Induction was discovered by American scientist Joseph Henry in the 1830s. English electrical experimenter, Michael Faraday, also discovered induction around the same time as Henry. Induction fields are standing waves that do not propagate. In radio, they are called "near" fields. The propagation, or radiation, of electromagnetic fields was discoverd by English mathematician and physicist, J.C. Maxwell, in the 1860s. Maxwell did not demonstrate the existence of electromagnetic radiation by experiment, but showed mathematically that the electrical laws discovered by Faraday result in the existence of radiation. Hertz proved the existence of radiation by experiment, and Marconi invented a practical method for using electromagnetic radiation for communications over long distances. Several experimenters observed radiation long before Hertz, but didn't know it. Henry himself observed what he thought was induction over very long distances, suggesting that what he was actually observing was radiation.
The earliest "wireless" communications devices were based on induction; but, since Marconi, wireless communications has been mostly based on radiation.
Radiation occurs at distances greater than !/2 wavelength from a vertical monopole. There is a transition point at about 1/6 wavelength where the magnitude of the near field and the radiating field (the far field) are about the same. At 1/2 wavelength from the antenna, there is very little near field.
The useful thing about the near field for Part 15 is that the field strength is very strong very near rhe antenna, even if the radiated power is very low.
As an example, the radiated power from a well-designed Part 15 AM system is about 50 uW at the upper end of the AM BCB if the antenna is at ground level. The field strength is about 2 mV/m at 30 meters. At the low end of the AM BCB, the same system would have a radiated power of only about 3 uW, but the field strength at 30 m is more than twice that at the upper end of the AM BCB. This is because 30 m is well in the near field at the lower end of the AM BCB, while at the upper end of the band, 30 m is near the transition region between the near field and the far field.
The increased signal strength in the near field can be obtained at lower fequencies. The results are very dramatic in the 160-190 kHz band, which is governed by Section 15.217 of the Rules. Section 15.217 allows a antenna system length of 15 meters, compared to 3 meters for Section 15.219. The maximum power input to the output stage is 1 W instead of 0.1 W for 15.219. Despite these antenna and power improvements, the range is considerably worse with 15.217 than with 15.219. The way the "lowfers" get any rang at all is by using CW instead of voice. However, in the near field, the performance of a 15.217 system is magnificent! At 30 m from the antenna, the field strength is about 400 mV/m.
The magnitude of the near field drops off a lot faster than the far field. In the far field, doubling the distance from the antenna causes the field strength to be cut by a factor of 2. In the near field, doubling the distance causes the field strength to be cut by a factor of 8. So, the near field drops very rapidly with distance. At 170 KHz, the signal is barely detectable at about 250 meters when AM modulation is used. This modest range could be of interest for some experimenters.
If anybody is interested in trying out this Part 15 "basement band," there are AM/FM/SW/LW receivers available from Radio Shack and other stores.
In 1975, my Part 15 friends and I tried
broadcasting AM on 170 kHz. The transmitter
used tubes and came from Popular Electronics,
I think. It was called "The Neglected Band
Transmitter." It ran one watt input on 170 kHz.
We had 2 problems. We ran it into a 50 foot
wire (the legal length at that time) so the signal
probably didn't go anywhere.
The second problem was that none of us had
any radios that tuned to 170 kHz.
Except for the guy who built the transmitter.
He had a Drake SW-4A, which was a very
good radio for that range.
We didn't get any reception reports.
Bruce, MICRO1690/1700
P.S. I just Googled "The Neglected Band
Transmitter." The first two hits on the
top of the page refer to the original article
from 1972. This is a 3 tube AM transmitter
for the 160 - 190 kHz range. Even though
my friend built it, there was no way to
tell how well it really worked, and unfortunately,
the transmitter is now long gone.
So in the case of a Talking House part 15 AM set up with the external ATU/antenna where elevating the antenna is not an option and the ground conductivity is poor, would it make more sense to set up shop on the low end of the MW band (540-800 kHz) and live off the near field? If you get 2 mV/m @ 30 meters in the near field at 1700 kHz, where would the 2 mV/m contour fall in the 540-800 kHz range? 630 and 700 are frequencies that might work in my area...in the daytime anyway.
Without the ability to elevate the antenna, I'm just wondering if I am wasting my time in the 1600-1700 kHz range if the near field is going to be the only really usable signal.
Ermi, I have gone back and read your info
on the near field a bunch of times. This is
interesting stuff. I have tried to use my
SS-Tran on 620 KHz in the past, which is
clear here during the day. I didn't have
much success, but I didn't try very hard.
A good loading coil for 620 would be big.
Sounds like a cool idea for a future project
just to try it because I have no idea what
the performance would be like in the real
world.
Best Wishes
Bruce, MICRO1690/1700
I tried 870 at my last location and it worked great. 870 got me out of the hash from some street lights, distribution transformers or something that was tearing things up at the 1000 - 1700 end, and I didn't note any loss of useful range.
Bonus was that morning and evening skywave was much less down there too.
Dear Forumers:
What I find interesting is that low frequency options have come under discussion, compared to the sense that recently, only the high end of the AM band has been promoted.
It seems to me that if an air coil is used then the high end of the band is strongly preferable because the low end of the band requires many more windings. More windings means more resistance and signal loss.
The Talking House uses a slug tuned coil, so the full number of turns are in use across the band and the tuning rods are pushed inside the coils as the frequency goes down.
When the auto tuner is adjusting the inductor for the low end the slugs are inserted to nearly the full length of the coil and at the high end of the band the slugs are almost completely withdrawn.
so are you saying it doesn't matter high or low on talking house b/c of same number of turns?
Are you saying it doesn't matter high or low on talking house because of same number of turns?
Yes, exactly - the configuration of the tuner uses the full number of windings on both coils for all frequencies. The iron powder rods are used to increase the uH of the tuner as the frequency goes down by mechanically inserting the rods farther and farther into the coils.
One thing to keep in mind is that at 1700 kHz, the 10 ft. antenna is about 0.017 wavelength, while at 540 kHz, it is only about 0.006 wavelength, which would be the equivalent of using a 3 ft. antenna at 1700 kHz...so basically at the top end of the band you are allowed three times the effective antenna length that you can have at the low end (even though in both cases they are 10 ft.). This will result in much more actual radiated power at the top.
Theoretically, this would be somewhat offset by the fact that ground conductivity at the low end is better that at the high end of the band, but I still think you are better off operating at the high end...
Looking at the Talking House coils they look like they are wound with "Litz" wire. If so this is a type of construction used normally in receiving circuits. "I think" but don't know for sure, and I'm open to clarification, that this type of circuit construction is normally not used for power gain as in a transmitter or antenna circuit but rather for voltage gain in receiver type circuits. Anyway this got me to thinking further about the differences in building to part 209 instead of 219. A designer for 209 only has to worry about radiated power. He could run 5 watts or 100 watts into the final tank circuit as long as the legal radiated power from the antenna is not exceeded. TH could design a power leveling (output feeds back control to a lower stage) circuit that stayed within the legal limits of 209 across the band and use as in-efficient a transmitter output circuit as they wanted. With 219 the 100 mw input limit requires the most effecient transmitter output circuit as possible since radiation is not limited. If I'm correct experimenters on this group should not waste to much time following certified designs as it's apples to oranges comparisons. Comments?? Dave
could run 5 watts or 100 watts into the final tank circuit as long as the legal radiated power from the antenna is not exceeded.
That appears to be the case in simple FM units like the Scosche unit and some of the other units I've seen. The final might be cabable of unacceptable power but radiation is controled, either by attenuation or with a phycally limited antenna - in some cases no more than a PCB trace.
Apparently there are a variety of opinions about the applicability of Litz wire in transmitting uses. This post and this post, among others suggest the possibility that Litz has desirable attributes.
this type of circuit construction is normally not used for power gain
In the TH application the coils are used for adding inductance.
Great refrences scwis! There should be a part 15 hall of fame and Phil B. ought to be the first inductee for his great transmitter, great customer service, and great posts.
What the near field does is that it increases the antenna gain close to the antenna. In the far field, the field strength increases by the factor of 2 when the distance is halved. This is an increase of 6 dB. Very close to the antenna, the signal increases by the factor of 8 when the distance is halved, which is 18 dB. The antenna gain in the near field increases 18 - 6 = 12 dB each time the distance between the antenna and the receiver is halved. In the far field, there is no change in the antenna gain with distance, provided that there is no attenuation of the signal due to ground loss.
In an electrically-short antenna, such as is used in Part 15 AM, the loss resistance due to ground loss, loading coil loss, and transmitter output impedance loss, is much more than the radiation resistance, and the radiation resistance is proportional to the radiated power. At 1700 kHz, the radiation resistance of a 3 m vertical monopole over ground is about 0.13 ohms. The radiation resistance is proportional to the inverse square of the wavelength. At 530 kHz, the radiation resistance for the same antenna is about 0.013 ohms, which is 1/10 of the radiation resistance at 1700 kHz. The lower radiation resistance results in lower radiated power at 530 kHz. However, in the near field, there will be some distance from the antenna where the field strength is greater than the field strength at 1700 kHz at the same distance.
The transition point between the near field and the far field is at about 1/6 wavelength, or more exactly, at 1/(2*Pi) wavelength. At 1700 kHz, the transition point is at about 28.1 meters from the antenna. At this distance, the antenna gain is nearly the same as in the far field. As closer distances are approached, the maximum antenna gain of 12 dB/(half distance) is approched. Because, at 1700 kHz, the transition point is close to the antenna, no significant near field antenna gain is observed, exceipt at distances of a few tens of feet.
At 530 kHz, the transition point between the near and far fields is at 90 m. At 30 m, the field strength is more than twice as high as the field strength at 30 m when using 1700 kHz, despite the fact that the radiated power at 1700 kHs is much higher than at 530 kHz.
At 170 KHz, the transition point is 281 m. At 30 m, the receiver is so close to the antenna (as a fraction of wavelength) that the field strength is enormous. A detectable signal is heard to nearly the transition point.
Ermi I hope I got this right but it looks a 219 rig at the high end of the band is best for range but if you want to use the low end then a 209 field strength certified transmitter would give the best coverage there. ??