TV Antenna Terms Glossary - G to P

Jack

A jack is a female connector.

Kilowatt (see Watt)

Log-Periodic antenna (Log Periodic Dipole Array, LPDA)

The LPDA has several dipoles arranged in echelon and criss-cross fed from the front. The name comes from the geometric growth, which is logarithmic.

Log periodic antenna diagram

This is a very wideband antenna of medium gain. Usually only about three of the dipoles are carrying much current. The other elements are mostly inactive. As frequency increases, the active elements “move” toward the front of the array. Most VHF antennas are LPDAs.

TV LPDAs come in two types: straight and Vee. The Vee type (LPVA) has a very slightly higher gain for channels 6-12. The straight type has nulls 90° to each side that can be used to cancel out a ghost, but the Vee type has less overall radiation to the sides.

LPVA (see Log-Periodic antenna)

Log-Yag antenna

This hybrid antenna is an LPDA with some Yagi elements added. It is a good wideband antenna and is a common VHF antenna.

Long-wire antenna

A long-wire antenna is long, but of no specific length or geometry. Long-wire antennas are used for long distance reception on the AM and short-wave bands, but are seldom used above 20 megahertz. This is not a good TV antenna because it is unpredictable and usually has low gain.

Loop antenna

Although a loop antenna will work at any frequency (if the loop is the right size), for TVs it is only common as a UHF antenna. It is a weak antenna, comparable to a single dipole.

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Matching transformer (see balun)

Mismatch between antenna and feed-line - An antenna has an innate impedance measured in ohms. If this value is the same as the transmission line’s characteristic impedance then the energy captured by the antenna will be efficiently transferred into the transmission line. Otherwise a portion of the signal will be reflected (retransmitted). The greater the degree of numerical mismatch, the more that will be rejected by the line.

For channels where the net gain is far less than the raw gain, the antenna is badly matched.

The antenna impedance is usually not just resistive, but also includes a reactive component (which means there seems to be a capacitor or coil in series with the antenna) and the reactance tends to be the bigger problem. There are ways to cancel out this reactance, and also adjust the resistance. But making the antenna better matched at one channel will make it worse matched at other channels, and there is rarely an overall improvement.

Presently there is no hardware commonly available to change the match, except for some indoor antennas. To perfect the match for a given channel on an outdoor antenna requires the services of an engineer.

Multi-path interference

This is a problem which, if severe, can prevent DTV reception even if the signal is strong. The signal is reaching the antenna by more than one path due to reflections off of structures, and diffraction around hills and sometimes trees.

There are two distinct categories of multi-path interference. The first is “short delay” multi-path, delays of less than about 20 nanoseconds. On analog channels there will be no visible ghosts.

Short delay multi-path

This is always caused by something directly in front of the antenna. One common cause is a tree in front of the antenna. There will be chaotically overlapped signals behind a tree. This will mainly affect UHF reception. The solution is to relocate the antenna (or cut down the tree). If the antenna stays behind the tree, you will likely see dropouts on UHF channels when the wind blows. And that’s for strong-signal areas. In weak-signal areas you will likely get no UHF reception at all behind a tree.

The other common cause is an irregular horizon line (structures and trees in the distance). These will cause overlapping fields, which will result in a regular pattern of strong and weak spots. For UHF, moving the antenna right or left 900mm or so can make a huge difference. Moving the antenna is usually the solution. Unfortunately a strong spot for one channel can be a weak spot for a different channel. The same phenomenon happens for VHF, but since wavelengths are ten times as big, the strong and weak spots are ten time further apart, so moving the antenna to a strong spot is often too far to be practical. For VHF the solution is usually a bigger antenna.

Long delay multi-path

This is caused by a large obstruction like a hill or a large building. If you tune in an analog channel close in frequency and from the same direction, you will see ghost images. The offending signals are approaching the antenna mostly from the sides, but also maybe from the rear. Actually all analog images have these ghosts, but without the direct path blocked they are too dim to see.

One solution is to move the antenna to a new spot where signals from the offending directions are less strong. A move of 6m or more will likely be necessary.

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