ANTENTOP- 01- 2018 # 022

Antenna Installations: Fact and Fiction

Antenna Installations: Fact and Fiction

Considering that antennas are daily familiars in the life of the average TV technician, it is quite surprising that they are also among the least understood items with which he deals. Some common misconceptions and half truths concerning them have come to the author's attention in the experience he has had with antenna installations.

By Jack Beever, Jerrold Electronics Corp

Credit Line: Radio and TV News, # 4 1959, pp.: 42, 43, 124.

These misunderstandings can cause real trouble for the installer faced with decisions a knotty problem involving signal interception. With respect to the common beliefs listed here, concerning how many do you know the full truth?

 Note from IG: I have been working with antennas installation for 20 years and I agreed with the author. I have to add that in the present times there are more myths then truth...

 FICTION: The front to back ratio of an antenna is the ratio between the sensitivity to a signal from a station in front of the antenna and the sensitivity to a signal from a station to the rear of the antenna.

 FACT: This half-truth can be a damaging one if it is taken literally. Strictly speaking, the front to back ratio is based upon reception of a single signal originated from one point. The antenna is first oriented so that it is picking up maximum signal from this source while facing it. Then the antenna is rotated through exactly a half turn- 180 degrees- and pickup of the same signal is measured. The front- to- back ratio is the relationship between these two readings.

 Antenna patterns are such that it is possible for a relatively small shift in orientation-say a shift of 10 degrees- to produce a very great apparent change in this ratio, often by as much as 5 to 20 times. These changes, depending on the particular radiation pattern and the installation, may be more important than an arbitrary ratio.

Radio and TV News, # 4 1959

Front Cover

Another point to remember is that stated front to back ratios must be considered at specific frequencies, since antenna radiation patterns invariably change with frequency.

 The technician can be check this ratio himself if the antenna is mounted on a rotator, or can be conveniently rotated on its mast, and a steady signal is available. A word of caution: this method can only be depended on when no significant reflections are present, such as are caused by large metallic structures at some distance or small ones very close.

FICTION: A YAGI antenna has many times the gain of a tuned dipole, such as pair of "rabbit- ears."

FACT: If you call three times "many," then it's true. The best YAGI antennas show gains no greater than 12 dB over tuned dipoles, which is four times the voltage. The average cut-to-channel YAGI will have from 8 to 10 dB gain which is from 2.5 to 3.16 times. The big advantages is the often because the YAGI gets put in a better place- up on the mast.

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