As I posted in another thread, I've been researching the sensitivity of various FM receivers. Any statement with regard to the range of a Part 15 FM transmitter that doesn't take into account the specifications of the receiving equipment at the other end - and the most important spec is sensivitity - is just meaningless.
Although I've just started this task, I have some reasonably accurate numbers on high end home receivers, and good car receivers can approach, at least in FM sensitivty, the specs of those home receivers.
So let's do some analysis on the current Part 15 FM rules, which allow a field strength of 250uv/m at 3 meters.
The best home receivers (along with the best car receivers), have a useable mono sensitivity of around 1uv, a mono sensitivity with 30-50 db quieting of 1.5uv, and stereo sensitivity of 15uv with 30-50 db quieting.
Field strength is linearly proportional to the distance between the transmitting antenna and the receiving antenna. So, the field strength of a legal Part 15 signal at 200 feet (about 60 meters) would be 12.5uv/m, line of sight. However, the signal level at the input to the receiver would be around 5uv (math for this factor done elsewhere on the Part15.us site). That's easily within the ability of these receivers for a mono signal - a stereo signal would likely be too noisy, and the receiver would either switch to mono automatically (most car receivers do this) or you would have to switch to mono mode manually. In fact, you would need a field strength of around 40uv/m to receive a quiet, stereo signal with separation - that would translate to a range of 18.75 meters or roughly 60 feet.
Moving to 400 feet, the field strength of a legal Part 15 signal would be 6.25uv/m. That would translate to an input signal of around 2.5uv. You would still hear a mono signal with full quieting on the best home and car receivers.
Moving outwards to 800 feet, the field strength would be 3.125uv/m, and an input signal of 1.25uv. You would be able to hear a mono signal, but it would be weak and could be noisy. You would still have full quieting for a mono signal at 600 feet (with a field strength of 4.69uv/m and an input signal of 1.875uv).
It is likely that any discussion with the FCC and the NAB and others about increased field strength for Part 15 FM will focus on the potential for interference. So it is imperative that we look at the maximum ranges that any field strength can produce. But that's not going to be the typical range that most people will get - most will not have McIntosh home receivers, although it is likely that they will have a reasonable car receiver. I would like to propose that we define a new term - useable range - which also defines a typical home receiver and its sensitivity, and takes into account obstructions (i.e., if your antenna is indoors, a signal has to travel through walls to get to it, reducing its field strength, not to mention other buildings and even topographic features that might get in the way). It's the useable range that we, as Part 15 broadcasters, are most interested in. And we likely have to define both the useable mono range and useable stereo range.
Moving along a bit further, the Canadian maximum legal field strength for FM is 1000uv/m at 3 meters. You merely multiply the above ranges by 4 to get the Canadian equivalents. For example, at 600 feet, with the U.S. maximum, you can easily receive a mono signal with full quieting on a high end home or car receiver - in Canada, that translates to 2400 feet. You would be able to receive a very weak and potentially noisy mono signal at 3200 feet. And a stereo signal with quieting and separation at about 240 feet. But just a word of caution for Canadian readers - those numbers are for line of sight. In the real word, the further you get from the transmitting antenna, the more likely there will be obstructions between it and your antenna (trees, buildings, hills, etc.). And those obstructions will attenuate the signal field strength and reduce the range.
Still not earth shattering numbers, but significantly better than in the U.S. And the sky hasn't fallen in here yet. So I do think that there is potential to increase the U.S. field strength maximums if the requests are reasonable, and backed up by solid technical information re potential interference, as well as maximum and useable ranges.
Great work Artisan looking at the receivers and their quality as a vital component in the low power FM system.
I really woke up to the receiver issue when I discovered that the TECSUN PL-310 clearly receives a certified FM signal over a wide area both indoors and outdoors while other radios cannot get a hint of signal.
Lesson # 1, a part 15 FM broadcast station wants his audience to have the very best receivers.
At the moment I have been shopping for a new yard radio for listening outdoors, and noticed that many models do not publish their technical specifications, just like Artisan mentioned, and this seems to be a clue that these radios are poor.
I have also decided to push for another condition for any part 15 FM station that wants to reach an audience... the transmitting antenna MUST be located in outdoor open space raised ABOVE obstructions. Hate to say it, but anything else is toy time and not serious.
I've been silently reading the math results presented here and at RadioInsight.com at this topic
http://radioinsight.com/community/topic/fm-pirate-in-norwich-on-87-9
I believe that as the crew here at part15.us continue to move forward, the crew over at HB will continue to move things backward again.
There are several sites related to home consumer transmission devices such as part 15 related intentional radiators, of the two that focus mainly on part 15 for micro-radio stations, one is for CLPFM and the other is dead set against it. It's like a tug of war between here and there. You gain three steps here and lose 5 steps over there. My experiences over there, is His word is the only words that count. That ego is going to make things quite difficult, when you try to present your side and you have someone presenting their opinion that contradicts your points.
How are you go to win on a federal level when someone who claims to be an SBE Engineer argues and attempts to prove you wrong? After a while, it will be like pulling hens teeth and bashing your head against the wall. Your points may be accurate facts, but just like over at RadioInsight.com you're going to have someone who claims they are better educated than you and twist your facts into confusion. Confusion that will undoubtedly lead many to give up and give into those trolls.
By the way who is David? You must be a member here.
Bruce.
The nice thing about specifying maximum field strength is that the type of antenna can be left open. You can achieve identical field strengths with relatively low input power transmitters and good antennas as well as higher power transmitters with poor, such as indoor, antennas.
MrBruce had the right idea before anyone else... have people receive the station on computer stream and put Part 15 FM relay stations in many locations. It's the way to do it.
The deficit-disorder trolls who pestered MrBruce reveal something about themselves everytime they do something negative... they have spare time because of empty lives. If they were the grand successes they claim to be they'd never have time to annoy MrBruce or anyone else.
MrBruce it would help you to learn how to work with the mentally disabled. You can't allow their words to mess with your feelings.
David is ArtisanRadio.
I'm not too worried about Hobby Bill and similar individuals. I've dealt with their like before. In fact, I've managed and fired people like them for deliberately and erroneously throwing up sand and dirt to obscure the real facts. If we keep things simple, scientific and factual, and don't allow ourselves to wander off on unimportant tangents (i.e., getting dragged down into the dirt with them), they will only end up looking silly at the end. There are many, many more of us who know what we're doing than them.
It's the FCC and the NAB that we need to focus in on. To alleviate the concerns of the NAB (and thus, the FCC) with regards to interference. To show those two bodies that if this additional class of license is allowed, that it might actually revive overall interest in radio and keep it healthy. Which would lead to more listeners, and more revenue. I have plenty of ideas in that area.
I do think we need to think usable range here and probably for Stereo as we are trying to attract listeners who are in their homes. If your talking 1000 meters your talking 3200 Feet or just around a quarter mile. And yes I don't think we should throw away the inside antenna users though we know we can do better with an outside antenna. Remember the whole part of Micro Broadcasting is to make it affordable to those who want to enjoy the hobby. And again we have some data from Canada as to the interference that was or was not caused. Plus not every car transmitter sold in Canada is going the max range so too I think if we categorize it slightly differently we should have a better result against someone driving and jamming other stations.
Now we know what a 500mW transmitter into a rubber duck can do and the field strength. Are we still looking at something close to 6708 inside I think I'll have to keep re reading this but I have a slightly better idea of what is going on here.
The number one key to local success is to plant those part 15 transmitters in buildings where many families live, like I did with my network. I have one local building that was once an elementary school converted into an apartment building. That building is 3 stories high, has 10 apartments per floor, that is 30 apartments in ONE structure! Now, located just outside this building is 5 additional private homes within 20 feet of the front of this structure to the south and west. In the northern direction unfortunately is a children's park with a play scape. Not much of a listener's spot in that direction. To the east is 3 additional homes within range of the FM signal. Now do the math, that was ONE FM transmitter using an indoor horizontal dipole attached to the vertical antenna of the MS100, the vertical portion of antenna was fully collapsed though, we were transmitting a horizontally polarized signal, even though we added an antenna to the MS100, some may say we altered the FCC certification by doing this, but our intentions was not to get more gain by doing so, our intentions were to be horizontally polarized and you can't do that with a single telescopic antenna laying flat. Don't ask me why, but we tried doing that and the signal did not appear to be horizontally polarized by doing that.
David, it appears Edward over at RadioInsight who is really not anyone I know personally is sticking up for you and me as well, it appears, so yes we had many supporters for our station and people are saddened that we called it quits. But rather than have everyone go through this bullshi* with RFBurns also known as Ray F. Burns at HB, I decided it was best to call it quits. I'll say it again, I felt it was best for the innocent people that were involved. If it was just my house involved, I would not have quit, but then, that would mean me using a powerful transmitter just to get that service area back and we know, that is illegal, so, that is why I called it quits.
If you walked into the room that was once studio A in my house, there is nothing left that resembles or indicates that WXTZ ever existed, with the exception of a very large library of 45's, LPs, Cassette tapes and CDs.
Ya know? I never, ever, wish anyone bad luck, but then there's always that first time, where I hope and pray, Ray F. Burns training wheels fall off of his little biddy tricycle and he goes BOOM!!!
Bruce.
Do the sensitivity numbers consider adjacent channel interference? What about co channel?
Full power stations that use SCA carriers get an increase in their allowable deviation. That means a first adjacent channel maxed out class C far away using multiple SCA carriers could still be putting energy into a channel that sounds clear at a micro power transmitter location. So a first adjacent channel station could cause some on channel noise.
If the goal is range, forgetting stereo would give the micro power signal higher power density which would help it win if the capture effect came into play.
I know a rubber duck is nice because it is rugged, but if indoor coverage is a goal, a circular polarized antenna would greatly benefit the micro power transmission. Circular polarization puts energy into the horizontal plane and with a 90 degree phase shift to that energy into the vertical plane. If a diehard micro power fan were trying to get the signal they could use a circular polarized antenna indoors and they would get more signal. For most normal listeners the V polarized part of the signal would probably penetrate the building better (studs are vertical). But outdoor TV/FM antennas tend to be H polarized, so we need a H component to reach them.
Signals in dBµ were observed on the meter of a Tecsun PL880 receiver tuned to a local FM station on 90.3 MHz (18" whip receive antenna length, vertically polarized, height about 6 feet above the earth). Three locations along a radial line from the transmit antenna were used: in front of, inside, and behind my home. My home is of brick veneer construction, single-story ranch.
Front yard: 56 dBµ, varying about +/-3 dB across a ten foot perpendicular bisector of the radial
Centered inside the home: 56 dBµ, same variation as above
Back yard: 56 dBµ, same variation as above.
A legal Part 15 FM station would not produce fields as great as the above, but the variation produced by changes in the receive antenna location would be the same.
So the "building penetration" loss at VHF, per se, is not a serious issue as concerns these types of structures.
Good information Rich. Thanks.
As for interference, I believe that the manufacturer numbers are best case. Selectivity (rejection of adjacent channel interference) is definitely a factor that has to be taken into consideration, particularly in crowded band conditions.
Mono signals will definitely get the most range. Even if you broadcast in stereo, the range in which you can actually receive in stereo is limited, given the factor of about 10 in sensitivity that these high end home and car receivers have. I have no reason to believe that this factor will change with mid range receivers or portables either.
I broadcast up here in Canada in mono (I experimented with stereo and found that it did get less noise-free range, even when you were receiving in mono). I even stream in mono. It maximizes the number of listeners I can have on my stream (I run my own server and pay for the bandwidth). Plus, a lot of my programming is in mono, and isn't all that affected even if the source material is in stereo (right now the programming consists of 50s and 60s obscure teenage-oriented pop music - 'The Best Music You've Probably Never Heard' as my slogan goes).
The VHF field readings found by Rich (Post#10) are better than many would have expected, which of course comes as comforting news.
To learn that typical, common, or regular houses are essentially transparent to VHF/FM signals improves the outlook for Part 15 FM work in ordinary neighborhoods.
Of course there are types of construction and obstacles that more or less attenuate FM signals, and composing a table of such data invites yet another side-study to predict signal penetration in given situations.
Now big of an antenna are you talking about? A Transmitter like the SainSonic AX-05B does have a TNC connector where when the desplay of the transmitter faces you you hold the antenna in your hand and put it into the TNC connector and screw counter-clockwise. I probably could get a male TNC connector and have it fed into some RG8 coax into a circular antenna on top of my desk beside the window if they make such an antenna. Then if we get the permit I could run the 500mW and it may go a mile if I were to have been the one doing the experiment. Also in the experiment I saw it was Stereo and dropped off like a Rock and I mean a Rock after the 1/4 mile range. It was like the meant for that TX ot only to 1/4 mile. The FCC would have Loved it because if they were to give us that 1 mile they can rest asure that there is No Signal after X feet. I've never have expected a tx to go from nice Stereo to dead static within a few inches past the mark but that was what happened. The experimenter ever went further to see if the signal came back and it did not. Now if that TX was outside maybe a few feet up yea I am sure it would had went further but the experiment was meant for a rubber duck (or any inside antenna).
The experimenter could have not done mono because the TX was only a Stereo FM Transmitter. But again I was very happy with what I saw with it. How much further do you think it would have went if your saying Mono would have made all the difference?
Worst came to worst and I'd not want to TX Mono but I suppose I could have done a show half in extended Range Mono and half in Stereo. That would be if we get this thing passed. Would it also help to make sure our TX's don't cause interference with nearby channels? That I too don't know if that would make that much difference. Then I'd have to ask would an Album Rocker listen to a Mono signal?
Somewhere I mentioned that a new translator is operating on MY frequency at 101.5 MHz, but I can't hear that translator on my ordinary indoor radios, so it hasn't been a problem.
But then I thought about the TECSUN PL-310 with its vastly superior FM radio, and WAS able to hear the translator sitting at my desk, but ONLY if I touched the telescopic antenna with one hand.
Tonight I took the TECSUN outdoors to see if the reception of the translator is any different, but it's exactly the same out there as it is indoors, confirming Rich's finding in Post # 10.
A whole world opens up with membership in the ALPB.
Just what do you have planned? Is this gonna be a secret only for us ALPB members to know in order to keep the Johnny Punks from getting on FM? Well just Email me or call my Rockline Tomorrow as I'll be Live. I'd be interested to know just what this is if your not willing to give us any info here.