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Ok, I’ll start this post by saying right off I’m by no means expert at RF, and this is *not* something I’d expect to stand up in court to contest a measurement done by the FCC.
What I’m looking at is something to give a “ballpark” check to try and make sure a transmitter/antenna isn’t putting out thousands of times more power than it should be. Having read some accounts of where the FCC snagged people as pirates, usually it didn’t seem to be “was using 110% of the power allowed for part15”.. It seemed to usually be more like a thousand or thousands of times the allowed amounts.
Ok, I’ll start this post by saying right off I’m by no means expert at RF, and this is *not* something I’d expect to stand up in court to contest a measurement done by the FCC.
What I’m looking at is something to give a “ballpark” check to try and make sure a transmitter/antenna isn’t putting out thousands of times more power than it should be. Having read some accounts of where the FCC snagged people as pirates, usually it didn’t seem to be “was using 110% of the power allowed for part15”.. It seemed to usually be more like a thousand or thousands of times the allowed amounts.
So this is just an attempt to *try* to stick close to legal power using measurements that don’t take gear that runs thousands of bucks. I may be way off with the technical premises here, and I figure that any shortcomings in the attempt at a method I’m going to outline here will be pointed out, and that’s fine. This isn’t any sort of “official verification method”, it’s just a kind of rude and crude attempt at “reasonable means” to verify that an FM xmitter/antenna combo isn’t putting out so much that it’s basically just looking for trouble.
There is one instrument I’m familiar with that measures in microvolts, and that’s the S-meter on a reciever. I gather it’s by no means official, but s9 on a reciever is *approximately* 50 microvolts. “Multiband” radios, or some old stereo recievers have s-meters. Not real hard to find one cheap off ebay or a second-hand store or yardsale. You might already have one around or know someone who does. A portable that can run off batteries would be better, but you use what you can find.
Now the chart on the Ramsey site about halfway down this page:
http://www.ramseyelectronics.com/resource/default.asp?page=fcc
.. lists 63 microvolts as the allowable power level at 12 meters (around 40 ft).. So if the antenna on the radio you’re using to check the signal is around a meter or so long (a little stiff wire taped to it could make up the difference if it’s shorter), the signal at about 40 ft from the xmitter should be just a tiny bit more than it takes to get an s9.
The s-meter hypothetically needs 50 microvolts to give an s-9 reading, 63 microvolts are allowed, one could do math to figure out exactly how many more feet to go out to get an exact s-9, but I leave that 13 microvolts as a “safety margin”, personally.
So if you’re “pinning the meter” (well over the s9) at 40 ft.. you’re too hot. If you’re getting a “solid 9” at say 300 ft? You’re way over.
What to do if you’re over? Well, you’ll need to attenuate the signal a bit. Maybe cobble together a simple bandpass filter and try ones with more “poles” until you bring it down to s-9 at 40 feet. If that didn’t do it, maybe a longer (and more convenient for your studio) cable run to a better antenna location. And just keep adding such things till it does comply (or go with a different xmitter circuit).
Not saying this would stand up in court as “proof” or be useful in contesting a reading done by the FCC if someone complains about you and they come to check.. But hopefully if someone is *trying* to comply and can explain what they did to check the range and etc on their station, perhaps the inspecting agents might be more inclined to advice or warnings instead of a fine. No way to know on that for sure, but a sensible person who is trying to run legal does what they can to cooperate/comply.
Now, whether it’s a practical amount of power to use when trying to actually serve even a small listener base.. Well, I’d say it’s possible, but not using the same approach as the local big fm stations. AM looks like a better bet, since 100 mw is going to be considerably more signal than is allowed on FM legally.
But there’s my very rough method for at least hopefully not being in severe violations on radiated power for part 15 FM. Now the more tech savvy folks here can go ahead and rip it up and spot the holes in it, I don’t mind. It might lead to a better idea of how to do it, but that was my approach to trying to stay in complaince with gear I had handy.
Daniel
Total posts : 45366Experiment…Experiment….Experiment
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
Total posts : 45366I just checked my AM “signal strength” with a Yeasu FRG-100 receiver. The S meter indicates a signal of +20 over S9 which would seem to indicate that if it is calibrated as you suggest that I am way over the legal limit.
I don’t think I am since I am using a SSTRAN AMT-3000 and I have measured the DC input power of this unit to be 93 mW. and am using a base coil loaded 3 meter antenna mounted horizontally on the ceiling of my basement. It appears that I am OK with Part 15.219 rules (which don’t apply to field strength)..
So why the high reading? I think it is because my receiving antenna is a long center fed dipole which is installed outdoors about 50 feet away from my transmit antenna. I can receive 5 kW. broadcast stations on AM from hundreds of miles away in the daytime so this setup really “hears” well. Another confounding factor is that my receiver and RG8/U transmission line (receive) are within feet of the transmit antenna and may be picking up some of the AM signal by leakage. In summary, I do not trust my S meter to gauge my signal strength since the effects of the antenna and feedline in presenting the uV/meter field strength as uV at the antenna connection on the receiver are not known.
Your idea may have merit but my experiment would suggest it does not work for my installation.
It would be interesting if others with S meters could present their data.
Neil
Total posts : 45366Hi Neil
All I was talking about was for FM with its 250 microvolt at 3 meters limit on radiated power.
But with an AM band rig you *should* see higher readings than I’m talking about here. AM part 15 is allowed 100 milliwatts which is a lot more power to radiate than is allowed on FM.
AM at present has much simpler rules that way for complaince.
Also antenna length on the reciever is definitely going to be a factor, since a long dipole for SWL has a lot more “capture area” than the 1 meter long antenna that seems to be specified for the FCC test for FM transmitters.
FM transmitters under current rules (at least so far as I can understand the rules) don’t run with anywhere near 100 milliwatts. FM is limited to no more than a certain signal strength at a certain range, as opposed to how many milliwatts the xmitter puts out.
Dan
Total posts : 45366[quote=Rattan]Hi Neil
FM transmitters under current rules (at least so far as I can understand the rules) don’t run with anywhere near 100 milliwatts. FM is limited to no more than a certain signal strength at a certain range, as opposed to how many milliwatts the xmitter puts out.
Dan
[/quote]
That is correct. There are some who will tell you that under test site conditions that the radiated output power to create the 250uV/M at 3 meters is in the range from nano-watts to less than 25mW. However IMHO, the FCC will not be knocking down doors with the BATF and Mod Squad at those power levels unless the operator is interfering with the licensed broadcasts of another service, TV included for those who like 87.9.
Total posts : 45366[quote=wdcx]
There are some who will tell you that under test site conditions that the radiated output power to create the 250uV/M at 3 meters is in the range from nano-watts to less than 25mW.[/quote]
Actually the source for that nanowatt value is the FCC. Please see their document at http://www.fcc.gov/Bureaus/Mass_Media/Databases/documents_collection/910724.pdfThe 0.01 microwatts they note there for Part 15 FM is the same as 10 nanowatts.
//
Total posts : 45366Hi Dan,
In your original post you mentioned both AM and FM and I thought perhaps you were considering both and that my AM experience might be useful. You are right about AM having more power to radiate but the efficiency of the short AM antenna is very low and very little of the power available to the antenna radiates. FM antennas are much more efficient so much less power is needed to produce the equivalent field strength compared to AM. I think the FCC considered this and also the easily constructed gain antennas for FM and therefore to limit FM range they limited the field strength rather than specifying the power of the transmitter. As others mentioned, FM with a modest gain antenna (1/4 wave gnd plane) will produce the legal field with nanowatt range powers.
Do you think that the receiving antenna and line could give the same problems for accurate FM measurements that I experienced on AM? Your S meter idea may be useful for “ballpark” measurements if somehow it could be calibrated with a known field or the antenna characteristics were known. For example we would have to know how the antenna converts the uV/m field strength to uV at the receiver input. Maybe if the field strength of a broadcast station were known with some accuracy at a certain location this could serve as a rough calibration. Any thoughts on this?
Neil
Total posts : 45366[quote=radio8z]Hi Dan,
……Do you think that the receiving antenna and line could give the same problems for accurate FM measurements that I experienced on AM? Your S meter idea may be useful for “ballpark” measurements if somehow it could be calibrated with a known field or the antenna characteristics were known. For example we would have to know how the antenna converts the uV/m field strength to uV at the receiver input. Maybe if the field strength of a broadcast station were known with some accuracy at a certain location this could serve as a rough calibration. Any thoughts on this?
Neil
[/quote]Hi Neil,
You may already know this, if so just bang me in the head. For FCC and other compliance measurements, calibrated antennas are employed with an accompaning antenna correction factors (ACF) in dB. The antennas are calibrated against a reference antenna on an open area test site.
The antennas can be adjustable dipoles, log periodics, conical, etc.. The FCC has long prefered dipoles although under certain circumstances they will allow other types of antennas as long as one can document and corrolate to a dipole.So measuremets of field strength are made by measuring the energy at a specified distance impinging on the antenna, the ACF +/- in dB and cable loss are factored into the final measurement. A field strength meter (FSM) or spectrum analyzer is employed.
So to answer your question: yes, a receiving antenna can adversly effect the measurement. A gain antenna like a yagi would result in a higher reading.
Total posts : 45366Hi John,
I don”t know the specific details but what you wrote makes sense to me. I think Dan’s idea is worth discussion just as long as we understand the limitations. His proposed method may not result in super accurate data but it may be close enough if done carefully to give some assurance that the signals are not grossly out of bounds. It is probably equivalent to my recommendation of limiting the signal to 200 feet or so. Not too technically correct but workable.
Neil
Total posts : 45366It definitely would have some effect on FM measurements as well if one used a larger reciever antenna, Neil. Not sure how to precisely determine how much it would, but I’d bet on it.
As I understand the spec for FM, it is 250 microvolts with a 1 meter long antenna located 3 meters from the transmitting antenna, basically. They also have other field strengths they can use at other ranges. I’d assume for the purpose of keeping people from being able to just put an FM xmitter antenna too high up to be measured by that method.
Now, the device they’re using has to be a reciever of some sort, since I believe it’s tuned to your frequency. If it wasn’t, it’d also be picking up whatever else was on the air nearby which might include regular broadcast stations, cell phone towers, tons of stuff.
I have to admit, I’m a little shakey on what s-meters *really* measure anyway. Microvolts at the detector stage? I just don’t know. Others here know much more about that matter than I do.
Now, so far as I understand the matter, AM is not required to comply in the same fashion as FM. But I’m purely a layman, not an engineer and certainly not an expert on FCC matters.
AM has it’s own restrictions, as you mentioned.
For the measuring idea I mentioned (using a reciever s-meter) to actually be near accurate, yes, one would pretty much have to have someone with a calibrated s-meter like they talk about in that spec, and compare the readings. Even then I’m sure it wouldn’t stand up in court or anything. My idea was strictly for a quick rough and ready check to try and see if they were drastically going over the limits for FM part15.
The readings I mentioned would not have much bearing on a compliant AM station so far as I know.
Not to say an s-meter reading off a nearby reciever doesn’t have any uses, though. As a quick relative check of if your station is “getting out” normally, it’s handy. Like if you noticed the s-meter reading was lower than normal, and you haven’t changed the reciever antenna, then something could be wrong at your xmitter antenna and it’d be a good idea to check everything. I think having some sort of reciever on to hear how your signal actually sounds is always good anyway. It can sound great on the headphones off your mixing board, but what’s going out can still be messed up for any of a number of reasons.
Dan
Total posts : 45366Dan,
Thanks for your comments. As I stated your idea has merit and could be workable. You are apparently aware of some of the technical questions but there is no harm in trying this. From my readings I find that it is not uncommon for part 15 broadcasters to set their deviation (FM) by comparing their received audio with that of broadcast stations. As is your proposal for field strength on FM, it may not be exact but maybe it can be close enough. If you do any experiments, I hope you report to the board what you find.
I was never a professional broadcast engineer or techincian but I was the “Chief Engineer” for a carrier current station at the Univ. of Cincinnati. I recall the first rule of broadcast engineering which is “Monitor your on the air signal” and I am pleased that you understand this. I don’t know how many know the story but last week an open mike bathroom conversation got on the air for 90 seconds because someone wasn’t monitoring. This isn’t really an engineering issue but someone was asleep at the switch.
Neil
Total posts : 45366Dan you are correct. In a recent post where I was jokingly refering to an el-cheapo FSM, you can see the FCC agent telescoping the 1 meter antenna on his FSM/receiver. Naturally he’s more tha 3 meters away from the source, so if he measures more than 250 uV (43.5 dBuV) he has ya!. Now let’s get back to using other calibrated tunable receivers that provide readouts in dBuV, have quasi-peak detectors, etc….hmmmmmmm
WDCX AM1610 Part 15
John
Owner-Operator-Chief Engineer-Program Manager
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