As we soon learn here at Part 15 School a loading coil can bring a 3-meter vertical antenna to resonance.
What we know is that a particular number of turns-of-wire around a round, square or triangular coil form wired in series between the transmitter and antenna achieves resonance, thereby extending the signal range.
Certain variables determine the actual length of coil wire, such as thickness of wire, space between winds across the surface of the coil, and diameter of the coil.
Let's imagine that a single coil reaches resonance with 50-feet of wire.
But we decide to have three coils connected in parrallel. Doing so would require each coil to be wound with 150-feet of wire. Am I right?
Big question: why would we do that?
Answer: Because we wonder if it would perform better than using one coil alone.
What do we know now: We don't know if it would matter. But thinking about it makes one curious.
Such is the mental activity of a part 15er during a Labor Day weekend.
Your proposal is interesting and here's some more about coils in parallel. The inductance (and reactance if the frequency doesn't change) of inductors connected in parallel is similar to the resistance of parallel resistors in that the equivalent inductance is L = 1/(1/L1 + 1/L2 + 1/L3 + ....) so in your example of three coils in parallel, the inductance of each coil would have to be greater than the equivalent inductance. There is another factor, the mutual inductance, which changes the equivalent inductance but I am ignoring this for now.
The amount of wire will not be a linear relationship so your example of 50 feet vs. 150 feet is likely not correct. The inductance is proportional to the square of the number of turns and the total length of wire is proportional to the number of turns so the inductance is somewhat proportional to the length of wire (simplifying by ignoring coil dimensions).
Let's say we want to replace a 10 Hy inductor which has ten turns and ten feet of wire with three separate coils. Each would need an inductance of 30 Hy so when in parallel the equivalent is 10 Hy. All things being equal, a 30 Hy inductor will need sqrt(30/10) or 1.73 times 10 turns which is 17.3 turns and in our example would need 17.3 feet of wire. for each coil, which is 52 feet total which is 5 times the length needed for the single original coil.
Ain't math fun!
Neil
The "mutual inductance" makes sense now that you've mentioned it, I hadn't recognized the fact that the proximity of the coils to each other would affect the sum of them.
The matter of dealing in lengths of wire not mathematically evenly derived from a single instance is not intuitive and bends the brain in three directions.
By saying that the idea is "interesting" do you mean that it might contribute something worthwhile to performance?
It could give the antenna structure a nice 3-D appearance.
Carl asked "By saying that the idea is "interesting" do you mean that it might contribute something worthwhile to performance?"
This phrasing was because you had an idea about a configuration of coils which is not the norm for this application and I found it interesting and worth a look because there might be an advantage to this since it is a different way to load an antenna. I don't think I proved anything re advantage or not but it was interesting enough to pursue based on the wire length question.
People who work with technology tend to use tried and true approaches when designing circuits and systems and can easily miss another possibly better approach. I have learned to listen to ideas beyond convention because, in my experience, every now and then a better solution is found in this manner. That's why it is "interesting".
Neil
Then I got to wondering what would happen if one of the several coils was fed in reverse, the RF entering at the top of the coil and connected to the antenna at the bottom of the coil, thus throwing it out-of-phase with the other two coils, causing..... what?
Because of the "mutual inductance" the coil relations with one another would be not unlike that of a transformer.
Could it be used to shape patterns? That is, to govern the directional characteristics of the antenna?
The imagination knows no bounds.
At the ALPB meeting the discussion about loading coils for AM and antenna configurations was brought up after I said that I was getting my Talking House transmitter back from repair. Plus I have a home brew ATU for it that I'll be using with the original wire instead of using the internal tuning circuit as it was discovered that more of the 100mW input is actually getting to the antenna with the home brew ATU. This is because it uses better coils for it.
I with my curiosity blurted out “what happens if someone made a AM dipole with two loading coils one for the hot end and instead of using a ground for your transmitter simply use a loading coil. Both ends of the antenna would be 5 Ft. or you could use 2 4 ft sections to make an 8 ft antenna which would actually be better for me as my Landlord wants to have things sort of short and neat and will allow me to have an antenna so song as I don't drill holes. There is already the flat adapters made for Satellite T and guess what they have the F connectors on them. So if need be I could get the antenna outside higher and my home brew ATU is water proof.
My question was if it would make the range improve ever further with my AM dipole at 8 Ft. Or what about an AM J pole antenna? What would that do to the Talking House transmitter with a home brew ATU?
I want to use AM as a backup for when we have those temperature inversions and you know me I'm off the air on FM when this happens. But at least AM I'm still on air and maybe can gain range and listeners as I'm telling people about the Tecsun Raio's for $45 that may revive my station on AM and also has the super sensitive FM reception it takes to pull in weak FM stations like mine.
TheLegacy, I think that a coil between the transmitter ground and the earth ground would act like a resistor and reduce the effectiveness of the ground side of the antenna.
To get the most bang for the buck the transmitter needs to be as direct to ground as possible which is measured by the lower impedance the better... in other words 1-ohm would be better than 5-ohms....
In fact I don't think there's any way that coils in serial configuration could ever provide an advantage with the short antenna, which is why my suggestion calls for coils in parallel, which would lower the impedance of the coil portion of the antenna.
Keep experimenting no matter what they say. That's what I do.
"... In fact I don't think there's any way that coils in serial configuration could ever provide an advantage with the short antenna, which is why my suggestion calls for coils in parallel, which would lower the impedance of the coil portion of the antenna. ..."
The purpose of the loading coil(s) is to offset the capacitive reactance of the electrically short radiating conductors legally permitted by FCC §15.219(b), so as to achieve resonance of that antenna system -- which for other things equal maximizes its radiation/coverage.
Various numbers/configurations/locations of loading coils contribute different and very significant r-f losses to such systems even if each of those combinations produces a resonant antenna system.
" ... Keep experimenting no matter what they say. That's what I do."
Very good. Hopefully your doing so you will lead you to the knowledge you seek, whether or not it supports your present thoughts about this.
Rich submitted: "Various numbers/configurations/locations of loading coils contribute different and very significant r-f losses to such systems even if each of those combinations produces a resonant antenna system."
From where I sit that sounds like there's no gain, but worse than that, there's a predictable loss by employing multi-coils in parallel on a short-stub AM radio antenna.
This website leads nowhere but failure.
"From where I sit that sounds like there's no gain, but worse than that, there's a predictable loss by employing multi-coils in parallel on a short-stub AM radio antenna. This website leads nowhere but failure."
_________
Possibly not.
Your real-world (and scientifically-based) experiments may prove otherwise, should you choose to conduct them.
Rich having stated: "Your real-world (and scientifically-based) experiments may prove otherwise, should you choose to conduct them."
Having previously said that combined coils would cause loss it seems ingenuine that Mr. Rich actually believes that experiments, whether conducted in the real-world or anywhere else, would or could produce stunningly surprising results.
On the visceral level I feel we are being trifled with.
Carl I think you misunderstood what I was talking about. What I'm proposing is instead of a monopole AM antenna to use the same type of antenna configuration that is used presently for FM. You could have it horizontal or vertical picture it like a T sort of like those wires that come with your FM stereo receiver. Now both ends of the T would have to separate loading coils one for the right side of the dipole one for the left side of the dipole. One side is hot one side is the ground side of the coax. Now between each element you have a loading coil one on the right side and one on the left side. You could then place your dipole so that its vertical so you have a loading coil at the top and a loading coil at the bottom no ground would be used except for the type of ground you would have on an FM dipole which is none. Now I'm curious as to what type of range you might get.
Even Tim and Moby could gain a little ranch with this as he has his ground wire dangling in the air. Instead he could put a loading coil on that ground. It should make it resonate if you do it right which would then act like a dipole.
TheLegacy, your complex antenna description is not clear to my head and what I need is a picture drawing of how it would be designed.
The difficulty of describing three-dimensional antenna builds with words alone keeps all of us from being understood when we try to describe antenna systems.
I do not understand what you have described.
CLIP from Reply 12: ... You could have it horizontal or vertical picture it like a T sort of like those wires that come with your FM stereo receiver. Now both ends of the T would have to separate loading coils one for the right side of the dipole one for the left side of the dipole. One side is hot one side is the ground side of the coax. ...
Just to note that _neither_ conductor of a coaxial cable (OR a balanced transmission line) is either "hot" or "ground."
Both conductors of both such transmission lines convey/pass equal r-f currents that are 180 degrees out of phase with each other.
Okay let me try and describe this a little better. Picture a peace of coax 1 end will have a F connector or PL 259 connector. The other end will have the outer Shield peeled back so that you can see the inner conductor. Now the inner conductor will be connected 2 a coil then the coil goes to a 4-foot rod. The left side of the coax will have its Shield connected to another coil then A-Rod of 4 foot connected to that coil. So you'll have 1 rod facing right the other Rod facing left sort of like a t-shape. if you turn the dipole if you turn the dipole vertically you have 2 rods one going up with a coil at the bottom one going down in the coil would be at the top with the red pointing down. I hope this describes it plainly enough