Can someone provide a full description of how a quarterwave vertical antenna
with radials works? Length of radials is also a quarterwave.

I find that many books give a good description of antennas like the Yagi,
and then suddenly become very vague when describing the quarterwave
vertical. Books
refer to image theory where an image of the radiating element is produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

What difference does it make if the radials are in free space or in the
ground? Some articles claim that the radials tend not radiate because they
cancel out, while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this contradiction?

The vertical element is usually called the radiating element. How well do
the radials radiate? The same magnitude of current flows into the vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have a
radiating element? What happens to the SWR?

David wrote:
Can someone provide a full description of how a quarterwave vertical antenna
with radials works? Length of radials is also a quarterwave.

I find that many books give a good description of antennas like the Yagi,
and then suddenly become very vague when describing the quarterwave
vertical. Books
refer to image theory where an image of the radiating element is produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

What difference does it make if the radials are in free space or in the
ground? Some articles claim that the radials tend not radiate because they
cancel out, while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this contradiction?

The vertical element is usually called the radiating element. How well do
the radials radiate? The same magnitude of current flows into the vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have a
radiating element? What happens to the SWR?

Picture a half wave dipole, with a balanced feed. Two elements
perform the radiation and there is zero voltage swing at the exact
center of the dipole (though there is peak resonant current passing
through the center).

Now, cut that dipole exactly in half, and place a mirror at the half
way point. Half of the balanced feed line can be replaced by an
unbalanced (coaxial) feed line of half the impedance, since two of
those, with their shields connected and the center conductors out of
phase, would make a balanced feed line.

The radiation from the quarter wave half of the dipole is reflected by
the mirror to produce an an image of the missing half of the dipole.
The radials at the end of the quarter wave dipole act as the mirror.
This effect is pretty efficient as long as the radials are at least
1/4 wavelength long.

On Sun, 9 Jul 2006 20:57:18 +0100, "David" nospam@nospam wrote:

Can someone provide a full description of how a quarterwave vertical antenna
with radials works? Length of radials is also a quarterwave.

Hi David,

Someone can, and someone already has, but that hasn't helped you has
it? The following statements suggest so:

I find that many books give a good description of antennas like the Yagi,
and then suddenly become very vague when describing the quarterwave
vertical.

It isn't vague, unless you've been saddled with poor references. On
the other hand there is not much to say when you are working with
elementary monopoles and dipoles. Yagis, in this sense, have much to
be discussed.

Books
refer to image theory where an image of the radiating element is produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

Radials being the "other half" simply reveals that the monopole
(especially when elevated) is a vertical dipole.


What difference does it make if the radials are in free space or in the
ground?

About 3dB.

Some articles claim that the radials tend not radiate because they
cancel out,

All parts of an antenna radiates, the radials' contributions cancel -
at a distance.

while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this contradiction?

Poor references. The radials simply serve for drivepoint Z
consideration (we already agree that their contribution to radiation
cancel). For all practical purposes, the "ground plane" would have to
extend out 5 to more wavelengths to affect the lobe characteristics of
radiation.

The vertical element is usually called the radiating element. How well do
the radials radiate?

Perfectly, or as well as the "radiator" presuming they all exhibit
similar construction.

The same magnitude of current flows into the vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too much
of a gap for them to form a ground plane?

They are simply not long enough, and certainly don't exhibit near the
coverage (the gap you describe) as does a plane of metal (or
seawater).

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have a
radiating element?

A poor one, but given the wheel of fortune, the feedline could make up
the difference.

What happens to the SWR?

It usually goes ballistic, but again, with ground nearby, you could be
heating worms and find the SWR at a comfortable value.

73's
Richard Clark, KB7QHC

David wrote:
Can someone provide a full description of how a quarterwave vertical antenna
with radials works?

Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
--
73, Cecil http://www.qsl.net/w5dxp

"John Popelish" wrote in message
...
David wrote:
Can someone provide a full description of how a quarterwave vertical
antenna
with radials works? Length of radials is also a quarterwave.

I find that many books give a good description of antennas like the
Yagi,
and then suddenly become very vague when describing the quarterwave
vertical. Books
refer to image theory where an image of the radiating element is
produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

What difference does it make if the radials are in free space or in the
ground? Some articles claim that the radials tend not radiate because
they
cancel out, while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this
contradiction?

The vertical element is usually called the radiating element. How well
do
the radials radiate? The same magnitude of current flows into the
vertical
element as the radials, although the current into the radials is split.

A normal ground plane is a large sheet of metal that reflects the radio
wave
emitted by the radiating element. If there are four radials, each a
quarterwave long, do the radials form a ground plane? Or is there too
much
of a gap for them to form a ground plane?

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have
a
radiating element? What happens to the SWR?

Picture a half wave dipole, with a balanced feed. Two elements
perform the radiation and there is zero voltage swing at the exact
center of the dipole (though there is peak resonant current passing
through the center).

Now, cut that dipole exactly in half, and place a mirror at the half
way point. Half of the balanced feed line can be replaced by an
unbalanced (coaxial) feed line of half the impedance, since two of
those, with their shields connected and the center conductors out of
phase, would make a balanced feed line.

The radiation from the quarter wave half of the dipole is reflected by
the mirror to produce an an image of the missing half of the dipole.
The radials at the end of the quarter wave dipole act as the mirror.
This effect is pretty efficient as long as the radials are at least
1/4 wavelength long.

"John Popelish" wrote in message
...

\

Picture a half wave dipole, with a balanced feed. Two elements
perform the radiation and there is zero voltage swing at the exact
center of the dipole (though there is peak resonant current passing
through the center).

Now, cut that dipole exactly in half, and place a mirror at the half
way point. Half of the balanced feed line can be replaced by an
unbalanced (coaxial) feed line of half the impedance, since two of
those, with their shields connected and the center conductors out of
phase, would make a balanced feed line.

The radiation from the quarter wave half of the dipole is reflected by
the mirror to produce an an image of the missing half of the dipole.
The radials at the end of the quarter wave dipole act as the mirror.
This effect is pretty efficient as long as the radials are at least
1/4 wavelength long.

My experience with Navy UHF (225 - 400 MHz) antennas bears this out. There
are two vertically polarized omni antennas that appear in great numbers:
AT-150, which is a true dipole, fed with coax through an internal balun, and
the AS-390, which is a quarter-wave whip with eight "spider-leg" radials.
It is fed directly. They perform equally well and the system designer's
choice is generally based on mounting considerations.

There are over a dozen UHF antennas, some in stacked combinations called
"stovepipes", but of the single-unit antennas, the AT-150 and the AS-390 are
among the most common.

Image theory is for a perfect groundplane e.g. large area metal sheet. The
wave emitted by the vertical radiating element is reflected by the ground
plane.

Image theory as I see it follows. Wave emitted by vertical element is the
incident wave that hits ground plane, inducing currents in the ground plane.
Currents flowing in skin depth of ground plane emit a wave of opposite
polarity to cancel out the wave at the boundary of the ground plane, thus
making the electric field in the ground plane zero. The wave of opposite
polarity is the reflected wave. The reflected wave appears to be coming from
an image antenna. Image theory is a mathematical model for solving antenna
simulations where there is a monopole over a ground plane.

How do the radials reflect the wave? If they are not a good enough ground
plane because of the gap, how do they reflect? I cannot see the transition
from ground plane to radials, when looking at image theory.

David wrote:
Can someone provide a full description of how a quarterwave vertical antenna
with radials works? Length of radials is also a quarterwave.

When you connect a source or feedline to this antenna, the same amount
of current which flows into the vertical flows into the radials. First
consider one which is well above the ground. The current in each pair of
radials flows in physically opposite directions. So the radiation from
the radials cancels completely at right angles to the radials, and
nearly completely in other directions. The vertical acts like a dipole
except with half the length and twice the current, resulting in the same
pattern and field strength as a dipole.

If the radials are buried, the current into the radials spreads into the
ground. Current through the ground results in loss due to the ground's
resistance. Therefore many radials are required to force the majority of
current to flow in the wires rather than the ground. This is
particularly important close to the vertical where the current density
is high.

I find that many books give a good description of antennas like the Yagi,
and then suddenly become very vague when describing the quarterwave
vertical. Books
refer to image theory where an image of the radiating element is produced by
the radials, and show a spear shape going into the ground. Some say the
radials are the other half of a dipole.

That's probably because the authors don't understand some fundamental
principles, or else they oversimplify to the point where the explanation
isn't correct. Radials are nothing more nor less than conductors
carrying current, and radiate accordingly. But they're placed and fed so
the radiation nearly cancels.

What difference does it make if the radials are in free space or in the
ground? Some articles claim that the radials tend not radiate because they
cancel out, while other other articles claim that the radials simulate a
ground plane and reflect the radio wave. Can you explain this contradiction?

It's a lousy explanation of what's going on, written by someone who
doesn't really understand. When the radiation from the vertical strikes
the ground, it's reflected. If the ground were perfectly conductive,
flat, and infinite in extent, it would be like a mirror. But real ground
isn't any of these things, so a mirror is a very poor representation.
The reflection from the ground causes the formation of a vertical
radiation pattern which looks very different from what you'd get from a
perfect, mirror-like ground, with the exception that salt water does
approximate a mirror reasonably well. Except at high radiation angles,
this reflection takes place well beyond any radials, so the radials
don't contribute at all except at high angles.

The vertical element is usually called the radiating element. How well do
the radials radiate? The same magnitude of current flows into the vertical
element as the radials, although the current into the radials is split.

Correct. See above.

A normal ground plane is a large sheet of metal that reflects the radio wave
emitted by the radiating element.

"Normal"? Where have you seen an antenna mounted over a metal ground
plane many wavelengths in diameter? Perhaps a UHF antenna in the middle
of the top of a car, but that's about it.

If there are four radials, each a
quarterwave long, do the radials form a ground plane?

Radials do not form a flat metal conductor many wavelengths in diameter,
if that's what you're asking. And they don't reflect the radiation from
the vertical, either.

Or is there too much
of a gap for them to form a ground plane?

They're much too short to reflect the radiation. Or are there other
properties you require for something to qualify as a "ground plane"?

If the radials are disconnected and taken away, with the vertical
quarterwave element still connected to centre conductor, do I still have a
radiating element?

Yes. Whatever current you put into the element, an equal current flows
elsewhere. If the element is connected to a coax transmission line, it
flows down the outside of the coax, so the coax radiates just like the
element. If you just plug it into a coax connector on a transmitter, the
current flows out of the connector onto the outside of the transmitter,
so it and the path to the Earth radiate just like the element. Current
on a conductor creates radiation. It doesn't matter one bit whether you
declare the conductor to be "ground", a "ground plane", or a "transmitter".

What happens to the SWR?

You now have an asymmetrical antenna. One "half" is the vertical and the
other is whatever conductor the return current flows on. The SWR will
almost certainly be different than it was for a typical ground plane
antenna.

Roy Lewallen, W7EL

The contradiction over antenna radials continues. One posting says that the
radials acts as a mirror and reflect the wave, another post says the radials
do not reflect - that the radials are simply positioned so that the
radiation from them cancels out.

On Mon, 10 Jul 2006 00:11:25 +0100, "David" nospam@nospam wrote:

The contradiction over antenna radials continues. One posting says that the
radials acts as a mirror and reflect the wave, another post says the radials
do not reflect - that the radials are simply positioned so that the
radiation from them cancels out.

Hi David,

Well, this is not an opinion based outcome, and interpretation is even
less forgiving.

Radials that "act as a mirror" are fantasy for radial lengths less
than 5 wavelengths at less than several hundred in count. Simple
geometry and trig are suitable to observe this.

Radials that "are simply positioned" certainly outnumber those that
are not. A vertical with two radials is sufficient to do the job, and
simply positioning them at 180° to one another is enough to insure
their radiation from canceling at a distance.

Now, when we regard the first claim in light of the second, it is
amazing how much mirror-like quality those two radials have (which
sort of puts the bronx cheer to the mirror claim).

73's
Richard Clark, KB7QHC