Multiband Quads
THE quad is reputedly a one-band antenna and, whereas dipole-type beamelements are commonly made to work on several wavebands, the normal “multiband” version of the quad uses separate loops for each band. This seems to be due to a misunderstanding about the properties of loops.
Some years ago the author made a rough theoretical estimate of the gain of a 14 mc quad and got a figure of 6.5 db. For the same beam energized at 21 mc the calculated gain was also about 6.5 db, and this led to the design of a reversible two-band beam with loops 17' square, open wire stubs 36' long, and matching on both bands, without retuning, into 100 yards of untuned line. This beam, centered on VK, has given consistently good results over a long period and reports, relative to other phone stations, have been, if anything, better on 21 mc than on 14 mc. Later developments have included several kinds of three-band arrays. The two band system described above turns into a bi-square at 28 mc; the mounting of two quads at right angles on the same pole or tree gives all around coverage by beam switching; and the use of a colinear pair of quads gives increased gain plus electrical beam rotation. We also discovered that loops can be distorted into a wide variety of shapes, such as triangular, without noticeably affecting their radiating properties. Comparisons have been made between three modes of operation, namely with a parasitic reflector, a parasitic director, and both elements driven.
As multiband beam-elements, loops have two main electrical advantages over dipoles. Firstly the radiation resistance is higher which means there is less objection to the use of long resonant feeders or stubs, and secondly if the feeder or stub is made the right length as in fig. 1 its lower end remains a point of low voltage on all bands thereby removing another objection to the use of resonant feeders. This also allows, if required, the connection with acceptable matching on each band, of a long non-resonant feeder. One of the author's arrangements uses another property of loops to obtain tri-band matching without the use of long resonant lines. This enables a 21 mc loop to be resonated at 14 mc with a minimum of loading, thus keeping losses and the inevitable restriction of bandwidth to a minimum.
Facts About Loops
Figure 1 shows a loop plus a tuning stub, which together resonate at frequencies in the region of 7, 14, 21 and 28 mc with a voltage node at each end of the system. The resonant frequencies depend, to some extent, on how much of the wire is in the loop and how much in the stub, and are not in exact harmonic ratio, but this can be ignored for the moment.
Figure 2 shows the current distributions round the loop when this contains 1/2, 3/4, 1, 1 1/2 or 2 wavelengths of wire. For a loop 17' square this corresponds to excitation at 7, 10.5, 14, 21 and 28 mc respectively.
