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There have been many questions regarding how to select an antenna for
scanning. This is a Readers Digest version of antennas, meant to give
new users some idea of the different antennas and their good and bad
points. At the end, I give some specific recommendations on how to build a
general purpose VHF/UHF antenna system. This is not an all inclusive book
on antennas, but rather a guide for novices in their quest to learn more
about their scanning hobby. Whats the best antenna? There would seem to
be as many different answers, as there are people asking the question.
Everybody has a favorite band that they listen to, and different antennas
work better on different frequencies. But the fact is, the general
principles remain the same for everybody. The object of an antenna SYSTEM,
is to deliver as much signal as possible to the antenna jack on the back of
the radio at the frequency youre listening to. Notice I wrote system; the
antenna is a system comprised of several parts: the antenna, the lead-in
cable, and the mast or tower to hold it up.
Antennas
There are MANY different and very good antennas on the market. There
are also some bad ones out there. Antennas are where the most compromises
must be made. If you have unlimited room , and very deep pockets, you can
put up an antenna farm with a different antenna and radio for each band
that you listen to, and not have to compromise. But if youre like the
rest of us, you WILL compromise. If you only listen to one band, then
your best bet is an antenna designed specifically for that band. Something
like a 1/4 wave ground plane, or a 1/2 wave dipole antenna is a good choice
for omni-directional listening at a low cost. It will have the added
benefit of being less efficient out of band, which if you live in a dense
signal environment, may attenuate those out-of-band intermod producing
signals somewhat.
1/4 Wave Ground Plane Antenna
This is a single band vertically polarized antenna that offers about 3dB of
gain in a relatively narrow frequency range. Its major benefits are its
low cost and small size. The ground plane isolates your antenna from
having to be coupled to earth ground at a specific multiple of the
wavelength, by simulating ground with the radially mounted elements around
the bottom. A car mounted antenna is typically a 1/4 wave that uses the
body of the car for its ground plane. There are some versions that have
several vertical elements (like the R.S. Allband with 3) but each
vertical element will only be resonant in one band. While it will receive
signals in all bands , it will only be efficient in the 3 bands that the
vertical elements are cut to resonate at. Radio Shacks all-band (they
used to call it a tri-band) is a good antenna if you have a limited number
of bands you listen to. Its reasonably priced and gives good performance
on 3 popular frequency bands and is usable on the other bands as well, and
best of all it is inexpensive.
Discone Antenna
This is a relative of the 1/4 wave ground plane antenna optimized for wide
frequency bandwidth. It offers 0dB of gain, on frequencies from about
120-1300MHz, and with a vertical element on top, it is usable down to about
30MHz. Gain is achieved by compressing the radiation pattern into a
donut shape with little of the signal radiating upwards or downwards,
concentrating the pattern perpendicular to the vertical axis of the
antenna. ItÕs called a discone because it is comprised of two parts, the
disc, a group of elements parallel to the ground around the top, and the
cone, the diagonal radial elements around the bottom. These could be made
from a solid metal disc and a cone shaped sheet metal radial, and perform
the same, but the wind loading would be increased. The Diamond D130J and
the Sigma SE1300 are good discones for general purpose scanning. My
personal experience with the Radio Shack discone antenna at home, has shown
that it is not a very good implementation of the discone design, and should
be avoided. It is too fragile and does not work below 100MHz. I believe
that the discone is the best all band antenna, it really works. I don't
see ANY other type of omni-directional antenna usable for TRANSMITTING on
ALL VHF and UHF ham bands (50, 144, 220, 432, 900, & 1200 MHz) like the
discone is. It would be usable on all frequencies in between too, but
that's illegal, if you're transmitting as an amateur. I have built many
UHF data and voice links (among other things) for the US Govt over the
years and we use discone antennas for the 225-400MHz military UHF band; the
VSWR is consistent and low across the entire band.
1/2 Wave Dipole Antenna
This is also a single band antenna that offers 2dB of gain in a relatively
narrow frequency range. The dipole antenna is the standard against which
gain is measured on all antennas, and it is twice as long as a 1/4 wave
antenna. It has balanced signal and ground sides, which means that the
coax feed is in the center of the antenna. The center conductor is hooked
to the top half and the shield connects to the lower half. It requires a
balun to connect it to coax cable, although there are feed techniques that
can do the job of matching the antenna to the 50 ohm coax. It is fairly
large for the frequency it's tuned to, and like the ground plane antenna,
it isolates your antenna from having to be coupled to earth ground at a
specific multiple of the wavelength, by simulating ground with the lower
half of the antenna. The dipole can be oriented either vertically or
horizontally.
Yagi Beam Antenna
Named after it's inventors Mr Yagi and Mr Uda (the second guy always loses
out, and I forget their first names), theYagi-Uda parasitic array is
another single band antenna. It offers 10-20dB of gain and 10-30dB of
front-to-back isolation in a relatively narrow frequency range. It is a
group of dipoles all the same length, connected to a boom, to hold them a
specific distance apart. It offers excellent gain, and front-to-back
isolation, and a narrow beam width that it will receive from. The gain is
determined by how many elements are used as directors, and is achieved by
limiting how many directions a signal can be received from. Like a
magnifying glass focusing the sun, the smaller the spot the hotter it gets.
The most useful feature of a beam antenna, is that the can be rotated to
null out a signal you do not want or maximizing the one you do want. You
will need a rotor to point it in the right direction; if you want to listen
in more than one direction. The down side is, it will only have gain in a
narrow frequency range of about +/-1% of the center frequency, which would
be beneficial in a dense signal environment to attenuate those intermod
producing signals you do not want, or if you only listen on one band. It
is most commonly used by commercial and amateur operators, since it is an
inexpensive and very efficient type of antenna for single band, point to
point, communication in the VHF/UHF range.
Log Periodic Beam Antenna
The Logarithmicly Periodic Dipole Array is a beam antenna optimized for
wide frequency bandwidth. It offers 5-15dB of gain with a moderate
10-15dB of front-to back ratio; the beam width is fairly wide when compared
to a Yagi. It is a group of dipoles of decreasing size (with the longest
in back and the smallest in front), connected to a boom, to hold them a
specific distance apart. The tapering of the elements is what gives it
the wide frequency range, by always providing an element that resonates
near the frequency that your operating on. It is most commonly used in TV
antennas, where operation on many frequencies is required. The down side
is that the LPDA can be fairly large for a VHF/UHF antenna. There are
commercial versions available that provide general coverage. Create Labs
makes two models for $200-350. EEB and Ham Radio Outlet both have them in
their catalogs.
TV Antennas
A TV antenna is NOT a very good scanner antenna because it is optimized
only for the TV bands. If you look closely at a TV antenna you will notice
that the taper of the elements is not uniform. There will be several long
ones (Chan 2-6 at 54-88MHz) then several medium long ones, usually
interspersed with the long ones (Chan 7-13 at 175-216MHz), and then a bunch
of short ones, all the same length (UHF 470-812MHz). The missing elements
are for the frequencies that a scanner user wants, but are not in the TV
band, so they are not included in the design. If the frequencies that you
do listen to are close to the TV bands, then after re-orienting a TV
antenna to vertical polarization, it may work, but IÕll bet it doesnÕt work
very well. Another problem is, the UHF elements on a TV antenna are ALWAYS
a Yagi design. The reception range that they advertise is only on one
channel (my guess is around Chan 35 at 600MHz) and the gain falls off the
farther you get from that center channel. There are no scanner type
signals anywhere near this frequency and a Yagi is a tuned frequency
antenna. You will NEVER see a gain Vs frequency plot of any TV antenna
from the manufacturer. This is why TV antennas make lousy scanner
antennas. If you want to use a wide band UHF TV antenna, try a 4-bay bow
tie, it has about 6dB of gain, a 15dB front-to-back ratio and resonates
across a wide frequency range. The whole antenna just needs to be
re-oriented to vertical polarization. On VHF, donÕt bother. I am not
aware of any true broad band LPDA TV antenna, they are always optimized
only for TV frequencies, due to the lack of elements resonating in the
88-175MHz and 216-470MHz bands.
Lead-in Cable
It doesn't matter how good your antenna is, if you are feeding it with
lossy COAX.
The loss that a COAX has, is determined by many factors, most having to do
with the density and effectiveness of the shield and the dielectric (the
insulator in the center). If the shield is not very good, more of the
signal will be lost before it gets to your radio, and you will be
susceptible to multipath distortion in strong signal environments. If the
dielectric is made wrong, the impedance of the COAX will vary. The problem
is the quality of the cable itself, this is something that you can't see or
discern from the specs, RG-8 is RG-8 right? WRONG, because it is a
manufacturing process, if the machines that manufacture it are not set-up
properly, or the materials used are sub-standard, the result will be BAD
COAX. Cheap COAX may have a 98% shield in itÕs specs but it won't be
consistent across the entire length of the cable. Sometimes there will be
areas several feet long with large gaps with only a few strands in the
shield. There is no way to tell from outside the cable that something is
wrong, without using an expensive network analyzer setup. The same for
the dielectric, if it's uneven, the characteristic impedance of the COAX
will vary tremendously. The result is you just won't hear as many signals
at the higher frequencies. That is why you should always buy name brand
cable from a reputable manufacturer. There is a reason the cable is cheap.
Frequency is the other MAJOR contributing factor in determining your
losses. The higher the frequency, the higher the loss. If you only listen
to the California Highway Patrol, in the 39-45MHz band, then a VHF 1/4 wave
ground plane antenna and any kind of COAX will do, such as RG58, which is
easily routed, and not very expensive. The same is true if you listen only
to railroads or police/fire in the 150MHz band. A single band 1/4 or 1/2
wave and the better RG8 cable will be plenty good enough. But if you
listen to the 800MHz trunked band you can significantly improve your
reception, by just using better COAX. I use Heliax on my system for
maximum performance. I've included a chart of some common 50 ohm coax and
their loss at different frequencies for comparison.
These are from the Belden Wire and Cable catalog, except for the Heliax.
Losses in dB per 100Ft
50MHz 100MHz 500MHz 900MHz
---------- ----------- ----------- -----------
RG-58A/U 3.3 4.9 13.3 20.0
RG-8/U 1.2 1.8 4.7 6.7
Belden 9913 0.9 1.4 2.9 4.2
1/2Ó Heliax 0.56 0.83 2.0 2.8
Note: The losses scale proportionally with length. Half as long, half the
loss in dB.
The Mast or Tower
There is no substitute for height, until you've cleared the obstructions
around your house. After you have cleared any obstructions, more height
will give only slightly improved range. Remember line-of-sight won't
change much with another 10 or 20 feet of elevation. It takes 1000s or
10000s of feet of elevation to really make a difference.
I don't know much about lightning protection, since here is Silicon valley
we rarely ever get any lightning, maybe once every 5 years. I saw some
just last winter, or was it the winter before. Anyway don't forget to
ground your system as appropriate.
My Recommendations
I think most of us listen on many different bands, so a single band
antenna just won't cut it. A single antenna, with good performance on
all bands is the best solution. I personally have chosen a Diamond D130J
discone antenna, and I am very pleased with its performance. The Sigma
SE1300 has the same performance but costs a little more. I recommend
RG8/U or Belden 9913 low loss COAX as your best bet, the 9913 is much more
expensive. I got my Heliax, surplus and cheap. I swept it using an HP
network analyzer to be sure it was still good. For a mast, no more than
10-20ft above the roof is necessary to clear most obstructions, such as
neighbors houses and trees. More than that will give only a marginal
improvement in receiving range and make the installation more difficult and
dangerous.
Multiple radios
I've connected all of my radios to one antenna with the use of a 4-way
splitter. This is not a cheap R.S. splitter but a commercial quality
hybrid type from Mini Circuits with guaranteed specs from 1 to 1000MHz. I
lose 7dB of signal but it's worth it to have only one antenna and cable.
My ICOM 7100 and both PRO2006s all run from the same antenna and I have an
extra spigot that I can use for testing my PRO-43 or any other radio.
If you want more in-depth information, try the ARRL Antenna Handbooks
(there's one specifically for VHF/UHF) and check your local library.
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