One of the many places I saw a reference to inductive top-loading is in a Radio Shack antenna book, which is written for ordinary consumers and hobbyists, and not for antenna experts. This is why I was surprised that PhilB, whom I considered to be a radio expert, did not know about it. I actually thought that all radio engineers knew about inductive top-loading. Obviously, I was wrong.
As I already posted several times, any real inductor has capacitance as well as inductance. The bigger the dimensions of the inductor, the larger its capacitace is. It is only the theoretical "lumped constant" inductor that is defined to have no capacitance; but this is a mathematical concept, and the lumped constant inductor does not actually exist. A real inductor placed on top of a vertical monopole does not require any vertical antenna section above it in order to achieve resonance because the inductor itself provides the needed capacitance that a higher antenna section would provide. Current does flow through the inductor, and has a return path through the outer surface of the inductor because of its self-capacitance. The top-loading coil is not just a piece of metal providing capacitance, but it is also an inductor that resonates the antenna system. No additional inductance is needed for resonance in an antenna system with a top-loading coil.
In an e-mail excange I had some time ago with Bob Pease, the well-known columnist in Electronic Design, Bob, in his laconic way, called me, a "good explainer." I now realize that I don't deserve Bob's praise, because I was not able to explain inductive top loading even to someone with as impressive credentials as PhilB, who certainly would have more than enough capacity to comprehend the subject.
I'll try some more to give an explanation, because I really want PhilB to get it (if HE doesn't who will?): The RF current flow through the top-loading coil is not uniform. For a cylindrical vertical coil, the maximum current is at the bottom and goes to zero at the top. Maybe this is the hangup that prevents understanding. Perhaps PhilB thinks that, for an inductor to work, the current must be uniform from one end of the coil to the other. This is a false assumption based on the "lumped constant" concept. To simplify the calculations, most circuit analysis in electrical engineering does not consider spatial dimensions, and all components are assumed to be points of zero dimension, with "wires" of zero length inter-connecting them. The lumped constant assumption does not work with antennas, as is seen by the fact that the current along an antenna wire is not necessarily uniform.
The inductance needed to get resonance with a top-loading coil is considerably higher than for a center-loading or base-loading coil. This is becuse the non-uniform current distribution in the top-loading coil provides more inductance from the bottom of the coil (where the higher current is) than the top. One of the websites mentioned by PhilB says that top-loading is not practiclal at lower frequencies because of the high inductance required. I have used top-loading down to 1300 kHz, indicating that it can be used for Part 15 AM.
This is about the best I can do without illustrations. I hope that my explanation of inductive top-loading is clearer now.
It's more stuff I didn't know.
Somewhere around here Neil put in
something about electron flow in the
antenna and the return path. Do you
guys know what thread this was in?
My eyesight has been worse the last
few weeks. I've had 8 operations and
I'm getting tired of the problems. Anyway,
I love all this stuff - it just takes me a
million years to read it. And I also miss
things fairly often. So if you guys know where that
electron thing is - let me know. Because
I'm many decades behind ALL OF YOU.
Best Wishes Always To All You Guys
Bruce, MICRO1690/1700
I just found the part Neil wrote about the
flow of electrons.
Thanks!
Bruce, MICRO1690/1700
Mentioned herein was the idea of putting the transmitter at the top of the antenna. That brought to mind something totally different but an interesting side note.
Imagine a vertical tower, grounded at the base. You connect the ground side of your transmitter antenna cable to the base of the tower. Radials are extended horizontally out from the tower. They are insulated from the tower and connected to the hot side of the transmitter antenna cable. Any antenna structures at the top of the tower serve as top loading. This idea was used for the lower Ham bands, 80 and 160 meters. Of course there was some tuning involved to make it resonate.
Talk about a radiating ground!
Saw this in a QST antenna book, a Ham Radio magazine.
Ermi,
OK, let’s try for clarity.
Let’s go back to the beginning. Here is my concept of the antenna you are talking about.
- A vertical antenna (3 meters?)
- Transmitter feed point is at the bottom between the antenna and ground.
- There is an inductor at the very top with the bottom terminal connected to the top of the antenna and the top terminal unconnected (no wire at all above the inductor)
- There are no other loading coils anywhere in the antenna below the top inductor.
- The antenna is tunable to resonance by varying the inductance of the top inductor.
Please correct any assumptions I made that are wrong.
You said you have been experimenting with a number of top inductor loaded antennas, presumably, as I have described above. Could you pick one from your experiments and describe it in more detail? The following parameters will help: frequency, antenna length, inductance of the top coil, and approximate physical dimensions of the coil.
Have you done field strength comparisons with any other more conventional type of antenna?
Once this is nailed down, we may have a more solid base for discussion.
"My eyesight has been worse the last
few weeks. I've had 8 operations and
I'm getting tired of the problems. Anyway,
I love all this stuff - it just takes me a
million years to read it. And I also miss
things fairly often."
Sorry to hear that. So, your computer doesn't have Text-To-Speech capability? I thought there was a government edict that all commercial OS' have to have some minimum amount of Universal Access controls.
I use a Mac, so I'm sort of used to having Text-To-Speech and screen reader capabilities, and voice command controls (Speech Recognition) which comes with a plethora of commands, plus you can create custom ones and speakable items. Since Snow Leopard came out, VoiceOver has been dramatically improved. You can adjust almost everything to accommodate low vision, hearing impairments, etc., etc.
Because of my CTS I often use the SR commands, and have taught the computer a bunch of custom ones. When my eyes are feeling strained, I can use the screen reader to zoom plus read the text aloud.
All that stuff comes standard under modern versions of OS X.
Ken, that's a brilliant idea.
Yes Bruce, you have a text to speech on your computer!
If your using XP, then just go to: start/all programs/accessories/accessibility/narrator - it's a small program and will open right up.
If you're using a different version then XP, the path to the program will be very simular
mram1500,
I have considered modeling the antenna you described using NEC. The topology is complicated, so I would want to simplify it by modeling the antenna as a grounded vertical monopole next to and near four ungrounded horizontal radials joined at the middle as a cross, with an additional segment containing the signal source joining the bottom of the monopole to the center of the radials. I think that this is substantially the antenna you described.
Far from ground, this configuration is an ordinary ground-plane antenna. With the monopole grounded at the bottom, and the radials close to and parallel to the ground, I would expect most of the current from the signal source to flow by capacitive coupling from the radials to ground. A small amount of the current from the signal source would flow by capacitive coupling from the radials to the monople, causing current flow along the monopole, thus producing radiation from the monopole.
At first glance, this looks like a very inefficient antenna configuration because most of the transmitter power would be absorbed by the ground.
I am wondering if this is the actual configuration that was published in the ARRL Antenna Book, or if there is something missing. The edition I have on hand (1970) does not have the antenna you described.
I recently received a private inquiry about what I meant by the "self-capacitance" of a coil. The person guessed that I meant inter-winding capacitance. I actually meant the capacitance of the outer surface of the coil to the surrounding environment. A coil does have some inter-winding capacitance, but that is quite small compared to the capacitance due to the outer surface.
The standard reference on the subject of self-capacitance of coils (as well as coil Q) is R. G. Medhurst, "H. F. Resistance and Self-Capacitance of Single-Layer Solenoids," Wireless Engineer, February, 1947 p. 35, continued on p. 80, March, 1947.
"This article describes a simple and effective means of using a grounded tower, with or without top-mounted antennas, as an elevated ground-plane antenna for 80 and 160 meters."
The actual publication I read this article in is "More Wire Antenna Classics - Volume 2" which contains "more of the best articles from ARRL publications."
ISBN: 0-87259-770-9
First Edition-Third Printing, 2006
The article is on page 20 of chapter 7.
An "elevated ground plane antenna" is usually known as simply a "ground plane antenna," in which the radials are an appreciable distance above the ground. Therefore, the hot radials of the antenna described by mram1500 must be a significant distance over the ground, instead of right over the ground, as I envisioned at first.
The higher the radials are "elevated," the lower the loss due to RF current passing through the earth ground will be. So, I can see that this method is a viable means of getting an existing grounded tower to function as a vertical radiator.
I don't see any Part 15 AM application, however.
I received a personal e-mail saying that mram1500's elevated ground plane antenna does, indeed, have Part 15 AM applications, as long as you mount a 3 meter antenna on top of the tower.
Very funny. This is the first Part 15 joke I've seen. I just thought of another:
Q: How did the chicken get his Part 15 signal to cross the road?
A: By mounting his transmitter and antenna on a tower.
And how about this one?
Q: What has 100 milliwatts input?
A: A Part 15 AM transmitter.
Q: What has 100 milliwatts OUTPUT?
A: ALL Part 15 AM transmitters.
This could be the beginning of a new stand-up comedy routine!
Well first, it's not MY elevated ground plane antenna. The antenna is featured in one of many ARRL publications. Antennas interest me and this one was intriguing.
I found it interesting in that a properly designed radial system is not supposed to radiate to which the transmitter output is connected. Add to that the fact that the vertical is grounded, not connected to the transmitter "output" but rather, ground.
As you pointed out the antenna as shown is not a Part 15 antenna due to the physical size. But, if one could convince and Inspector that the transmitter is connected to the non-radiating ground counterpoise and the tower is not... Now there is a Part 15 joke!
I've even seen the 100 mW output boo boo on the sites of companies that sell certified AM Part 15 transmitters - a case of the marketing people not talking to the engineers, I reckon.
Hi Ermi! I was inspired by your post, so I came up with the extended version ...
A: He pecked at a piece of wire, and thought it would help, so flew to the top of the tower and twisted it onto the antenna ground and tower.
It worked very, very well, but when he flew back down, a fox was waiting for him and began to chase him around the tower, so he picked up a coil of wire and tried to escape by flying back to the top and twisting the coil onto his other wire.
It sort of worked, but unfortunately, when he flew back down, the fox caught him anyway ...