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Antenna Fundamentals part. 2

Antenna Fundamentals part. 2


Antennas are connected to transmitters or receivers through transmission lines. Since the antenna impedance is not a constant function of frequency, it cannot be matched to the transmission line at all frequencies. When the antenna impedance does not match the impedance of the transmission line usually 50 Ω or 75 Ω, reflections are formed at the connection to the antenna.

Waves that come from the source are reflected back down the transmission line reducing the ability to transmit power.

The VSWR, voltage standing wave ratio, is a measure of the mismatch. VSWR is the ratio of the maximum voltage to minimum voltage on the transmission line.

With an impedance mismatch, the VSWR is greater than one, indicating the presence of reflections. As the impedance at the end of the transmission line becomes higher - approaching open circuit, the VSWR approaches infinity, indicating that all the power is reflected.

This situation is similar to the incidence of a light beam at an interface between two media, such as air and water, in which some light is reflected and some goes into the water. VSWR reduces the amount of power transmitted to the antenna or reduces the signal from the antenna when it is used to receive signals. The change in VSWR and the proportion reflected are shown in Figure 9a and 9b, respectively, for a 50 Ω system, in which the load resistance is varied.

Another problem with connecting to antennas is signal unbalance caused by a ground plane. Figure 10a shows a dipole antenna connected to a source through a shielded cable. The shield is connected to the ground plane. Parasitic capacitance between the antenna and the ground plane causes some current to flow through the ground plane rather than through the shield. When this occurs, the current on the antenna is unbalanced, and the antenna loses efficiency. To correct this imbalance, a device called a balun balanced to unbalanced is used. A simple type of balun is shown in Figure 10b.

Here, the balun is comprised of a ferrite cylinder bead placed over the coaxial cable. The ferrite increases the impedance only for the common mode current and has no effect on the normal differential mode current in the cable. Consequently, the current that causes the unbalance is reduced, improving the operation of the antenna. For receiving antennas, the incoming signal may induce current on the shield that causes the unbalance. The ferrite bead reduces the current on the shield.

Figure 7. a E and H fields are disrupted by perfect conductor, b tangential E and normal H must be zero at boundary, c image of charge or current in conductor to satisfy boundary conditions.

Figure 8. a Dipole parallel and b perpendicular to conductor and image and c reflection of wave from dipole explained by considering effect of image.

Antennas are used to receive very small signals. It is therefore often necessary to use an amplifier to in

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