TV Antenna Terms Glossary - A to F

Pages: Previous «« 1 2 3 4 5

Elements

An antenna is made up of several elements that work together. They transfer energy back and forth between each other before the feed-line finally absorbs it. If one element is removed, the drop in performance is usually much worse than the decline in the element total would suggest.

The manufacturers can get very creative when it comes to counting the elements. Dipoles in an LPDA are usually counted as two elements. Sometimes each rod in a reflecting plane is counted as one. Real antenna people never do these things.

EMI (Electromagnetic Interference) (see Interference)

Back to Top

F/B (Front to Back Ratio)

This ratio tells how good the antenna is at rejecting signals from the rear. It is seldom truly important because interference seldom comes from the rear, but it can happen. This ratio is the gain factor in the forward direction divided by the gain factor from the rearward direction. But since gains and F/B ratios are usually given in dB, you normally get the F/B figure by subtracting the rearward gain from the forward gain (both in dB).

The F/B ratio is mostly a marketing tool, and is not a very useful number. Respectable antenna textbooks don’t define it. The true ratio often varies dramatically from channel to channel. This author has seen two common definitions for the F/B ratio. One uses the gain in the 180-degrees direction as the rearward gain. The other uses the X-degrees direction, where X is the rearward direction (from 90 to 270 degrees) with the most gain. It is usually impossible to tell which is being used if a radiation pattern is not available.

F-connectors

The F-connector is the only connector in common use for 75-ohm TV antenna systems.

There is a quick push-on version of the F-connector, but these are never reliable long term, even indoors. Even good connectors require protection from the weather. How long the cable lasts depends solely on how long you can keep water out of it. 3M Vinyl Electrical Tape is a good waterproofer. Cover the connectors completely. The better types of connectors are the crimp type and compression type.

Don’t buy the smaller crimp tools. They will never make reliable crimps. Most importantly, get the right fittings. There are two different sizes, one each for RG59 and RG6.

Fading

Depending on the terrain at your location, there are often two paths for the signal to follow to your antenna.

Signal Path

When the sun warms up the land, a warm air layer near the ground can add a third path. The warm air causes a bending called refraction, which is identical to the “mirage effect”.

Signal Path

Which of these paths will be the strongest is hard to predict. The ground-reflecting path is usually weak if the reflection point is forested. The bent path can be enhanced by focusing, making it stronger than the direct path.

These paths will add together at your antenna, and considering phase, subtraction is a possibility. Whether subtraction occurs depends on the length of the bent path. Since the warm air layer is always either growing (sun up) or shrinking (sun down) this path length is always changing. For UHF the path length need change by only ten inches to turn addition into subtraction. If both path signals are about the same strength, your DTV channel will dropout momentarily. If you see two dropouts that were N minutes apart then you will probably continue to see dropouts every N minutes. This is fading. There is no cure for it, but a stronger antenna will make it less likely.

The above is but one scenario for fading. There are many variations on this depending on the terrain.

Field strength meters

Sometimes readers ask where they can get a portable signal strength meter, thinking this will allow them to make objective studies of the signal. A signal strength meter is not as useful as you might think. The standard meter works well for analog stations. But for DTV, a strong meter reading is no guarantee that you have found a good reception spot if certain types of multi-path are present.

The ultimate arbiter of where the signal is good for your receiver is the receiver itself. This author recommends that the signal strength readout provided by the receiver be used to search for the best antenna spots. The quality of the result justifies finding ways around the two problems you will discover when you try to do this. The first problem is that your HDTV is probably not on the roof, thus you can’t see the signal readout. The second problem is the receiver’s meter probably reads zero when it can’t demodulate the signal. This second problem is nasty for people in fringe areas. There is portable signal strength meters made for DTV, but they are terribly expensive.

FM receiving antennas

The FM radio band is 88-108 MHz, a 20 MHz band adjacent to channel 6. Some VHF TV antennas work well over this band, but others don’t. There is no easy way to tell if a given TV antenna will work well for FM. To receive far away FM stations, you should probably get an FM Yagi.

Folded dipole

The terminal impedance of a folded dipole is four times that of a simple dipole of about the same dimensions. Other ratios are possible by using two rods of differing diameters, or by adding a third rod. The impedance of a freestanding half-wave folded dipole is 300 ohms, but the presence of nearby elements will usually lower this considerably.