Below are some "benchmark" values for the field strength at several distances from the r-f system described there.
These calculations are based first on a NEC-2 analysis to determine the FCC efficiency for the antenna, and then using that value with the FCC's propagation curves for the frequency and ground conductivity, to determine the distances. This is a proven process which applies even at these low powers.
Below are some "benchmark" values for the field strength at several distances from the r-f system described there.
These calculations are based first on a NEC-2 analysis to determine the FCC efficiency for the antenna, and then using that value with the FCC's propagation curves for the frequency and ground conductivity, to determine the distances. This is a proven process which applies even at these low powers.
This information should assist anyone wanting to know the coverage performance of an excellent Part 15 AM r-f system, as described.
DATA:
Frequency = 1700 kHz
Applied Power = 80 milliwatts (~output power of Part 15 AM tx)
Radiator = 3-meter total length including the conducting path from the tx chassis to the ground plane (antenna is ground-mounted)
Antenna System RF Resistance, Loading Coil plus Ground = 10 ohms
Ground Conductivity = 8.0 mS/m (typical value)
Radiation System FCC Efficiency = 21.12 mV/m at 1 mile for 1 kW applied
RESULTS:
Field Strength > Distance
2 mV/m > 0.09 miles (good signal)
1 mV/m > 0.18 miles (fair)
0.5 mV/m > 0.34 miles (noisy)
0.05 mV/m > 2.5 miles (very noisy)
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Rich,
Thanks for your time and effort to run the numbers.
Discussions of range achieved by part15'ers on AM seldom mention the conditions at the receiving site. With Rich's numbers, we can predict the open field performance which is probably most closely experienced in practice with a mobile receiver. Unfortunately, part15 broadcasts are not too useful for this mode since the listener will drive through the coverage area in minutes.
I don't want to be discouraging by saying what follows, but it is what it is. It is not likely that at home listeners will have the field strength at their receivers as strong as predicted or as strong as would a mobile receiver. Add to that problem the difficulty with noise on AM from light dimmers, computers, flourescents, etc. and the situation for practical home listening gets worse.
Rich's numbers show a rather limited range with ideal conditions. For reasons stated, the practical range is less.
There is nothing wrong with trying what you can to make a part15 AM system "serve a community", but I wanted to point out what you are up against.
I, and I am sure others here, would be interested hearing from those who have an idea of their real home based listening range rather than a calculated contour. All I can offer is that the maximum home listening range I ever achieved years ago was about 700 feet (using a KnightKit broadcaster which probably exceeded the 100 mW limit, and a 10 foot antenna).
Neil
Neil, I'm achieving about 2 blocks home range with the SSTRAN and SSTRAN antenna elevated just under 30 ft and 4 8 ft ground rods spaced about 30 ft apart around the outside of the house. My mobile range is about 3 miles with dead spots along the way. It's amazing how that strong signal just drops after the 2 blocks in any direction, yet the weak signal remains rather constant over the 3 miles. There's another subject here that I've never seen discussed much, and that is radiating your signal into power lines, communications lines, railroad tracks and even bodies of water for greater coverage. These effects take away the pure mathmatics of a radiated signal directly from an antenna source. Jim B
If you consider what I mentioned earlier, two blocks home range sounds good.
The sudden drop is probably due to moving from the near field (1/r^3 reduction) to the far field (1/r reduction). I forget the exact details, but I bet Rich could elaborate on this.
Regarding power lines, I transmit from an antenna in my basement since I am just using my system for personal stuff around the yard. My neighborhood has underground utilities and only my neighbor and I are on the same distribution transformer. Though I do not intentionally couple to the power lines and probably would not get through the transformer if I did, I do notice when I do a mobile check that if I am under a pole power line I can hear my signal about two miles away. The line is probably acting as a receive ant. in this case, but it does make one wonder if the lines could be legally exploited.
Neil
The sudden drop is probably due to moving from the near field (1/r^3 reduction) to the far field (1/r reduction). I forget the exact details, but I bet Rich could elaborate on this.
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The near field extends less than 20 feet from a 3-meter antenna on 1.65 MHz. If a sudden drop in far-field signal strength is noticed, it is most likely just related to the low radiated power, and the fact that field strength drops by at least 50% each time distance doubles.
For example, if a receiver needs a 1 mV/m field for usable performance, you can see from the numbers in my first post above that it would work OK about 0.18 miles from the tx, but moving it only a few hundred feet further away would put it in the zone of no coverage (as far as that receiver is concerned).
The same physics applies to AM broadcast stations, but with their high power level, the signal changes very slowly across any linear distance of several hundred feet, or even several miles.
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Thanks for commenting. Sounds reasonable to me.
Neil