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I asked this question earlier. Didn’t see it in the threads list. Reposting. What is the difference in field strength and antenna range for an FM Part 15? If one were to set up an 100mW-500mW FM tx dealy, (No limits on tx power, antenna length, ground lead etc. by the FCC) according to specs, follows the ONLY restrictions placed on FM’s whereby he/she limits its “field strength”, while mounting its antenna 30 feet up and achieves greater range, has he/she violated the FCC’s rules for FM Part 15’s?
Maybe a better question to ask is this; Are antenna field strength and antenna range one and the same? A closer look at the rules governing Part 15 FM’s doesn’t seem to indicate that they are. At least there is no wording in the language ANYWHERE which specifies that they are deemed to be the same. I have read posts where people assert “It is clear that the FCC intends for Part 15’ers to limit their ‘field strength’ to 3 meters”. That’s fine. But, that definition from a forum guy is indeed only an interpretation. And it is not what is stated by the FCC.
Are they both the same? How could they be considered the same when the FCC itself doesn’t define it them as such? What say you? Is it possible to reduce an antenna’s field strength while achieving greater range due to elevation. I have read posts by a few who claim to have done so. Figured I’d take the discussion a bit farther.
Total posts : 45366Field strength and antenna range are not the same thing but they are inter-related.
Antenna range refers to how far away the signal travels and can be heard.
Field strength is a numeric measurement of that antenna range at a specific distance.
If the field strength measurement exceeds the number set forth in the rules, the antenna range must be reduced until it matches or does not exceed the legal field strength at that distance.
Total posts : 45366Answer to your question on legality of the elevated antenna is it is legal as long as the field strength 3 meters in any direction from the antenna is less than or equal to 250 uV/m. Transmitter power output can be related to field strength but to keep this post reasonably short I will save this until asked.
There are big unknowns when trying to relate field strength to range in a part 15 FM station such as the receiver sensitivity and the amount of signal the receiver antenna delivers to the receiver.
These calculations are to illustrate what factors determine the range. It is possible to calculate the field strength at a given distance from a transmitter with a field of 250 uV/m at 3 meters as FS(uv/m) = (250 uV/m) x (3meters/distance(meters)). For example, the field strength at 100 meters is FS = 250 x (3/100) = 7.5 uV/m. Conversely, the distance (which is the range) can be calculated if the required field strength for a receiver/antenna to operate is known. Distance(meters) = (750)/(required field strength). If the receiver requires 15 uV/m then the range is = 750/15 = 50 meters.
The problem is that the required field strength at the receiver number is usually not known. An average or approximate number, say 7.5 uV/m can be used but the answer will be no better than this assumed number. Also, the path between the transmitter and receiver can alter the field strength in ways which are not easily predictable. If the transmitter antenna is elevated so there is nothing but “free space” between the transmitter and receiver antennas then the equations above can be used to estimate the range.
The point being is that the path unknowns between the transmitter antenna and the receiver and the unknowns regarding the receiver sensitivity make it very difficult to establish a firm number for the range.
Reports from hobbyists regarding their range experiences usually come from installations where the actual radiated field strength is not known so these are not reliable if compliance is your goal. The closest estimates for a compliant system would be from those who are using unmodified certified transmitters based on the assumption that the field strength from these is less than the legal maximum. This is not very scientific but is probably the best we can do with limited test equipment.
Neil
Total posts : 45366I’m still not sure if anyone has directly answered the question.
There are two components to that question, I believe. You can use any combination of transmitter power, length of antenna and antenna elevation as long as you only achieve the signal range that would correspond to a field strength of 250uv/m at 3 meters. That would typically mean 600-800 feet to a sensitive car receiver (no obstructions) or 200 feet (what the FCC uses) to a typical stereo receiver. I’ve posted some calculations in a recent thread, and others have posted more detailed calculations on determining how a given field strength translates to a signal strength at the receiver antenna input. Of course, it would be ideal if you had a field strength measurement device.
The second component is signal quality. You would also need to ensure that the various technical aspects of a Part 15 FM signal are met, such as bandwidth, harmonics, etc. The least problematic way to achieve that is to use an FCC certified transmitter.
Total posts : 45366A Field strength reading is an exact MEASUREMENT of your signal strength taken at a certain distance from your antenna. On Pt. 15 FM, that distance is 3 meters, and the maximum signal strength allowed is 250uv/m. You can stay within that limit using a 50 watt transmitter if you load it into a dummy load. But if you use an efficient antenna, like a dipole, you’ll probably have to bring your power down to only 5 or 10 milliwatts!
Let’s say you put your antenna on a 10 foot pole, and measure your signal to be not more than 250uv/meter at 3 meters, but you’re not happy with your signal.
So you raise that same antenna to 30 feet. Will you have a greater field strength reading 3 meters away from the antenna? No.
But will your signal go farther? Yes. Why? Because at 10 feet up, your signal is forced to travel through obstructions to get to listeners down the street. (Mostly, walls and roofs of the houses between yours and the house you’re trying to reach) By raising your antenna to 30 feet, you don’t have to cut through all the obstructions, which increases your signal at the receiver’s location. This means your “range” has also increased. But your signal strength 3 meters from your antenna, HAS NOT CHANGED, and you are still legal.So what is “Range”? My definition: Range is an estimate of how far away your station can reliably be heard. It is not an exact measurement, like field strength is. And it varies tremendously depending on who you ask. When some station owners want to know their range, they’ll jump into a car with the best FM radio they can find, and drive until they absolutely can’t hear it at all anymore. Then that distance becomes their “range”. But wait a minute. How reliable is your signal at that distance? How many listeners have the best car radios that exist? And are going to stay tuned in to your station when it becomes noisy and cuts in and out, until it can’t be heard anymore? I can only think of one….the station owner!
Then on the other hand, there’s “Joe Blow Typical Listener”, at home in front of his living room stereo. His stereo doesn’t have an antenna on it, because every other station in town has 100,000watts. (or so it seems) He doesn’t need an antenna! So Joe Blow Typical Listener can barely pick you up from across the street. He’s shocked when he finds out you’re right across the street. In HIS mind, your “range” is about 75 feet! And so it goes.
Assuming a legal field strength of 250uv/m measured at 3 meters, it’s probably best to expect “ranges” that have been quoted most often on this site — 600 – 800′ on a GOOD car radio with no obstructions, or 200′ or so on a more typical portable, or a home stereo. At least you won’t be disappointed. If you get more than that out of your station, you’ll be delighted! Hope this helps.
Total posts : 45366Ding, ding, ding… That is almost the exact same answer I received from a field agent (FCC inspector) in Texas via email to the question I posed on Saturday! (rlkocher) Your answer was almost word for word what the guy sent me. Your clarity on the issue was awesome. Appreciate all of the responses to the question. I’ll keep you all abreast of my experimenting with FM Part 15 tx’s. Love the forum!
Total posts : 45366The FCC says NOT to take field strength measurements in the near field. They also define the near field as 10X the antenna length or one wavelength, whichever is greater. SO, based upon that they cannot take accurate FS readings at 3M. One wavelength on the AM band is a pretty long distance, so how do you stay withing the rules, when the rules contradict each other?
Tommy J.
AM-580 The Bull!
Bellville, Texas
Total posts : 45366At my FM frequency my 1/4 wave antenna is only 28″ long. 10 x 28″ = 280″/12 = 23.3333 feet. 1 wavelength at my frequency would be around 9 3/4 feet. So according to your info, the FCC would want to take their reading at 10 times the antenna length. That would be at 23.3333 feet,in my case. With the sensitivity the FCC meters must have, I see no problem with them getting a fairly accurate reading at that distance. They wouldn’t be looking for 250uv/m at that distance tho…probably more like 100um/v. Upon reading that, they would figure the 3 meter field strength to be around 250uv/m, since field strength is inversely proportional to the distance from the antenna.
This is for FM. I don’t know what the proceedure is for AM….this whole thread has been about Pt. 15 FM. Maybe the answer you seek would be on an AM thread?
Total posts : 45366Indeed!Interesting take…
Total posts : 45366Yes, on FM there is little hope to get a legal Part-15 station that can reach more than a block or two. On AM, however, the rules are more liberal but seem to contradict themselves. My Potomac will only read 100uv and up, I’m pretty sure the field agents use the same meter. I still have a 100mw EDM transmitter, a 30w FM exciter, and a J-pole made from 3/4″ copper. The 30w will get me about 5+ miles with the j-pole clamped to a fence post. The EDM has never really been tested, but with the factory wire it was good for about 200ft. If I could legally get one mile on FM, I would do it in a heartbeat. I’m running carrier-current AM, but may be switching to a carrot on a stick soon.
Tommy J.
AM-580 The Bull!
Bellville, Texas
Total posts : 45366So at 24ft, outside the near-field on FM, the visiting agent has a FS meter that will give an accurate reading under 100uv? Wish someone would post what instrument that is, I have never seen one.
Tommy J.
AM-580 The Bull!
Total posts : 45366Hmmm…If your meter reads 100uv/m and over, the FCC is using the same meter, and the reading they’re looking at about 24′ out is around 100uv/m, maybe they figure if the meter moves at all, the station is illegal!
Seems funny — We have receivers today that will respond to 3 or 4 uv — so why don’t we have meters that will read less than 100uv? Isn’t that what the “s meters” on our receivers do? I’m lost too!
Total posts : 45366The Motorola Station Monitors we use in the shop (R2670 and R2670B) read down into the single digit uV. The noise floor is below -90dbm and the scale goes down to -110 dbm. We don’t use a calibrated antenna but as “rikocher” said “we have receivers that respond” to very small signals.
Total posts : 45366With my FIM-21, the noise floor is well above the 15uv/m I’m allowed with carrier-current. On the 100uv/m scale, I can turn my transmitter off and still get as high as 25uv/m readings, so how can you ever get a reading that says your legal? FCC says to go closer in and even calculated the signal at 100ft for me, but thats near-field, and there is no way thats correct. Maybe on FM its cleaner, isn’t 2uv a good useable signal on FM?
Tommy J.
AM-580 The Bull!
Total posts : 453662 uv (microvolts) is a very weak signal on both FM and AM.
2 mv (millivolts) is quite strong on both bands.
There are 1,000 microvolts in a millivolt!
Could it be millivolts and microvolts are getting mixed up in some of the measurements/estimates we’re talking about?
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