Practical_Antenna_Handbook_0071639586

C h a p t e r 1 3 : C u b i c a l Q u a d s a n d D e l t a L o o p s 327
especially in the case of multielement delta loops, where the elements are often suspended
from a long boom fixed-mounted near the top of the tallest tower in an antenna
installation.
Make no mistake—because of its triangular shape and acute interior angles, the
delta loop is a compromise compared to the square quad loop. Modeling the free-space
difference between a square loop and an equilateral triangle indicates about 0.25 dB
difference in the gain of the main lobe (in favor of the square loop) when the length of
each loop is adjusted for a purely resistive feedpoint impedance at the design frequency.
Nonetheless, the delta loop can do a passable job at providing low-angle gain on the
lower HF bands.
Diamond Loop
The diamond loop consists of a square loop rotated around its broadside axis by 45 degrees.
When fed at a top or bottom corner, as shown in Fig. 13.9, it delivers 1.5 dB more
forward gain in free space than a delta loop designed with the same rules. The extra
gain is the direct result of the greater spacing and better physical layout of the two inverted
vees that form the array. However, that’s not really a fair comparison, since the
height of the diamond loop is twice that of the delta loop for the same frequency. Thus,
the diamond loop is not usually a candidate for the lower HF bands, but single-element
and multielement versions might warrant consideration as fixed arrays at 20 m and
above if the user has a single tall support or can stretch a catenary line between two
such supports.
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Figure 13.9 Horizontally polarized diamond loop.