Just two things I see all the time that bug me.
1. On radio forums like this, when people are discussing Part 15 FM radio coverage, someone always says something like "xxx feet to a sensitive car radio" or "1/2 mile to a sensitive car radio". Can you just hear the husband talking to his wife "OK, honey, w've got to trade in the car. I need one with a more sensitive radio so I can sit in the driveway and listen to that little station the guy up the street is running out of his bedroom". This is a silly standard to reference. Car radio coverage means nothing to a radio statiion when your coverage is as small as Part 15 FM. A car driving through your neighborhood at 30 miles an hour travels at 44 feet per second. If you had a coverage of 300 feet a passing car would be in your signal range for 600 feet (300 either side of your antenna) which means he could hear you for about 13.6 seconds. If he slows down to 15 MPH he might hear you for about 27 seconds, but your programming would be drowned out by the horns honking behind him. I know it's because people don't want to reference their coverage to "the house next door, or maybe two over, if they're listening on any typical radio they may own in their house". 'Cause that doesn't sound very good!
2. People in forums who won't say or type "Ebay". Like it's some sort of banned swear word. I bet most of us have shopped there. People instead say completely stupid things, like "Evil-bay" "The place by the water" "I bought it over on the 'bay" or "a popular auction site" . It's just another online business. Call it what it is, for Pete's sake.
TIB
When you're talking about range, you need a reference point. Car radios are a good reference point, with most having sensitivity around 1uv. In contrast, portable radios are all over the map, and range with them tells you little. I know that when I talk about range to a car radio, I make no claims that that range is actually useful range - except when I was on Bowen Island and my target audience was the cars in the ferry lineup.
Ooh ooh, that's me, Eeeebay! I've been on forums where posting site names, anything big and commercial is frowned on, I think it's because people from that site might come in, or maybe it's a spam trigger on the site due to commercial messaging? I've adapted it to my language, except for insider words on the group, like SSTRAN, Spitfire, Ramsey, Blare.
Here's a curved ball for ya, the recent postings of the manuals for the Ramsey FM-25s, they have a sensitive radio coverage figure shown in a table to over 5000 feet for Part15 FM, at 250 uv/m, where the group here has said 800-1000 feet max to a sensitive radio.
I had an FM-10, and the most I got was near 1/2 mile with a wire dipole to a Superadio, II I think, had to get up on a small hill though, to get above obstructions. I didn't use it though, the sound of my FM-10 was terrible!
Seriously? Car radios are a horrible reference point. Just as bad, if not worse than home radios.
We presently own and drive 7 different vehicles, varying in age from 1951 to 2014. With everything from stock AM radios to high end dealer installed radio systems. Every one has completely different radio capabilities. My Part 15 AM comes in best in my 2014 Mustang. Our 2010 Escape is a close second. It comes in pretty crappy in my wife's 2013 Focus. Comes in good in our 1959 F100 with a 1970's am/fm cassette deck radio installed. A coworker who lived ONE BLOCK from us couldn't get our Part 15 at ALL in his Kia. A block away. Can't listen in the '64 Rahcnero 'cause the radio won't tune up to 1620.
My Mustang pulls in way more FM stations than my wife's Focus. The '59 is terrible for FM. When I was doing range testing on FM transmitters last year every modern car we owned lost the signal at a different place. This was in wide open space, no obstructions, line of site. The Mustang, Focus and Escape all lost the signal at a different spot.
Near as I can tell car radios are just as fickle as any in house radio.
When you find a specification for sensitivity of a car radio, the sensitivity spec of the radio does NOT take into account how the surrounding vehicle may attenuate that signal. The radio people don't install the radio in different models with different options and determine the specs for each model. What antenna does the car have? Some have a real rod antenna on the fender, some have a wire in the windshield. How many other electornic devices are installed in the car? Does the car have steel, aluminum, or composite body panels? The changes the ground plane and pattern of reception.
How many people know if they're car has a "good sensitive radio"? Is there a place people can go to look it up? Where might one find a chart of all possible radios installed in all manufactured automobiles as stock radios, for say, the last 10 years? How does a lay-person know if they have a sensitive car radio? And if they compare to someone elses car how do they know the difference isn't in the antenna or the surrounding parts of the car.
Just another silly way to try to gauge signal coverage that has no standard. Just as many (if not more) variables.
TIB
But if someone were to sit in the car they could listen to you from a crtain location. Part 15 and BETS-1 radio has to be listened to from one location.
The car is the best place to check for open space on the dial because they are the best tuners there are.....selectivity. sensitivity and image rejection of strong stations. Stations you can hear in a car you can never hear on a regular radio. Even a current Macintosh tuner will not work as good as a car. The differences mentioned doesn't change the fact that they are all much better than anything else.
And also reception is unidirectional unlike a home radio where you have to rotate the radio to a certain direction for different stations or change the antenna position for different FM stations.
Can you imagine driving in one direction and when you turn the corner you loose it....the reception is the same facing north south east or west.
Mark
I like to call it the "World Famous Ramsey Chart". It is clearly a fantasy. 250 uV/m will simply not go that far. At the far end of that chart, not even to a communications receiver with a directional antenna.
But I bet it helped them sell transmitters. And I bet a lot of people were really torqued off when their transmitters didn't go a block.
TIB
From an actual radio sensitivity study that I dug up.
Car radios - Average 1.3 uv, Best <1.0 uv, Worst 6.3 uv
Home/portable radios - Average 8 uv, Best 2.3 uv, Worst 28.2 uv
There was one car radio that had really horrible sensitivity, with the others being remarkably consistent. The home radios were all over the map, with many having really bad sensitivity.
Yes, there are variations in car radios too. But at the end of the day, you WILL get more than 200 feet with a good car radio (which is what this seems to be all about). And people being people, they'll only report the best range they've ever gotten, even if its driving up a mounting and hanging out the window with the transmitter.
The worst car radio still has better sensitivity than almost all home radios with few exceptions like the GE super etc. or a hi-fi tuner/reciever.
And with the majority of home radios the imaging from strong stations bleeding all over the band wipe out the weaker ones that on a car you get perfectly. AM the same thing. The $20 clock radio would have sensitivity of 28uV or worse I presume.
I was surprised that the average for home radios is 8uV....as there are a whole lot of lousy ones compared to good ones.
Mark
That was for a noisy, 26db signal. A clear signal (46db, almost but not quite city grade signal) for home/portables was: 46db SNR: Average 400 uv, Best 200 uv, Worst 1000 uv.
The Ramsey chart cited above is accurate, insomuch as it's a mathematical description of the inverse linear relationship between distance and field strength in free space. It does not take into consideration many real world factors that attenuate your signal, including obstructions, ground reflections (for installations near ground level) and even atmospheric conditions. Plus, it does not consider the fact that there is loss in inducing a signal into the antenna terminals of a receiver from a field. Some work was done on this a while ago here on the Forum (I believe by radio8z) and for the example used, that factor was around 2.5 (i.e., it took a field strength of 2.5 uv to induce a 1 uv signal).
If you want to be conservative, then you can divide the distances in the Ramsey chart by 3 and get the potential range of a legal Part 15 FM transmitter with a given receiver sensitivity in the absence of any other attenuating factor.
Of course, in real life, you're forced to live with the atmosphere, and there will be ground reflections and the like, even if you eliminate obstructions. You can minimize ground reflections by getting your transmitting antenna up very high, and you kill two birds with one stone, as you're also going to minimize the effect of even the most benign obstructions, such as bushes, trees, etc.
So you can get relatively close (within a linear factor, at least), with the appropriate installation, to our modified Ramsey chart (distances divided by 3), and in that sense, it can be somewhat useful.
As I hinted at in a previous post, I've found range reporting to be influenced primarily by human factors, rather than technical and scientific ones.
If a broadcaster is operating legally, then they tend to over-report their range. Yes, they've managed to obtain a somewhat noisy signal 800 feet in their car (or someone else's car), and so they report that, with the implication that's it's typical. What they don't report is that to a typical home radio, they're consistently, with a good signal, getting about 100-150 feet. Both are accurate, and with an accompanying description of the conditions under which the results were obtained, both are useful (despite personal opinion otherwise).
On the other hand, those operating illegally (which seems to be many of the members of the Part 15 Facebook group) tend to under report their ranges, in an effort to appear that they are closer to operating legally than they really are. So you'll have someone running a 200mw transmitter with an outside antenna report that they get 1/4 mile range or less. That would likely be true for a good, fully quieted signal into a middling radio, but what they don't say is that with a good car radio, you'll hear a less clear signal miles away (potentially causing interference).
Al this operates against a backdrop of personal bias, from those reporting ranges, to those operators reading the reports (and wanting the one-time ranges to be true ALL the time), as well as the somewhat biased peanut gallery (either positive or negative) that sits back and takes pot shots at the reports, depending on the results.
For example, we all know that Hobbybroadcaster is biased against Part 15 FM broadcasting. It's rare to see them even admit that it's possible to get more than 200 feet from a legal transmitter under ANY conditions. When pushed, and they have no choice, they'll admit that it IS possible, but then they'll come up with a 'but' ... but the range isn't useful (i.e., the car driving effect brought up by the original poster, which I like to call the HB argument, as it's been used over there many times) or whatever. Regardless, it doesn't invalidate the results, even though it might appear to (it's a good example of the 'red herring' falacious argument). The range WAS achieved.
That's why range reporting and the conclusions to be drawn from it always have to be taken with a grain of salt. Even that 200 foot range that is always being thrown about by the defenders of legal Part 15 FM - if you do the math, you'll see that with some 'typical' radios you're lucky to get reception across a room. Jeff (Station8) has developed a range reporting form that attempts to bring the activity more into the scientific arena and that can be found on the ALPB website. It's a good tool for those of us unfortunates who don't own tens of thousands of dollars in radio test equipment, because at the end of the day, it's field strength that really tells the tale.
The Ramsey Chart did get me excited about the possibilities, and I think it did make me experiment more with different antennas and putting the transmitter up high. I was thinking of someone listening to a distant public station like they said, but then coming across my broadcast.
We had a few open frequencies at the time, where nothing came in, except under skip conditions, and I really knew what was there, listening to the FM band in different places and with all kinds of radios at the time.
When I got my first digital tuned transmitter later, it was really cool to have my transmitter on a stable open frequency, and sit in a parking lot close by and tune frequencies one by one, and my station fit right in the open space, same as the neighbor stations and without interfering with them. I realized that anyone in the area could be doing the same thing, tuning in like I was.
I never used the original Ramsey FM 10 that much to do shows on, being embarrassed about the signal it was putting out, with an older model CD player plugged in. There were tones, stereo light flashing on and off, sibilence clipping, and listening with headphones, the channels sounded out of phase, not 180 degrees, but something less than that, and it was disconcerting in headphones.
As for listenable range, I see what you're getting at, and know the effect with my AM and FM over the years. I do use maximum range as one of the benchmarks to if I've had an improvement or not when testing. It's stronger at this one junction than it was before, I can hear it better along this road...
There is one undeniable fact though, FM or AM, but more pronounced on AM, where the local strength signal will go a certain distance, then drop to a "distant" signal strength, where it's narrow, having static, subject to other stations. It seems like that distant strenght can go on and on, many times the range and coverage area that the local signal has. I wouldn't be surprised if the max range was ten times more than the local range! That's on AM.
One time I was happy about getting this weak signal on a certain street, great success, people can listen to it in all these houses I'm passing, but then thought, wait a minute, who's going to be listening to this, my station that's half in static? Maybe DXers and hams, if there are any around, might find it.
Maybe what Part15 stations need is a signal map with two circles on it, reasonable range and a distant contour. I think MRAM had a map like that for his old location.
As you'll recall, last fall I tested several FM transmitters.
In one test I did a reception "range" test. I *believe* this was with the WH 3.0 transmitter (which was over the legal limit). I lost reception to my Tecsun portable within 250 measured feet if I remember right. This radio lists the FM sensitivity as "less than 2 uV"
This was in wide open space, in an open field with no obstructions. I imagine that Tecsun is more sensitive than most home radios. So, in wide open line of sight conditions, with a transmitter that was over the legal limit (I don't remember the amount over, but it was substantial) and the distance range actually measured with a tape measure, the signal was well gone before 300 feet. I'm going on memory here, but I can look up the numbers.
In that same day of testing I also used our three "modern" cars to gest the range. I don't believe I included this in my test results -- as it seemed more of a test of the radio receivers than the transmitter. This was on an open road with no buildings for a block and a half. I don't remember the distances but the Mustang picked it up the furthest, followed by the Escape, and the Focus coming in third. The magic marker lines I made on the curb are still there. The Focus had defintely lost it by 200 feet, the Escape went further and I believe the Mustang made it out to 300 feet or so. I should go try to measure again. But again, this was wide open, line of sight. 300 feet down a straight road is not very far. This was in three relatively modern cars -- 2014, 2013, 2010.
And the range tests should be valid for any legal FM transmitter. 250uV is 250uV no matter what transmitter it's coming out of, and no matter the antenna. To be legal, if you have a better antenna you need to generate less power. If you have a crappy antenna, you need more power. The end result of 250 uV should measure the same range regardless of the transmitter. The variable of course is that NONE of the transmitters I tested were "on the money" in field strength at 3 meters.
And of course nowadays all the car radios seem to be rated in dBf rather than uV just to frustrate us.
TIB
Based on Rich's past modeling of Part 15 FM, ground reflections appear to be the major limiting factor of ground installed systems. Too bad you can't get some height for additional testing. I saw substantial range increases with a transmitter mounted on a roof, as opposed to ground level.
Based on Rich's past modeling of Part 15 FM, ground reflections appear to be the major limiting factor of ground installed systems. Too bad you can't get some height for additional testing. I saw substantial range increases with a transmitter mounted on a roof, as opposed to ground level.
"Can you imagine driving in one direction and when you turn the corner you loose it....the reception is the same facing north south east or west."
Sure can, thats called an in-glass windshield antenna. My 82 C10 has it and will lose or gain stations depending on the direction I'm driving. The wife's 2010 Carolla does it to a somewhat lesser extent with the stubby rear facing antenna.
The Carolla has the better radio and will pickup the part 15 AM several miles out. A 2002 Lexus has the same results with fantastic reception.
Much to my surprise the aftermarket AudioVox that was previously in the truck ended up being the best radio of the 3, though the sound quality isn't near as good as the Delco I replaced it with. Looking at the maunual now it has a listed sensitivity of 1.5 uv on FM and 20 uv on AM.