There has been some discussion here about the use of elevated radials in Part 15 AM setups.
Some think that the lengths of those conductors need to be included in the limit given in FCC §15.219(b).
Others think that, because they produce very little far-field radiation, they should be exempt from §15.219(b).
The official position of the FCC on this matter is not widely known, if known at all.
Below is a post I made on another board (ham-related) that gives some background on the subject.
It might be of some interest to Part 15 AMers, where an elevated "source" (transmitter) is located at the base of a ~3-meter whip, with one or more elevated horizontal radials connected to the r-f ground terminal of the transmitter (no "ground lead").
Again -- we don't know how the FCC would view this, so potential users might want to factor that into their thinking/planning.
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In a radiating dipole, the current that flows from the 2-terminal source of the dipole onto one arm of the dipole originates on the other arm of the dipole (see graphic below).
When current flows toward the source it is 180 degrees out of phase with the current that flows away from that source. Yet the fields radiated by both arms of the dipole reinforce each other in space.
This is not the case for horizontal radials used in a ground plane antenna. Those single-conductor radials are driven at their physical center by ONE terminal of the source, which source is located at the bottom of the vertical conductor. Therefore the current flowing onto each half of that radial has the same relative phase as it flows along each half to the open ends of that conductor.
However the current on one half of the radial is flowing in a physical direction opposite to an equal current and phase flowing on the other half of the radial. So the radiation patterns from the two halves of that radial are out of phase, and (very nearly) cancel each other in the far field.
The antenna design described by Rich will be built and tested in the next few months, as I'm getting set to build outdoor antennas for AM part 15.
One of the antennas will be standard 10-foot vertical with ground radials, and another one will follow the instructions provided here for the Elevated Ground Radial System. I'll even add a tag saying "Antenna by Rich."
My Talking House ATU antenna is rooftop mounted. I tried placing 6 - 12 foot radials evenly spaced around the base of the antenna.
It did seem to improve the range a little but mostly it helped prevent detuning of the antenna system when the amount of moisture on the roof changed.
Printing Rich's Post # 1 "Elevated Radials for Part 15 AM Systems" I studied it over a warm bowl of old fashioned oatmeal with cranberries and honey, until I became confused.
The next step was to analyze my confusion until it broke down into questions.
The discussion opens with mention of an elevated transmitter with its 3-meter vertical whip, a familiar arrangement until we add elevated horizontal radials...
Vewing the illustration we discover the "whip" in this special exhibit is a vertical coaxial cable, the dipole floating above the top of the coax.
The coaxial cable has the appearance of a transmission line, measurement of which counts under 15.219.
It is understood that the dipole formed by the two horizontal radial wires delivers RF at the same relative phase along each half, thus reinforcing rather than canceling. Therefore it appears to be a horizontal dipole fed by a transmission line. The resulting pattern would be horizontal with lobes in opposite directions and nulls at the ends.
Also, I see a grounding symbol on the picture, but the article earlier states "no ground lead."
How wrong am I?
Dipoles don't have to be horizontal. This is simply a typical illustration of the most common installation.
As for the coaxial cable, this again being a typical illustration, wouldn't be needed as the "driven" side of the dipole could connect directly to the transmitter antenna terminal. The other half of the dipole would connect to a convenient transmitter ground point.
In the illustration you will notice RF flowing to the ground side of the dipole in addition to the outside of the coax. If the ground side of the dipole were illiminated and a current balun placed 1/4 wavelength from the point at which the shield ends, you would have an "end fed dipole" but we've gone down that road before.
In that case the antenna would not be in compliance due to its total length.
The original point of the post centered around equal and opposite radial currents cancelling out. In my case, the Talking House ATU is a tuner which resonates the 102" vertical radiator. My horizontal radials, equal and opposite each other, form the ground side of the antenna system. Due to the equal and opposite currents which cancel, increased field strength results by lowering the RF ground resistance as the radials themselves do not add to the far field signal. Rather, more of the RF is radiated by the vertical radiator as the applied signal is not lost to a poor RF ground.
The illustration in post 1 shows the current flow on a typical dipole antenna system used by hams. It is not an illustration showing the use of elevated horizontal radials in a Part 15 AM setup.
The text in post 1 tells the reason why a set of horizontal radials doesn't radiate (much), when a dipole does. The graphic of a dipole system helps to understand this point.
In the first part of my first post, I wrote, "It might be of some interest to Part 15 AMers, where an elevated "source" (transmitter) is located at the base of a ~3-meter whip, with one or more elevated horizontal radials connected to the r-f ground terminal of the transmitter (no 'ground lead')."
MRAM's post 5 above has a layout sketch of the system I described in words, in the paragraph above.
Below is a graphic showing the fields of elevated vertical antennas (such as ~3-m whips) driven against a set of elevated horizontal radials. Radiation from the horizontal radials is horizontally polarized, and shown at the right side of the graphic.
The subject of elevated radials for Part 15 AM was discussed at some length in http://www.part15.us/comment/29468 .
Thank you MRAM. Thank you Rich. Now I understand the comparative things being talked about.
It seems like now I understand.
I think i have heard someone mention this on Part 15.us a while ago but i would like to try a shorter radiator. Cut a 5 foot radiator then a 5 foot ground rod below that parially driven into the earth.
There would probably be a coil in the middle and a box for the transmitter, or in my case a blue bucket.
Would this be a dipole regardless of the coil in the middle?
I think it would be a form of dipole, an "Earth Piercing Dipole," but MRAM and Rich will know for sure.
I do think blue buckets vastly improve any antenna.
This thread will do for asking questions about a semi-elevated set of ground radials I will be adding to my AM 1550 system.
The radials will be "semi-elevated" because half of them will project forward on the ground from the front wall of the building, and the other half will follow the ceiling of the basement untill they exit the sides and rear of the building at ground level.
The AMT3000 is located indoors at the front window, with a loading coil feeding a connection with a 6-foot metal window frame with an added wire outdoors on the porch up to about 9-feet total antenna.
Presently a transmitter ground drops down the baseboard and is clamped to the steel I-beam in the basement, grounded to both the water pipe and the outdoor electric system ground rod. The ground attachment is necessary to cause the transmitter to properly tune according to PhilB's instruction for using a loading coil.
With only the ground and no radials the signal is solid within a close-to-the-house range, but drops off abruptly at about 200-feet out.
Once the radials are added, we will see how much better the transmission reaches beyond the present limit.
For curiosity, we will detach the ground clamp and see how radials alone compare to radials + ground clamp.
Member Blare posted: "This thread will do for asking questions about a semi-elevated set of ground radials I will be adding to my AM 1550 system." etc, etc.
Maybe I missed them, but what questions did you include in your "Hitching a Ride On the Thread" post (#10) in this thread?
Could such questions be those that you pose only to yourself?
I hoped the questions would be inplicit in the description; in wondering about the differences to be experienced between the several arrangements as described.
If no questions are detected no answers will arrive.
At first I thought today might be ideal to go outside and install ground radials.
The temperature was Zero-Fahrenheit, meaning "no temperature whatsoever," but it soon became One-degree-Fahrenheit which is super cold.
Meanwhile in Ragland, New Zealand, where Johny of Rag FM has temperatures in the Celsius range, it is much warmer.
Therefore measuring temperature in Celsius is more confortable than Fahrenheit.
Member Blare posted: The temperature was Zero-Fahrenheit, meaning "no temperature whatsoever," ... etc.
With due interest and research, hopefully you will come to recognize that zero degrees on both the Fahrenheit and Celsius scales are FAR above "no temperature whatsoever" -- which is Absolute Zero (-273.15 degrees Celsius).
Is that the same as ZERO degrees Kelvin?