Quarterwave vertical with radials
 

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?

Quarterwave vertical with radials
 

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.

Quarterwave vertical with radials
 

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

Quarterwave vertical with radials
 

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

Quarterwave vertical with radials
 

"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.

Quarterwave vertical with radials
 

"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.

Quarterwave vertical with radials
 

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.

Quarterwave vertical with radials
 

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

Quarterwave vertical with radials
 

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.

Quarterwave vertical with radials
 

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

Quarterwave vertical with radials
 

On Sun, 9 Jul 2006 23:53:40 +0100, "David" nospam@nospam wrote:
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.

Hi David,

Well, given your repetition of "ground plane," be cautioned that is
not one-and-the-same meaning for radials (even if they are called part
of a ground plane antenna).

73's
Richard Clark, KB7QHC

Quarterwave vertical with radials
 

Cecil Moore wrote:
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.

I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.

Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.

Learning this stuff can be a lot of fun. It can be frustrating, too.

Good Luck,

John
AB8WH

Quarterwave vertical with radials
 

"David" nospam@nospam wrote in message
...
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?

Effectively, yes.

A metallic surface (your "sheet of metal") can be replaced by a partially
metallic surface -- within limits. If you keep the size of any gap under
1/10 wavelength, the surface will appear solid. This I know from satellite
reflector work.

The use of four radials appears to be a compromise for using a solid
surface, but it obviously works. The RF sees these radial wires and behaves
like we want. I think adding more radials will always make a better
counterpoise, but I also think you reach the point of diminishing returns
pretty quickly. (We aren't the first ones to speculate about this, after
all :-)

Quarterwave vertical with radials
 

"Cecil Moore" wrote
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
==========================================

What everybody forgets about is that the velocity factor along
ground-mounted radial wires is about half of the free-space value.

Consequently, the 1/4-wave resonant length is crudely only half of the
elevated value.

On the other hand, the resonant length is very non-critical because Q
is very small - Q is only 2 or 3 and is even smaller at the high end
of the HF band.
----
Reg, G4FGQ

Quarterwave vertical with radials
 

Reg Edwards wrote:

What everybody forgets about is that the velocity factor along
ground-mounted radial wires is about half of the free-space value.

Consequently, the 1/4-wave resonant length is crudely only half of the
elevated value.

On the other hand, the resonant length is very non-critical because Q
is very small - Q is only 2 or 3 and is even smaller at the high end
of the HF band.

Actually, at HF and average ground, the velocity factor below the ground
is about 1/4 to 1/5 the free space value. And no resonance at all is
usually apparent because of the high loss.

EZNEC isn't among the "everybody" who's forgotten it. Choose any real
ground, open the Utilities menu and click Ground Info, and you'll see
the velocity factor along with other information. But it's seldom of any
practical use.

Roy Lewallen, W7EL

Quarterwave vertical with radials
 

Cecil Moore wrote:
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
jawod wrote:
I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.
Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.
Learning this stuff can be a lot of fun. It can be frustrating, too.

Just last month, with four elevated 40 meter radials 6 feet high, the
antenna was about 5 dB weaker than the very same antenna with 16
radials laid directly against soil.

This basic result repeated at three different soil locations on three
different bands, 160, 80, and 40, so it is not a fluke.

In my last quick measurement on 7MHz:

16 long radials directly on the earth (no attempt to make resonant
since they have very low Q) 0dB reference

8 long radials on the ground -1.3dB reference

4 long radials on the ground -3dB reference

4 resonant elevated radials at six feet -5.6dB reference

73 Tom

Quarterwave vertical with radials
 

Sometimes the more I learn the less I know.

I have dabbled a lot along these lines in the last few months. I have
determined my methods are faulty.

When I compare two or more antennas for gain I have no means to
measure the actual gain because I really don't know what the radiation
pattern is in real life.

However, comparing measured gains with calculated gains has given me
more confidence in the EZNEC calculations.

I have limited my test antenna to a 30 foot vertical with radials
consisting of electrical extent ion cords connected in parallel
stretched out on the ground. I seem to be manipulating the take off
angle and the impedance of the feed by adding and subtracting these
radials. The vertical seems to be more quiet (fewer signals) than a
dipole but pretty much the same strength on those it hears. The
reference dipole is the 40M section of my CushCraft A3S Beam at about
40 feet.

The only certain conclusions I have made are that getting high
confidence numbers about radials is a lot of work and probably beyond
my resources. The ARRL Antenna Handbook and EZNEC are usually right.

Usually right...
If you lie to EZNEC it will lie right back to you with an even bigger
lie. Be very careful with assumptions!

The Antenna Handbook...
There is still the unresolved issue of conjugate matching. I noted
last week or so that a copy of Walter Maxwell's book that retailed for
$19.95 went for about $75 on EBAY.

John W8CCW

On 10 Jul 2006 04:01:01 -0700, wrote:



Cecil Moore wrote:
Try the ARRL Antenna Book. In general, symetrical elevated
radials don't radiate. In general, ground mounted radials
are lossy.
jawod wrote:
I second this. ARRL Antenna Book:
Check pages 2-16 to 2-18 and "The Effects of Ground' which is Chapter 3.
All the answers you need are there.
Newsgroups can be helpful but sometimes only partly. A good text is
your best friend.
Learning this stuff can be a lot of fun. It can be frustrating, too.

Just last month, with four elevated 40 meter radials 6 feet high, the
antenna was about 5 dB weaker than the very same antenna with 16
radials laid directly against soil.

This basic result repeated at three different soil locations on three
different bands, 160, 80, and 40, so it is not a fluke.

In my last quick measurement on 7MHz:

16 long radials directly on the earth (no attempt to make resonant
since they have very low Q) 0dB reference

8 long radials on the ground -1.3dB reference

4 long radials on the ground -3dB reference

4 resonant elevated radials at six feet -5.6dB reference

73 Tom
John Ferrell W8CCW

Quarterwave vertical with radials
 

David wrote:
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.

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. Then imagine thin radial slots spread
around the vertical. Since these slots do not cross any current path
that is needed to produce the image, they have little effect on the
image. Widen those slots, and decrease the number of them, and
eventually you get to a ground radial system with only a few radials.
There has to be a transition point, where the radials are only a
poor approximation of the original disk. The question is, how well
must you approximate the disk to get a reasonable approximation of the
far field radiation pattern it would have helped produce?

Quarterwave vertical with radials
 

Sal M. Onella wrote:
"David" nospam@nospam wrote in message
...

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?


Effectively, yes.

A metallic surface (your "sheet of metal") can be replaced by a partially
metallic surface -- within limits. If you keep the size of any gap under
1/10 wavelength, the surface will appear solid. This I know from satellite
reflector work.

The use of four radials appears to be a compromise for using a solid
surface, but it obviously works. The RF sees these radial wires and behaves
like we want. I think adding more radials will always make a better
counterpoise, but I also think you reach the point of diminishing returns
pretty quickly. (We aren't the first ones to speculate about this, after
all :-)



Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from 4 or
more radials. EZNEC shows very little change in terminal impedance and
pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

Cheers,
John

Quarterwave vertical with radials
 

John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from 4 or
more radials.

Well, in theory yes, but in the real world , usually no. The reason
being the decoupling. Four or more radials will decouple the line
quite a bit better than two. I did tests adding radials to a VHF
ground plane, and I saw improvement with each addition of radials
I tried. Eight radials was a noticable improvement over four.
But I always put it down to the improved decoupling of the feedline,
rather than any big decrease in ground losses.
I imagine if you used separate decoupling sections to avoid feeder
radiation, the number of radials would matter little if any.
As far as elevated ground planes vs ground mount...Cecil does
have a point. It's common knowledge that a real low ground plane
generally sucks. You need many, many, more radials to equal
the ground loss of one at 1/2 wave up. While I don't doubt that
the low ground plane was beaten by the ground mount in Tom's test,
very few people actually run ground planes that low. If they do, they
can count on me to berate them for it.. IE: I often jumped on Cecil
for using one at appx 1/8 wave, and wondering why it didn't work too
well.
Thats too low, unless you have a lot of radials. In my observations
comparing ground planes, you really need to be at least 1/4 wave in
the air if you are going to use only four radials. Even then, thats not
optimum. At 1/4 wave up, 8-12 radials is closer to optimum. Four
radials at 1/4 wave is appx equal to about 60 on the ground.
By "optimum", I mean equals 120 radials on the ground...
Myself, I had a full length monopole on 40 m, with 32 ground radials.
It was rarely much better than my dipole on medium long paths of say
1500 miles. When I elevated the antenna to 1/4 wave, and used only
four radials, the performance was much better. Like day and night
really. So I agree, if you run an elevated GP, it needs to be up in the

air, or else you will need many radials. At 1/8 wave up, you need appx
60 radials to equal the 4 radials of the same antenna at 1/2 wave up.
I've heard many a tale of people running low band ground planes, real
low
to the ground, and having bad results. But you won't hear those bad
stories from the ones that run them at 1/4, 1/2 WL up.
MK

Quarterwave vertical with radials
 

John Popelish wrote:

Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .

It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

Roy Lewallen, W7EL

Quarterwave vertical with radials
 

John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from
4 or more radials. EZNEC shows very little change in terminal impedance
and pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

George Brown, the inventor of the ground plane antenna, found that only
two radials were necessary. But when his company went to sell it, the
marketing department decided that no one would buy a two-radial ground
plane antenna in the belief that it would be omnidirectional. So they
added two more to make it "look" more omnidirectional. The four-radial
ground plane persists to this day.

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Roy Lewallen, W7EL

Quarterwave vertical with radials
 

Roy Lewallen wrote:
John Popelish wrote:


Picture a half wave disk of metal as the ground plane, producing the
inverted image of the vertical. . .


It appears that what I've been writing the past few days either isn't
being read or isn't being believed. Among it is an explanation of why a
"ground plane" doesn't produce an "image" of the vertical.

Since you appear to continue to believe this, please explain the
mechanism by which you think a half wave disk produces an "image" of the
vertical.

The disk forms an image by allowing the electric field lines to
terminate perpendicular to the "mirror" surface on exactly the same
lines as if they were heading toward a lower half of a dipole, while
the radial currents in the "mirror" allow the magnetic field lines to
encircle the monopole in the same pattern they would form if the
missing half of the dipole were in position.

This same pattern of electric and magnetic fields above the "mirror"
produces (half of the) photons that the full dipole would have
produced. A half wave diameter disk is about the minimum size
"mirror" that will keep the field patterns close enough to those of
the dipole to launch those photons. A larger disk would do better,
but not a lot better.

Quarterwave vertical with radials
 

Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper tape
on a Duroid dielectric material, a lot like the window antenna John
described. An omnidirectional pattern was a requirement, and I was
concerned that either the flatness of the tape or the presence of the
dielectric might have some impact on the circularity of the pattern. So
I had it tested at a local lab. It was the most circular pattern they'd
ever seen, having about 1 dB maximum difference between any two directions.

Did this antenna include any provision to prevent current on the
outside of the feed line?

Which direction did the feed line exit the antenna?

Quarterwave vertical with radials
 

On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:

A larger disk would do better, but not a lot better.

Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.

the "mirror" produces (half of the) photons that the
full dipole would have produced.

Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

73's
Richard Clark, KB7QHC

Quarterwave vertical with radials
 

John Popelish wrote:
Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper
tape on a Duroid dielectric material, a lot like the window antenna
John described. An omnidirectional pattern was a requirement, and I
was concerned that either the flatness of the tape or the presence of
the dielectric might have some impact on the circularity of the
pattern. So I had it tested at a local lab. It was the most circular
pattern they'd ever seen, having about 1 dB maximum difference between
any two directions.


Did this antenna include any provision to prevent current on the outside
of the feed line?

Which direction did the feed line exit the antenna?


I don't know about Roy's antenna, but this subject has come up before,
and at the time I made a two meter vertical ground plane with only two
radials. No matter how I oriented the antenna, radially, I got the same
signal strength on my field-strength meter. And yes, I took precautions
to make sure the feedline wasn't radiating. (Many ferrite beads at
strategic places on the feedline to the point that feedline radiation
was undetectable.) If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.
73,
Tom Donaly, KA6RUH

Quarterwave vertical with radials
 

Tom Donaly wrote:
If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

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.
--
73, Cecil http://www.qsl.net/w5dxp

Quarterwave vertical with radials
 

Richard Clark wrote:
On Mon, 10 Jul 2006 14:27:05 -0400, John Popelish
wrote:


A larger disk would do better, but not a lot better.


Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.


the "mirror" produces (half of the) photons that the
full dipole would have produced.


Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

Do you deny the photonic nature of radio waves?

I just realized that the sentence you quoted s easily misinterpreted.
When I said "the "mirror" produces (half of the) photons that the
full dipole would have produced." I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line. I didn't mean that the mirror
produces half of the total photons that are radiated.

Quarterwave vertical with radials
 

George Brown was over precautious. Only one vertical radial is
needed. There is no loss in efficiency. The radiation pattern remains
sensibly the same.
----
Reg.

Quarterwave vertical with radials
 

Tom Donaly wrote:
John Popelish wrote:

Roy Lewallen wrote:

Just a few weeks ago, I designed what amounted to a two-radial ground
plane antenna as part of a consulting job. It was made from copper
tape on a Duroid dielectric material, a lot like the window antenna
John described. An omnidirectional pattern was a requirement, and I
was concerned that either the flatness of the tape or the presence of
the dielectric might have some impact on the circularity of the
pattern. So I had it tested at a local lab. It was the most circular
pattern they'd ever seen, having about 1 dB maximum difference
between any two directions.

Did this antenna include any provision to prevent current on the
outside of the feed line?

Which direction did the feed line exit the antenna?

I don't know about Roy's antenna, but this subject has come up before,
and at the time I made a two meter vertical ground plane with only two
radials. No matter how I oriented the antenna, radially, I got the same
signal strength on my field-strength meter. And yes, I took precautions
to make sure the feedline wasn't radiating. (Many ferrite beads at
strategic places on the feedline to the point that feedline radiation
was undetectable.) If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

I agree. The point is, that I wonder if Roy's antenna feed had this
precaution that reduces the radiation effect of the feed line, or if
feed line radiation was part of the antenna.

Quarterwave vertical with radials
 

Reg Edwards wrote:
George Brown was over precautious. Only one vertical radial is
needed. There is no loss in efficiency. The radiation pattern remains
sensibly the same.

8-D

Quarterwave vertical with radials
 

"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.
--
73, Cecil
=====================================

If they don't cancel-out each other in the near field then they don't
cancel-out each other in the far field either.

A pair of radials behave as a continuous dipole fed at its center via
a single wire. And it radiates.

A circular disk, diameter = 1/2 wavelength, fed at its centre
radiates.

But don't ask me what its radiation resistance is. It must be very
low.
----
Reg.

Quarterwave vertical with radials
 

Ok I am getting confused. You are saying that a groundplane will not
work as good a a ground mounted vertical ? At what angle are you
talking about? Are you more interested in working 500 miles or 6,000
miles?

Ron

Quarterwave vertical with radials
 

On Mon, 10 Jul 2006 17:57:42 -0400, John Popelish
wrote:

Hi John,

In fact a larger disk will actually raise the launch angle - hardly a
satisfactory mirror analogy.


the "mirror" produces (half of the) photons that the
full dipole would have produced.


Photons? This is CecilBabble. Mirrors as "productive" sources of
photons demonstrates the failure of analogies.

Do you deny the photonic nature of radio waves?

Hi John,

This last question is standard CecilBaiting at which he is a master.

I've made a career in photonics, so you will have to go some distance
to start offering a case that comes remotely close to their cross
application. Barring that, why introduce concepts that don't advance
the topic? The following is hardly any clearer by clinging to poor
metaphors:

I just realized that the sentence you quoted s easily misinterpreted.
When I said "the "mirror" produces (half of the) photons that the
full dipole would have produced." I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line. I didn't mean that the mirror
produces half of the total photons that are radiated.

73's
Richard Clark, KB7QHC

Quarterwave vertical with radials
 

Richard Clark wrote:
On Mon, 10 Jul 2006 17:57:42 -0400, John Popelish
wrote:

Do you deny the photonic nature of radio waves?


Hi John,

This last question is standard CecilBaiting at which he is a master.

I've made a career in photonics, so you will have to go some distance
to start offering a case that comes remotely close to their cross
application. Barring that, why introduce concepts that don't advance
the topic? The following is hardly any clearer by clinging to poor
metaphors:

I guess the perceived quality of any given metaphor depends on your
mental model of the rest of the universe. Antennas and photons work
for me. If they don't work for you, I have no problem with that.

Remember, it is Cecil, not me, who demands agreement or eternal verbal
torture.

Quarterwave vertical with radials
 

On Mon, 10 Jul 2006 19:40:59 -0400, John Popelish
wrote:
I guess the perceived quality of any given metaphor depends on your
mental model of the rest of the universe. Antennas and photons work
for me. If they don't work for you, I have no problem with that.

hi John,

It would seem that they "don't" work for you. I have no problem
shifting to a photonic dialog, but you have yet to emerge from a
rather muddy start.

Remember, it is Cecil, not me, who demands agreement or eternal verbal
torture.

I can torture with the best of them too. Choose your metaphors well
to avoid the embarrassment of Abu Graib.

73's
Richard Clark, KB7QHC

Quarterwave vertical with radials
 

On Mon, 10 Jul 2006 17:57:42 -0400, John Popelish
wrote:

I meant that half as many photons
are produced, compared to the full dipole antenna that produces the
same fields above the center line.

Hi John,

So, proceeding along your avowed lines of Photons, one of several
questions:
Presuming 100W radiated, how many photons would that be so that we
can talk about them by halves.

Yes, that is perhaps unfair, however it demonstrates how easily the
discussion can tumble for lack of quantifiables such as that original
offering of 100W.

Should we discuss how infinitesimal the energy is in a 40M photon?
(Easily accounts for why so many are needed for that same 100W.)

No, I suppose not.

Want to get into the problems of diffraction with object lenses that
measure less than a wavelength of the photon?

Hard to escape, and makes a mess of describing mirrors too, especially
when they are skeletal approximations as well.

I can offer more thread-busters when it comes to photonics, but that
is a slam dunk. Get us rolling on one ace proposition, and I will get
back to you in a couple of hours.

73's
Richard Clark, KB7QHC

Quarterwave vertical with radials
 

Roy Lewallen wrote:
John - KD5YI wrote:

Actually, on elevated antennas (as in the usual VHF setup), just two
quarter-wave radials 180 degrees apart is almost indistinguishable from
4 or more radials. EZNEC shows very little change in terminal impedance
and pattern by removing two radials from a 4 radial ground plane.

I once used copper tape on a window to make a ground plane vertical like
that for 70cm. It worked very well.

George Brown, the inventor of the ground plane antenna, found that only
two radials were necessary. But when his company went to sell it, the
marketing department decided that no one would buy a two-radial ground
plane antenna in the belief that it would be omnidirectional. So they
added two more to make it "look" more omnidirectional. The four-radial
ground plane persists to this day.

The real reason to use 4 radials or more is decoupling the feedline
shield.

Decoupling is very bad with two radials unless you get lucky with
feedline and/or mast length or use a decoupling aid like a common mode
choke.

On a commercial 47 Mhz GP I designed that had 4 radials, the radials
had to be isolated from the mounting and a ferrite decoupling sleeve
placed over the coax. I can't imagine how bad that problem would be
with only two radials.

73 Tom

Quarterwave vertical with radials
 

Cecil Moore wrote:
Tom Donaly wrote:
If you can bring yourself to think in terms of
current directions and far field superposition of waves, this
behavior shouldn't be that hard to understand.

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. The problem is the spatial distance is different unless exactly
broadside to the pair.

Even 4 radials has this problem, but the more radials the less of an
issue it is.

73 Tom

Quarterwave vertical with radials
 

Reg Edwards wrote:
George Brown was over precautious. Only one vertical radial is
needed. There is no loss in efficiency. The radiation pattern remains
sensibly the same.
----
Reg.

All you have to do is figure out how to decouple the feedline for less
cost than the cost of three additional radials and a tiny easy to build
choke.

Getting the feedline off a four radial GP is bad enough.