voltage fed vertical question
 

in a never ending quest to acquire antenna knowledge
i've been doing some reading of a paper back book
by a well know antenna guru who's now a sk.
he claims that if a vertical antenna is fed at it's base
with a parallel resonant l/c circuit and tapped on the
inductor to to get an appropriate swr match, it's
rf ground can be no more than a 3'x3' screen combined
with a rod in the ground.

in my mind, this seems to be something for nothing. if true
why do i read about the importance of having a number
of radials?

questions, comments, pronouncements from the gurus on high
most welcome. brickbats & complaints, etc 2&1 /dev/null

larry
kd5foy

larry d clark wrote:

in a never ending quest to acquire antenna knowledge
i've been doing some reading of a paper back book
by a well know antenna guru who's now a sk.
he claims that if a vertical antenna is fed at it's base
with a parallel resonant l/c circuit and tapped on the
inductor to to get an appropriate swr match, it's
rf ground can be no more than a 3'x3' screen combined
with a rod in the ground.

in my mind, this seems to be something for nothing. if true
why do i read about the importance of having a number
of radials?

questions, comments, pronouncements from the gurus on high
most welcome. brickbats & complaints, etc 2&1 /dev/null

Two words - coil loss. A center-fed 1/2WL vertical would
probably be better - no coil.
--
73, Cecil http://www.qsl.net/w5dxp



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Double check the antenna details. I suspect you are describing the
matching circuit for a 1/2 or 5/8 antenna, not a 1/4 wave antenna.

DD

larry d clark wrote:
in a never ending quest to acquire antenna knowledge
i've been doing some reading of a paper back book
by a well know antenna guru who's now a sk.
he claims that if a vertical antenna is fed at it's base
with a parallel resonant l/c circuit and tapped on the
inductor to to get an appropriate swr match, it's
rf ground can be no more than a 3'x3' screen combined
with a rod in the ground.

in my mind, this seems to be something for nothing. if true
why do i read about the importance of having a number
of radials?

questions, comments, pronouncements from the gurus on high
most welcome. brickbats & complaints, etc 2&1 /dev/null

larry
kd5foy

Been there, done that, many times. If the vertical is
a half wave, you can just use the shield of the coax
feeding it as the little bit of counterpoise you need.
In direct A/B comparisons to quarter wave verticals
over good ground screens, the no ground vertical
half wave is so close in performance you can't measure
the difference.

Coil loss is not an issue if you use the proper coil.

Rick N6RK


"larry d clark" wrote in message
. com...
in a never ending quest to acquire antenna knowledge
i've been doing some reading of a paper back book
by a well know antenna guru who's now a sk.
he claims that if a vertical antenna is fed at it's base
with a parallel resonant l/c circuit and tapped on the
inductor to to get an appropriate swr match, it's
rf ground can be no more than a 3'x3' screen combined
with a rod in the ground.

in my mind, this seems to be something for nothing. if true
why do i read about the importance of having a number
of radials?

questions, comments, pronouncements from the gurus on high
most welcome. brickbats & complaints, etc 2&1 /dev/null

larry
kd5foy

Cecil Moore wrote in message ...


Two words - coil loss. A center-fed 1/2WL vertical would
probably be better - no coil.

Probably fed, the coil loss is very low. The version I prefer is a
single turn coil, and a coax capacitor if needed. Very low coil
losses. I've used loads of the base fed half waves. Also have used a
center fed 1/2 wave. Never could notice any real difference between
the two in performance. The only advantage to a center fed I could
see, is possible use on other bands using a tuner. Many people do this
with the center fed 1/2 wave A-99 CB antennas. Trying to work multi
bands with a base fed antenna is not going to work very well. But I
don't recommend that anyway... MK

People like to simplify things, to make them easier to understand. They
read that radials are important, but don't understand why, and apply
that "sound bite" to all vertical antennas. (Or even to all antennas.)
This sort of binary thinking -- or rather, non-thinking, is commonly
applied to politics and a vast number of other fields besides antenna
theory.

When you put power into an antenna, a current flows out of one conductor
of the feedline to supply that power. An equal and opposite current
flows into the other conductor. In the case of a grounded vertical, this
means that whatever current flows into the base of the antenna also
flows through the ground -- where the feedline shield is connected. Due
to the resistance of the ground, this results in I^2 * R power loss. If
the antenna's radiation resistance is comparable to or lower than the
ground resistance, the fraction of applied power that's lost is
significant, so it's common to lower the ground resistance by using
radials. Radials become increasingly important as the vertical gets
shorter, because a short vertical has a lower radiation resistance.
However, the feedpoint radiation resistance of a half wave vertical is
very high -- typically higher than the ground resistance. For a given
power input, a relatively small current flows into the base of the
antenna, so very little current flows in the ground. Consequently, the
ground loss is low, and there's no need to decrease its resistance with
radials.

Roy Lewallen, W7EL

larry d clark wrote:

in a never ending quest to acquire antenna knowledge
i've been doing some reading of a paper back book
by a well know antenna guru who's now a sk.
he claims that if a vertical antenna is fed at it's base
with a parallel resonant l/c circuit and tapped on the
inductor to to get an appropriate swr match, it's
rf ground can be no more than a 3'x3' screen combined
with a rod in the ground.

in my mind, this seems to be something for nothing. if true
why do i read about the importance of having a number
of radials?

questions, comments, pronouncements from the gurus on high
most welcome. brickbats & complaints, etc 2&1 /dev/null

larry
kd5foy

When you put power into an antenna, a current flows out of one conductor
of the feedline to supply that power. An equal and opposite current
flows into the other conductor. In the case of a grounded vertical, this
means that whatever current flows into the base of the antenna also
flows through the ground -- where the feedline shield is connected. Due
to the resistance of the ground, this results in I^2 * R power loss. If
the antenna's radiation resistance is comparable to or lower than the
ground resistance, the fraction of applied power that's lost is
significant, so it's common to lower the ground resistance by using
radials. Radials become increasingly important as the vertical gets
shorter, because a short vertical has a lower radiation resistance.
However, the feedpoint radiation resistance of a half wave vertical is
very high -- typically higher than the ground resistance. For a given
power input, a relatively small current flows into the base of the
antenna, so very little current flows in the ground. Consequently, the
ground loss is low, and there's no need to decrease its resistance with
radials.

Roy Lewallen, W7EL

Roy, I've always maintained that when I don't know that the ground
resistance is zero, I want as little current flowing in it as possible. My
inverted "L" is a voltage fed half wave on 160 meters, about 70 feet up and
170 feet out courtesy of a couple of strategically placed Oaks . I have
measured that feed impedance as being in excess of 2600 ohms and feed it
with a remotely tuned "L" network. I used it for several years just fed
against 60 feet of 6 inch well casing, and then, bowing to conventional
wisdom as advertised on 1850 KHz, added an elevated counterpoise beneath the
whole thing. (About 12 feet in the air) I didn't notice any change in signal
reports, but that impedance sure changed a lot. Did I change something
besides the ground resistance?

Regards

W4ZCB

On Thu, 20 May 2004 08:40:47 -0700, Roy Lewallen
wrote:

However, the feedpoint radiation resistance of a half wave vertical is
very high -- typically higher than the ground resistance. For a given
power input, a relatively small current flows into the base of the
antenna, so very little current flows in the ground. Consequently, the
ground loss is low, and there's no need to decrease its resistance with
radials.

From a practical view point you are correct, but for those who wish to
nit-pick adding radials will, according to NEC, result in an increase
in performance.

At about 0.35-wavelength from the base of the ½-wave monopole the
ground current peaks ( Brown, Lewis and Eastein) . The ground
losses are just further away from the base of the antenna.

Modeling I found that comparing a full-bore ground system (120
half-wavelength radials) to a single eight-foot ground rod (average
ground) the difference reported was about 1 dB.

Considering the cost difference, for me, there are more practical ways
of gaining 1 dB.

Danny, K6MHE

On Thu, 20 May 2004 09:30:07 -0700, Dan Richardson
wrote:

Modeling I found that comparing a full-bore ground system (120
half-wavelength radials) to a single eight-foot ground rod (average
ground) the difference reported was about 1 dB.

Hi Danny,

I made the same comparison at 40M. The difference between 120
(quarterwave) radials and 1 amounts to 0.1dB Clip that one down by a
tenth and the difference climbs to an astronomical 0.3dB.

73's
Richard Clark, KB7QHC

In the inverted L or any antenna with a horizontal wire, there's
coupling between the wire and ground. The field from the horizontal wire
induces current in the ground under it. If the wire is low, the loss
produced by this current can be substantial. By putting an elevated wire
under the horizontal wire, you've changed this coupling to the ground,
plus you've introduced a new conductor into the antenna. Mutual coupling
between this conductor and the other wires will change the impedance.

Modeling will give a lot of insight into what all is going on.

Roy Lewallen, W7EL

Harold E. Johnson wrote:

Roy, I've always maintained that when I don't know that the ground
resistance is zero, I want as little current flowing in it as possible. My
inverted "L" is a voltage fed half wave on 160 meters, about 70 feet up and
170 feet out courtesy of a couple of strategically placed Oaks . I have
measured that feed impedance as being in excess of 2600 ohms and feed it
with a remotely tuned "L" network. I used it for several years just fed
against 60 feet of 6 inch well casing, and then, bowing to conventional
wisdom as advertised on 1850 KHz, added an elevated counterpoise beneath the
whole thing. (About 12 feet in the air) I didn't notice any change in signal
reports, but that impedance sure changed a lot. Did I change something
besides the ground resistance?

Regards

W4ZCB

A single ground rod, unless in sea water, has a resistance to mother earth
between 50 and 200 ohms. Let's take it to be 100 ohms.

Efficiency of a 1/4-wave vertical, feedpoint resistance = 37 ohms, is 27
percent.

Efficiency of a 5/8-wave vertical, feedpoint resistance = 50 ohms, is 33
percent.

Efficiency of a 1/2-wave vertical, feedpoint resistance = 2500 ohms, is 96
percent.

The difference in radiation pattern in a typical back yard, in the vertical
plane, is neither here nor there.

The 1/2-wave antenna also needs the most simple L and C matching network.

But I'd never recommend a ground rod anyway. Not worth the time, trouble
and expense unless extremely short of real estate at ground level.

Roy, the problem of choice lies in over-complication by too 'clever',
'knowledgeable' old-wives and gurus rather than under-complication.
----
Reg, G4FGQ

"Roy Lewallen" wrote in message
...
In the inverted L or any antenna with a horizontal wire, there's
coupling between the wire and ground. The field from the horizontal wire
induces current in the ground under it. If the wire is low, the loss
produced by this current can be substantial. By putting an elevated wire
under the horizontal wire, you've changed this coupling to the ground,
plus you've introduced a new conductor into the antenna. Mutual coupling
between this conductor and the other wires will change the impedance.

Modeling will give a lot of insight into what all is going on.

Roy Lewallen, W7EL

Thanks, don't know why I hadn't considered "the rest of the half wave". Not
many options other than what I have up, so will pass on the modeling. It's
REALLY a very decent performer on 160, 80 and 40, and unobtrusive in the
summer.

W4ZCB

On Thu, 20 May 2004 17:15:01 GMT, Richard Clark
wrote:

I made the same comparison at 40M. The difference between 120
(quarterwave) radials and 1 amounts to 0.1dB Clip that one down by a
tenth and the difference climbs to an astronomical 0.3dB.

Maybe the difference is the length of the radials. I used ½-wavelength
radials as the peak ground current is at 0.35-wavelength from the base
of the monopole - ¼-wavelength radials would be too short to reach
that area.

Danny

Be careful about making generalizations about this. The position of the
peak current depends on frequency and the ground characteristics. I
believe it's also a function of the height of the vertical. In some
cases there's no real peak at all, but an exponential-looking decay of
current from the base of the vertical outward. This, incidentally, was
experimentally measured and documented by Brown, Lewis, and Epstein in 1937.

Roy Lewallen, W7EL

Dan Richardson wrote:

Maybe the difference is the length of the radials. I used ½-wavelength
radials as the peak ground current is at 0.35-wavelength from the base
of the monopole - ¼-wavelength radials would be too short to reach
that area.

Danny

On Thu, 20 May 2004 19:49:28 -0700, Roy Lewallen
wrote:

Be careful about making generalizations about this. The position of the
peak current depends on frequency and the ground characteristics. I
believe it's also a function of the height of the vertical. In some
cases there's no real peak at all, but an exponential-looking decay of
current from the base of the vertical outward. This, incidentally, was
experimentally measured and documented by Brown, Lewis, and Epstein in 1937.

Roy Lewallen, W7EL


Interesting. I went back to the model and took a look at the current
in the radials.

My model was a 1/2-wave monopole using 120 1/2-wavelength buried
radials. The frequency was 3.6 MHz.

EZNEC (Version 4) reported the peak radial current at about
0.41-wavelength from the base of the antenna. I made two runs. One
using poor ground and one using average ground. The peak current
location was the same in both.

This still leads me to believe that the difference in gain reported
between what Richard had modeled and I found (0.1 dB vs 1.0 dB) is due
to the length of the radials.

In ether case adding that much wire (15,840 feet) for so little gain
sure doesn't seem worthwhile.

73
Danny, K6MHE

In ether case adding that much wire (15,840 feet) for so little gain
sure doesn't seem worthwhile.

73
Danny, K6MHE

========================

Danny, I quite agree.

The current-carrying cross-sectional area of the Earth is enormous at
distances from the antenna base of 1/4-wavelength and greater. Regardless
even of very poor soil resistivity, loss in the soil is sensibly zero.

Furthermore, propagation velocity in the soil is MUCH less than the free
space velocity and I am of the opinion that computer models give a very
distorted picture of what actually happens.

At distances of the order of 1/8 free-space wavelength practically all of
the current flows in the soil. Shallow-buried radials might just as well not
be there. The copper is better used to increase the number of short
radials.

But an increase in the number of short radials is a waste of copper anyway
when the number of radials is already very large.

What B,L&E were doing with 120 radials at MF in 1937 is hardly relevant. I
understand they forgot to determine ground conductivity - an indication they
didn't fully appreciate what they were about. As they were the first in the
field to make such measurements this omission is understandable. But at HF,
soil characteristics are considerably different - factors which computer
model users do not feed into their models.

Computerised antenna model users are inclined to suffer from delusions of
accuracy - drowning, unaware, in a sea of uncertainties.

But there's no real harm done! ;o)
----
Reg.