|
Antenna Primer Part III:
Copyright
by
Clem Small KR6A
To
select an antenna to fit your needs, you should consider the frequency, or
frequencies, on which you want it to perform, whether it should be directional
or relatively nondirectional, and if you wish to utilize it primarily for DX or
for close-in communication paths. You should also consider the potential sites
you have available for the antenna, and the antenna’s cost.
Except
for the cost factor, we’ll discuss these considerations below. For cost, just
keep in mind that building your own antenna can often save you the major portion
of the expense of an antenna as compared to buying a ready-made, commercially
constructed antenna.
Some
Easy, General-Purpose Solutions
Many
Monitoring Times readers want one of their antennas to support listening
from the LF band on up to the HF band, and maybe even cover a bit of the lower
end of the VHF band. Such an antenna would serve to monitor LF signals, beacons,
the AM broadcast band, tropical bands, HF utilities, ham radio HF bands, and
more.
For
decent performance, convenience, modest cost, and ease of construction it is
hard to beat a random-length wire antenna for the application just described. If
long enough, it will have nulls (directions of reduced response), but they are
not deep for antennas at practical heights. This antenna’s response is
modestly non-directional. Building your own random-length antenna was covered in
this column two months back.
For
general local listening on the VHF-UHF bands, a groundplane antenna gives good
response toward the horizon in all compass directions. It is perhaps the best
solution for catching the action in your area. We’ll discuss making these
useful antennas below.
Some
Other Antenna Solutions
Perhaps
you’d like a random-length antenna, but don’t have the space. Then an active
antenna is often a good alternative. Active antennas are small enough to sit on
your operating table, simple to operate, and will usually give reception equal
to a 50 ft or longer random-length wire antenna. On the downside, they will cost
more than a random-length wire, and can be subject to intermodulation distortion
and overloading from strong stations.
If
you want to listen primarily to one frequency, or to one band, a good solution
for general monitoring is often a halfwave dipole cut to the frequency utilized.
For reception on MF or HF, it should function reasonably well over an entire
band. Building your own dipole was covered in this column last month.
Mounting
your horizontal antennas approximately a quarter wavelength (246/freq. in MHz)
above ground tends to favor close-in communication paths, whereas mounting them
a half wavelength (492/freq. in MHz) above ground gives better DX performance.
Keep
in mind that on MF and most of the HF band received noise is high enough that,
except for directional antennas, most antennas tend to yield similar quality of
reception. As long as the antenna is large enough, or efficient enough, to
capture sufficient signal for decent reception, then more signal strength is of
little value. This means that on MF and the lower portions of the HF band,
except for directional antennas, expensive or complex receiving antennas are
generally no more effective than a random-length wire. If received noise is
man-made, then a horizontally-polarized antenna may be less noisy than a
vertically-polarized one.
It
is true that directional antennas increase signal strength in their favored
direction. But more importantly for HF and lower frequency reception, they
reduce interfering signals and noise in other directions. This makes an
important difference in reception if you are bothered by antenna-received
interference. At the higher HF frequencies and above, where received noise is
generally lower, a directional antenna’s greater signal strength output is
quite important in bringing weak signals above the noise level. HF beams, such
as the Yagi-Uda or quad, are excellent choices from about 15 MHz through UHF,
and can be rotated to direct your antenna’s response in various directions.
Phased beams are sometimes useful, but generally cannot be rotated.
If
You Transmit
For
transmitting, the concept of antenna reciprocity tells us that an antenna’s
gain, feedpoint impedance, radiation pattern, and most other of its variables
are the same, whether the antenna is transmitting or receiving. But, it is
important to realize that a transmitting antenna and its feedline must be
capable of handling the transmitter power fed to them.
Active
antennas contain a small RF receiving amplifier, thus they cannot be used for
transmitting. Desk-top loops, and Beverage antennas are not very useful for
transmitting.
Groundplane
antennas are practical and useful from microwave frequencies on down into the HF
band. Their low vertical radiation-reception gives good all-around local
coverage for HF and higher frequencies, and also great DX performance on HF.
Fig. 1 gives the essentials for building one. For lower frequencies where the
elements are several feet long, one method of construction is to make the
antenna from wires terminated in antenna insulators for connecting to mounting
ropes. The antenna is then tied in position with the top held high by a post or
tree limb, and the radials tied to shorter posts, or other points, with ropes.
For
antennas mounted outside, don’t forget lightning-induced damage protection.
The minimum is to never use them in weather likely to produce lightning, and
disconnect and ground them when they are not in use.
Of
Course There’s More Than This
I
may have mentioned some antennas with which you are not familiar. I wish
there were space enough to define them. On the other hand, there are many
good antenna designs which I haven’t mentioned at all. If you find that the
above discussion whets your appetite, I’d like to urge you to look at some
books on antennas. Then select the one that most appeals to you, and cover it in
some detail.
W6SAI’s
HF Antenna Handbook is a good introduction, especially for hams, with
directions for building a variety of antennas. Joe Carr’s Practical Antenna
Handbook (4th edition) is excellent, and covers a very-wide range of topics,
plus it includes lots of information on building and testing your own antennas.
The ARRL Antenna Book is both technically excellent and filled with many
practical plans for building amateur-radio antennas.
Many
more good antenna books are available. Some libraries carry one or more books
about antennas. To buy books, check both new and used book stores, internet
bookstores, Ebay, and radio and electronic supply house catalogs.
This
article first appeared in Monitoring Times, April 2002 "Antenna Topics"
|
