antenna works well when you have lots of neighbors who are fairly
nearby, thus forming a kind of wireless hub-and-spoke network. (Note too, a
vertical transmits equally
in all directions using vertical polarization, an obvious but important point.)
It is also usually easier to place a vertical at a much greater height than
a beam from a purely logistical standpoint. (Verticals have far less wind
loading than beams.) And, as with all antennas, height is everything! So, the
odds tend to favor verticals...
Here is one place where verticals can give you misleading information. Verticals
can "hear" very well, but that doesn't mean that they have enough power to reach
all the stations listed in the "heard" list. One reason for this is the
omni-directional transmission. It tends to dilute the power by spreading it
out. A beam, on the other hand, can almost always reach the stations it
hears... Ah, decisions, decisions...
enerally vertical antennas come in three "flavors," although there are many
varieties of this class of antenna: the center-fed vertical, the ground
plane vertical, and the stub-tuned, or J-Pole, vertical. The center-fed and
the J-Pole offer the lowest angle of radiation. This means that they will
transmit, on average, over a greater distance. However, both types are free
space radiators, making then easily detuned by nearby objects, particularly
metal objects, even their own feed lines! The ground-plane is less sensitive
to this type of coupling, but its angle of radiation is not as low as the
other two mentioned above.
nother choice for an antenna is the loop
and multiple loops, otherwise known as a beam. The single loop is effective
for connections along an axis, for example, North and South of your location.
Make sure that you feed the loop on its side to obtain the correct vertical
polarization since all VHF packet transmissions are vertically polarized.
If you would like to see if you have room for a loop and what type of design
might be best for you, link to the Loop Calculator Page...
Figuring Practical Loop Designs
very popular antenna is the Yagi
usually of 3 or 4 elements, although they can be up to 11 or more. ( Again,
make sure that the elements are positioned in a vertical alignment to
achieve the correct vertical polarization for VHF/UHF packet.) Gain for this
antenna ranges from 4-5 db to 10 db or more, depending on the number of
elements. It can be used as either a fixed antenna
or a rotatable one. Most hams purchase this type of antenna since its
popularity has brought the price well within reach of the average ham.
(But, it is not difficult to build if you want to try your hand at
the classical "boom and element" construction techniques.)
nother very interesting and somewhat unusual
antenna is the Quagi
. This hybrid combines elements from both the
quad and Yagi antennas. It seems to find more application at the UHF
frequencies, though, probably because it is easier to feed the driver
as a loop as opposed to a vertical element. The back two elements,
driver and reflector, are quad loops while the directors are verticals
like the Yagi. Its use is very similiar to the Yagi, a high gain beam
which can be fixed or rotatable.
nd all of these VHF antennas should be fed
with the lowest loss coax cable that you can afford. Sometimes, you
can feed them with 300 ohm ribbon line, but these types of antennas
are usually of the homebrew type and frequently require matching devices
at both ends of the line... 50 Ohm coax is the standard for today's use!
hat about HF packet antennas? Although there
is not a great deal of activity on HF packet, there is
than you might think. And again in general, there is no
particular antenna that will
present any special advantages over the normal ones used for voice
transmissions, such as: the dipole, the loop, the J-pole, and the beam.
Any of the above that can be used for voice, will work equally well for
packet. In fact, many HF packet operators share their entire radio station
modes of operation, including the antennas, switching between voice and
packet as occaision demands. (A very versatile and inexpensive antenna
is the multi-band horizontal
which works quite well on 14.105Mhz LSB, the
frequency. Here's a
sample in real time...)
As with all outdoor antennas, make sure that you have a ground system that
will carry static build-up and lighting strikes to earth, especially
if you have a high vertical mounted near or on your house!
o, when all is said and done, you have to decide
what antenna is going to do what job; what is the capability of each
antenna, and with whom each antenna is going to communicate. Whether you
have 5 or 6 running, or are just putting up your first one, the antenna
is the front-end of your communications system, and perhaps its most
important single component. When you can answer all of the above questions,
you will know the best antenna(s) for you and your station.