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Guide to Scanner Antennasby 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. 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. |
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