television antenna types

HDTV Antennas
An antenna made for analog TV will work fine for DTV. There is nothing different about an antenna for DTV or HDTV. Unscrupulous people have labeled their antennas “HDTV Antennas” as a marketing ploy. The honest antenna makers have had to re-label their products likewise to avoid losing sales.
Some terms
Gain – a measure of how much signal the antenna will collect.
Beam width – how directional an antenna is.
Bandwidth – how the gain varies with frequency. A narrowband antenna will receive some channels well, but other channels poorly.
This is an easy page. But if you want, you can skip or skim the rest of this page without compromising your understanding of the pages that follow.
The Dipole
This is the simplest TV antenna. Variations on the dipole are the bowtie (which has wider bandwidth), the folded-dipole (which can solve an efficiency problem) and the loop (a variation on the folded dipole). All four have the same gain and the same radiation field: a torroid (doughnut shape). The gain is generally 2.15 dBi. “dBi” means “dB of improvement over an isotropic radiator”, which is an antenna that radiates equally in all directions. This sounds like a discussion of transmitting antennas, and it could be. An antenna will have the same gain when receiving as when transmitting, and also the same radiation pattern.
The dipole has positive gain because it does not radiate equally in all directions. This is a universal truth. To get more gain, an antenna must radiate in fewer directions. Imagine a spherical balloon. Now press on it from opposite sides with a finger of each hand. Push in until your fingers meet. The result looks like the torroid above. But more importantly, the balloon expanded in the other directions. A-hah! Gain! That’s the way antennas work.
Keep this balloon analogy in mind. More complicated antennas work by reducing radiation in most directions. They distort the balloon considerably, but the volume of the balloon remains constant.
Another rating system for antennas uses dBd, which means dB of improvement over a dipole antenna. To convert dBd to dBi, just add 2.15. Antenna makers specify their gains in dB. They actually mean dBd, but given the way they exaggerate their claims, dBi is usually closer to the truth.
In the US, TV antennas are always horizontal. If you rotate an antenna about the forward axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the angle. In other words, when the antenna elements are vertical, no signal is received because TV signals have horizontal polarization.

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