| 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.
AntennasThere 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 AntennaThis 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 AntennaThis 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 AntennaThis 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 AntennaNamed 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 AntennaThe 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 AntennasA 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 CableIt 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 TowerThere 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 RecommendationsI 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 radiosI'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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