wrote:
Cecil Moore wrote:
It's pretty easy to understand. Any two radials,
180 degrees apart and high enough, should theoretically
cancel each other's radiation in the far field.

Not true.
There is always an angle and direction where the fields do not fully
cancel.

Funny, I don't see "fully cancel" anywhere in my posting.
I probably should have said "tend to cancel".

A free space vertical with horizontal radials in EZNEC
has horizontal radiation more than 40 dB down from the
vertical radiation. That's a high degree of cancellation.
--
73, Cecil http://www.qsl.net/w5dxp

Cecil Moore wrote:

Funny, I don't see "fully cancel" anywhere in my posting.
I probably should have said "tend to cancel".

A free space vertical with horizontal radials in EZNEC
has horizontal radiation more than 40 dB down from the
vertical radiation. That's a high degree of cancellation.


The issue is the horizontal opposing radials only have that degree of
cancellation for perfectly horizontal directions.

You will be able to see your statement isn't true if you place the
antenna in freespace and look at pattern distortion at various
elevation angles. For example, the 2-d plot is skewed 2.11 dB from
being circular at - 45 and +45 degrees elevation. The skewing gets
worse at larger angles from the plane of the radials.

If the radials were REALLY radiating -40dB in all directions as you
wrongly assume, there would NOT be significant FS change in the azimuth
pattern at various elevations.

You looked at horizontal radiation, but the horizontal radials peak
radiation is vertically polarized and nearly off the radial's ends.
(Just like in a dipole pattern.)

The radials do indeed radiate enough to change the pattern a
significant amount (but not at zero degrees), but the largest problem
is decoupling the feedline shield. The fewer radials are used, the
bigger the problem becomes.

There are VERY good reasons everyone settled on four radials, and it
isn't the old wive's tale about making the antenna look good. Four
radials is a reasonable compromise between excessive common mode
problems and tolerable common mode feedline current problems, pattern,
and cost.

Don't feel bad though Cecil. Many people miss this point, even card
carrying Mensa members.

73 Tom

On 11 Jul 2006 02:22:15 -0700, wrote:

There are VERY good reasons everyone settled on four radials, and it
isn't the old wive's tale about making the antenna look good. Four
radials is a reasonable compromise between excessive common mode
problems and tolerable common mode feedline current problems, pattern,
and cost.

I don't think that is a "wive's tale".

From George Brown's (one of the co-inventors of the ground-plane)
book "and part of which I was" the following quote about the ground
plane antenna:

".... In our initial experiments we found that only two horizontal
rods (ground rods) functions just as well as four. Many people from
the Broadcast Sales organization came by to view our test and they
always expressed doubts as to the ability to radiate uniformity when
only two ground rods were used. To quiet them, we used four ground
rods for a while, thus stilling the criticism. When the antenna became
really popular, we did not dare confess to our ruse."

Danny, K6MHE

What's the matter with 3, equi-spaced radials?

Be economical. Save a radial! It looks better too. And there are no
arguments about directionality.
----
Reg.

On Tue, 11 Jul 2006 14:35:34 +0100, "Reg Edwards"
wrote:

What's the matter with 3, equi-spaced radials?

Be economical. Save a radial! It looks better too. And there are no
arguments about directionality.
----
Reg.

Modeling such an arrangement gave no real noticeable difference
between using three or four radials.

Danny, K6MHE

"Dan Richardson wrote
wrote:

What's the matter with 3, equi-spaced radials?

Be economical. Save a radial! It looks better too. And there are
no
arguments about directionality.
----
Reg.

Modeling such an arrangement gave no real noticeable difference
between using three or four radials.
=========================================
Of course it didn't. That's the point I was making. The number of
radials, from 1 to N, is immaterial.

As N increases there will be a slight improvement in radiating
efficiency. The N loss resistances are all in parallel as seen by the
feedline.
----
Reg.

Reg Edwards wrote:
Of course it didn't. That's the point I was making. The number of
radials, from 1 to N, is immaterial.

Guess it depends upon one's definition of "immaterial".
One horizontal radial will certainly radiate more
horizontal radiation than two opposing horizontal radials.
--
73, Cecil http://www.qsl.net/w5dxp

wrote:
Cecil Moore wrote:
A free space vertical with horizontal radials in EZNEC
has horizontal radiation more than 40 dB down from the
vertical radiation. That's a high degree of cancellation.

The issue is the horizontal opposing radials only have that degree of
cancellation for perfectly horizontal directions.

That's the issue? Something that no one has ever asserted
otherwise?

You will be able to see your statement isn't true if you place the
antenna in freespace and look at pattern distortion at various
elevation angles. For example, the 2-d plot is skewed 2.11 dB from
being circular at - 45 and +45 degrees elevation. The skewing gets
worse at larger angles from the plane of the radials.

Just ran that test. There was 0.02 dB difference at +45 and -45.

If the radials were REALLY radiating -40dB in all directions as you
wrongly assume, there would NOT be significant FS change in the azimuth
pattern at various elevations.

There is no significant FS change according to EZNEC.

You looked at horizontal radiation, but the horizontal radials peak
radiation is vertically polarized and nearly off the radial's ends.
(Just like in a dipole pattern.)

Unfortunately for that argument, the radiating currents in
a dipole are in phase, i.e. designed for maximum radiation. The
radiating currents in symmetrical radials are 180 degrees
out of phase, i.e. designed for minimum radiation.
--
73, Cecil http://www.qsl.net/w5dxp

One of the earlier postings suggested that the quarterwave vertical antenna
with radials was elementary and easy to understand. I have never found this
antenna easy to understand.

RF experts on this newsgroup cannot agree on whether i) the radials reflect
the wave or ii) the field from the radials cancels out. The standard
academic books show that the principle behind the vertical ground plane
antenna is that the vertical radiating element emits the wave, and is
reflected by the ground plane.

You can view a conductor as having current pushed through it by a RF source,
or the current can be induced in the conductor by the wave. This is a
boundary condition in Maxwell's equations, referred to in theory of
transmission lines and guided waves.

You can view the radials as reflecting the wave and having current induced
in them, or they can have current pushed through them by the RF source. This
is probably the same thing, due to the arrangement of all antenna parts
forming the antenna impedance. In image theory, the impedance comes from
both the self impedance and the mutual impedance.

It appears that a single counterpoise wire is connected to the RF ground
side to provide a conductor for that side and be a form of dipole. If a
proper RF ground is not provided, the result may be RF in the shack e.g. the
RF tries to return via mains wiring. Does connecting several wires make the
RF ground side less live i.e. occupying a larger area to be more of a
reflector and thus dissipative? If a RF ground is live, it can be dangerous
to touch it. Do you increase the area of RF ground to make it less dangerous
to touch e.g. radials under a carpet when relatives and pets are about?

The theory behind the quarterwave vertical is the monopole above a ground
plane, where the ground plane reflects the wave emitted by the vertical.
The monopole is explained using image theory. In practice, the ground plane
is
replaced by radials. Do the radials reflect the wave then?

The reflecting element on a Yagi manages to reflect most of the wave. The
reflecting element on a Yagi is a parasitic element that has an impedance
to cause the wave emitted by the driven element to flow in a particular
direction. A Yagi normally has only one reflector. Although the reflector
is in the near field of the Yagi, can a comparison be made with the radials
of
a quarterwave vertical antenna? The reflector on a Yagi is usually a thin
tube with lots of air (gap) around it. Even though it occupies a small
area, it still manages to reflect most of the wave. Yagi has a Front to
Back
ratio in dB.

Radials can be tuned. Some antennas have loading coils in the radials.


Antenna theory is often about wires and metallic items reflecting waves,
and the phase of the reflected wave. The phase of the reflected wave can be
constructive or destructive, affecting the impedance of the antenna. If an
antenna is mounted too close to the ground, the reflected wave cancels out
the emitted wave.

Because a ground plane reflects the wave, the impedance of an antenna can
vary with height.

Parastic elements on a Yagi have a mutual impedance to each other. Would
you regard the radials on a quarterwave vertical as having a mutual
impedance?

The radials increase the conductivity below the radiating element,
decreasing ground losses. The radials are regarded as a finite or imperfect
ground plane.

References:
"Antenna Theory and Design" by Warren Stutzman and Gary Thiele. pages 66 to
68. Practical monopole with radial wires to simulate a ground plane.
"Antenna Engineering Handbook" by Richard C. Johnson. Radials suppress
currents from flowing on outside of coax. p 28. If the ground is imperfect,
the perfect reflected image is mutiplied by a complex ground reflection
coefficient. The ground has a mutual impedance.
"Antenna Theory" by Professor Constantine Balanis. Second Edition p 165. A
ground plane formed by a perfect conductor completely reflects the wave. If
the ground is finite i.e. not as conductive, it still reflects the wave but
not as well. The conductivity determines the quality of the reflection.

All antennas consist of conductors which have current conducted to them
from sources and induced in them by coupling to fields from other
conductors or other parts of the same conductor. These currents create
fields. Ground plane antennas work exactly the same as all others. In
that way they're simple to understand.

Yes, you can view it this way or that, with various degrees of accuracy
and inaccuracy. The problem is that people begin to believe that the
alternate views are really what happens, rather than attempts at
simplifying and understanding things. Before you know it, you've got
mirrors, "ground" high above the Earth, impossible reflections, and
other dubious concepts which end up leading people farther and farther
from really understanding the basic principles involved.

Roy Lewallen, W7EL

David wrote:
One of the earlier postings suggested that the quarterwave vertical antenna
with radials was elementary and easy to understand. I have never found this
antenna easy to understand.

RF experts on this newsgroup cannot agree on whether i) the radials reflect
the wave or ii) the field from the radials cancels out. The standard
academic books show that the principle behind the vertical ground plane
antenna is that the vertical radiating element emits the wave, and is
reflected by the ground plane.

You can view a conductor as having current pushed through it by a RF source,
or the current can be induced in the conductor by the wave. This is a
boundary condition in Maxwell's equations, referred to in theory of
transmission lines and guided waves.

You can view the radials as reflecting the wave and having current induced
in them, or they can have current pushed through them by the RF source. This
is probably the same thing, due to the arrangement of all antenna parts
forming the antenna impedance. In image theory, the impedance comes from
both the self impedance and the mutual impedance.
. . .