Total posts : 45366
[quote:b1850d9763]Would the coil resistance value make much difference? That’s probably measuring DC resistance, which is not the critical factor in the influence of the coil on the efficiency of the radiator.[/quote:b1850d9763]
This takes a rather complex answer. The real purpose of the loading coil is to cancel all or most of the capacitive reactance of this very short radiator. The resistive term (eg, the radiation resistance) of the base input impedance of a 3-meter vertical radiator on 1,000 kHz is less than 0.1 ohms, and its (capacitive) reactance term is several thousand ohms. Even if the transmitter can drive very low values of load resistance, the high reactance in this short antenna will produce high load SWR to the transmitter, and the transmitter cannot deliver its rated power into such a load.
The reactance term needs to be zero before the transmitter can see a perfect match. That can be done by a suitable loading coil, but such a coil also can add several ohms of DC resistance in series with the antenna current. As in any series circuit, the larger resistance of the loading coil (several ohms) will dissipate much more of the available power than will the radiation resistance of the short antenna (<0.1 ohm).
[quote:b1850d9763]It has also been observed that Part 15 AM broadcast reception is more influenced by groundwave and near field radiation than the far field radiation the NEC apps analyze, which would seem to limit the applicability of the simulation in real world scenarios.[/quote:b1850d9763]
The far-field boundary for a 3-meter vertical on 1,000 kHz is well within 50 feet, so this really is not a factor for distant signals in this application. In any case, my analysis first used NEC-2 to generate the radiation pattern shape and peak gain of this Part 15 radiator when the “ground terminal” of the transmitter is connected to a perfectly conducting Earth plane through a typical DC resistance. I used 5 ohms, which is no doubt optimistic for Part 15 stations. [AM broadcast station antennas with 120 radials each 1/4-wave long typically have around 1 or 2 ohms of DC resistance to real Earth.]
The NEC-2 program also outputs field strength at 1 mile for 1 kW of input power over this perfect Earth. This corresponds to the FCC’s “efficiency” value for MW broadcast verticals vs height, which was based on measured data from 1937 (George Brown, et al).
Then I used the efficiency of the Part 15 radiator with the FCC’s propagation curves for the frequency and a ground conductivity to determine the field strengths and distances you repeated below, for 100 milliwatts of power in the radiator.
So by this process the calculated values are as “real world” as possible without actually duplicating the assumed conditions in real hardware, and making the measurements with a real field strength meter.
[quote:b1850d9763]Field Strength –> Distance
2mV/m –> 0.087 miles (very good signal)
0.5mV/m –> 0.333 miles (OK signal)
0.1mV/m –> 1.523 miles (marginal, depending on receiver and local noise level)
WOW! That’s pretty encouraging if NEC shows far field radiation of that strength. When ground wave and near field are added, and a good Q factor coil is used for matching, a pretty satisfying range should be attainable![/quote:b1850d9763]
My calculation already gives the end result, but it will be optimistic because the radiator will not have the full 100 milliwatts in it, and losses in the ground system of a Part 15 AM probably are not as low as my assumption for it. In fact I have a paper from Carl Smith showing about 95% system losses for a 3-meter vertical on 1,000 kHz using a ground consisting of 30 radials each of 0.15 wavelengths. This ground system is no doubt better than has been installed at any Part 15 AM station.