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Hello All,
Richard Fry recently posted on another board theoretical field strength and usable range estimates based on his professional expertise and on reliable software. He posted in part:
>>>There have been posts here and elsewhere reporting unexpectedly great coverage distances by certain “Part 15” AM systems.
Below is a calculation representing about the greatest consistent range that can be expected from a legal Part 15 AM system, over terrain with the highest ground conductivity shown in the FCC’S M-3 chart for the continental US.
Hello All,
Richard Fry recently posted on another board theoretical field strength and usable range estimates based on his professional expertise and on reliable software. He posted in part:
>>>There have been posts here and elsewhere reporting unexpectedly great coverage distances by certain “Part 15” AM systems.
Below is a calculation representing about the greatest consistent range that can be expected from a legal Part 15 AM system, over terrain with the highest ground conductivity shown in the FCC’S M-3 chart for the continental US.
DATA:
Frequency = 1600 kHz
Applied Power = 80 mW
Radiator = 3-meter resonant vertical with base at ground level
Coil Resistance = 2 ohms
R-F Ground Resistance = 10 ohms
Ground Conductivity = 30.0 mS/m
Radiation at 1 mile = 0.2 mV/mRESULTS:
Contour level > Distance to contour
5.000 mV/m > 0.0340 miles (good signal to cheap, indoor radio)
2.000 mV/m > 0.0850 miles (acceptable, but somewhat noisy signal to cheap indoor radio)
0.100 mV/m > 1.5865 miles (acceptable, but noisy signal to a good car radio in an area with no overhead wires, no local electrical interference, and no co-channel interference) >>>>
I have been experimenting with Part 15 AM systems since 1959 and have been on and off the air with three transmitters since then. My purpose here is to affirm that his calculations agree with my experience to an almost astonishing accuracy. Theoretical calculations are not absolutely accurate and do not account for the many variables encountered in actual practice, but I disagree with those who say that theory does not apply to practice. Theory guides practice and Mr. Fry has provided a basis on which to predict realistic real world performance of compliant Part 15 AM transmitters.
My recent experience with a SSTRAN AMT-3000 unit which runs at a measured 93 mW. input (compliant with Part 15.219 in this regard) is reasonably close to the range predictions of Mr. Fry based on his calculations.
Claims of range far exceeding the theoretical limits and the range obtained in my experience are not consistant with compliance with the rules. If you have or are considering a Part 15 operation, the numbers cited by Mr. Fry are realistic for a compliant Part 15 AM system.
I hope this information aids those who are just starting or are thinking of starting AM unlicensed broadcasting by presenting a realistic assessment of what to expect.
Neil
—
Thread links from scwis:
Rich’s post quoted by Neil: http://part15.us/node/511
Additional similar post from Rich: http://part15.us/node/270
Rich’s Blog with PDF links about range: http://part15.us/node/850Legally operated low power transmitters don’t go very far. AM transmitters generally go less than a mile, and FM a few hundred feet.
FM is especially bad, because the final judge of Part 15 compliance is how far your signal goes – ouch!
However, in the interest of getting as close to the theoretical maximum as possible I was wondering if Mr. Neil and Mr. Fry could help us noodle through the answers to some questions that I have.
We know that adding a base loading inductor to the three-meter vertical radiator can improve matching to the transmitter, and help approach the maximum range. Transmitters like the Rangemaster use a wound toroid inside the transmitter and a 108″ whip vertical radiator, the manufacturer of the SSTRAN includes a diagram for an external air coil and a larger diameter copper pipe vertical radiator.
I was hoping our most technically sophisticated contributors could help us answer these questions:
For a toroid:
What is the ideal toroid size and material mix for the core? Amidon, (http://www.amidoncorp.com/) et al, Part numbers much appreciated.
What is the ideal wire size for winding the toroid coil?
How is the 50 ohm tap for the ground connection at the ground end of the coil winding best determined?
For an air coil:
What is the ideal coil form diameter for a base loading inductor to properly load a three-meter vertical element made of one inch OD copper pipe operating in the upper end of the AM BCB?
What is the ideal coil winding spacing for a base loading inductor to optimize Q for the above use?
And for fine tuning:
Which is better for fine tuning either the toroid or air coil system – series or parallel variable capacitor? WHat is a good connection schematic for the whole tuning and loading system?
At you convenience, of course, gentlemen 🙂
Experimental broadcasting for a better tomorrow!
Total posts : 45366Your questions are good, and would best be addressed in a research project by the manufacturers of 100 mW AM transmitters for the Part 15 market, or by some other interested business that could finance doing so, and then recoup their expenses by offering a “system” answer.
The optimum components in the antenna system are dependent on what load impedance the transmitter is capable of working into, and the antenna installation environment (r-f impedance in the ground connection, for example).
//
Total posts : 45366I swear – we go over this every so often, and someone always says “Legally – you cant go X feet/miles/yards/inches/whatever”
Here’s a fact – and it’s a fact to me because i did it.
Manteca Local Radio utilizing a single 98mW transmitter, a inductor, and a 9′ bit of copper antenna. All mounted on a mast, and grounded to said mast, while the mast was grounded to a couple of copper rods and a cold water pipe.
This transmitter covered the entire town (small town – look it up on yahoo maps for eact range – Manteca California)
With 2 FCC agents in town, and one I was in constant contact with – who had past the house so many times to make sure I was running a 9.81′ radiator in total.
This is real, this isn’t something I did on a calculator. I’m no RF engineer, but the guy who designed and set all this up was a Physics Professor at Stanford University, and one hell of an RF engineer.
Take what you experience and use it.
Take theory, and put it to a real test.
If you rely on theory, or you rely on what people say: you are a fool.
Experience things first hand.
Total posts : 45366I’m surprised that an engineer would suppose that an AM signal has a cut/dried coverage area under this stated set of limitations, when the lower the frequency on the band, the more groundwave propagation comes into play.
I haven’t set up a station yet but for the groundwave reasons find that broadcasting on 600 khz in my area is preferable to finding something open on x-band.
AM propagaton is more variable under different conditions than FM for no other reason than the frequency propagation conderations alone. You can have a set input, a set antenna height, a set power output and still propagate farther than you figure is standard–depending on conditions.
You have all the time in the world to stay logged on to this site for hours at a time, to receive email updates of new forum posts, to post in the forums, publish PDF files, and do the math to continue to share your opinions of what can’t be done.
BUT
It’s just far too much trouble and effort for you to answer constructive questions that might in some way provide assistance?
Not even the time to make some suggestions that might work in a theoretical ideal system with the standard assumptions in place (e.g., 50 ohms transmitter load impedance, 80 ohms RF impedance in the ground connection, 20 ohms ground loss – all taken from WA7CS RF Calculator Page, a source to which you so often refer)?
Total posts : 45366Given equal radiated powers, and other things equal, then the groundwave field at 600 kHz is greater than than at 1700 kHz.
But a 3-meter radiator cannot radiate the same power at 600 kHz as at 1,700 kHz, other things equal, because the radiation resistance of a 3-meter vertical drops as the frequency goes lower. At 600 kHz it is extremely low. That is why most operators favor the high end of the band.
//
Total posts : 45366[quote=scwis]You have all the time in the world to stay logged on to this site for hours at a time, to receive email updates of new forum posts, to post in the forums, publish PDF files, and do the math to continue to share your opinions of what can’t be done.[/quote]Actually my posts show what can be done with certain equipment configurations and operating parameters. Some view that as my saying nothing else is possible, but numbers I post are only a benchmark for people to use or ignore, as they wish.
The reason I’m logged on so long is to give detailed responses to the topics posted here by others.
[quote]Not even the time to make some suggestions that might work in a theoretical ideal system with the standard assumptions in place (e.g., 50 ohms transmitter load impedance, 80 ohms RF impedance in the ground connection, 20 ohms ground loss – all taken from WA7CS RF Calculator Page, a source to which you so often refer)?[/quote]Anybody can download and run those programs. There are programs by that author (Reg Edwards, who is NOT WA7CS) to calculate the inductance, form factor, Q etc of a Part 15 coil, in fact. So that already is available if anybody wants them.However the numbers posted in Neil’s quote of mine above, and most others I post, are based on my own NEC-2 analysis to determine the radiation efficiency of the antenna, and my own use of the FCC’s published groundwave propagation curves for medium wave.
//
Total posts : 45366Hello all,
SCWIS has asked some really good questions and good answers can lead to practically useful application. I will have to think about some of them before chiming in except for a couple where I can offer some information.
Bear with me if you already know this but for those who don’t, the reason we include a series inductance for the 3 meter antenna is because the antenna appears as a small capacitor (high reactance) in series with a small resistance (radiation resistance). For the transmitter to effectively deliver power to the antenna, the capacitive reactance needs to be reduced or cancelled. The torroid or other loading coil provides this function but there is bad news. The RF resistance of the inductor adds resistance and thus losses to the signal that we are trying to get to the antenna. If you look at the resistance of the coil (2 ohms or so) compared to the radiation resistance ( tenths of an ohm) you conclude that the coil wastes most of the power. But there is another factor which reduces the importance of the inductor resistance. The effective RF resistance of a “good” ground is tens of ohms which also consumes power. I conclude that since the ground resistance is the major waster of power that it doesn’t make much practical difference whether the inductor resistance is 2 or 4 ohms. Adding a couple of ohms to a system with a resistance of 20 or 30 ohms has little effect. So, it appears to me that either a torroid with higher loss or a coil with less loss is not an important factor and either should work essentially the same. Not just any RF choke is suitable since most of them are used to block rather than pass the energy and thus have high RF resistances. That’s why we try to wind high Q coils with large wire.
Regarding the coil form factor and Q, again bear with me, there are some little mentioned things going on with coils. The Q is essentially a measure of the energy loss in the coil, though most of us think in terms of bandwidth. (Technically Q is the ratio of energy stored per 1/2 cycle over energy lost per 1/2 cycle….high Q implies low loss). Coils also have capacitance which appears from turn to turn of the winding. This leads to what is known as self resonance. Form factors are based partly on keeping this self resonant frequency above the intended operating frequency of the system. I wound a coil on a 4 inch PVC form 17 inches long (about 200 uH.) intending to use it on part 15 AM. I measured the self resonant frequency to be 3.5 MHz. It would work OK for AM frequencies but I was a little concerned that this is near the second harmonic of the X band and may allow the second harmonic to radiate. This work is still in progress. I have read various articles about form factor and what we are trying to do is to control the stray capacitance and Q. I don’t know if there is one good answer to the form factor question, but it seems that many authors favor a 1:1 up to about 4:1 (L/D).
I know this post is getting long so just one more comment. I started this thread to confirm, based upon my experiences, that the ranges that Mr. Fry cited are realistic given the conditions of his model. Of course you can do better and you can do worse, but they appear to me to be good ballpark indicators of what to expect. The only “hassle” about legality comes from the fact that Mr. Fry modeled the system based on part 15.219 restricted antenna and ground conditions. MLR is correct that the legality has been covered in depth so please view my post as nothing more than a qualitative physical confirmation of the expected performance of a particular transmitter power working into a particular antenna and ground.
Neil
Total posts : 45366Ya, I see what you’re saying about antenna limits, but I still want to do the groundwave thing. Seems to me I should be able to make an adjustment electrically/electronically to the antenna, and it seems to me that with a length, resonant, of some fraction of the desired wavelength, I should be able to come out ahead on that somewhere.
You don’t think so, maybe?
Total posts : 45366[quote=Clara-Listensprechen]Ya, I see what you’re saying about antenna limits, but I still want to do the groundwave thing. Seems to me I should be able to make an adjustment electrically/electronically to the antenna, and it seems to me that with a length, resonant, of some fraction of the desired wavelength, I should be able to come out ahead on that somewhere. You don’t think so, maybe?[/quote]
Here are some numbers comparing Part 15 AM antenna system radiation efficiencies for 600 kHz and 1,650 kHz. The antenna system has these parameters in common at both frequencies…— Radiator: 1/2″ OD x 9 foot vertical copper pipe, with short ground lead.
— Overall Radiating Length: 9.84 feet (3 meters), including the complete conductor length to r-f ground. The Part 15 tx is located a few inches above the earth, and the bottom of its “ground lead” connects directly to a buried r-f ground using no “ground wire.”
— R-F Ground Resistance: 20 ohms
Here are the numbers applying to each frequency…
At 1,650 kHz
Antenna Base Impedance: 0.113 – j2,575 ohms
Coil Resistance: 2 ohms
Radiation Efficiency: 0.51%At 600 kHz
Antenna Base Impedance: 0.015 – j7,100 ohms
Coil Resistance: 4 ohms (coil R higher due to more reactance at 600 kHz)
Radiation Efficiency: 0.06%This difference in radiation efficiencies means that on 1,650 kHz this system will radiate 8-1/2 times more power than at 600 kHz, for a given applied transmitter power. That will result in about 2.9 times more field strength for the system on 1,650 kHz, other things equal.
Groundwave propagation losses are lower at 600 kHz than at 1,650 kHz, but at the power levels radiated by Part 15 AM stations, that doesn’t overcome the benefit of the higher power that is radiated by the 1,650 kHz system.
//NOTE: This message appears below radio8z’s post, but it’s NOT directed specifically at radio8z, or anybody else – it’s for EVERYBODY!
Part15.us was created to encourage, and let me repeat that for emphasis ENCOURAGE, legal, low power broadcast experimentation.
Got that? ENCOURAGE.
We all know that anyone with a credit card can set up a pirate operation in about 20 minutes that will cover 20 miles in high fidelity stereo, and no amount of curmudgeonly admonishment will dissuade a person who wants to do that.
The only hope I see of getting folks to operate legally is to give beginners and other experimenters some possible hope of getting some kind of useful range. I agree wholeheartedly that we must be realistic (if I didn’t certain posts would not be here, I am admin, after all) but the tone of realism needs to be positive and encouraging.
Positive and encouraging.
Call me a Polly Anna if you wish, but I still believe in, and have experienced, useful range with legal methods. As I have posted a number of times before, useful requires that your audience be densely distributed and close by (apartment complex, row houses, etc.) but it can be done.
I’m going to continue, and to encourage others to continue, to pursue that goal.
Hey, there’s that “E” word again!
No need for any “all I am trying to do” responses, because what you are trying to do is your business. What I am trying to do is maintain a discussion board that is positive and encouraging, and that’s what I will do.
Short version – the tone of some of the posts on the forums needs to change. Posts that are positive and encouraging are welcome, no matter what the content or message. Posts that stray too far from the benchmark of positive and encouraging will be considered suspect. If I see a suspect post I will write the author and discuss the matter. I’m not here to delete stuff, I’m here to help.
How else do we have any hope of steering people in the right direction?
Experimental broadcasting for a better tomorrow!
Total posts : 45366Efficiencies can be improved by taking measures as simple as impedence matching, or actually CUTTING the length from 3 meters to a length better resonant with 600 kHz and mitigate that electronically (loading, tuning, trimming, whatever).
Have you got numbers for some antenna length other than 3 meters that would be resonant with 600 kHz?
==========
I did some quick rough number crunching and find that 3 meters isn’t even close to being resonant with 600 kHz, and we’re cut down to 1/64th wave getting down to 12 feet and change.
Going to 1/128th wave takes me down to 6 feet and change. More precisely 6 feet, 1 inch and 1/8th.
Total posts : 45366scwis,
Nevermind…got it working.
Neil
Total posts : 45366I’ve been focusing on going low end rather than high end (frequency) not just because I want to focus on groundwave propagation but also because of the issues you raise with stray capacitance in RF coils. The lower the operating frequency, the more negligible those factors become.
Of course there are antenna limitations, and the lower the frequency of operation, the further away one gets from a decent fraction-wave length when talking about a mast type radiator.
Where can I find discussions of dipole type antennae regarding the low end of the spectrum? Seems to me horizontal rather than vertical would actually be prefered when focusing on groundwave propagation anyway. A quarter-wave antenna for 600 kHz is a handleable 195 feet.
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