So now I and a number of others are minions?
Trolling?
I have no interest in feeding RFBurns's ego. I attempted to show that both his and PhilB's attitudes have a place here, but obviously there is no room in his world view for anyone else but himself.
Into the kill file for all of RFBurn's posts - kerplunk!
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?
I'd like to make a loading coil out of superconducting materials, and keep it packed in dry ice...would that lower the resistance?
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?
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:
http://www.part15.us/index.php?q=node/1535
Neil
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 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:
http://sstran.com/public/Compariston%20of%20base,center-loaded_and%20top_hats.pdf 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.
Ermi,
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.
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.
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.
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.
Ermi,
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. http://strobbe.eu/on7yd/136ant/#IndTop 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. http://www.nonstopsystems.com/radio/antenna-loading-coils.pdf 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. http://www.qsl.net/on7yd/indtop.htm 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.
Please note our guidelines ask that participants post only their own work.
This extends to re-posting the content of other users' posts. An occasional line or two, when essential for clarity, is fine. Re-posting paragraphs from others' posts is not OK.
There seems to be a sudden increase in this practice and it needs to stop.