November 1966 QST
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
Author Pappenfus presents in this article
an alternative antenna for people operating at long wavelengths who do not particularly
want or are prohibited from having a Yagi or similar structure. At 80 meters, for
instance, a Yagi is only a little smaller than a football field - or so it seems.
The sight of such a structure towering over a neighborhood house is to a Ham what
the face of an ugly baby is to its mamma (something only a mother could love, per
the old yarn). A conical monopole antenna may be a reasonable compromise. The conical
monopole antenna is a base-fed vertical antenna having an omni-directional pattern
in azimuth but with an elevation pattern that keeps most of the energy down close
to the horizon, where it belongs for long-distance transmission.
The Conical Monopole Antenna: Four-to-One Frequency Coverage with
Commercial version of the conical monopole
used by the U.S. Navy and other government services.
By E. W. Pappenfus,* WB6LOH
It is important to concentrate your transmitter power into the proper beam if
you wish to deliver the best signal to the other fellow's receiving antenna. This
has logically led to the popularity of the Yagi beam antenna on the higher-frequency
amateur bands. A beam antenna for the 80-meter band should have a 140-foot reflector
and a 77-foot boom on a 250-foot tower. This makes the beam antenna impractical
for the 80-meter band, and even for 40-meter operation a full-size Yagi is a forbidding
structure to the neighbor's narrow-minded view - even a well-trained XYL might view
such a monster beam with alarm. There is no easy solution to the need for a good
DX antenna at low frequency, but the conical monopole antenna may be of interest
to the more eager radio amateur as a more practical solution. The conical monopole
antenna is a base-fed vertical antenna that has an omni-directional pattern in azimuth
but with an elevation (vertical plane) pattern that keeps most of the energy down
close to the horizon, where it belongs for long-distance transmission. This is important
as will be shown in the following table, giving the one-hop distances for an assumed
radio ray at various angles above the horizon.
News releases on the new WWV mention the use
of "conical monopole" antennas, and the same antenna has been seen at many military
installations. While the antenna is possibly a bit "rich" for the blood of most
hams, it is still interesting to know how it is constructed. The antenna was developed
and is sold by Granger Associates.
Fig. 1 - (A) Top view of the conical monopole antenna for
3.5 through 14 Mc. (B) Side view of conical monopole at section A-A. Note that grounding
stubs, b, connect to short radial wires, a. Wires c run up the sides of the supporting
Fig. 2 - (A) Top view of the antenna top hat. The steel
plate is held to the 2 X 4
spokes by wood screws. (B) Side view through section
The above distances are based upon an assumed height of the virtual reflection
point in the ionosphere at 180 miles. It is evident from the table that it is important
to concentrate the radiated energy from the transmitter at low angles. Even when
two-hop transmission paths are assumed, the maximum of the elevation plane beam
should be held down "near the deck." For a path between New York and London, it
is desirable to radiate most of the energy below 8 degrees for a good two-hop path.
The Handbook1 shows that both horizontal dipoles and beams should be
about one wavelength above ground for low-angle radiation, and even with this height,
the maximum radiation is at 15 degrees with essentially zero right along the earth.
The above discussion of vertical plane patterns shows why a vertical antenna may
frequently out-perform a horizontal beam antenna. Another important consideration
of Yagi and dipole antennas is their very narrow-band characteristic. It is usually
hard to cover even one amateur band effectively without high v.s.w.r. using these
The Conical Monopole
How would you like a good low-angle antenna that would cover not just one, but
three bands and that is only about 0.17 wavelength high? The conical monopole is
such an antenna. It is big compared with a dipole but then it is unfair to compare
a sailboat with an ocean liner, since the performance is much improved with the
big one. The conical monopole antenna consists of two hexagonal cones joined at
the bases. The lower cone, including an impedance-matching stub to improve the impedance
over the operating frequency range, is fed from the 50-ohm transmission line. To
simplify construction, the cones are simulated with wire elements to form a cage.
In commercial versions, the central tower, supporting the cages, is a metal tower
connected to ground, but the antenna described here uses a telephone pole with six
wires running down the pole connecting to the ground system. A pole is used because
no guying is needed and an old pole may be easier to find than a metal tower. Thus,
the antenna is at d.c. ground and this protects the station from lightning damage.
Fig. 1 shows the overall dimensions for a conical monopole antenna that
will cover the 80-, 40-, and 20-meter bands with a v.s.w.r, of less than 2.5 to
1. Unfortunately, the best impedance match to 50 ohms is in the range of 10 to 12
Mc., which is of no interest to the ham. The base of the cones is 31 feet across
the diagonal. The antenna is supported by a telephone pole about 48 feet long (five
feet of it in the ground) so no guying is needed. A guyed metal tower or wood -4
X 4 could be used if desired. The top cone is made up of 12 wires, 2 at each corner.
The bottom cone has 3 additional wires added to each face of the cone to better
simulate a solid cone. The sectional view of Fig. 1 shows the outside wires,
two of the six radial wires a, grounding stubs b, and pole wires c. The radial wires
and grounding shunt wires make up a shorting stub connected across the transmission
line that feeds the outside cage at the bottom of the lower cone. A ground radial
system consisting of 60 ground radials 62 feet long connects to the sheath of the
transmission line, to the six matching stub down-leads and the six wires running
down the pole.
A small flat-top (see Fig. 2) at the top of the upper cone is supported
by 2 X 4s screwed to the pole with lag screws. A galvanized steel 16-gauge plate
at the top stabilizes the top hat and provides an easy termination for the cage
wires and the pole wires. All antenna wire is 10-gauge soft copper or Copperweld
wire. The Copperweld wire is hard to bend and keep straight, but it is much stronger
than copper and the cost is much less. A staple can be used to fasten the two cage
wires to each of the spokes, preferably on top near the end of each spoke so the
peripheral wire d can be soldered to the two cage wires at each spoke. The top-hat
assembly should be done on the ground before the pole is erected. However, climbing
lugs on the pole will permit assembly and soldering in the air, if desired. A propane
torch is very handy for soldering the wire.
Fig. 3 - Details of the central spoke assembly.
The central spoke assembly supports the widest part of the antenna at a height
of 17 feet 3 inches above the ground. Select straight and clear 16-foot 2 X 4s for
the spokes. These are cut off to extend 15 feet 6 inches from the center of the
pole. Gate hinges fastened to the under sides of the spokes and to the pole with
wood screws support the spokes at the center; the outer ends are held up by the
upper cage wires. Cage wires spread to four inches apart at the end of the spokes
where they are soldered to the peripheral wire. A copper plate is cut as shown in
the detail of Fig. 3 to hold the cage and peripheral wires. The copper plate
is cut out of sheet copper with tabs similar to the kind found on solder lugs. These
tabs are bent over the cage wires and soldered in place. The plate is fastened to
the spoke and then the peripheral wire is soldered in place. It should have some
slack so that when the lower cage wires are soldered in place, there will not be
excessive tension on the peripheral wire and the spokes. In addition, spoke wires
(a in Fig. 1) must be soldered to the peripheral wire and to the pole wires
at the pole. The stub wires (b in Fig. 1) should also be soldered in place.
At the conclusion of all of the soldering and screw-fastening to the spokes, the
top cone should be nicely aligned and tensioned. If it is not symmetrical at this
time, it should be adjusted. This would be a good time to check the dimensions -
an accuracy of ± one inch should be sufficient. The three additional wires on each
face of the bottom cone are soldered to the peripheral wire spaced equally from
Fig. 4 - Top and side views of the bottom feed ring. For
clarity, not all of the pole wires and grounding details are shown.
At the bottom of the lower cone (Fig. 4) six one-inch diameter copper pipes
with ends flattened form a ring to which the 30 wires of the lower cone are attached.
Heating the tube ends will make it easier to flatten and bend them. Bronze bolts
3/8 inch in diameter are ideal for holding the lower ring together. Before bolting
the ring together, fasten the insulators to the ring using loop of wire going around
the bronze bolts and placed between the flattened sections of the pipe. Similar
loops of wire connect the insulators to the turnbuckles and 1/4-inch hooks screwed
to the pole complete the tensioning arrangement at the base of the antenna. It might
be simpler to drill all of the holes after the pipes are bolted together. Now is
the last chance to adjust the tension of the wires so it is important to carefully
position the feed ring by blocking it up from the ground and carefully tightening
the turnbuckles. The wires are then fed through the holes in the copper pipes, wrapped
back around the pipe and twisted back on themselves preparatory to soldering. The
blocks are then removed and the turnbuckles are tightened to make the whole structure
rigid. If all wire lengths are okay, older the wires to the feed ring. Two one-inch
copper straps connect from the feed line to the feed ring. Both ends of the strap
are carefully soldered to make good electrical connections to the coax and to feed
ring, respectively. If solid coaxial cable is used, the end must be carefully wrapped
with electrical tape to prevent the entry of moisture.
Two guy lines of polyethylene (water-ski rope) stabilize the antenna and keep
it from twisting (see Fig. 1.).
About 4200 feet of wire is used in the ground system. Luckily, it does not have
to be copper. Galvanized No. 10 steel wire is almost as efficient and much cheaper
to use. If desired, the ground wires can be laid along the surface rather than being
buried. If burial is desired, a small garden plow will reduce the amount of coolie
Each ground radial is stretched out from the pole and anchored to a temporary
stake. The grass and underbrush should be cleared away so the wire will be flat
on the ground. It can be held down with large staples driven into the ground which
will hold the ground wire in place until the growth of vegetation binds the wires
in place. Five foot by 3/8 inch diameter galvanized rods are driven into the ground
at the end of every third radial where the radial is soldered or clamped to the
rod. A circular wire ties all of the ground rods and remaining radials together
as shown in Fig. 4.
Fig. 5 - Radiation pattern for (A) 80 meters and (B) 20
meters. Solid patterns are for conical monopole over perfectly conducting ground;
dashed, for average soil.
After all of that work, what do you have? The performance can best be shown in
the elevation plane patterns given in Fig. 5. The dotted curves are typical
for average soil conditions. The specified ground screen will improve the patterns
by about 1 db. at low angles. It is easy to see how effectively the antenna concentrates
energy at low angles for long one-hop path. It is not very effective for 100 miles
but for this local work, any old horizontal antenna is adequate, and v.h.f. is a
better answer. The radiation pattern is not too good on the 20-meter band where
radiation is too high above the horizon, but the 40-meter pattern is almost as good
as on 80.
lf it is desired to use this antenna for 40-, 20-, and 10-meter operation, then
all dimensions should be multiplied by 0.543. However, a horizontal beam is usually
a better choice. Only a few amateurs will have the space and the ambition for building
this antenna, but for those who do, it will greatly improve communication.
Parts List of Major Items
4200 ft. No. 10 galvanized wire
900 ft. No. 10 copper or
6 10-inch turnbuckles
6 3/8 inch bronze bolts and nuts insulators,
6 to 9 inches long
15 ft. one-inch copper pipe
6 screw hooks, 1/4 X 6 inches
2 copper straps, 1 X 26 inches
3 2 X 4s, 5 feet long
6 2 X -4s, 16 feet
1 polyethylene rope, as needed
6 gate hinges
1 16-gauge galvanized
steel, 18-inches diameter
20 galvanized or copper-plated ground rods, 5-feet
1 The Radio Amateur's Handbook, 42nd edition, Fig. 14-1
Posted May 5, 2021(original