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“Here’s another digression: Part 15 AM can be used across all of the broadcast band, and even slightly below. Part 15 AM operators stick to the upper end of the band because of the much higher antenna efficiency there. However, the near field extends a lot further at the lower end of the range because of the longer wavelengths there. Maybe Part 15 AM is not hopelessly bad at the low frequencies.”
…happens to be something I was already thinking on and why the current direction of the discussion has been of particular interest. One lambda at 1700 khz is about 176 meters.. while one lambda at say 530 khz is about 565 meters.
Now using the formula Rich quotes from Balanis of lambda/2*pi, that’d give the outer boundary for the near field as approx 92 ft for 1700 khz and approx 295 ft at 530 khz.
Using Ermi’s calculation of 0983634*(lambda), it’d be approx 57 ft for 1700 khz and about 182 ft for 530 khz.
Either way, it’s a difference of over 300% in the radius of the near field, which is where (as Ermi Roos noted) it is not unusual for good signals to be obtained. Also, if I recall correctly, 300 ft (very close to Rich’s formula when applied to 530 khz) is what the “Talking House” claims as it’s minumum distance. Or if you take the 295 ft that Rich’s formula yields and the 183 ft that Ermi Roos’s formula gives (at 530 khz) and “split the difference”, you’d be at about the 250 ft the FCC estimates for a part 15 AM.
Now, as I understand what has been said about the near field, it would be where we’d expect reasonable reception with the infamous “cheap portable radio”. A good car receiver is considerably more sensitive and could pick up the signal at a greater distance, BUT…as it moves progressively further from the near field and out beyond the “radiating hemisphere” that Trainotti places at lambda/2, the signal (if it can be received at all) will be primarilly groundwave. And since the part 15 AM radiator is extremely short, any propagation much beyond the near field “bubble” would be more dependent on the ground of the part15 station than on the length of the very short radiator?
Am I understanding the basics correctly here, or am I way off base?
I may have an anecdotal point from the testing of the kit which would seem to support the idea of near field increasing as frequency is lowered (and wavelength increases). When I first powered up the transmitter I tried the high frequency end of the AM band. Reception on a very poor clock radio on the other end of the house was so-so, but that may have been due to the radio picking up strong local stations all the way to the top of the band (it’s not very selective, it’s a GPX clock radio that has seen better days). In any case, I was picking up hum and other stations on more than one receiver in the house, not due to the SStran, since the hum was present even when the transmitter was off. As I tuned down the band, the hum also decreased and the “bleed” from the local stations all became negligible at about 1 mhz. All the local stations are at 1230 khz and up, so I started thinking at least in my area the mid to lower end of the band might be better in terms of having less chance of interfering with people trying to listen to the local commercial stations. But the cheap clock radio definitely seemed to receive better at the lower frequency as well. Might have been due to other factors, but with this discussion I’m thinking it might be because it would be (by Ermi’s formula) close to the edge of the distance for near field at the high end of the band, but well inside the near field at 1 mhz. (1 mhz near field = approx 97 ft by Ermi’s formula)
In any case, definitely a lot of good food for thought in this topic.