Vertical Antennas - Do Radials Radiate?

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David Ratledge:
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?

 

Phil Barnett:
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

Lee A Crocker:
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

Bill Savage:
OK then, do the "radials" of a ground plane antenna radiate?

Dan:
"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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