November 1966 QST
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. All copyrights are hereby acknowledged.
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.
all available vintage
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
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. 14-1The 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
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.
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
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. 1.).
Fig. 3 - Details of the central spoke
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 labor.
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 communication.
Parts List of Major Items
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.
4200 ft. No. 10 galvanized wire
900 ft. No. 10 copper or Copperweld
6 10-inch turnbuckles
6 3/8 inch bronze bolts and nuts
insulators, 6 to 9 inches long
15 ft. one-inch copper pipe
screw hooks, 1/4 X 6 inches
2 copper straps, 1 X 26 inches
2 X 4s, 5 feet long
6 2 X -4s, 16 feet long
1 polyethylene rope,
6 gate hinges
1 16-gauge galvanized steel, 18-inches
20 galvanized or copper-plated ground rods, 5-feet long