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.
of Contents]These articles are scanned and OCRed from old editions of the
ARRL's QST magazine. Here is a list of the
QST articles I have already posted. As time permits, I will
be glad to scan articles for you. All copyrights (if any) are hereby acknowledged.
See all available
vintage QST articles.
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,* 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 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.
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.
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 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.
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. 2 - (A) Top view of the antenna top hat. The steel plate is held to the 2
spokes 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.
Two guy lines of polyethylene (water-ski rope) stabilize the antenna and keep it from twisting (see Fig.
Fig. 3 - Details of the central spoke assembly.
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.
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
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 ground; dashed,
for average soil.
Parts List of Major Items
4200 ft. No. 10 galvanized wire
ft. No. 10 copper or Copperweld wire
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
3 2 X 4s, 5 feet long
6 2 X -4s, 16 feet long
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 long