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one direction. Other responses produce a fairly full response broadside to the line of the antenna. Such an antenna pattern might be useful if one wanted to concentrate radiation in an easterly or westerly direction (producing a good null in the opposite direction) and still have a reasonable amount of north-south response. When the "short" side is reduced to either 7/16 or 3/8 A, primarily broadside radiation takes place. Such a response would be ideal for the fellow who moans that he can't work anyone in a direction broadside to the line of direction of his lo ng-wire antenna. The patterns shown are only exactly true for a frequency or band where the antenna lengths are the electrical lengths shown. They do change with frequency. However, as a general approximation, they still will retain the same general shapes when, for instance, the 3A antenna is operated on a frequency such that its electrical length is I V,A. As the total electrical length of the antenna is made shorter, varying the feed point placement has less effect upon the horizontal pattern. It probably is not worthwhile to experiment with asymmetrical feed to vary the antenna pattern when the antenna electrical length is less than IA. Feed Point Impedance As the feed point is varied, the impedance presented will also vary. The range of variation depends upon such factors as the antenna element/wavelength diameter ratio and can go from 50 ohms to a few thousand ohms with thin-wire antennas. One could simply feed the antenna with a resonant line such as 450 ohm open wire or twinlead and employ an antenna coupler at the transmitter end to derive a low impedance, non-reactive load for the transmitter. A transmatch coupler will easily handle the range of impedances encountered, for instance, and provides a handy means for multiband coupling to the antenna. For those interested in single band operation and/or using a non-resonant feedline, the antenna impedance that must be matched can be calculated without too much complication. To a close degree, the feed point impedance of an asymmetrically fed antenna is one-half the combined impedance of two symmetrically fed antennas whose half length is equal, respectively, to the length of each side of the asymmetrical antenna. Some examples should make this calculation clear. Suppose that an asymmetrically fed antenna is used having a "short" side of ~A and 64 a "long" side of 3~A. The feed point impedance will be one-half the added feed point impedances of a symmetrical ~A and 6 ~A (I3/2A) antenna. The center impedance of a I>A antenna is about 70 ohms and that of an odd multiple I>A is the same, or also 70 ohms for the 13/2A antenna. The combined impedance is 140 ohms, half of which is simply 70 ohms-the same as for an ordinary I>A dipole. In this special case, an ordinary 50 or 70 ohm coaxial line can be used to feed the antenna directly, with the addition of a I : I balun, if desired, to preserve the feed-point balance. As another example, slightly more complex, consider the situation if an asymmetrical antenna were used having a "short" side of 3/8A and a "long" side of 3 1/8A . To find the feed-point impedance, it is first necessary to determine the feed-point impedance of a symmetrical 3/4A and 6~A antenna. These can be determined from the graphs shown in Fig. 3. The curve is used which corresponds to the wire diameter/wavelength ratio of the actual asymmetrical antenna. In this case, a A/IOOO ratio is assumed, which is slightly large for a wire antenna operated on ten meters, for instance, but close enough to produce meaningful results. Then, the center impedance of the 3/4A symmetrical antenna is found to be about 500 ohms resistive and +500 ohms reactive (500 + j500). The center impedance of the symmetrical 6 ~A antenna cannot be read directly, but since the impedance values repeat every wavelength, the impedance value of the 6~A antenna is approximately the same as a 1~ or2 ~A antenna. From the graphs, the impedance value is ISO ohms resistive and -500 ohms reactive. The center impedance of the asymmetrical antenna is: Z 1>(500+j500+150-j500)=1'(650)=325 ohms. In this case, the reactive portions of the two symmetrical antennas are equal in magnitude but opposite in sign, so they simply cancel. The resultant resistive 325 ohm feedpoint impedance would offer a good match to a 300 ohm twinlead feedline, or the antenna could be fed via a 4 : I balun with 75 ohm line, thus taking care ofthe impedance matching problem and preserving the feed-point balance simultaneously. The choice of other "short" and "long" lengths for the asymmetrical antenna may not, of course, offer such simple feed-point impedances. In such a case, one can either build a matching network to allow coupling to the antenna by a non-resonant feedline or accept some slight change in the radiation 73 MAGAZINE
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WORLD'S LARGEST INDEPENDENT HAM MAG
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