It is important to use a good quality antenna from a reputable manufacturer, with due consideration of the location, channel or frequency and power of transmitters you wish to receive. Most antennas consist of a main boom with several straight rods across it. The more rods (elements), the higher the gain and the sharper the antenna directivity becomes. If desired transmitters are different directions from your location, a rotator is essential for best results.
TV and FM transmissions in North America are required to have horizontal polarization. This means the rods will be effective if horizontal. Some TV and most FM transmissions have additional vertical polarized energy. To receive this energy, the rods will have to be vertical, as is the case with an automobile antenna. Vertically polarized radio waves are more prone to reflection, leading to multipath. Your antenna should be carefully positioned to obtain optimum reception on all television channels. Sometimes this may mean a high gain antenna with mast head amplifier (booster). Failure to fit a good quality antenna often leads to poor reception on one or more channels. As television reception can vary dramatically over distances of a few feet or even a few inches, the precise location of your antenna can have a big influence on the quality of reception you obtain on each of the channels. A good way to mount the antenna is on a crank shaped, stand off pole, allowing sideways movement of the antenna during installation. Vertically polarized antennas must be fixed to the pole using an end mount behind the reflector like the antennas in Fig 1 & 2, the U shaped support bracket on some larger antennas interferes with the antenna's pick up pattern when mounted vertically.
If all the transmitters are in the same range of frequencies, have the same power and location (rare in most of the world), a skilled antenna contractor should position the antenna so that all channels are received equally well. In technical terms, he should measure the sound and vision components of each television channel on a meter, and the vision to sound ratio should be close to 10:1 on each of the channels. If it isn't, you may get 'buzzing' on captions, interference patterns on pictures, as well as errors on captions or teletext. In extreme cases, it may not be possible to obtain acceptable reception of all channels using a single antenna. One solution may be to use two antennas - one carefully adjusted for good pictures on, one channel, the other optimised for reception of the others. Two separate downleads would be needed, selected by means of a special change over switch fitted near the TV set.
Outdoor antennas do not last indefinitely and if your reception has begun to deteriorate, it could be due to corrosion of the cable connections inside the junction box. The antenna may even have been knocked off alignment or broken by strong winds or a rather heavy bird! Inspect the antenna for mechanical damage, and check the cable downlead to make sure it has not deteriorated or been broken. Sunlight can cause it to become brittle, while the ingress of water causes a large loss of signal to the TV set. antennas and downleads may need replacing more often in exposed or industrial areas.
Ghosting on a TV picture is caused by reflected signals, also called multipath signals, from hills, tall buildings, cranes, trees, etc, arriving at your antenna a tiny fraction of a second after the direct signal from the transmitter.
These delayed or ghost signals appear to the right of the main image on the screen. If you are in the USA, Digital TV is particularly sensitive to corruption by multipath. A good directional antenna is needed to reject the unwanted signals. If these come from the general direction in which the antenna is pointing, the more complicated type of antenna design more elements or a complicated reflector array at the back can be effective. If the ghost signals are from the sides and rear of the antenna, the log periodic type or the `Grid' type made from a rectangular grid of wires with four X shaped elements on one side, are often better. See Antenna Types for information on various antennas.
Trees both attenuate and reflect radio waves. In experiments I performed near Anchorage prior to beginning broadcasting on Channel 5 (near 79 MHz) I noted vastly increased multipath for vertically polarized signals. See an explanation of Polarization here. For this reason, I determined that KYES should only broadcast H Pol signals. A few tests indicated to me that most multipath energy was being returned from trunks of trees in the foothills of the mountains, not from the higher mountains as I had expected.
FM sound broadcasting is usually mixed, V and H pol, so as to better deliver to automobile aerials, which are vertically polarized. If your FM reception is fuzzy due to multipath, especially if wooded mountains are nearby, and the station transmits both H and V pol energy, a good H Pol outside antenna should clear up some of the fuzz simply because it rejects V pol signals. Television is usually Horizontal only, although in the US mixed polarization is permitted and sometimes used, and in some countries some stations are V pol only.
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