I just received a message from a reader on another board with some interesting questions prompted by this subject. The responses are posted here, as they may interest others.
Q1 (paraphrased): How would the performance of a system using a transmitter with 100 mW d-c input power to the final r-f amplifier, a 2.99-meter whip antenna, a 0.01-meter ground lead connected only to the negative d-c common of the transmitter, and no other conductors compare to that permitted by FCC §15.209?
Q2: Would it make any difference in performance if the system described in Q1 was elevated?
The table and chart below were generated some time ago, and give some insight into this. They are based on the use of the ground connection losses and other conditions shown.
FCC §15.209 permits a maximum field of 24,000/(Freq in kHz) at a distance of 30 meters from the transmit antenna -- which for 1650 kHz is about 14.55 µV/m. Note that a 14.55 µV/m, 1650 kHz field is well below the ambient r-f noise level in most locations outside of a screen room.
The performance of the configuration described in Q1 would be very poor compared to a compliant §15.219 system using a short conductor to a ground rod, as the antenna system would have such high reactance that nearly all the power available from the transmitter would be dissipated as heat, even if the antenna system could be tuned to resonance. Probably it would perform better than a system compliant with FCC §15.209, though.
The height of a Part 15 AM transmitter and its attached whip -- by itself -- has no significant effect on field intensity. What does have a large effect on the field intensity of an elevated system is the radiation produced by the conductor(s) other than the 3-m whip, leading away from the transmitter. They, along with the 3-m whip form an off-center fed dipole, which has much better radiation efficiency than the 3-m whip with a very short ground lead, alone. This is easy to see in the NEC plot (2nd graphic below).
Some of what's said in the very interesting discussion & charts translates clearly to my understanding... but other detail escapes me as I try to percieve the total message.
In Q1 the setup per 15.219 describes a .01-meter ground lead connected only to DC common of the transmitter.
That translates to mean that the .01 meter ground lead IS NOT connected to an actual ground.
But the charts show varieties of radials and ground rods not previously introduced.
As a secondary nit-pick I would ask if a "ground-lead" is not grounded, how can it be called a "ground-lead"?
Thanks for the comments. Responses:
... the .01 meter ground lead IS NOT connected to an actual ground. But the charts show varieties of radials and ground rods not previously introduced.
The next to last paragraph of my opening post has my comment about the likely performance of the system described in Q1. I put those graphics together quite a while ago. While the graphics don't directly address the system in Q1, their information is pertinent to the context of my opening post.
As a secondary nit-pick I would ask if a "ground-lead" is not grounded, how can it be called a "ground-lead"?
Good point. It isn't a ground lead then, just an arm of a short, off-center-fed dipole. I used the term "ground lead" in my reply because that was the term used in the question I was answering. Apologies.
Rich, it is good to have asked things that actually made sense.
It is fair to admit that your postings over the past years have brought me to read and carefully re-read what you have posted to be certain that what I end up saying in response correctly fits the discussion.
It takes a blizzard.
My questions and Rich's clarifications over a couple of incidental details should not distract from a major point put forward by the opening post...
What Rich reveals is paramount in understanding what might be called a heretofore hidden secret of part 15 AM antenna systems...
It's the interconnecting audio and power conductors reaching up to an elevated installation that result in superior performance of elevated antennas by providing unintended radiation.
Previously some have believed that the elevation in itself improved outreach.
Much discussion has taken place over extended ground leads which have come to be taboo when they result in breaking the 3-meter rule, but now we can wonder if long power/audio cables might actually be more or less the same as a long ground lead?