ANTENTOP- 01- 2018 # 022

Antenna Installations: Fact and Fiction 

FICTION: Impedance matching between antenna and load is not too important-you can't see the difference.

FACT: if you have more signals than you need, you may not see the effects of mismatch in terms of signal strength, because the set has A.G.C. You may, however, see the secondary effects of mismatch, since a mismatch always causes reflections in the line. These reflections can produce line ghosts- close-in- ghosts that may be so near the primary image as to look smear.

Also, the tuner response will change when it sees a mismatch at the input causing degraded picture resolution on monochrome and kill all kinds of difficulties in color reception.

FICTION: It takes an antenna with a very narrow forward lobe to eliminate a ghost.

FACT: Not so. The ghost elimination depends on the antenna pattern having a sharp null, which may be oriented to ghost source, without moving the pickup lobe too far from the best reception angle. At the high band, for the example, an ordinary broadband conical antenna may do better than a YAGI type, because the conical has three major pickup lobes at the high band. (See radiation patterns, Figure 1) Also there are several null points from which to choose. This permits considerable flexibility in orienting the antenna so that it may select the desired signal while rejecting the reflected one.

Figure 1 Antenna Pattern for YAGI and Conical TV Antenna

If a solid sheet of metal is used as the screen, the edges of the sheet radiate, and the pattern of radiation is practically circular, so the same end is practically circular, so the same end result ensues. This is not mere theory. We have seen it tried more than once.

FICTION: An antenna may be shielded from interference with a screen made of metal mesh or sheet metal.

 FACT: It won't work. Take the metal mesh- for instance, poultry netting- and place it between a radiation source and an antenna. The incoming radiation impinges on the screen and starts currents circulating in the latter. Since the screen is not loaded- no current is being taken from it- almost all the energy it intercepts is re-radiated- and it re- radiated on both sides of the screen. The idea of the screen is thus defeated.

 If a solid sheet of metal is used as the screen, the edges of the sheet radiate, and the pattern of radiation is practically circular, so the same end is practically circular, so the same end result ensues.

In one case a screen bigger than a billboard was erected to cut off co-channel interference. It actually seemed to make matters worse.

 FICTION: If you stack two antennas you'll get the twice the signal voltage of one.

 FACT: If you manage to attain a perfect job of stacking two antennas, you'll gain only a 40 per-cent increase in voltage. What you actually double when you add another antenna is signal power. Double power across a fixed impedance, in this case almost always 300 ohms, results in a voltage increase of 3 dB, which is about 40 per-cent. Since it is almost impossible to get full efficiency, you'll do well to get 30 per-cent in practice.

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