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AM Antenna distributed capacitance idea

T

Tom Wells

OK in the ground lead thread we have the concept of radiation efficiency, loading coils, and capacitive top loading on the table.
If we can increase the capacity of the antenna, we can have better current flow at the input, no?

So instead of a single element radiator, solid or hollow regardless of OD, what happens if we use a chunk of litzendraught wire?
Litz wire is very fine stranded, multi-multiconductor wire favored for LW and MW coil winding in the old days.
Lets say you could pack 100 separate, insulated conductors into a 3 meter pole only 1 inch wide.
Wouldn't this have well more capacitance than any single conductor, and wouldn't this be evenly distributed along the length
of the antenna instead of lumped?
I can visualize that the current flow at the end of the antenna when a "hat" is used will
be something, while in the idea I propose the current naturally is zero at the end.
I have to think it would increase bandwidth at least.


How would the opposing fields packed together react on SWR and would they likely cancel in place?
Might the field effect be additive?
Would you expect any different results than a single conductor?

I would think this has been tried, but I've never heard of it.
 
Quoting Terman in RADIO ENGINEERS' HANDBOOK, 1943 Edition ...

\\ Practical litz conductors are very effective at frequencies below 500 kc, but as the frequency becomes higher the benefits obtained are less. This is because irregularities in stranding and capacity between the individual strands result in failure to realize the ideal conditions. //

another author writes:

\\ If the strands are insulated (Litz wire) and frequency is increased, eventually we reach a point where the conductor resistance increases and we are much worse off than we would be with braided or stranded wire. The upper limit point is generally around 1 MHz for Litz wire. //

The capacitance that is most important in an electrically short, medium wave vertical monopole is its capacitance to the earth. That capacitance needs an offsetting inductance at the feedpoint in order to bring the antenna system to resonance and maximize system efficiency. So the higher that capacitance is, the more it reduces the loss in the needed inductance.

Adding a symmetrical arrangement of horizontal wires at the top of a 3-meter conductor used as a Part 15 AM monopole adds capacitance, and also increase the r-f current flowing at and near the top of the monopole. Both of those are useful at increasing system efficiency, but the horizontal wires might not be seen as permissible under 15.219(b).

Increasing the OD of the conductor used for the monopole adds capacitance even with no horizontal conductors. Here are some calculated values for a base-driven, 3-meter monopole at 1610 kHz:

OD, inches Impedance, ohms

1/4 0.1077 -j 3460
3/4 0.1077 -j 2809
3 0.1077 -j 1990

The first term in these impedance values is the radiation resistance of the conductor at that frequency. Notice that it remains constant regardless of the radiator OD. This is the only part of the complex impedance that can produce useful radiation.

The reactive ( -j) term decreases with increasing OD, indicating that the input capacitance is increasing. The applicable equation here is Xc = 1 / (2 * pi * f * C), where f = frequency in Hz, and C = capacitance in Farads.

It would be interesting to learn anyone's real-world results when using litz wire for a Part 15 AM antenna. If using litz wire adds capacitance to the earth then it might be worthwhile.
 
The benefit of using Litz wire is to improve the Q of RF coils by reducing the skin effect and the proximity effect. The larger diameter wire (with hundreds of strands) can be used for improved Part 15 AM loading coils, but it is very expensive. From my own experiments, I have noticed that the Q of coils made with Litz wire is better than with enamel magnet wire with the same total cross-section. This is true even at the top of the AM broadcast band, but the improvement is greater at lower frequencies.

I don't see any reason to use Litz wire for linear antenna elements. Solid copper pipe is a lot better for that kind of use.
 
Ermi Roos said:
... I don't see any reason to use Litz wire for linear antenna elements. Solid copper pipe is a lot better for that kind of use.
Click to expand...

Due to "skin effect," r-f current flows only on and very near the outer surface of a conductor. So mechanical considerations aside and in the practical world, a hollow tube (pipe) performs as well as a solid one of the same OD, and costs a lot less.
 
"Increasing the OD of the conductor used for the monopole adds capacitance even with no horizontal conductors. Here are some calculated values for a base-driven, 3-meter monopole at 1610 kHz:"

OD, inches Impedance, ohms

1/4 0.1077 -j 3460
3/4 0.1077 -j 2809
3 0.1077 -j 1990

I just measured a steel 55 gal. drum. (23"x34" approx.) 3 of them stacked and welded together would be within the 3-meter rule in "length". (They don't mention dia. ;) )

How would a 23" dia. conductor work? Just curious..
 
Offgridkindaguy said:
How would a 23" dia. conductor work?
Click to expand...

NEC shows about 0.057 -j 835 Ω for a base-fed, 23" W x 102" H cylinder on 1610 kHz.

This configuration would need a smaller series inductance to resonate the radiator, and would have a relatively wide SWR bandwidth. The smaller inductor could have less r-f loss than those needed to resonate smaller-diameter radiators.

OTOH the radiation resistance of a 1/2" OD conductor just meeting the 118" radiating length permitted by 15.219(b) is about 0.1 Ω, which performance in the complete antenna system probably would more than offset the r-f loss of the larger coil needed to resonate it (assuming the same loss in the r-f ground system of each).
//
 
Thank you, Mr Fry..

Another question.. Same scenario..

How much would/does the larger diameter radiator affect the radiation pattern? More/Less far field?

Again, just curious..

Thanks in advance.
 
Offgridkindaguy said:
How much would/does the larger diameter radiator affect the radiation pattern? More/Less far field?
Click to expand...

The far-field shape of the radiation pattern on a given frequency will be the same for radiators of equal length. It is roughly in the shape of a donut cut in half across its widest dimension.

The gain of the complete antenna system may vary (probably insignificantly), depending on the losses in the loading coil and r-f ground system in each case.
 
Hello, I'm not a common poster here, but I have been stepping in and looking around semi-regular.
Anyway, I have only the vaguest idea about what you're all talking about.. but if my uneducated, inexperienced mind is comprehending the topic correctly it sounds like your saying there is a potential of doubling an antennas efficiency by utilizing the inner "skin" as well as the outer skin..

But isn't a standard whip hollow? So wouldn't the current be running along both the inner and outer surface of the whip anyway?

I know I'm missing something - big.. but I just wanted to comment because I was trying to grasp the concept of what your talking about.. it just sounded intriguing -so please excuse my ignorance.

Can someone per chance point me to a idiots guide to 1/4 wave antenna theory?.. and I mean an idiots guide for dummies, not a technical document intended for someone who already knows their stuff. I only graduated kindergarten.


Oh, by the way, Hello Mr Fry... and everyone else to
 
Best quick antenna course ever is the "ARRL antenna handbook"

I haven't had time to try out my hairbrained idea, but the idea is cause current to flow all the way to the end of the the 3m cylinder, then be induced into what electrically appears to be continuing length, hidden within the physical length.

This could be some lumped L and C "inside" , or brought back out the bottom and experimentally tuned.
 
Tom Wells said:
Best quick antenna course ever is the "ARRL antenna handbook"

I haven't had time to try out my hairbrained idea, but the idea is cause current to flow all the way to the end of the the 3m cylinder, then be induced into what electrically appears to be continuing length, hidden within the physical length.

This could be some lumped L and C "inside" , or brought back out the bottom and experimentally tuned.
Click to expand...

Something I've recently thought of....

I wonder what would happen if you wound a helical coil that was 3 meters long (taking care to not let the coils touch), and house it inside a hollow non-conducting cylindrical tube, like something wooden for example? (Might "look" better to use something metal like copper, but I wonder if you'd need to "insulate" (even if it's only space) the coil from the inside surface of the tube, and I wonder if the two metal things would interact negatively with teach other. On the other hand, an inspector wouldn't see the coil, as it would be hidden inside the structure.)
 
tfcwings said:
I wonder what would happen if you wound a helical coil that was 3 meters long ... etc
Click to expand...

tfcwings: Your idea shows a lot of creativity and imagination, which is valuable.

If you'd like to read a conclusion about this, please check the graphic from an antenna engineering textbook at the link below.

As it turns out, for a given application a self-resonant, coiled wire used as a monopole radiator doesn't perform much differently than a straight wire of the same end-end length used with a "loading coil" at its base.

http://i62.photobucket.com/albums/h85/rfry-100/Helically-woundVertical.gif
 
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