I worked in one for several years testing my designs but this was at 2.4 and
5.6GHz.
Long before that it was as a range tech for a defense contractor
designing/building/installing military EME arrays of yagis as well as
troposcatter antennas used during Vietnam. Sometimes the same yagi designs
were used, just in different configurations.
It was all about measurement, measurement, measurement as the slightest
inconsistency had to be examined, reasoned why, and solved. Over and over
and over.
Carl
Carl
KM1H
----- Original Message -----
From: "Charlie Cunningham" <charlie-cunningham@nc.rr.com>
To: "'DAVID CUTHBERT'" <telegrapher9@gmail.com>; "'ZR'" <zr@jeremy.mv.com>
Cc: "'Donald Chester'" <k4kyv@hotmail.com>; <topband@contesting.com>
Sent: Monday, December 17, 2012 6:31 PM
Subject: RE: Topband: Ground mounted 1/2 and 1/4 wave verticals (was GAP) -
A Hoot! - A "Dream"?
Wouldn't it be something if we had a fully instrumented spherical anechoic
chamber large to enclose a 160 m vertical, or inverted L or TEE and their
radial systems - and then we could answer some of these challenginq
questions by MEASUREMENT? !!! :-)
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of DAVID
CUTHBERT
Sent: Monday, December 17, 2012 5:23 PM
To: ZR
Cc: Donald Chester; topband@contesting.com
Subject: Re: Topband: Ground mounted 1/2 and 1/4 wave verticals (was GAP)
Carl,
What we do in the near-field to control ground loss affects the far-field
signal equally at all elevations. Therefore there is no need to measure
far-field field strength at more than one elevation.
We have control of the near-field and anything we do in that region shows
up
as a change in input impedance.
Dave WX7G
On Dec 17, 2012 3:08 PM, "DAVID CUTHBERT" <telegrapher9@gmail.com> wrote:
Carl. I quantified ground loss in the near field. Now it's your turn.
Numbers please, not adjectives or hand waving.
Dave WX7G
On Dec 17, 2012 2:59 PM, "ZR" <zr@jeremy.mv.com> wrote:
Because youre still stuck in neutral and are measuring/calculating
nothing of interest.
The loss is determined at various elevation angles at a sufficient
distance by field strength.
Get a helicopter.
Carl
KM1H
----- Original Message ----- From: "DAVID CUTHBERT" <
telegrapher9@gmail.com>
To: "Donald Chester" <k4kyv@hotmail.com>
Cc: <topband@contesting.com>
Sent: Monday, December 17, 2012 1:53 PM
Subject: Re: Topband: GAP Vertical Question
Where is the 40-60% claimed ground loss?
I get 4%.
On Dec 17, 2012 6:12 AM, "DAVID CUTHBERT" <telegrapher9@gmail.com>
wrote:
*Half wavelength vertical ground loss*
Let's see if we can quantify the conduction losses of a 1.8 MHz
half wavelength vertical connected to average earth via a ground
rod. This paper by N6LF shows one skin depth at 1.8 MHz to be 6
meters.
http://www.antennasbyn6lf.com/**files/ground_skin_depth_and_**
wavelength.pdf<http://www.antennasbyn6lf.com/files/ground_skin_dept
h_and_wavelength.pdf>
Let's assume the current magnitude in the ground mirrors that of
the antenna. Driving the antenna at the base such that the current
at the antenna center is 1 amp, the ground current 40 meters away
from the antenna is 1 amp. The 1 amp of ground current passes
through a section of earth having an effective depth of of 6
meters. For a 1 meter radial length and
40 meters from the antenna the section has dimensions of 1 meter X
6 meters X 250 meters (250 meters is the circumference). Given a
resistivity of
200
ohms/meter the resistance of this section is 200/(6 X 250) = 0.13
ohms.
The
loss in this section is 0.13 watts. Using NEC we see with the base
current set to give 1 amp at the antenna center the power into the
antenna is
100
watts.
Closer to the base of the antenna the effective ground resistance
increases due to the smaller circumference. Closer to the antenna
the current decreases. Roughly Integrating the ground loss from the
base to the
80 meters away gives a total ground loss of 4 watts. The no-radial
ground loss is 5 watts and the antenna gain is reduced by
10LOG(100/96) = 0.2 dB from the full radial case.
How about ground loss due to the induced E-field in the ground? I
believe this is accounted for in the previous calculation. I ran a
NEC simulation to explore this. The two cases were a 266' vertical
fed against thirty 3'
radials and thirty 133' radials. The radials are 0.05' above medium
ground.
The NEC Average Gain was compared for the two cases and showed a
difference of 0.06 dB.
Dave WX7G
On Sun, Dec 16, 2012 at 6:42 PM, Donald Chester <k4kyv@hotmail.com>
wrote:
Then, why do broadcast stations that use vertical towers at
approximately a half wavelength, purchase valuable real estate and
spend thousands of dollars for the copper to install from 120 to
240 or more radials, each usually a half wave or more in length?
See G. H. Brown: "Ground Systems as a Factor in Antenna
Efficiency", IRE Proceedings, June 1937 p. 753. Brown
demonstrated that the distribution of earth currents and ground
losses is such that the region of maximum current and loss occurs
at a distance of about 0.35 wavelengths from the base of a ground
mounted half wave vertical antenna, which was verified
experimentally.
There is zero loss at the base of the antenna itself, since there
is no base current because the antenna a fed at a current node.
An rf ammeter inserted in the ground lead, as well as one inserted
in in the antenna lead attached to the insulated base of the
radiator will read zero. The ground losses occur farther out from
the base of the antenna. Low effective earth resistance provided
by a good ground system is ABSOLUTELY NECESSARY for vertical
antennas of ANY height if one expects good radiation efficiency.
The claim that no ground system is needed for a half wave vertical
is nothing more than a long-standing popular misconception.
This topic prompted me to dig out and review an anecdote I recall
reading in my decades-old copy of CQ magazine's Vertical Antenna
Handbook, by USNR Capt. Paul H. Lee, K6TS (1974). He reported
receiving mail from a ham who had made the "discovery" that he
could tune and operate a half wave vertical without a ground
system, feeding it by a parallel tuned tank circuit whose lower
end is grounded. Since an rf ammeter in the ground lead showed
no current, he could dispense with the ground system and its loss.
He suggested to the Capt. that he should "discover the new world
of half verticals with no ground system".
Quoting from the text (p. 84):
"The correspondent's claim... is true ONLY IF HE IS CONTENT TO
THROW AWAY FROM 40 TO 80 PER CENT OF HIS RADIATED POWER IN THE
FORM OF EARTH LOSSES.
(the correspondent) stated, 'The ZL's call ME, when I use my
half wave vertical!' This is not surprising, in view of the fact
that the half wave's vertical pattern has a lower main lobe angle
than a quarter wave would have... However, he would hit the ZL's
even harder if he would put in a ground system. Of course, the
half wave vertical is not dependent on a ground plane, however
lossy or efficient, for the condition of RESONANCE, since it is
resonant in itself because of its half wave length.
However,
IT IS DEPENDENT ON A GROUND PLANE FOR ITS EFFICIENCY OF RADIATION,
as is any vertical antenna...'
Don k4kyv
>Given that a half wave vertical has a base impedance of over 1000
>ohms
and a single ground rod in dirt is 100 ohms at most not a single
radial is needed to obtain close to 100% radiation >efficiency.
> Dave WX7G
> And this statement is based on what? Publications,
> measurements, modeling?
>
> I have built a number of 1/2 wave verticals without radials and
> >
compared
> them to 1/4 wave verticals with radials. They are
> indistinguishable in performance and certainly do not exhibit
> substantial ground losses AFAIK...
>
> Rick N6RK
>I can think of NO earthly reason,that makes ANY electromagnetic
>sense
to me, as antenna engineer fo placing a radial system under the
end of a vertical 1/2 wave antenna - "earth-worms" not
>withstanding!
>It's CURRENT that "warms the earthworms"! NOT electric field
>intensity!
>...the ground system does NOT act as a "shield" from the "lossy
earth"
nor protect the "earth-worms"! There is absolutely NO reason to
require a radial system under a 1/2 wave vertical antenna.
>Such an antenna will operate just fine on its own in free-space.
>Consider this - to deliver 1000 watts to a 1/4 wave vertical with
>a
REALLY GOOD ground system and a driving point impedance of say 40
ohms would require 5 amps of RF current delivered to the >antenna
system and ground. Todeliver that same 1000 watts to an end-fed
vertical of
2000-4000
ohms real would require an antenna current, at the fed endof 0.5
-0.7 amps! It's the CURRENT >that produces the losses in the "lossy
earth"
and
"warms the earth worms". At worst, for the 1/2 wave end fed
vertical - a simple ground rodshould be just fine, and the earth
worms should be >quite comfortable, and the antenna will work
VERY well!! Of course it will be
250-260 feet tall!
>Charlie,K4OTV
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______________________________**_________________
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