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1-10 of 39 messages
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Vertical Antennas - Do Radials Radiate?
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by W4DDR on November 30, 2008
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I am wondering if the radials of a vertical antenna, either ground-mounted or elevated, radiate any part of a transmitted signal and how much. If they don't, I am wondering why not.
Here is why I began wondering about this:
In thinking about a basic dipole, it is constructed of two 1/4 wavelength wires joined in the middle by the feedline with (assuming the feedline is coax) one wire connected to the center conductor and the other wire connected to the shield.
Since rf is ac there is no definitive positive or negative side to a dipole and both sections of wire radiate the signal. This seems further illustrated by any good technical resource on antennas that shows current and voltage distributions along a dipole as they are along the entire 1/2 wavelength of the antenna and not just along the individual 1/4 wavelength halves. A dipole then does not have a "hot" 1/4 wavelength side that does all the signal radiating and receiving while the other 1/4 wavelength side serves only as a ground to work against. Perhaps in reality both sides of a dipole take turns being the hot side and the ground side, but because of the alternating nature of rf the two sides switch these roles back and forth so quickly that for all practical purposes it is easiest to think of the antenna as a whole and not worry about this fact.
When I think about vertical antennas with radials and compare them to dipoles I get confused. A vertical antenna is constructed of a 1/4 wavelength section mounted in a vertical orientation joined via a feedline to radials mounted in a horizontal orientation. The vertical section is connected to the center conductor of the feedline with the radials connected to the shield. The general idea is that the radials should be at least 1/4 wavelength long. With an elevated vertical the radials must be precisely tuned, but with a ground-mounted vertical this isn't so critical due to the detuning effects of the ground. To me, this sounds like just a different configuration of the basic dipole. In fact, it is common to hear it said that the radials are the "other half of the antenna." That being the case, shouldn't the radials be an equal partner in radiating the signal in just the same way as the "other half" of a dipole?
Am I completely misunderstanding how antennas radiate signals?
Or am I just misunderstanding how verticals differ from dipoles?
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RE: Vertical Antennas - Do Radials Radiate?
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by AI4OF on November 30, 2008
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The radial field doesn't radiate because the currents involved flow in all directions, cancelling each other out as far as signal radiation goes.
Here's an interesting collection of wisdom about radials.
http://lists.contesting.com/_topband/2002-04/msg00010.html
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RE: Vertical Antennas - Do Radials Radiate?
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by W9OY on November 30, 2008
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The thing that makes a radiator "radiate" is the acceleration of electrons in the radiator. The radiator in a vertical is the vertical element. As fields are produced by the vertical, those fields induce currents in the "ground". Those currents need to be returned to the feed line. The radials act as a means to couple the feed line into the ground providing a low (lower) loss return for the currents that are induced into the ground. The more current you can recover to the feed line the more efficient your vertical "system" is. The image of the "vertical" really isn't like some schematic picture of a rod like image extending into the ground. The fields of the antenna excite basically some amorphous volume of earth below the antenna. That's why you need a radial "field" to couple into that volume.
It is really not adequate to think of a vertical as half a dipole. In fact if you center feed a half wave vertical (or even end feed a half wave vertical) you will still induce current into the ground, and you will still need some means to return that current back to the feed line. This is why the notion that an end fed half wave doesn't need any radials is a bunch of malarkey.
This I think is a pretty good description of how an antenna radiates.
http://www.arrl.org/tis/info/whyantradiates.html
73 W9OY
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RE: Vertical Antennas - Do Radials Radiate?
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by K3AN on November 30, 2008
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OK then, do the "radials" of a ground plane antenna radiate?
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RE: Vertical Antennas - Do Radials Radiate?
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by N3OX on November 30, 2008
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"OK then, do the "radials" of a ground plane antenna radiate? "
The short answer is, basically, no.
They have current flowing in them, tapering to nothing out toward the end, and they have an electromagnetic field around them, but if you go off a very, very long distance from the antenna (so far that the antenna element lengths might as well be zero) and you look at the current distribution out there, you'll see that the radials carry equal and opposite current and basically lay right on top of each other (from your extremely distant vantage point).
So the "far field" radiation from the radials cancels out.
There's nothing to cancel the radiation from the vertical current though, so you look at the antenna from a far distance and you see this little tiny point emitting essentially vertically polarized radiation.
To be fair, perfect cancellation isn't quite what happens, because strictly speaking, no matter how far you get from the antenna, there's still a little phase difference among various parts of the antenna, and it emits a little bit of horizontally polarized "radials radiation."
EZNEC shows, for a VHF ground plane with four radials, that the horizontal radiation pattern has many lobes (I think eight) but the strongest they get are about 54dB down from the main vertically polarized radiation. That's not perfect cancellation, but it's close. The more radials you have, the better the cancellation gets, and the weaker the horizontally polarized "radials radiation" gets.
In the real world, the horizontal radiation could easily be much stronger than that, because good cancellation is extremely sensitive to the exact current balance. So if you make one radial a little short or its end insulator is a little wetter in the rain than the others, and the perfect balance will be upset, and you'll get more radiation from the radials, but it will still be much, much weaker than the vertically polarized radiation from the vertical.
The radials on a ground plane antenna serve an important role as sort of a "reservoir" for charge, but in the elevated case it's sort of a resonant reservoir to make it easy to get the electrons in and out of it. You want the electrons to happily slosh through the feedpoint at the frequency at which you're trying to drive them. They go in and out of the reservoir, and electromagnetic fields build up around them consistent with their motion and distribution along the elements, but those fields don't really contribute to radiation.
73
Dan
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RE: Vertical Antennas - Do Radials Radiate?
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by KI8DJ on November 30, 2008
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What about elevated tuned radials,do they also not radiate. Gary ki8dj
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RE: Vertical Antennas - Do Radials Radiate?
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by W8JI on November 30, 2008
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The radials, like any conductor with current DO radiate. Don't let anyone tell you they do not!! The question is always how MUCH they radiate.
Consider two radials equal in length and 180 degrees in azimuth from each other. Many think or claim they don't radiate, but they certainly do. They always radiate substantially in the near field, and significantly in the far field in SOME directions and angles.
So the question is always how much do they radiate compared to other things like the thing we call the antenna. Assume you have a 1/4 wave vertical with two radials. Each radial has 1/2 the current of the antenna, and so close to the radials the field from each can be substantial. The electric field is strong, there is so much voltage you can light a florescent lamp near the radials. The magnetic field is strong. The radiation field is also strong.
If you move away from the radials broadside to them, at a distance where the length of the radials is small compared to the distance you are at, the radiation field will be small. But if you get around towards the ends of the radials they radiate qite well!! Not as good as the vertical, but still substantial.
If you add more radials the fields get weaker, especially the far field radiation. The more radials the antenna has the weaker the fields... electric, magnetic, and radiation... become. This is because the voltage and current is distributed between all the radials.
It is not correct to think or say "radials don't radiate" because it clearly depends on how far we are from the radial, how many there are, and what direction we are talking about.
The correct answer is they radiate, but the amount depends on distance, direction, and the current and voltage in the radials compared to other things like the antenna. Sometimes, like when you have 20 or more radials, they have no significant radiation. Sometimes, like when we have four or less, they can have significant radiation.
73 Tom
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RE: Vertical Antennas - Do Radials Radiate?
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by VK1OD on November 30, 2008
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Lets just review the meaning of 'radiate'.
Elecromagnetic radiation describes the propagation of energy outwards from the source as time varying electric and magnetic fields together. In free space, the electric and magnetic fields are in a particular combination, and the power flux density (ie the energy per unit time per surface area) decays as the inverse of the square of distance.
Close to an antenna, other fields will also be observed. A large distance from the antenna, only the radiation fields will be observed (because the other fields decay more quickly).
Radiation is the result of the current that flows in the conductors.
The current that flows on a conductor varies in amplitude and phase along the conductor's length.
In free space, the net radiation in a given direction is the result of the combination of the the incremental currents' magnitude, phase and direction (in three dimensions).
If you were to consider only the currents in a set of symmetric paired ground plane radials (resonant or not), the effect of the incremental currents will result in very low radiation in many directions, an approximate cancellation. The more pairs of symmetric radials, the more thorough that cancellation.
In practice, a ground plane antenna using just four radials has sufficiently low net contribution to radiation from the currents in the radials that there is almost no asymmetric distortion of the pattern.
So, in summary, in free space, with sufficient pairs of symmetric radials, the contribution of currents in the radials is almost zero, NOT zero, but so low as to be insignificant.
Closer to ground, and in the presence of other objects (conducting and non conducting), the cancellation is less perfect. Nevertheless, a VHF ground plane with four symmetric radials mounted clear of other objects is close enough to the free space model to regard that it has *insignificant* radiation due to the currents on the radials.
Buried radials, or even elevated radials anywhere near ground are another issue.
Owen
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RE: Vertical Antennas - Do Radials Radiate?
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by W4DDR on November 30, 2008
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The discussion so far has suggested two additional related questions I feel I should add to the mix.
If, as AI4OF suggests, radials do not radiate because the currents cancel each other out, should this be counted as a loss of potential signal (because what goes up the feedline that isn't radiated is a loss)?
On the other hand, if as N3OX and VK1OD suggests, there is some small amount of radiation from radials, especially those on or buried in the ground, should this be counted as a reduction or even loss of potential signal (because anything radiated from a wire on or under the ground probably isn't going to contribute much to long-distance communication)?
Actually, maybe there is a third and even more important question from a practical standpoint. Regardless of which of the two above is correct, would the reduction or loss of signal be enough to measure or even matter?
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RE: Vertical Antennas - Do Radials Radiate?
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by W7ETA on November 30, 2008
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You can read more, in addition to the great prose presented here, at W4RNL's site, He posted a lot of info on verticals, radials and capacity hats.
73
Bob
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