Television Antenna FAQs Television Antenna FAQs
A television antenna is basically a low tech item. It has a series of elements cut to specific lengths that are related to the frequency of the signal it is designed to receive. Each element is a pair of metal (usually aluminum) rods sticking out from the boom.
The real difference between one brand and another, assuming similar gain and front to back ratio, will be how well anodized the metal is and quality of construction. As the metal oxidizes and the rivets rust, the active elements lose contact and the performance suffers. Antennas do wear out with age if they are mounted outdoors. Indoors they will last a lot longer.
Types of Antennas
Antennas are available for VHF, UHF, or VHF/UHF. If you don't need to receive UHF channels (the ones above 13) over the air, then don't waste money on a VHF/UHF combo. The FM radio band falls between channels 6 and 7 on the VHF band, so a VHF antenna will usually work for FM as well.
There are also channel cut antennas. These are antennas specifically designed to receive one channel. They are sometimes used by cable companies to pick up local stations. A large electronics supply house should be able to get them for you. The only use for one of these in a home set up is in the case of a single station located in a different direction from all the others that is difficult to pick up. You might use a two antenna set up (see below) in that case. Directional FM antennas are a variation of a channel cut antenna designed to pick up the FM band.
Television antennas are directional. They should be pointed at the transmitter of the station you are trying to receive. That is the purpose of having a rotator. In case you don't know, the tips of the elements need to be angled toward the transmitter. Think of the elements as V's with the twin top of the V aiming towards the source.
Antennas have a specification called gain. This is a measure of how well the antenna will pull in a signal at a specific frequency. There is another specification called front-to-back-ratio. That is how much better the antenna receives a signal coming from the direction the antenna is pointed compared to a signal coming from the opposite direction. Front to back ratio is what makes an antenna directional.
Many antennas do not have these specifications on the box or in the literature. If you see these specifications, just remember that generally the larger the number the better, but since these measurements are affected by frequency, you may not be able to compare between two different manufacturers' numbers. Generally the bigger the antenna and the more elements, the higher the gain and front to back ratio. Keep in mind, though, that a combination VHF/UHF/FM antenna will have more elements than a UHF only or VHF only antenna of the same gain.
Questions and Answers:
Q: What do the milage ratings on antennas mean?
A: Not much. These ratings are very much like the lux ratings on camcorders. Each manufacturer does their own tests and makes some sort of rating, but the real life performance is so dependant on external factors that the mileage ratings are almost meaningless. If you live on the plains, the mile rating might actually be a reasonable figure, but if there is anything resembling a hill or trees or tall building between you and the transmitter, all bets are off. Antenna height is more of a factor in what you will receive.
Q: How high should I mount my antenna?
A: As far above local terrain as reasonable. A difference of a few feet is probably not much help. If you can put your antenna on top of a nearby hill, then you may actually be accomplishing something, but the difference between 1 foot off the roof and 5 feet is probably not worth it. Remember with large antennas, a short mast is better because of wind loads. If your house is on top of a hill, then stay off the roof; put the antenna in your attic or in the back yard. Reception is pretty much line of sight, so try to locate your antenna where it has a clear view to the horizon in the direction it needs to point.
Q: Should I get a rotator for my antenna?
A: It depends. Your antenna should be pointed at the station you are trying to receive. If you have stations located in different directions from your house, then a rotator might be useful. If you live where there is only one nearby city with television stations, then a rotator is probably useless because television transmitters are usually all located in the same place in one city (i.e., the tallest building).
If you have more than one television and more than one person watching, you may run into a problem with a rotator. If the station you want to watch is located in one direction and the station your kid, spouse or significant other is watching is located in another direction, you have a conflict. Which way to turn the antenna? It may be cheaper and cause less trouble to get two or three antennas and leave them pointed in the different directions you need. You can then combine the signals from these antennas into one downlead.
This arrangement should work provided there aren't some strong local stations that will reflect off buildings and hills. That will tend to cause ghosts. If you mount the two antennas on the same pole, you will need about 4 to 5 feet of vertical separation. If you mount them on separate poles ,just don't point them toward each other and keep them a few feet apart. If you have problems with ghosting in this sort of set up, you may have to separate the antenna downleads into two and use a switch box. The professional trick would be to use filters, but that's a bit over the top for this discussion.
Q: Should I get a preamp for my antenna?
A: It probably wouldn't hurt, and it may help. If you live fairly close to the local television transmitters, then an amplifier probably isn't needed. However, if you split your signal several times, the signal loss starts to be a problem. If you have two VCRs, three TVs and an FM receiver all connected to one antenna, a preamp is really a good idea. An unusually long downlead, greater than 50 feet, may benefit from a preamp also. If your reception is marginal then a preamp is probably a good idea. Just remember, a good preamp is not a substitute for a good antenna. Sometimes, a preamplifier does more harm than good. It is possible for the preamplifier to saturate on a strong signal and produce intermodulation products that degrade reception on adjacent channels. So if you get good strong signals without a preamp, then leave well enough alone.
Q: What kind of preamp should I get?
A: You should get the type of preamp that comes in two pieces. One is the actual preamp and mounts up on the pole with the antenna. The other is the power supply and mounts inside, out of the weather. The idea here is to amplify the signal as close to the antenna as possible so that you are not amplifying noise picked up by the downlead. There are many different types of preamp available. Some are for VHF only, some UHF only, some VHF and UHF, and in differing amounts of gain from 6 or 7 db to 30 db. Some have FM traps built in to prevent interference from strong local FM stations. There are even single channel preamps designed to be used with channel cut antennas. Those are usually for cable TV headends.
In multiple antenna situations, you should use only one preamp. Combine the antenna leads before the preamp. There are techniques for combining signals from multiple preamps, but that is well beyond the scope of an FAQ list. Also, keep in mind that UHF frequencies are attenuated much more quickly than VHF.
You may need a preamp for UHF stations even though VHF comes in loud and clear. A good UHF antenna may give you 8 or 10 db gain, and 100 feet of RG-59 can easily lose most of that in attenuation at those frequencies (see chart below).
Q: What sort of downlead should I use?
A: The best thing to use is good quality 75 ohm coax. RG-59 is the standard, inexpensive choice in this area. Many folks prefer RG-6 because of better shielding. Oddly enough, 300 ohm twinlead under ideal conditions may have less loss than coax. The trouble is that conditions are almost never ideal and coax is far superior to twinlead under most conditions (except pinching). There is also another option called shielded twinlead. It's expensive, hard to find, and hard to work with, so it's not really worth it. It is mentioned here in the interest of completeness. Whatever you get, make sure you get good quality cable and connectors. The few dollars you save here are going to cause you lots of grief in the long run, especially if you get cheap connectors.
Below is a chart of attenuation for coax downlead. This chart is for Belden cable and represents typical loss from "good" cable. Inexpensive cable can easily be much worse. Notice that UHF frequencies are attenuated much more than VHF.
|Connector||Cable||60 MHz||200 MHz||500 MHz|
|Parts||Gauge||Channel 2||Channel 13||Channel 18|
|RG-59U||20 AWG Conductor||1.8||3.8||6.0|
|RG-6U||18 AWG Conductor||1.5||3.1||5.0|
|RG-111U||14 AWG Conductor||1.0||2.0||3.3|
|Mini RG-59/u||26 AWG Conductor||4.0||7.8||12.4|
Q: If the antenna has 300 ohm connections and the coax is 75ohm, what do I have to do to hook them together?
A: Many antennas come with a 300 to 75 ohm transformer for just this purpose. It has a short piece of twinlead on one end and a standard coax connector on the other. If you are not using a preamp, then that is all you need. If one doesn't come with the antenna, they are available for a few dollars (less than $5) wherever you buy the antenna. If you are using a mast mounted preamp, some of them have a 300 ohm input and a 75 ohm output. This eliminates the need for the transformer at the antenna. You might save it to use on a television that doesn't have a 75 ohm connector.
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