The Conical Monopole Antenna
November 1966 QST Article
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
November 1966 QST
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.
The Conical Monopole Antenna
Four-to-One Frequency Coverage with a Vertical
Commercial version of the conical monopole used by the U.
S. Navy and other government services.
By E. W. Pappenfus,*
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 140foot 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.
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.
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 antennas.
1 The Radio Amateur's Handbook,
42nd edition, Fig. 14-1
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.
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.
Fig. 1 - (A) Top view of
the conical monopole antenna for 3.5 through 14 Mc.
view of conical monopole at section A-A. Note that
stubs, b, connect to short radial wires, a. Wires c
run up the
sides of the supporting 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.
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 spokes.
Fig. 2 - (A) Top view of the
antenna top hat. The steel plate is held to the 2 X 4
by wood screws. (B) Side view through section B-B.
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.
guy lines of polyethylene (water-ski rope) stabilize the antenna
and keep it from twisting (see Fig. 1.).
Fig. 3 - Details of the central
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.
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
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.
Fig. 4 - Top and side views of the bottom feed ring. For clarity,
not all of the pole
wires and grounding details are shown.
Fig. 5 - Radiation pattern for (A) 80 meters and (B) 20 meters.
Solid patterns ore
for conical monopole over perfectly conducting
for average soil.
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
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 long
1 polyethylene rope, as needed
6 gate hinges
galvanized steel, 18-inches diameter
20 galvanized or copper-plated
ground rods, 5-feet long