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ANTENNA TOPICS
BUYING, BUILDING AND UNDERSTANDING ANTENNAS
Clem Small, KR6A
clemsmall@monitoringtimes.
Alternatives to Elevated, Outdoor Antennas
Some folks who would love to get into
a radio hobby feel thwarted that they
have no place to put up an elevated,
outdoor antenna. The good news is that you don’t
actually have to have an elevated, outdoor antenna
to enjoy shortwave listening, broadcast-band
DXing, amateur radio, logging low-frequency
beacons, or the many other kinds of fascinating
radio hobbies that people enjoy. Indoor antennas
often can be a good interface between your
radio and the multitude of radio signals that are
present around us 24 hours every day.
To begin, let’s be up front about it and admit
that indoor antennas usually don’t perform as
well as elevated, outdoor antennas. And if the
building in which you use them has lots of metal
in its construction, that may lead to very poor
performance. But, in many locations, indoor
antennas do work, and, in some instances, they
work quite well.
❖ Some Useful Indoor
Antennas
First, let’s consider some quick and easy
indoor antennas. Sometimes these will surprise
you with their performance. For UHF or VHF
scanning, almost any length of wire 12 inches (.3
meters) or so will often work quite well. For HF
and lower frequencies, a length of wire at least
15 feet (about 4.5 meters) or more in length can
be laid out on the floor next to the wall, under a
rug, or in the attic or overhead crawl space.
If your receiver has no antenna input connector,
it may work okay to connect your antenna
to the receiver’s whip antenna if it has one. Also,
it sometimes works to wrap several turns of wire
around the receiver’s case, and connect the wire
to your antenna.
In buildings which shield their interior
from signals, putting a long whip or metal flag
pole out a window or on a balcony as an antenna
may bring good results. Just putting a wire out
the window and letting it hang down toward
the ground can be useful for temporary installations.
Small, tunable, table-top loops and active
antennas are especially popular with folks like
AM broadcast band DXers and lowfers who
listen on the MF band and lower in frequency.
Table-top loops are noted for their ability to
reduce interference through the use of the deep
nulls (directions of low response) in their reception
patterns. Active antennas, depending on
the model, may cover from LF and lower, and
on up to the microwave frequencies. And they
often rival long, outdoor antennas in their ability
to pull in stations. Their weakness is that they
may lose sensitivity, or produce inter-modulation
distortion in the presence of very strong
signals.
There are larger (a yard or a meter or so in
diameter), single-turn, tunable loops available
which can be used indoors. MFJ* offers two
models, both remotely tunable: model 1786
tunes from 10 to 30 MHz, and model 1788 tunes
from 7 to 30 MHz. The Bilal Company** has
models for several ham bands. Bilal antennas
are tuned at the antenna rather than remotely,
and cover mainly the ham bands. I have used
Bilal loops and the no-longer-produced AEA
tunable loop (of the same general type as the
MFJ models) indoors. Both performed surprisingly
well.
These larger loop antennas can be used for
transmitting as well as for receiving. If you use
these or any other antenna indoors for transmitting,
they should be kept well away from other
objects, people and pets. I would recommend
only low power for any antenna used indoors.
Scientists are concerned about the effects of
exposing humans and animals to radiation from
antennas. You should familiarize yourself with
recommendations for safety in this regard if you
use any antenna for transmitting at other than
low power levels.
❖ Underground Antennas
There’s a rather unusual alternative to elevated
antennas, and those are antennas placed
on the ground, underground, or even under
water! Of course, all underground wires and
connections must be well insulated and weather
proofed.
At one time, when radio communication
was concerned primarily with signals in the
MF band and lower in frequency, these antennas
enjoyed a limited popularity. The lower the
frequency the better these antennas perform, and
the deeper they can be “planted.” They do have
a degree of immunity from lightning damage.
And they are reported to be less susceptible to
received noise than are elevated antennas.
And, yes, they do work, although they
certainly don’t perform as well as elevated
antennas. But they do come in handy for some
applications. For instance, they may be used as
emergency backup antennas due to the fact that
they are resistant to damage from disasters such
as tornados, heavy ice snow storms, or terrorist
bombing. I have used HF antennas buried an
inch or so underground for local communication
and received decent signal reports. I suppose the
earth I buried them in was “average,” and for
resonance the antennas were shorter by a third
than elevated antennas. So a half wavelength
antenna was 312 ft. divided by the frequency
(MHz) (or 95m/frequency MHz in meters).
Fig. 1. The wiring diagram for the circuit of a simple active antenna (A), and a pictorial diagram
of the antenna circuit showing how it snaps onto its battery (B).
❖ Let’s Build an Active
Antenna
The active antenna shown in fig. 1 is easy
to make, portable, and a good performer on the
LF, MF and HF bands.
The antenna is constructed on a snap-on
connector board which has been removed from
a discarded 9-volt battery. The connector board
62 MONITORING TIMES January 2005