Ultimate Part 15 AM Installation

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Ignorance: -is where someone or something is uninformed. This should not be confused with being unintelligent, as one's level of intelligence and level of education or general awareness are not the same. The word "Ignorant" is an adjective describing a person in the state of being unaware. ...

Ignorance is the opening necessary for those with a nefarious agenda to overcome the productive work of many.

Just because we don't possess the experience or vision of someone who has a different and diverse experience, doesn't make their opinion any less valid. To express one's self to that end would be a display of ignorance.

When we loose our temper, no matter how valid our argument, we lose the contest of intellect and will. Holding "court" requires the revealing of all evidence, regardless of how palatable the revelation of evidence. Let the jury decide the outcome. We are by participation named the jury. I get to decide for myself, the outcome.

Regardless of the label anyone wears, those who are intellectually impoverished can be nourished with the smallest tidbit or drop of drink. Get a life. Get over it. They are the statements that spring from the most important questions ever asked: What? and So what?

Marsh Johnson, Sr. - North Bend, Oregon, USA

Loading Coil

I'd like to make a loading coil out of superconducting materials, and keep it packed in dry ice...would that lower the resistance?

"Modulating carriers since the '80s"

Another Coil Question

The coil at very low temperature should exhibit an interesting value shift, but I don't know exactly.

BUT, I have yet another coil question. What would happen with a very large diameter coil, perhaps 36" diameter?

Carl Blare

Coils and Archives

When the coil diameter is made larger more wire or tubing is required for a given inductance so it becomes a trade off between gains realized by diameter, wire size, and winding spacing versus losses caused by the longer conductor. The optimum length to diameter ratio for a single layer inductor in terms of Q is generally accepted to be between 2:1 and 1:1.

Many posts about coils can be found in this site's archives via the search function.

One very good thread with a lot of information presented by Ermi Roos and others is here:


Loading coil Q and capacitance

A superconductive coil would have no loss, and therefore infinite Q. There have been superconductive coils for some special purposes, especially for generating super-high magnetic fields, for several decades. Superconductive coils are simply not economically feasible for radio. So, about the best material we have for radio coils is copper wire or tubing.

One reason I like top-loading coils is that self-capacitance degrades the Q of bottom-loading and base-loading coils, but improves the performance of top-loading coils. Large physical size increases the self-capacitance of coils, which is good for top-loading. Large top-loading coils have considerable performance advantages over center-loading and base-loading coils.

I'm back ...

I apologize for not responding sooner. I was working on a tabulation of model results for all the typical Part 15 AM antenna schemes (not easy). Here are the results:,center-loaded_and%20top... Look particularly at the column titled “NEC Max Gain (dBi)”. Positive is better. All results are negative, so look for the ones that are less negative (generally top to bottom).

My original post, way back at the beginning, spoke of the ultimate AM antenna as being a base-loaded 3m antenna with many long ground radials. I said "Stop fretting about top-hats, center-loading, etc. They don't help much and are either legally questionable or physically impractical." Top hats are legally questionable. Center loading coils create a cumbersome antenna that is hard to fabricate such that it will stand up to constant wind loading. Then there is the center-loaded antenna with top-hat which is both legally questionable and cumbersome.

I certainly agree that there is ample room for experimentation. There are two overlapping avenues for experimentation: legal and technical.

The real “Ultimate Part 15 AM Installation” is the best performing transmitter/antenna installation that is technically optimum, practical and legal. The easy winner for best gain in the (redacted - let's not discuss people who aren't participating) era was the elevated base-loaded antenna, not legal in today’s reality.

Top-loading coil


If you were being humorous, let us know and ignore the following.

Maybe the reason NEC doesn't model a "top loading coil" per your expectations is because a coil at the top of a vertical antenna reduces to a lump of metal. The top of the inductor is open circuit, so it can't act as an inductor in the circuit. Regardless of the various vertical antenna configurations, the "top" of the antenna has zero current, wherever the "top" may be (could be the ends of top hat radials). A coil at the top would probably model as a capacitive top hat if you were able to accurately capture the physical dimensions of the coil.

Center-loaded vertical antennas work because the coil resonates with the antenna capacitance substantially ABOVE the coil. You can place the coil anywhere along the vertical length of the antenna, but the coil will need to have higher inductance to resonate with the diminished capacitance as you place the coil higher. With the coil at the top, the capacitance of the antenna above the coil is zero, so the coil inductance would be infinite.

I think there is general confusion about the term "top-loaded vertical" . The term "top-loaded" equates to loading the antenna with a capacitance top-hat, and doesn't refer to where a loading coil is places.


This thread and the one about improving the AM25 contains so much useful information they will be printed and added to the file folder.

Call it old school, call it new school, I call it Home School College, and the learning is provided by a team of knowledgeable teachers.

Carl Blare

Displacement current and top-loading coils

PhilB seems to be saying that inductively top-loaded short antennas don't exist. Frankly, I find that statement to be astonishing. Surely, PhilB must have run into references to inductively top-loaded antennas at numerous times during his apparently extensive radio career.

Phil's confusion seems to arise from a notion that, since the top end of a top-loading coil is open-circuited, there must be no RF current passing through the inductor. This notion is incorrect. Any physical inductor (one of finite size) has capacitance as well as inductance. A larger inductor (in dimensions of length) has more capacitance than a smaller one. There is a return path for for the current in the top-loading inductor through the "displacement current" in the capacitance of the inductor. The top-loading inductor can also be tuned to resonate with the antenna (and coil) capacitance at the operating frequency.

I assure you that there is nothing at all novel in what I just said. Resonant inductively top-loaded antennas are almost as old as radio itself. As an example, see Figure 28 on page 795 of Terman's Radio Engineers' Handbook, 1943, which illustrates the use a coil at the top of a vertical monopole.

In order to try to convince you further, let me try to illustrate the conversion of an inverted-L antenna into a resonant top-loaded antenna. The inverted-L antenna is resonant when the total of the vertical and horizontal sections is about a quater-wave long. The inverted-L will still be resonant if the horizontal section is formed into a horizontal loop (DDRR configuration) with an open-circuited end. Two loops (with an open circuit at the end of the second loop) will also be resonant. The horizontal section can actually be formed into any number of loops to form an inductively top-loading antenna with an open-circuited top end, although the total length of the wire may not be a quarter wavelength when many loops are used.

A Different Variation

With the small physical sizes involved, a transmitter could be placed on top of the antenna.

How would such a placement's performance differ, given any of the loading coil arrangements being discussed, from a normal bottom location.

Carl Blare

A return-current path is needed for any electrical circuit

That's why it's called a "circuit!"

In the usual vertical monopole over ground, the RF current from the hot side of the transmitter output flows up along the vertical antenna conductor, and then flows to earth ground due to capacitive displacement current, and returns along the ground to the cold side of the transmitter output.

There is no particular reason why you can't connect the bottomg of a vertical monopole to earth ground, and place the transmitter on top; PROVIDED that that a return path for RF displacement current exists from the cold side of the transmitter output to ground.

If the transmitter is housed in a sizeable metal box, and the cold side of the transmitter output is connected to said metal box, there will be displacement current between the transmitter box and earth ground, thus completing the RF circuit.

A loading coil will still be needed for resonance, which can be contained inside the transmitter box, or somewhere along the vertical antenna conductor.

If the transmitter box is big enough, you can get four times the radiation resistance compared with the normal vertical whip, since the antenna current will be roughly uniform along the length of the vertical conductor. Placing the loading coil inside the transmitter box might provide an excuse for making the box as big as possible. I don't know if the FCC would buy it (most likely, the agent would simply laugh).

A transmitter on top of a vertical monopole would certainly be unusual, and I would not do it myself, but there are no physical laws that would prevent this from being done.

Displacement current and top-loading coils


You totally lost me there.

First, I don’t have a copy of Terman's Radio Engineers' Handbook, 1943, as I suspect few or none of the readers have. It would be much better if you could cite some Internet sources.

I have searched the Internet for “ inductive top-loaded” antennas. One that I found uses the term “inductive top-loaded” to describe a vertical antenna with a loading coil placed at various points along the vertical dimension. On this page there is a graph showing the Gain vs. Loading Coil Height for a 10 meter vertical antenna. The graph shows the peak gain is at 5m (center loaded) and the gain drops off the chart as the loading coil is continually raised to above 9 meters. The text says: “The loading coil has to be resonant to the antenna capacitance of the upper part (C2) what means that the inductance has to be larger as the coil is placed higher”. That implies that the loading coil would need infinite inductance to resonate with zero capacitance above it.

Another web site I found illustrates 6 different vertical antenna configurations, including a loading coil at the very top. It says: “Top loading: Top loading, if done inductively, requires the greatest amount inductance. On the lower frequency bands, inductive top loading may not be practical. The reason is that the inductance of the coil will be massive and to keep the antenna short will require both larger coil material and coil diameter”. Nothing further is said about this, so the contribution from this site is technically meaningless other than what is implied in the quote.

Yet another web site talks about inserting a “top loading coil” at the junction of the vertical and horizontal sections of an inverted L antenna. This web site is again fairly meaningless because it doesn’t provide details, but it does illustrate a viable loading scheme because the “top” loading coil actually resonates with the capacitance of the horizontal wire beyond the coil.

You said:

Phil's confusion seems to arise from a notion that, since the top end of a top-loading coil is open-circuited, there must be no RF current passing through the inductor. This notion is incorrect. Any physical inductor (one of finite size) has capacitance as well as inductance. A larger inductor (in dimensions of length) has more capacitance than a smaller one. There is a return path for for the current in the top-loading inductor through the "displacement current" in the capacitance of the inductor. The top-loading inductor can also be tuned to resonate with the antenna (and coil) capacitance at the operating frequency.

This wording seems to simply say that a top loading coil has capacitance as one of its properties. Displacement current relates to capacitors. It’s a somewhat obscure term to use here in a less technical part 15 forum, but simply refers to the AC current flowing between the plates of a capacitor where there is no conductive current. So, everything you are saying seems to point to capacitance, which is basically what I said in my previous post. The coil is effectively just a lump of wire at the top of the antenna, but it does have capacitance. In effect, it is acting as a capacitive top hat, not a resonating inductor. The only way it could be acting as a resonating inductor would be if it had a huge inductance that would resonate with the “stray” capacitance above. At AM broadcast frequencies, the inductance would be awesome to behold.

Finally, I will throw in, for consideration, your statement:

Surely, PhilB must have run into references to inductively top-loaded antennas at numerous times during his apparently extensive radio career.

There is an insinuation in there that I will refrain from commenting on in compliance with scwis’s wishes. I will simply say that my credentials include a bachelors and a masters in electrical engineering and 9 patents. My career has been in digital computer design, but I still have the skills to evaluate AC electrical circuits.

Interesting link to the Radio Engineers Handbook, 1943

Rich Powers Part15, Take 2..

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Experimental broadcasting for a better tomorrow!

More on inductive top-loading

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.

This all sounds good to me.

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.
Bruce, MICRO1690/1700


And Now For Something Totally Different

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.

by MRAM 1500 

Charter Member - Association of Low Power Broadcasters

Chairman - ALPB

More on inductive top-loading


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.

TTS and SR

"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.

All the best, Ken N. KF7PLC

Ken, that's a brilliant idea.

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

Rich Powers Part15, Take 2..

Grounded monopole with hot radials


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.

Self-capacitence of a coil

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.

From QST June 1994

"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.

by MRAM 1500 

Charter Member - Association of Low Power Broadcasters

Chairman - ALPB

Elevated ground plane antenna

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.

Part 15 AM humor

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!

Can't Take Credit For This One

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!

by MRAM 1500 

Charter Member - Association of Low Power Broadcasters

Chairman - ALPB

Did you hear the one about...

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.

Experimental broadcasting for a better tomorrow!

Q: How did the chicken get his Part 15 signal to cross the road?

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 ...

All the best, Ken N. KF7PLC

The End?

This time, the chicken finally gave up the tower and escaped to the safety of a Real Chicken Shed with an even taller tower, through a hole in the fence.

However, the fox stealthily followed the chicken through the very same hole, threatening all the other chickens in The Shed.

But a cow who liked chickens, came with bags of hay, chased out the fox, and stood guard while the chickens stuffed hey into the hole to repair the fence.

The fox, who wasn't interested in cows, decided to go on his way, looking for other prey, and the chicken was safe and happy to be able to crow to many, many more chickens and cows ..... THE END?

Epilog: We think some other farm animals may have recycled the parts left behind, moving them to smaller Sheds with no towers and perhaps fewer foxes.

All the best, Ken N. KF7PLC


Topic locked