TV Antenna Terms Glossary - A to F

Balun wire positioning

If your balun has distinct wires (pigtails) instead of ribbon cable for its 300-ohm connection then read on:

For twin-lead to be 300 ohms the spacing between the wires must be about 12 times the wire diameter (more if there is insulation). If this is not true for the 300-ohm balun’s wires, the effect is equivalent to a small “point capacitance”. If the spacing is less than 12 diameters then the point capacitance will be positive, otherwise it will be negative.

The consequence of the point capacitance is hard to predict. Most likely it will reduce the antenna’s net gain on some channels, but it could actually improve the antenna on other channels. The effect is probably a few tenths of a decibel for UHF, insignificant for VHF.

Here is a rational approach to this dilemma: -

Judiciously make a list of your must-have UHF channels.
Use the receiver to determine the weakest station on the list.
Note the signal strength of this station three times:

With the balun wires spread wide apart
With the balun wires pinched together
With the balun wires in a middle position

Leave the wires in the best position.

Bandwidth

A wideband antenna will pick up all the channel in a band, while a narrowband antenna will receive a few channels well but most channels poorly. (note: The TV spectrum is in 3 bands: VHF low, VHF high, and UHF.)

Beam width

High-gain antennas always have narrow beams. This can be good or bad, but it is an inescapable truth.

The beam width is normally measured to the “half-power points”. That is, the beam width is the number of degrees between the points where the gain is 3 dB less than for the antenna’s strongest direction.

Beam Width2
The antenna’s maximum gain can be found from the beam width using the formula:

G=41000/(A*B) where: -

1. G is the raw gain factor (relative to isotropic, not in dB)

2. A is the beam width, in degrees, in the elevation plane

3. B is the beam width, in degrees, in the azimuth plane

This is an approximate formula, but it tends to be highly accurate for common, one-directional TV antennas.

Some conclusions can be drawn from this equation:

1. For any given channel, whichever antenna has the highest raw gain will have the narrowest beam. (This applies to antennas with “searchlight” like patterns like the phased array or a Yagi.)

2. It is often pointless to try to distinguish weak-signal problems from multi-path problems. An antenna that improves on one will also improve on the other.