Practical_Antenna_Handbook_0071639586

326 P a r t I V : D i r e c t i o n a l H i g h - F r e q u e n c y A n t e n n a A r r a y s
used on 80 m, but the boom length and minimum tower height requirements for that
band typically lead to greater mechanical complexity, such as the use of trusses to
strengthen the boom. Regardless of how it is supported, the two lower corners of the
delta loop itself are usually tied off with nonconducting cordage to nearby trees or
dedicated anchors.
Like the quad, the delta loop can be fed anywhere along the loop, with the ratio of
horizontally to vertically polarized radiation changing accordingly. Feeding at the top
or at the midpoint of the bottom wire results in a horizontally polarized loop. Moving
the feedpoint halfway (or l/4) along the total wire distance between those two points
results in a vertically polarized loop. The feedpoint for vertical polarization is therefore
l/4 down from the top on either side; if the loop is an equilateral triangle, that’s equivalent
to l/12 above either bottom corner, or ¼ the length of a side above one of the two
bottom corners, as shown in Fig. 13.8.
Assuming the delta loop is an equilateral triangle, its total height is approximately
l/(3.5), or 41 ft at 7 Mhz and 81 ft at 3.5 Mhz. For the loop to retain any semblance of its
inherent free-space broadside gain, the bottom should be no closer to the earth than,
say, l/8, so in practice the top support for a delta loop should be at least 70 ft high for
40 m and at least120 ft high for 80 m.
Like Yagis and cubical quads, most users of a two-element delta loop prefer the
parasitic element to be a reflector rather than a director. As with the quad, the reflector
loop should be about 3 or 4 percent longer than the resonant length for the desired center
frequency. At HF the earth will be close to the bottom of the loop, so experimentation
to find the exact length that works best likely will be required. Some users use relays in
protective enclosures at each loop to switch extra wire in and out of either or both of the
two elements as a way of reversing the pattern or of having optimum lengths at two
widely separated frequencies in the 3.5-MHz band.
Because one wavelength of wire is split among only three sides, instead of four,
typically the bottom leg of an HF delta loop will be closer to the ground than the bottom
leg of a quad for the same support height. However, in many cases, the delta loop is
favored because the user has a single support that is higher than any other support—
Tie-off
ropes
I
Boom
I
d
I
3d
4
Direction of
maximum radiation
Feedline d /4 =
12
d
Driven element I
I
Reflector
I
Tie-off ropes
Figure 13.8 Vertically polarized two-element delta loop.