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!
** Another case of not understanding the antenna or the purpose and handling
of its current maximum. Some antenna engineer.
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
** Its not the base that is the problem. The current has to be dealt with no
matter where it is located on the vertical conductor or its electrical
length. For want of a better word its image has to be a perfect conductor
for the antenna system as a whole to be 100% efficient. It is also the
current and its efficiency that determine the power radiated at the lowest
angles. Excessive losses and that 10db becomes a simple glaring in your face
reality.
Carl
KM1H
----- Original Message -----
From: "Donald Chester" <k4kyv@hotmail.com>
To: <topband@contesting.com>
Sent: Sunday, December 16, 2012 8:42 PM
Subject: Re: Topband: GAP Vertical Question
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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