Fig, 6, Variations in both resistance and reactance along transmission line with SWR of 2,0 over any half-wave interval are shown here. Source of line is at left, andl load is toward right. At 0 and Y* wavelength points, resistance is lowest and reactance is zero. At % wavelength points, resistance is highest but reactance is also zero here. All other points have intermediate resistance, but also reactance. Patterns are similar for other vatues of SWR but variations are more extreme when SWR is higher. With extremely high SWR, resistance approaches infinite values, midpoint of the wire as well.
A half-wave length of wire is two quarter-wave lengths back to back, But a quarter-wave length of transmission line open-circuited at one end will appear to be a short circuit at the other end. This would indicate that a half-wave length of wire open at each end should have an apparent short circuit at its midpoint—and except for the fact that the wire does have some "losses" (the radiated energy is lost so far as the standing wave upon the wire is concerned), it well might! The impedance is very low at the midpoint, at any rate. This low impedance provides a feedpoint for the antenna which matches a low-impedance feedline, and is known as a "current-fed" system.
The low-impedance points produced by quarter-wave transformation of the open circuits at the antenna ends will coincide only when the operating frequency is such as to make the antenna an exact half-wave long, electrically, If operating frequency departs even slightly from this one single spot, the two low-impedance points will not coincide, although the impedance at the center will still be low over an appreciable band of frequencies.
As we saw in Fig. 6, when even a moderate SWR exists upon a transmission line the impedance is pure resistance only at exact quarter-wave intervals along the line. At all other points reactance is present also. An antenna, being a pair of quarter-wave transformers, operates with a very high SWR upon its structure, and ihe variations shown in Fig, 6, are exaggerated still more in this case. This means that if operating frequency is not precisely that for which the antenna was cut, the antenna cannot be a pure resistance to its feedline. Again, reactance will remain low ever an appreciable spread of frequency. Bandwidth depends upon the particular antenna design; some have wider usable bands than others.
Outside this usable band of feedpoint impedances, the mismatch between antenna and feedline is so great that more power is reflected than is radiated. We say that the antenna in such a situation "refuses to load,"*
For any single length of wire fed at its midpoint, there is a single lowest frequency
(*Ed. note: A purist will quarrel with this terminology. An antenna does not "load/' Itpresentsa load to the transmitter, The transmitter will refuse to load into a mismatch. The antenna will refuse to radiateJ)
at which it can operate in this manner; that^s the frequency at which the wire is electrically one half-wave long. If operation at twice this frequency is attempted, then each side of the antenna acts as a half-wave repeater rather than a quarter-wave transformer and it will refuse to load.
As we go up in frequency to three times the original value, though, we reach a point at which each half is three quarler-waves long, and the wire again acts as a pair of transformers. The loading conditions of the first frequency are essentially repeated here, and the antenna radiates nicely.
At four times the original frequency, we have repeaters again instead of transformers, In fact, at any even harmonic of the first or "design" frequency the halves will act as repeaters and a minimum of energy will be accepted. However, at any odd harmonic of the design frequency the halves will act as transformers and operation will be similar to that at the design frequency. The radiation pattern will, of course, be different at each frequency because of phasing interaction— but the antenna will accept and radiate the energy from the feediine.
The antenna we've been examining is, of course, the familiar half-wave dipole which appears in many variations such as the folded dipole, the inverted vee, etc. I he same principles apply to all; at the design frequency and odd harmonics operation is excellent, and at even harmonics essentially no energy is accepted or radiated. Such an antenna is said to reject even-harmonics. If cut for operation at 7 MHz, it can also be used on 21 MHz.
Current feed is not the only possible way to feed an antenna. It could be fed at a high voltage point instead, with a transmission line of sufficiently high impedance. This would change things, since the high voltage point is at one end of the line. At the second harmonic, now, the line is a full wave long-and the antenna is still matched. At the third, the length is one and a half waves, and the matched condition persists. An end-fed half-wave, then, will accept all harmonics either-odd or even. Often called a ^Zepp" antenna (such antennas were used on early Zeppelin airships), this type of antenna is popular with operators who like to use all bands and have only limited space in which to erect antennas.
Similar operation can be achieved with current feed by connecting several antennas in parallel and feeding them all at once fron>
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