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A thing Rich said in another thread about AM and FM part 15 ranges got me interested enough to do some math. Calculating back through the forumal he gave for half-wave dipoles, yes the allowed radiated power does come out to about 11 nanowatts. 11.4329etc nanowatts.
That can be found by taking the E=SQRT(49.2*P)/R formula he quoted , where P is the power in watts, R is the distance in meters. If you solve for P, it looks like P=(E*R)^2 /49.2
Put in .00025 for the power, and 3 M for the distance and you’ll see. 11.4 nanowatts. Rich has been talking very straight on that. It *sounds* like an impossibly small amount of “juice” to work with or measure effectively. *However*.. FM home receivers haven’t degraded near as much over the years as the AM section in the standard home entertainment system or whatever.
According to a bit I ran across over on the Ramsey site,
“A quick glance at stereo FM receiver specifications shows typical sensitivity of 1.7 µV before considering high-gain antennas or preamplifiers.”
Now, I haven’t checked FM receiver specs yet. But just assuming for the moment that number is somewhere near correct..
Working out the equation to find E at 400 meters (1/4 mile) we get 1.875 uV. Which would be enough to get at least some sort of a signal through assuming that number is typical of a consumer electronics home entertainment system of reasonable quality. Actually, it would be about 440 meters for 1.7 uV. 1,443 ft, approx.
There are some other things to consider as well. FM is line of sight, so antenna elavation is important. But there’s also no restriction on ground lead lengths and almost any FM transmitter is going to be a lot more than it would take to get that 1.875 uV at a quarter mile. So a resonant, well tuned antenna at a reasonable height with a good electrical ground is both practical and not “grey area”.
IF 1/4 mile to actual home stereos is enough for what you want to do. For those wanting to do a little neighborhood/community station, it could be just the ticket. That’s 4 (average length) city blocks out in every direction, which could be quite effective.
So 1/4 mile is not an impossible distance. A whole mile (1600 meters), well the field strength would be only .4 uV and I don’t know what would pick that up or how well, but you might hear *something* on a really good receiver with maybe a gain antenna on the roof.
So we aren’t talking *miles* of range for FM part15, but it’s not as incredibly short as it first seems, except to small cheap portables, where the legal range is going to be short. Maybe Rich or someone else here knows typical sensitivities for portables, car radio and etc? For barely audible to acceptable to decent reception?
The other thing to consider with FM is that there are such things as high gain receiving antennas for it. Old TV antennas are often going to corrosion on rooftops, some of them even those old “deep fringe range behemoths” that had insane amounts of gain. Typically 300 ohm, just like many home receivers for FM. Transmit power is limited, but high gain receiving antennas can make a world of difference to get to listeners who really want to hear you within a reasonable distance.
Another problem is that the FM band is pretty congested in some areas, so FM wouldn’t be a great solution for everyone everywhere. But rural and small-town areas it could work.
Then the big problem.. How the heck do you measure it to even come close to being in compliance to power levels so tiny you can’t see them on a regular hobbyist bench meter. I’ve tried to think of a direct and simple way to do it, and been frustrated. But I was trying to figure out how to measure 250uV at 3 m with a 1m andtenna.. If we step back a ways from the “problem” a practical solution becomes clear. In fact, lets step back 1/4 mile. If the antenna is mounted high enough to be in the clear so reflections from large metal objects and etc are reduced, then the line of sight signal into say a simple dipole antenna is going to be at least pretty close to the formula.
So get the antenna up, then attenuate the signal at the transmitter end until it is receivable at 1/4 mile to a receiver with around 1.7 – 1.9 sensitivity, and you’re at least real close. Check with everyone you know who lives nearby. If they’re within 1/4 mile and have a good receiver and they can pick you up, cool. If they are 1/2 mile to a mile away and getting you clear (without any heroic antenna efforts on their part), then you’re over power. If nobody without an external antenna can get you clearly at 1/2 mile , and 1/4 mile they can, then you know that you’re at least close. Might be running as much as twice the allowed amount, but if you look at the NOUO listings, that isn’t what they’re usually busting people for.
Now, if you run *all* of about any transmitter on the planet up to that antenna, you’re going to be overpower.. So chop it down a bit. First, maybe put in a 5 or 7 pole lowpass filter right after the xmitter so you aren’t messing up the aircraft bands and causing RFI on people’s tv. That’ll trim off a little power while cleaning up your signal. Then put in a simple resistive attenuator right after the filter, and chop the signal down until you’re getting that 1/4 mile range to a reasonable FM stereo (preferably one where the manual didn’t get thrown out and you can check the specs for the sensitivity).. Then check and make sure you aren’t going past that with clear copy unless it’s to a good receiver with an outdoor antenna with gain pointed your way.
And you should at least be *real* close. Not hundreds or thousands of times more power than you should be using like the people that get busted every day.
Now, all of that might be too much bother to some people for 1/4 mile range of FM. But to some that want to run a little neighborhood station, it could be worth the work.
Okay, anybody have some thoughts or input on the idea?
Daniel
Total posts : 45366I forgot to mention what Rich said that got me to thinking.. The part where he said that the range was very similar for part15 FM and part15 AM with that 200 ft to a cheap radio. With all the talk about formulas and distance and so on, it got me to thinking, so I looked into it more. And also to check his math in hopes he might have been mistaken.. But I’ve yet to *ever* see Rich’s math be off the mark.
But it got me into calculating it out, seeing that he was right about the 11 nanowatts, and then checking to see what 11 nanowatts could actually do. It doesn’t appear to be near as restrictive as we’ve often been saying here.
1/4 mile of range to home stereos (at least reasonable quality ones) or a bit more with outdoor gain antennas on the *receiver* end, and it could be practical for a nice neighborhood/community application if there’s a clear frequency or few in your area.
Anyway, that was the statement Rich made in passing that got me to thinking and pulling out the calculator. Thanks, Rich!
Daniel
Total posts : 45366Daniel,
Your posts are nice, thought provoking, reads. It is good that you are approaching this from a theory based perspective which you translate into practical results.
One slight correction: You wrote “Put in .00025 for the power”; it should read ” Put in .00025 for the field strength in volts/meter”.
Regarding measuring field strength, one way might be to get inside an FM receiver and measure the IF voltage before the limiter, which is a problem because limiting is done in the IF amplifiers so an unlimited signal will also be weak. This is not simple since the AGC will have to be defeated and an accurate means of measuring the voltage at 10.7 MHz. would need to be present. Maybe it would be better to attenuate the signal before the antenna terminals so the receiver doesn’t limit. Then the DC voltage at the discriminator will indicate signal strength. The other problem is establishing the calibration on the antenna. Not an easy task. No wonder real FS meters cost $$$$$.
It might be possible to get a calibration from theory by using a tx. with a known power output, attenuate the RF down to 11 nW with a resistive network, feed it to a 1/2 wave dipole accounting for feedline loss, and use the radiated field for range checking or FS meter calibration. This is the most feasible approach for a modestly equipped electronics hobbyist that I can suggest.
This is just some random brainstorming which might give someone an idea.
Neil
Total posts : 45366Thanks for the correction, Neil. I was thinking some of it through while typing it, and working out the numbers to doublecheck as I went along and you know how that goes for typing. LOL
I wasn’t thinking of trying to measure the signal strength at the receiver, or using a claibrated tx (since I have no idea where I’d get one or how to measure if it was actually putting out 11 nanowatts, that’s the problem in the first place).. My logic was since it’s possible to predict the likely range of a signal like we were doing the past few days for AM, that using the best sample group of receivers one can find in the area and seing if the reception was consistent with what legal power *should* be able to practically acheive, one could get a ballpark idea. Since if it’s even twice as much as 11 nw, the signal will go about twice as far and obviously one is over power. While a NOUO is *possible* for a signal that’s say, less than twice what it should be for FM, I don’t think I’ve ever seen one for a station that close.
Still brainstorming on it. The method I outlined would be crude and a bit approximate. But no more so than “if it doesn’t get further than 200 ft, it should be ok”.. Checking it with several receivers in the neighborhood to get at least something like a sample group, the inconsistincies between individual receiver characteristics would even out a bit.
I like the idea of having something like a meter for it too, but I haven’t figured out anything achievable with simple hobbyist gear that would come anywhere near a professional calibrated FSM, so I started thinking of other ways one could at least tell if they were reasonably near compliance or hundreds of times too much power. (which would be easy to do with even a few mw on FM, as Rich has often pointed out).
Daniel
Total posts : 45366Daniel wrote: “…’A quick glance at stereo FM receiver specifications shows typical sensitivity of 1.7 µV before considering high-gain antennas or preamplifiers.’ …Working out the equation to find E at 400 meters (1/4 mile) we get 1.875 uV. Which would be enough to get at least some sort of a signal through assuming that number is typical of a consumer electronics home entertainment system of reasonable quality. Actually, it would be about 440 meters for 1.7 uV. 1,443 ft, approx. … So get the antenna up, then attenuate the signal at the transmitter end until it is receivable at 1/4 mile to a receiver with around 1.7 – 1.9 sensitivity, and you’re at least real close.”
______________Daniel,
You’ve done some good thinking here. That 1.7 µV quoted sensitivity is about right for a good FM receiver, but probably it applies to monaural reception with a S/N level that is fairly acceptable, although not as quiet as the receiver can do. Stereo reception with a good S/N takes a lot more r-f signal input at the receiver (maybe 10 or more times as much as mono). I don’t know what the r-f sensitivity of an FM Walkman is, but I expect it isn’t this good.
Some other thoughts. The sensitivity spec given in µV for a receiver applies to the voltage across its antenna input terminals, which if that is an “F” connector, probably have a 75 ohm input impedance.
Note that this specification is not the same number as the field strength produced by the transmit system.
Also the receive antenna isn’t perfectly efficient at converting a received field strength into a voltage at its output connector. That performance depends on the design/adjustment of that antenna, and the operating frequency.
So when using a 1/2-wave dipole receiving antenna on 98 MHz, and allowing 4 dB for its cable and balun loss, the transmit system will need to produce about 5.5 µV/m of field strength in the area where the receiving antenna is placed, in order for the receiver to see 1.7 µV across its input terminals. That will shorten up the useful path lengths that you were expecting in your analysis.
Also, field strengths often are specified as XX µV/per meter, but that doesn’t mean that the receiving antenna always should be one meter long. The receiving antenna should be resonant at the receive frequency, which typically for a 1/2-wave, thin-wire dipole at 98 MHz would be about 1.45 m overall length.
You are correct about line-of-sight, reflection-free paths being necessary for best FM reception. An FM receiver/antenna system located inside a home, and blocked by several other homes/structures between it and the transmit antenna can see a reduction 50% or more in received field strength compared to an unobstructed, reflection-free path.
Maybe these added perspectives may help in experimenting with these ideas.
Rich
Total posts : 45366Daniel,
Your approach to this is good. I know that I tend to try to reduce things to equations, yet I have enough practical experience to know that theoretical models often have to include assumptions about parameters which may be no more than educated guesses. My background in servo control system analysis taught me this.
Sometimes, the best we can do is get reasonably close to what theory predicts in applications where there are unknowns. Your approach is good in this regard. As you said, following your suggestions may not be exact but it should be fairly close. If it yields results that are within a factor of two of the legal limit that would be an excellent approximation. I know it is much better than sticking an antenna in the air and just wishing for the best.
Neil
Total posts : 45366I am using an EDM at a calculated output of 9.8mw(from power in to final stage) fed with75 feet of RG-8(vf=66) into a quarterwave ground plane.Can you calculating sleuths tell me what amount of attenuation you would put in line to stay reasonably compliant? Thanks!
Regards,Lee
Total posts : 45366I was thinking more of receivers with a balanced 300 ohm antenna connection and feeding it from something simple and inexpensive to provide since many people never bother to attach an antenna to the actual receiver of a home stereo system.
What I was thinking is an antenna I’ve built and tuned before so a neighbor could pick up my little station with his stereo. I built the “slim jim” variation of a jpole from 300 ohm twin-lead. Very cost effective as a giveaway sort of item. They *theoretically* have a gain of around 6 db and were originally made for balanced line, though on 2M fm, they are usually fed with coax and as such a balun is needed in that case. So no balun loss *if* the receiver has 300 ohm terminals.
Another inexpensive option would be a standard folded dipole made of 300 ohm twin lead, which also would not require a balun if the receiver has 300 ohm terminals.
Cable run in either case on the receiver end would be very short.
Jpole type antennas tune sharper than simple folded dipoles, but might be worth it for the bit of gain they could give the receiver. Best way to find out would be A/B comparison.
Good point on the receiving antenna also needing to be in the clear for testing the reception at 1/4 mile. Upper floors and attic windows come to mind.
Daniel
Total posts : 45366I wouldn’t even know where to begin to calculate it, Lee.
My approach would be more likely to build a simple pi attenuator on a solderless breadboard with a fixed resistor on the xmitter end of what the xmitter expects to see, and parallel it with a potentiometer.. then on the antenna end, about what the antenna should have as loading to get a good match, and another potentiometer in parallel with that resistor. Then start with maybe a 500k pot between those to finish the pi. If that ends up not being enough, go to a bigger pot. But basically make all the resistances adjustable till you figure out what will do the job.
Tinker and tweak until the actual range the transmitter is getting is in the right ballpark, and everything seems to be working right. Basically bring up the resistance between the xmitter side and antenna side until it’s about close, then adjust the resistance in the other two legs to get what the xmitter and antenna need, then touch up the resistance on the leg between the two until the range is right, repeat as many times as it takes. When you’ve got it “close enough for jazz”, measure the resistance in each leg and build a little box to do the job with fixed resistances, maybe a little trimmer in series with the fixed resistor on the leg between the xmitter loading resistor and the antenna loading resistor, so you can touch it up later without jury rigging the whole mess together again.
I’m sure it could be done with a T or L network too, but the Pi is usually what I’ve used for matching audio gear where the actual impedance isn’t precisely known with a simple resistive network.
Oh, and a tip.. take the cans/covers off the backs of the pots for the experiment. They could add a bit of capacitance into the circuit at FM frequencies that you do *not* want when trying to come up with a purely resistive attenuator. Negligible at audio frequencies, maybe even AM BCB, but not so negligible around 100 Mhz.
That’s how *I* would do it anyway, but I’m a crude junkbox sort of guy. There’s probably a sliderule/calculator/emulation approach that’s considerably more elegant, but with all the variables involved, I wouldn’t know where to start with that. So I’d head straight to nuts and bolts and see if I could find settings that work and then take the values off those and build a nice tidy version into any small metal box with (mostly) fixed resitances and short lead lengths.
Daniel
Total posts : 45366Maybe Rich can come up with the answer to this one,Im sure of it!
The impedences are all 50 ohms. A quarter wave is also real close to 50 ohms,so a simple attenuator can be built as you say. In fact I have a nifty program which will spit out any db loss and impedance attenuator values I want.I need to know how much attenuation to go from my transmitter (9.8mw) thru the feedline(about 3db loss?) and to the quarter wave antenna to get the equivalent of 11 nanowatts referenced to a half wave. A quarter wave has less gain than a half wave,so how much power should be applied to the antenna? How many db loss should the pad have? I have waaayyyyy toooooo much range as it is……
Regards,Lee
Total posts : 45366Lee wrote: “How about a quarter wave? I am using an EDM at a calculated output of 9.8mw (from power in to final stage) fed with75 feet of RG-8 (vf=66) into a quarterwave ground plane.Can you calculating sleuths tell me what amount of attenuation you would put in line to stay reasonably compliant?
__________Hi, Lee –
The “vf,” or velocity factor of your coax line really has no bearing on this answer. All that matters here is the r-f output power of the tx, the antenna VSWR, the r-f power lost in that length of that coax, and the peak gain of the transmit antenna.
A 1/4-wave ground plane antenna has about the same peak gain as a 1/2-wave dipole, and a 75-foot length of RG-8 (e.g., Belden 9913) has a power loss of about one decibel into a 1.5 VSWR load (antenna) at 98 MHz.
A power loss of one decibel in the coax feeding this ground plane antenna means that the maximum power output of an FM tx could be about one decibel more than the ~11.43 nanowatts (nW) that a matched, 1/2-wave dipole itself needs to radiate the maximum Part 15 FM legal field. That power ratio of +1 dB is a multiplier of about 1.26 X.
So instead of the tx power output of a compliant Part 15 system with a zero loss coax cable and resonant 1/2-wave dipole being ~11.43 nW, it would rise to ~14.4 nW with your scenario.
If your tx output power really is 9.8 mW, it still would need to be reduced by about 58 decibels in order to generate a compliant (legal) Part 15 FM field with the antenna/coax components you have described.
Rich
Total posts : 45366Lee,
I ran a calculation based on your data.
11 nW referenced to 9.8 mW is -59.5 dB. Accounting for the 3 dB loss in the line, the attenuator should have a loss of 56.5 dB.
dB calculation:
dB = 10 log (11E-9W/9.8E-3W) = -59.5 dB
This will deliver 11 nW to the antenna feedpoint. From a search on “antenna gain” I find that the gain of a 1/4 ground plane depends upon who you ask. Even my reference books give conflicting data. Not very satisfying is it?
If the gain of the ground plane antenna is not 0 dBd then we need to adjust the attenuator loss accordingly.
Edit: Rich posted while I was posting so I didn’t see his. It appears that we are saying the same thing though a bit differently.
Neil
Total posts : 45366Daniel mentioned removing the metal cover from a pot used in an RF attenuator. This is not a bad suggestion but there is more involved.
One thing which is often overlooked when constructing a RF attenuator is the need for adequate shielding. A “rats nest” circuit will make a decent antenna and if the power is high enough it can produce quite a distant signal even though it attenuates a thru signal. At VHF, designing and building a good attenuator is a tradeoff between shielding and introducing stray L and C, while at the same time preventing unwanted radiation.
At the least I suggest that the attenuator be placed in a metal enclosure connected to circuit ground in order to stop it from radiating a signal. Double sided PC board pieces soldered together was my choice, but any metal box of the right size will work. Keep all the leads as short as possible.
Also, don’t use just any resistors. I recommend carbon composition if you can get them. Metal film also works, but avoid the very common wire wound units. Too much inductance.
Without scaring anyone away from doing this, I hope I gave a bit of information which will be useful. Do a bit of homework and you will find it is not that difficult to build a good attenuator once you know some of the traps to avoid. You will find that it involves a bit more than just a couple of resistors, but it is not hard to do.
Neil
Total posts : 45366WOW,thats a lot of attenuation! So here we go,I am building a 50 ohm in and output pi pad with 58db loss,in a metal box. I will re-test the range afterwards and report my findings. I am a good tech but never learned the math. I can build or repair most anything. I just cant engineer it very well…thanks again!
Regards,Lee
Total posts : 45366Lee,
Glad to help. For brevity, I didn’t get too detailed in my previous post. so I’ll add something here for you to consider.
Yes, that is a lot of attenuation and it is likely that if you do not add an internal shield between the input and the output that stray coupling between in and out inside the box will reduce the attenuation quite a bit. If you use a tee network, run the center resistor through a hole drilled in a metal shield. Install the shield perpendicular to the resistor in the box and ground it to the box. This directs the signal only through the resistor rather than allowing coupling around it.
A while ago I built a 5 step attenuator doing this. With the shields I described it worked like a champ at 146 MHz.
I hope you will report back the results of your test. You may be disappointed in the range which results but it will be interesting to read your report.
Neil
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