mode do not cancel. This means that the transmission line itself will radiate (becoming effectively part of the antenna)and also that the two poles of the dipole antenna will also be driven asymmetrically, violating the premise of theiranalysis to this point and changing the performance in some way.A balance mismatch on either end of the transmission line is likely to lead to excitation of an unbalanced mode.The unbalanced current component in this case may be supplied by the earth ground, in which case the ground wireitself can radiate. The unbalanced current component is frequently unintentionally rectified, leading to the generationof harmonic spurs. For these reasons and the ones mentioned in the last paragraph, unbalanced modes are undesirableand should be suppressed. This can be accomplished with a balanced/unbalanced transformer or a ‘balun’.Baluns come in a number of configurations. Figure 2.14 shows two simple examples. In the first, a transformeris used to match an unbalanced transmitter to a balanced transmission line.In the second, an RF low-pass filter(represented by a ferrite ring) is added where the antenna meets the transmission line. The filter suppresses any net RFcurrent on the transmission line associated with the unbalanced mode without affecting the balanced mode.It is important not to confuse the concept of the balanced or unbalanced nature of an antenna with its polarity. Allantennas have a polarity, which refers to the sense or phase angle of the current driven on the antenna in relationshipto the sense or phase angle of the current at the transmitter or some other reference point. Antenna phasing is a crucialconcept at the root of antenna arrays, which are covered in the next chapter. Baluns do not affect or have anything todo with polarity or phasing.2.6References2.7Problems48
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