Double Delta, was not simply "pulled out of the hat" but is based upon two well established and proven antenna forms. Figure 1 shows these forms. The antenna forms of Fig. 1 (A) may not appear immediately familiar, but it is really just half a terminated rhombic antenna which is vertically oriented, and is frequently referred to as a Delta antenna, it radiates mainly at high radiation angles, and is particularly useful for short-skip work on the lower-frequency bands. In its proper rhombic form and to provide some directivity and gain, the dimensions of the antenna have to be fairly large — at least a 60 ft center height on 40 meters, for instance. However, because the terminating resistor makes the antenna non-resonant, it will accept power over a broad range of frequencies even when its physical dimensions are contracted. The terminating resistor value must only be chosen for given physical dimensions of the antenna. The radiation pattern of the antenna will change as its dimensions are reduced to the point where a small loop configuration is reached. Generally, the vertical plane radiation partern becomes much broader, and for a given loop or delta size on a given frequency, the horizontal plane radiation may be mainly broadside or in line with the physical plane of the antenna. Such an antenna could be used alone with reduced dimensions, but one real disadvantage would be the radiation pattern change with frequency.
The antenna form of Fig. 1 (B) is that of a conventional turnstile antenna which consists of two half-wave folded dipoles phased so that an omnidirectional horizontal radiation pattern is produced. The necessary phasing is often done with a separate phasing line, but it can also be done by proper interconnection of the dipole feed terminals as shown. When done in this manner, the phasing will remain correct over whatever range the basic dipole antennas can be operated.
Figure 2 shows how the basic two foregoing antenna ideas are combined to form the Double Delta antenna. As can be seen, it
consists of two single Delta antennas oriented and phased like tlie dipole antennas in a turnstile configuration. Although the radiation pattern of the single Delta antenna will change with frequency (depending upon dimensions), the turnstile type phasing makes the Double Delta retain an essentially omnidirectional radiation pattern. The use of two Delta antennas further broadens the frequency bandwidth of the overall antenna system. Although a separate terminating resistor could be used in each Delta, the use of a single resistor was found to be sufficient.
No specific dimensions, even in terms of wavelength, was mentioned in the foregoing discussion because, in fact, no hard and fast dimensions can be given. The following section of this article describes a Double Delta antenna which I constructed for use on 80 and 40 meters, but the dimensions shown are by no means the only possible ones. Although studies could not be made of a large variety of antenna dimensions, it would appear that good efficiency will be achieved as Song as the total lineal length of each Delta element (sides plus bottom section) is made about Vi wave on the lowest frequency band to be used. The antenna will work with shorter dimensions, but the efficiency will suffer. Probably an absolute lower limit would be to make the lineal length i/8 wave at the lowest operating frequency. Even with these restrictions, the antenna height required is only a fraction of that of other designs. The ratio of total base length to height (not side length) at the center should not be made greater than a 1:1 ratio. This great latitude in choosing the antenna dimensions is partly possible because the terminating resistor can be "tailored" to suit a given impedance transmission Sine, and one is not dependent solely upon the antenna configuration to establish the feed point impedance.
Practical "Double Delta" Example
Figure 3 shows the dimensions of one "Double Delta" design which I tried for 80 and 40 meter operation. The total lineai length of each Delta was made about 'A wave on 80 meters. The base length to height ratio was chosen such that the height was about
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