April 1973 Popular Electronics
Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954  April 1985. All copyrights are hereby acknowledged.

The April 1973 issue of
Popular Electronics concentrated on antennas for everything from citizens band (CB)
radios to televisions and amateur radio. This article covers a compact, multiband Ham
antenna with a minimal level of skill and components required. Construction tips are
given. It includes the popular 10, 15, 20, 40, and 80meter bands. Standing wave ratio
(SWR) plots are shown for all four bands.
MultiDipole Antenna Has Low SWR and Uses No Traps
By Richard A. Yommus, W2DMK
Fig. 1  Method of constructing the multidipole antenna. If space is at
a premium, use inverted V approach.
Many hams encounter problems in erecting just one antenna, so the thought of needing antennas
for five bands seems out of the question. However, with about 70 feet of antenna space (somewhat
less if the inverted V configuration is used) a fiveband (75, 40, 20, 15, and 10 meters)
antenna can be erected. The antenna has a separate dipole for each band and does not use any
traps.
As shown in Fig. 1, the antenna is a Singlefeed, fourband, separate dipole system, using
a conventional, commercially available balun transformer (1:1, 75 ohms) for a symmetrical
radiated pattern on these bands. The dipoles are individually cut to any given frequency within
a band using the equation L = 468/F, where L is in feet and F in MHz. For general use, the
dipoles should be resonant at the center of each band.
On 40 and 20 meters, the antenna maintains a fairly consistent SWR of less than 2:1 (see
Fig. 2), while on 15 meters, the curve is flatter with hardly any change in SWR from the low
end to the high end of the band. On the 10meter band, the usable bandwidth is about 500 kHz
on either side of resonance before the SWR becomes excessive.
Using multiple dipoles with one feedline is a common practice among hams, but the fact
still remains that, for 80meter operation, it takes between 120 and 135 feet of wire to radiate
effectively. Using the popular traptype antenna on 7580 meters, the overall length usually
exceeds 100 feet with an extremely narrow operating frequency range.
The coaxial feedline does not represent a full wavelength electrically on the 40meter
band. At 7.15 MHz, the physical length of the coax is determined by (492/F) times 2 times
VF, where F is in MHz and VF is 0.66 when RG59 or RG11U, 75ohm coax is used. For 7.15 MHz,
then the length is 90' 6". The flat top overall length at 7.15 MHz is determined from 468/F
or 65' 6". This is divided by two to give 32' 9". By adding the coax length of 90' 6" to the
32' 9" of the divided flat top, a resonant length of 123' 3" is obtained, which represents
a half wave of slightly below 3.85 MHz.
Fig. 2  Standing wave ratios for the antenna on various bands. On 40 and
20 meters, SWR is fairly consistent; but on 15 meters, it is relatively flat.
The transmission line can be fed from the pi network of a transmitter or transceiver without
the aid of an additional antenna coupler, although a coupler could tune out any reactance.
The SWR is excellent for about 200 kHz of the 75meter phone band, and does not exceed
2:1 at this point. On this band only, the inner and outer conductors of the coax are tied
together at the transmitter end. In the event that operation is desired on the low end of
80 meters, a length of wire can be used to resonate at the desired frequency. The wire is
then switched out when operation at the high end is desired. This additional length of wire
can be simulated in an antenna coupler or by a simple L network. The radiated pattern on 75
meters is essentially omnidirectional with both vertical and horizontal polarization.
Construction. The 40meter antenna should be made of copperclad steel
wire to provide strength. Both the wire and plastic spacers used in the antenna can be obtained
from a length of 450ohm open TV transmission line, or commercially available air dielectric
transmission lines. Each end of the 40meter dipole is connected to an insulator, while the
center is tied to the connectors on the balun. The remaining dipoles are suspended from the
40meter dipole with the plastic spacers that come with the transmission line. Short lengths
of plastic rod may also be used. In most cases, it is sufficient to heat the wire and push
it into the plastic, where it will be firmly gripped when the wire cools. The spacers are
about 6" apart along the lines. The center of each dipole is connected in parallel with the
one above it, to a lug on the balun.
The coax transmission line should be kept away from buildings, trees, power lines, metal
surfaces, etc., when being fed as a resonant line. For the same reason, it should also be
outdoors as much as possible. Keep the transmission line at right angles to the flat top dipoles
if possible.
The balun transformer has no appreciable loss when connected as described here and appears
as a small inductance in series with the coax. Since the differential voltage across it is
very small, there is no possibility of its burning out.
To experiment with keeping the SWR fairly consistent from one band to another, add about
3' of 75ohm coax to the 90' 6" length. Before trimming the coax, make sure that all dipoles
are resonant at the center of each band. Then, trim off 6" of coax at a time, until the SWR
becomes consistent on each band. The extra length of coax will not impair operation on 75
meters; but it will shift the frequency slightly lower than 3.850 MHz.
Posted October 24, 2017
