Antenna Primer:
(Found on Web)
Guide to Scanner Antennas
by Mike Diaz
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. Whets 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 you're 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 you're 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. All band
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. It's 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 Logarithmically 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. (plus it's horizontally polarized-noted ScanDiego.com) 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
doesn't 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 it's 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.