wrote:
Maybe reading one of your own posts will jog your memory a bit. In it
we see you VERY CLEARLY stated two radials would cancel each other's
radiation.

Two radials do indeed cancel each other's radiation to a large
extent. But I certainly didn't say there were two and only two
radials in the entire system. I missed four days of postings.
--
73, Cecil http://www.qsl.net/w5dxp

"Roy Lewallen" wrote in message
...

snip

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.



snip

Which prompts me to ask a question: If a quarterwave vertical antenna has
many radials only a few feet above the ground, and these radials could be
made progressively longer and longer, does the antenna eventually fail to
"know" where the ground is? How long is "very long" to bring this about (if
it happens)?

I kicked some numbers around. By the formula two times
antenna_length-squared divided by wavelength [2D^2/lambda], I make the
far-field distance for a 14 MHz quarterwave vertical be only 2.5 meters
[less than a quarter wavelength] ... but typical radials are already longer
than that, aren't they? So this isn't a near-field/far-field boundary
issue, is it?

Tom Ring wrote:
wrote:

Maybe reading one of your own posts will jog your memory a bit. In it
we see you VERY CLEARLY stated two radials would cancel each other's
radiation.

73 Tom


Won't matter, he'll have an explanation for it, and it will be anyone's
fault but his.

tom
K0TAR

You were right Tom.

He came up with one and it wasn't his fault.

wrote:
Tom Ring wrote:
Won't matter, he'll have an explanation for it, and it will be anyone's
fault but his.

You were right Tom.
He came up with one and it wasn't his fault.

On the contrary, I confessed it was my fault for missing
four days of postings and thus missing the "two and only
two radial" context.
--
73, Cecil http://www.qsl.net/w5dxp

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.
. . .

"Not everything that can be counted counts, and not everything that
counts can be counted."
- Albert Einstein (1879-1955)


Zen


Roy Lewallen wrote:


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.
. . .

The vertical element is connected to the centre conductor (RF live). The
radials are connected to 0V on the transceiver.

If only the vertical is connected, the antenna still radiates although not
as well. If only the
radials are connected, the antenna does not radiate because the radials are
connected to 0V and not a varying voltage.

With both vertical and radials connected, the vertical element radiates the
wave. The wave is reflected by the radials as boundary condition of
Maxwell's equations. The reflection induces a current in the radials. This
current has a standing wave on it.

Do you think the above is correct? All parts of the antenna form the
impedance. Without radials, the impedance is poor and the vertical element
does not radiate well.

Other explanations say that displacement currents go through the air and
terminate on the radials. The displacement currents then becomes conduction
current in the radials. Displacement current is another anomaly with
electromagnetic theory.

I notice that two people have simulated the vertical antenna with radials
using EZNEC, and obtained different results. One simulation shows that the
radials radiate, the other shows that they do not.

The vertical element is connected to the centre conductor (RF live). The
radials are connected to 0V on the transceiver.

No. They're connected to the shield/braid of the feedline. There's
no assurance that this point will be at "0V" with respect to anything
in particular except itself, and in particular it usually won't be at
0 volts with respect to the transceiver's chassis / output jack
(except perhaps momentarily, twice per RF cycle).

If only the vertical is connected, the antenna still radiates although not
as well.

.... because the outside of the feedline will tend to act as a
poorly-tuned radial/counterpoise.

If only the
radials are connected, the antenna does not radiate because the radials are
connected to 0V and not a varying voltage.

With both vertical and radials connected, the vertical element radiates the
wave. The wave is reflected by the radials as boundary condition of
Maxwell's equations. The reflection induces a current in the radials. This
current has a standing wave on it.

Do you think the above is correct?

Not really, no. It's a mistake to think that the radials "are
connected to 0V and not a varying voltage". You're falling into the
trap of thinking that "ground" is some sort of magical "zero volt"
reference which is the same everywhere. That isn't true even at DC,
and it's certainly not true at RF!

All parts of the antenna form the
impedance. Without radials, the impedance is poor and the vertical element
does not radiate well.

The vertical element radiates very well indeed... it'll radiate all of
the power which is fed into it, except for a small amount of loss.

The problem isn't that it doesn't radiate. The problem is that it's
difficult to feed power into it, much of the time.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

On Tue, 18 Jul 2006 20:34:32 +0100, "David" nospam@nospam wrote:

If only the vertical is connected, the antenna still radiates although not
as well. If only the
radials are connected, the antenna does not radiate because the radials are
connected to 0V and not a varying voltage.
snip
I notice that two people have simulated the vertical antenna with radials
using EZNEC, and obtained different results. One simulation shows that the
radials radiate, the other shows that they do not.

Hi Dave,

Your statements above show a serious problem with understanding the
operation of antennas.

The radials are not potted plants merely arranged along the ground (or
in the air) to give a sense of symmetry and balance.

You would go further to engage more in dialogue rather than simply
posting statements. Much of the utility of radials has been
discussed, revisited, and rehashed to no apparent effect against what
you offer above. The last sentence is outrageously wrong for any of a
number of reasons (or proof of some pretty stupid simulation).

73's
Richard Clark, KB7QHC

Agreed, the centre junction of the radials is not always at 0V. Current
flows along the coax braid on the inside, meaning that the inside part of
the coax braid and radials junction can be any voltage. The radials have a
voltage gradient along them because of the standing wave. Because the return
current flows on the inside of the coax braid, it is normally safe to touch
or go near the outside of the braid. For permanent low installations in a
public area, coax should be used instead of twin feeder. The fact that the
return current flows on the inside of the braid gives coax its shielding
properties.