TV Antenna Terms Glossary - A to F

DC Block

Some splitters will pass DC through all 3 legs. If such a splitter came between the power injector and the mast-mounted amplifier then the DC would go some place unintended. Some antennas and some TVs are effective short circuits for DC. That short circuit would kill the DC at the amplifier, and thus the amplifier would not work.

A DC Block is just a coupling with a capacitor inside that will not pass DC.. Putting the DC Block in the line with the “short circuit” will enable the amplifier to work properly.

Some splitters have a DC Block built into one side. These are always clearly labeled.

Decibels

Decibels (dB) are commonly used to describe gain or loss in circuits. The number of decibels is found from:

Gain in dB = 10*log(gain factor) or:-

Decibel Gain

In some situations this is more complicated than using gain or loss factors. But in many situations, decibels are simpler. For example, suppose 10 feet of cable loses 1 dB of signal. To figure the loss in a longer cable, just add 1 dB for every 10 feet. In general, decibels let you add or subtract instead of multiply or divide. There are some special numbers you might want to memorize:

20 dB = gain factor of 100
10 dB = gain factor of 10
3 dB = gain factor of 2 (actually 1.995)
0 dB = no gain or loss
-1 dB = a 20% loss of signal
-3 dB = a 50% loss of signal
-10 dB = a 90% loss of signal

Diffraction (The bending of radio waves)

Diffraction is the ability of a wave to bend into the shadow created by an obstruction.

Diplexer (see Splitters/Combiners/Diplexers)

Dipole Antenna

The most basic of all antennas, the dipole is popular for TV reception because of its predictability. A freestanding half-wave dipole has a torroidal (doughnut-shaped) reception pattern, and has a terminal impedance of 75 ohms. But if this 75-ohm antenna is connected directly to 75-ohm coaxial cable without a 1-to-1 balun, some signal picked up by the dipole will flow onto the outside of the shield conductor where it will be retransmitted and lost. (Also some signal picked up by the shield will flow onto the dipole, disrupting the gain, the radiation pattern, and the impedance. This could be an improvement, but more likely it is the opposite.)

The dipole is 75-ohms at only one frequency. For other frequencies its terminal impedance will include some reactance, which will prevent a good match with the feed-line. See impedance. See “Mismatch between antenna and feed-line”. Making the dipole diameter larger minimizes the reactance, giving the dipole a larger bandwidth. The dipoles with the largest bandwidth are double-cone dipoles, but bowtie dipoles are somewhat effective at the same thing.

Directivity

Directivity is a measure of the directionality of the antenna. Numerically, the directivity is equal to the raw gain for TV antennas. See Beam width.

Director (see Yagi antennas)

Distribution amplifier (see Antenna amplifiers)

Driven elements (see Yagi antennas)

Dropouts (weak signals)

On DTV channels you will never see snow, ghosts, or interference, but you will see dropouts. When the signal is corrupted or becomes too weak, you will see “macro-block errors” (parts of the screen that are shifted or obviously wrong), sound dropouts lasting a few seconds, or image freezes lasting a few seconds. All of these errors are crude, unsubtle errors. If these are not present, your image is perfect.

The causes of dropouts are: -