Prior to the advent of personal
computers and handy-dandy antenna design software like
EZNEC, determining the effects of varying
parameters - element spacing, angles and length, ground plane distance and
extent, feedpoint impedance, the presence of conductive structures, etc. - it
was necessary to make a series of often complex mathematical calculations and
ultimately perform real-world measurement. Huge amounts of time would be
invested in the design and verification process. It has been know for a long
time that the distance an antenna sits above a ground plane has a significant
effect on the radiation pattern - particularly the vertical pattern. The
information provided in this 1954 Radio & TV News magazine article
undoubtedly required many hours to assimilate and required someone (author
William Harrison) with a lot of knowledge in the science/art of antennas. While
some empirical testing is still needed for critical applications, in most cases
these days the results of computer simulations suffice for determining the
viability of a particular antenna design and installation. Nearly every month
Joel Hallas (W1ZR) in his "The Doctor Is In" QST column publishes at
least one antenna analysis in response to a reader's question that includes
graphs produced by EZNEC simulations.
The Ground Plane Grows Up
Fig. 1 - The six-foot parabolic reflector, mounted on the side
of the building, is used to illuminate an antenna under measurement. A 5/8 wave
vertical is mounted in center of ground plane. The complete assembly is rotated
from inside the building where pattern is recorded. The antenna laboratory is located
on top of Mt. Lee in Hollywood, 1900 feet above sea level from which level it is
possible to make radiation measurements with minimum disturbance due to surrounding
terrain or structures. See text for discussion of the results obtained during antenna
By William H. Harrison, W6ULD
A new slant on a ground plane antenna that increases efficiency on higher frequencies.
During the past few years a great deal of interest has been centered on the quarter-wave
ground-plane antenna. As many know, the important advantages of this type of antenna
are low-angle radiation, ease of feeding, small space requirements, and relatively
low cost. The purpose of this article is to show how it is possible to further improve
the antenna by doing nothing more than going straight up a few more feet.
We will first consider the angle of radiation. Most transmissions on 20 meters
take place at angles between 6 and 17 degrees, 15 meters between 4 and 14 degrees,
and 10 meters at angles below 10 degrees. Anything above these angles is useful
only for short skip contacts. How many of you have had difficulty working those
DX stations but find that you have obtained excellent reports during short skip
sessions? The reason is that the antenna is radiating a good portion of the power
at the higher angles. The quarter-wave vertical antenna radiation pattern follows
quite closely a cosine curve from the horizon and concentrates much of the energy
at the low angles; however, at angles over 20 degrees there is still a great deal
of energy radiated that is of little value. The antenna to be described is non-directional
and tends to concentrate practically all of the radiation below 20 degrees. The
vertical plane pattern is quite similar to that of a well designed rhombic. As the
height of a quarter-wave antenna is increased, the energy radiated becomes more
concentrated at the lower angles. This continues with increased height to 5/8 wavelength
(0.625λ), as may be seen in Fig. 2. If the length is further increased, some
of the energy in the low angle lobe begins to form in a lobe at a much higher angle,
and power at the desired angle is lost. With the 5/8 wave radiator a small high-angle
lobe is present, however it is a relatively small portion of the total radiated
energy. Information taken from the "FCC Standards of Good Engineering Practice"
indicates that maximum radiation along the horizontal takes place with a 5/8 wave
Let us now consider feeding the 5/8 wave radiator. The quarter-wave vertical
antenna may be fed directly with 52-ohm coax because it has a base resistance of
40 to 50 ohms and a fairly low reactance. Measurements made on several such quarter-wave
antennas indicate that they can be fed directly and will maintain a standing wave
ratio on 52-ohm coax of about 1.5. As the height is increased the base resistance
of the antenna increases to a point around 0.42 wavelength where it reaches a maximum
and then decreases with a further increase in height. At a height of 5/8 wavelength
the base resistance is again near 52 ohms, which makes it simple to feed. The resistance
and reactance curves shown in Fig. 3 indicate the impedance for various heights
of radiators and points of actual measurement.
The 5/8 wave antenna on 20 meters is approximately 44 feet and measurements were
taken on either side of this value. It was found that the base impedance is slightly
higher than indicated in various texts possibly because of a difference in base
capacities or antenna cross-section dimensions. The height was increased to 47 feet
where the resistive component is 50 ohms, making this a desirable height. The measured
reactance is low at this height (60 ohms capacitive) which can be tuned out by a
small series coil as indicated in Fig. 6. If the coil is not used the system will
still work favorably as the measured standing wave ratio is only 1.5 to 1. By using
the little coil, in series with the coax feeding the base of the antenna, the v.s.w.r.
was reduced to approximately 1.05 to 1.
When working an antenna against ground, the ground system is very important.
Broadcast stations are required to install a ground system of at least 120 radials
1/4 wavelength or greater. The main reason is to provide maximum radiation at the
ground level. Without the extensive ground system much of the energy radiated at
the extremely low angles is lost in absorption. In this case low angles are meant
to represent those below 5 degrees. While we as hams are not particularly interested
in such low radiation angles, yet it is necessary to work the antenna system against
ground, and the better the ground system the less energy will be lost. At our former
home in Tempe, Arizona, I installed a ground system in the back yard before the
grass was sown. It consisted of 120 radials 35 feet long, using #16 galvanized iron
Fig. 2 - Vertical radiation patterns for different heights of
Fig. 3 - Resistance and reactance components of impedance between
tower base and ground system of a vertical tube mast.
Fig. 4 - Unattenuated field intensity for various heights of
vertical antenna measured at a distance of one mile from a one kilowatt transmitter.
See text for complete details.
Fig. 5 - Photo shows the antenna which was used to make the pattern
measurements shown in Fig. 7 A. This unit has an element length of 5/8 wavelength
on either side of the feedpoint.
Fig. 6 - Antenna base and mount used to support the 20·meter.
47-foot vertical antenna. Bonding strap is used to insure good electrical connection
between components. Note grounded coax.
Fig. 7 - Vertical plane radiation patterns. (A) Pattern measured
with antenna shown in Fig. 5. (B) Same antenna as shown in Fig. 5. except element
length reduced to 1/4 wavelength on either side of the feedpoint. (C) Horizontal
dipole antenna mounted 1/4 wavelength above ground, and (D) mounted 1/2 wavelength
above ground. (E) Comparison of 1/4 and 5/8 wave vertical antenna patterns. Pattern
details and measuring techniques are discussed in text.
The wires were buried 1 to 2 inches. This task wore out two hoes and the neighbors'
curiosity. The coax line was buried at the same time making a neat installation
with only the vertical antenna and mount showing above the ground. Two sets of guy
wires were used. I would like to mention that I have found it desirable to use two
egg insulators at the point where each guy is attached to the tower as well as breaking
up the guys every tenth wavelength with another egg insulator. This gives assurance
the guys are not connected to the tower electrically which would de-tune the system.
The ground system used with my present antenna consists of 16 radials ranging from
25 to 45 feet running out in all directions from the base of the tower. One may
plant as many as the wife will permit; however, make certain to use at least 4 radials.
Information available in various texts verifies the fact that added low angle
radiation is obtained with the 5/8 wave radiator; however it was desired to construct
a miniature 5/8 wave radiator as well as a quarter-wave vertical antenna system
and actually make radiation measurements. If the frequency is increased to 1000
mc. the radiators become 7.4 and 2.96 inches respectively, making them easy to mount
and rotate so that a radiation pattern may be obtained. The two antennas (individually)
were originally mounted in a ground-plane which consisted of sheet iron approximately
a yard square. The whole assembly was then rotated and patterns taken to determine
the directivity of the antennas in the vertical plane. Due to the small size of
the ground-plane the results did not resemble the desired patterns. The major lobes
were forced up away from the ground-plane and minor lobes developed off the back.
The data we are interested in is based on an infinite ground-plane. It was, however,
possible to use this same ground-plane to make patterns of horizontal antennas (mounted
parallel to the ground-plane) because the voltage vector cannot be supported on
the ground-plane and the radiation is driven up away from the ground, as indicated
by the patterns shown in Figs. 7C and 7D for horizontal antenna mounted a quarter
wavelength and a half wavelength above ground respectively. The final patterns for
the 5/8 and 1/4 wave vertical antennas, shown in Figs. 7 A and 7B, were taken by
using duplicate elements working against one another instead of against the ground-plane,
that is, a complete dipole was used (see Fig. 5) so that the pattern would not be
dependent upon a ground-plane of finite size. The image antenna in this case is
not a mere image but the real thing and radiates equally well, thus duplicating
the pattern of the top half of the antenna below the imaginary horizon. The results
are identical to the published patterns of broadcast engineering firms, except of
course in duplicate.
It would be interesting to make a study of the effects of the size of a ground-plane
with respect to the antenna height and also the effect of tapering the ground-plane
wires to form different sizes of conic ground-planes as are used by many hams. All
of these features affect the angle of radiation, and of course the height of the
system above the actual ground will also make some difference. In the case of 10
and 15 meters, due to the small height, it might be desirable to place the antenna
in a ground-plane located above ground (depending on the individual circumstances)
so that the antenna system will be in the clear.
Now we get down to the facts of how much improvement can be expected. The relative
merits of the two antennas may be compared by comparing their field intensities
at the desired radiation angles. This information is available in the FCC publication
mentioned previously. Based on 1 kilowatt input to each antenna, the quarter-wave
element produces a field of approximately 195 millivolts per meter at one mile,
while the 5/8 wave radiator under the same conditions provides about 275 millivolts
per meter. (See Fig. 4.) This is the field produced at zero degrees. The difference
in the two fields is reduced as the angle is increased, as can be seen by the patterns
in Figs. 7A and 7B. They become equal in magnitude at about 20 degrees but, as mentioned
earlier, long distance communications on 20 meters takes place at an average angle
of 12 degrees, 9 degrees on 15 meters, and about 5 degrees on 10 meters.
Based on the field strengths just given, we get a power gain of (275/195)2
= 2.0 (power gain is proportional to the square of the voltage ratio). However,
because the antenna length was increased slightly beyond 5/8 wavelength (47 feet)
to improve the impedance match to the coax line, the field strength is reduced to
approximately 263 millivolts per meter. This value gives a gain over a quarter-wave
vertical of 1.8 on the horizon and at a vertical angle of 10 degrees, 1.6. In terms
of db, the two gains are respectively 2.56 db and 2 db.
The 5/8 wave radiator has another advantage of producing maximum radiation at
a height of 3/8 wavelength above the ground which reduces loss due to absorption
of energy by house, garage, trees, etc. Maximum radiation from an antenna takes
place at the current maximums, and in this case we have a current maximum located
at a considerable height; i.e., one-quarter wavelength from the top. On 20 meters,
this amounts to a height of 30 feet.
If you have the opportunity of comparing the vertical 5/8 wave antenna with another
antenna, do so with a station which is located a considerable distance. I recall
an instance several years ago when a friend informed me that his ground-plane on
40 meters was not as good as his little low horizontal job. He had made checks with
a station located about 60 or 70 miles away. Further checks were made from my location
under similar conditions and what he had said proved to be true. After studying
the matter it was seen that the ground wave in either case has long since been attenuated
so ground-wave communications between the two stations was not possible. Communications
therefore were via sky wave, which in the case of the ground-plane was not possible
because of the low radiation angles causing the signal to skip right over the receiving
station. The energy from the horizontal, however, was concentrated at very high
angles (see Fig. 7C) so that some of the energy was hitting the ionosphere at the
proper angle and returning to the receiving station. Comparing the same two antennas
at 2000 miles, the ground-plane was found to be far superior to the horizontal.
I have found that a thirty-foot telescoping pole, normally used to support TV
antennas, makes an excellent bottom portion of the 5/8 wave radiator for twenty
meters. This pole is available for approximately fifteen dollars. Several lengths
of thin wall steel conduit that telescope inside the smallest section of the 30
footer can be added to bring the total length to 47 feet. The complete assembly
is light (40 pounds) and can be put up by two people, first figuring what the length
of the guy wires should be and installing two of them while the antenna is on the
ground. While one fellow holds the base down the other fellow can walk it up into
position. Once in position the base is secured so that the base man is free to pull
up on the third guy.
The coil is made with an inside diameter of 1/2 inch, #12 wire, wound 8 turns
per inch. For the 20-meter band, the antenna should be 47 feet high and the coil
should consist of 12 turns. A coil of 10 turns should be used with a 31-foot antenna
for the 15-meter band. For the 10 meter band, use a coil of 8 turns with a 23-foot
The 47-foot vertical was used during the field day contest producing good reports
from the east coast while running only 30 watts. A 579X report was received from
Australia. The 5/8 wave radiator is nothing new but definitely something that has
been overlooked by the hams. It adds that extra punch you've been looking for at
a very reasonable cost.
Appreciation is extended to Microwave Engineering Company for granting permission
to photograph and use their equipment to make the pattern measurements.
Posted April 23, 2020