.
UNQUOTE
When I read Rich's now infamous 11 nanowatt analysis, I tend to forget that he assumes a theoretical free standing dipole, literally in the middle of free space outside.
It is probably impractical to model the "real world" problems of what Rich calls QUOTE obstruction losses, and signal reflections from other conductors and surfaces along the radio path UNQUOTE
I think Rich points out that transmission line losses are typically negligible for very short runs. There may also be some (probably small) losses from how the antenna is physically or structurally mounted. But my own experience (as quoted above) is that these obstruction losses and signal reflections presumably created by my own house can have a huge impact on transmitting basically inside the house and around the yard.
In the set up I described, I experimented with a couple of FCC Certified P15 FM xmtrs (C. Crane and Belkin Tunecast II) plugged into a PC audio source in my home office. The receivers were Sony boomboxes: a Sony CFD-F10 and a Sony CFD-S500. I could not receive anything in the Kitchen that was 3 meters (10 feet) away. (The car radio also could not receive anything, in the driveway, another 30 feet away past the Kitchen, or maybe 40 - 50 feet from the xmtr.) The extent of the obstruction losses from some intervening closets, doors, windows and hallways surprised me. The house is not a "steel cage" mobile home. It has conventional stud framing with sheet rock interior walls and vinyl exterior and asphalt shingles. There are probably a lot of electic and Cat 5 wires along with plumbing and ductwork for the heating system inside those stud cavities. We are in a very rural area, so there are no other structues anywhere near the house.
Moving around the xmtr did allow reception in the Kitchen, but not outside.
I would imagine that obstruction / reflection losses can huge for our very low powered P15 FM devices. Certainly they have a big impact for my P15 devices. However the obstruction / reflection losses at my house seem to have no noticeable impact on the distant (30 - 50+ miles) broadcast FM signals that I receive inside the house without a problem. So presumably the distant FM stations are hitting the house's exterior (and interior) with a signal strength much higher than the 250 microVolts / meter at 3 meters that is permitted for P15 FM.
I realize I could go crazy with a "proper antenna" (which might negate P15 Certification) and I don't want to roof mount or make anything to visible (inside or outside) with my yardcasting due to the family's sense of neatness. I suspect that in my situation that I have described, it would take an awful lot more than 11 nanowatts to achieve a listenable signal in the Kitchen 3 meters away.
And maybe this sould be a separate thread. But I would certainly welcome any helpful feed back on this whole issue of obstruction losses and signal reflections.
Hi, RH -
Your Part 15 FM transmitters may be meeting Part 15 by allowing the antenna to radiate far less than the legal limit.
IMO, a Part 15 FM tx radiating the maximum legal field should be receivable on a decent receiver in most locations of a typical home with the construction you described -- especially only 10 feet away.
As a test could you take it outside, and see how far away you can receive it over a clear path on a car radio? If it is not at least 200-300 feet then I'd suspect that your Part 15 txs are radiating considerably less than Part 15 allows.
//
About a year ago the NAB commissioned a lab study on the field strengths of many of the FM part 15 transmitters. Here's the link:
http://www.nab.org/xert/corpcomm/NAB_Part15_Study.pdf
In this you will find the list of the units tested and a graph which shows how their field strengths compared with the FCC limit of 48 dBuV/m (equivalent to 251 uV/m) at 3 meters.
Calculating backwards, the CCrane unit produced a field strength of 100 uV/m so if this is representative of your unit it is hard to explain the very short range you reported. Even with reflections, moving the receiver around should produce a signal.
The data presentation in the report makes it difficult to compare the field strengths as we are used to discussing since the graph is in dBuV/m at 3 meters. Since dB is a log function, the data are compressed and a unit with double the field strength of another will appear only 6 dBuV/m higher. Though it doesn't appear to be the case on the graph, there are very large differences in FS of the devices tested.
If you are interested, you can pick the dBuV/m off the tables or graphs and convert to uV/m.
uV/m = 10^-6 (10^(dBUv/20))
For example, 48 dBuV/m converts as
uV/m = 10^-6 (10^(48/20)) = 251 uV/m.
That being said, the results of Rich's suggestion of a outdoor "free range" test may be interesting.
Neil
Hi Rich,
Your point about the device being BELOW the P15 LIMIT is well taken. Obviously there "is no glory" for a mass market consumer manufacturer to exactly hit the P15 limits on the nose, and the manufacturers can have the usual corporate conservatism and design the device to be well below the limit and still sell plenty of the units to a consumer that plugs their iPod into the transmitter and places it on the car front seat next to them, namely a distance of less than 6 feet away from the receiving radio.
(I suppose it is more likely that the manufacturer of an hobby product like the SSTran or the Rangemaster that I read about in this forum probably wants to try to engineer as close to the limit as they safely can, given the market of the type of hobbyist who participates in this forum typically wants to push right up to the legal limit. But this thread is not about AM.)
Neil,
Thank you for "translating" the NAB study. I had seen it referenced in this forum last year, but your math makes it far more readable and relevant to this forum and discussion.
While I cannot argue with physics or math or $10,000++ field strength meters, in my own simplisic A-B comparison across the kitchen a year or so ago, the C Crane (Device 5 in the NAB study) positively whipped the Belkin Tune Cast II (Device 4 in the NAB study) in terms of vastly superior range. Paradoxically, the NAB study would suggest the opposite result. My informal comparison was at 98.9 where the NAB claims a 51.7 dBuV/m (which exceeds P15 limits of about 48 dBuV/m that you calculated) for the Belkin vs 41.7 for the Crane. Go figure. I would have probably expected CCrane to come pretty close to the legal limit, since they seem to market to "radio enthusiasts" and I would have expected Belkin to play conservative for the 6 foot car range crowd, since this is a small sideline for Belkin's vast array of PC acccessories.
Maybe either I (or the NAB) had a Device that was not consistent with the "normal" quality manufacturing run. Or my house's obstructions and reflections are really unusual.
I would like to know which P15 FM device really does come closest to the legal limit, but at this point I am not sure that I would rely on or trust the NAB study, given my personal experience.
For both of you, the suggestion of a outdoor "field test" (like Rattan did to start this thread) is an excellent one. I know that my time schedule won't let me do that anytime soon, but if I ever do, I will report back. For now it will be armchair stuff.
Any other thoughts on the obstructions and reflections? Or a Certified device that exactly hits the FCC limits?
I've done extensive testing on the BA-1404 and other low power FM devices, and frankly if you design a transmitter so that it hits the FCC limit, it won't go very far. I would expect most certified devices to be well under the limit, since production variations can easily cause the level to vary significantly, and nobody wants a $1M fine (remember Behringer)? There is always the possibility that you could get a "hot" unit that is right on the limit, or a low unit that is putting out a weak signal. By the way, the output spectrum of a BA-1404 is pretty dirty unless you put significant filtering after it.
If you're trying to use Part 15 FM as a low power broadcast system (not just trying to reach your iPod), I don't see much hope unless you can put a network of these things on the air. A small town that could afford to mount transmitters on every other light pole and distribute audio to them could make it work, as suggested by another poster. But most individuals couldn't arrange something like that. For network operation, I would suggest tuning every other transmitter one channel up (or down). We have a regular broadcast station here in Northern Illinois (more than one, actually) that uses adjacent channels (e.g. 96.9 and 97.1 FM).
FCC rules allow 50 uV/m at 15M, which is about 50 feet. That signal level is what the FCC considers adequate for reception in a rural area, where there is low ambient noise and interference. It's not a lot to work with.
My personal recommendation would be to mount a certified transmitter with its supplied antenna up high and in the clear outdoors, just like you would do for AM. In other words, try to minimize the losses going through walls, wiring, etc. That's still not going to go very far at all.
What I would really like to see is some sort of low power Part 15 broadcast operation in the 2.4 GHz band using digital modulation. You can use up to a watt in that band. Under the right conditions, you should be able to cover a small town, etc. But there doesn't exist any standardized format for transmission and reception that I know of. On the other hand, the idea of broadcasting is that ordinary people can easily recieve your signal-- if it requires a special receiver or antenna, who's gonna listen?
Seems like AM is the way to go... though I fear IBOC interference will be the death knell for Part 15 stations.
WEAK-AM
Classical Music and More!
My idea is a town-wide Mesh network using ISM Bands (900MHz/2.4GHz) as the client access and UNII (5.8GHz) as the backbone for the mesh system. the user would log into 2.4GHz or 900MHz to get the stream and it would be transported to each node in the system via 5.8GHz also some form of optical (Free Space Laser) Data link between nodes as a backup or backbone to a mountaintop tower to link systems in different towns together. add to that system a part 15 AM/FM tx at each node as a suppliment.
as a side not it would be interesting to see the FCC in action trying DF each node in such a system 🙂
Thank You,
Rev. Robert P. Chrysafis
Universal Life Ministries
> http://www.ulc.org
Moderator Hunterdonfree
> http://groups.yahoo.com/group/hunterdonfree <
Your idea is good and I read where this is being developed. I don't have the reference in front of me but I think it was describe in Scientific American within the last 9 months.
This would involve the use of unlicensed repeater stations placed so that the signals would propagate through the mesh very much as is done with the internet. The address tags would route the traffic to the destination. Pretty clever isn't it.
I know for a fact that a major telecommunications firm was working on this about 5 years ago so they could provide phone service without the need for wires. Since I have been out of the loop for this time, I don't know where it has gone, but then the problem was that they could not manufacture their microwave product with a high enough production yield to be able to launch the product.
Part 15 allows operation in the 900 MHz band and also the microwave band. There are available many such modular devices which an experimenter could apply to realize your idea. I am now investigating 933 MHz. transmitter and receiver modules (Linx Technologies) which are available in the $30 range for some other uses, but they do support analog and digital transmission and could be used as you propose.
Your idea is good and is an interesting read. Thanks for sharing this.
Neil
Just got my new C.Crane FM transmitter and did some serious testing. Range from indoors was about 200 feet as expected,mounted temporarily 35 feet up its range increased to about 800 feet. Now I will get a nice enclosure and run some wire up the tower and mount it permanently with no fear of a violation(I hope!!) Now for the biggest suprise,this C.Crane is by far the best sounding unit I have tested. It is very clean and true in tone,no crashing of the highs into the pilot. They have done their homework and filtered it well. It blows away my old Ramsey and the EDM,which I have retired due to its high output power(60 blocks away at 35 feet). Now I am thinking of ways to put up a bunch of these certified FM units and link the studio to them. I wont give up on FM 'cause I love it so much. I was looking at the Linx modules,sold only in tubes of 40 for the tx and 25 for the rx. If anyone buys a bunch,I'd like a few...
Regards,Lee
http://www.freewebs.com/wilcomlabs/index.htm
Lee's report coupled with Daniel's might provide some hope for legal FM communitycasting. It seems, through experimenting, that a means to extend the range of a certified FM transmitter is elevation. Lee's data would indicate that his FM system range is approaching what is reasonable to expect from AM.
As researchers say as they close their published reports, "Though this research suggests blah, blah, blah, more experimentation is needed". More is needed but so far it looks good.
BTW Lee, I haven't checked for a while but the Linx stuff was available from Digikey. Linx is rather restrictive in their sales policies so go elsewhere for their products. They are well engineered and well fabricated and I recommend them to hobbyists looking to experiment with part 15 devices outside the broadcast bands.
Neil