Pamantasan ng Lungsod ng Maynila College of Engineering and Technology Electronics Engineering Department
A PROPOSAL FOR
Antenna Design and Construction
Submitted by:
Espedillon, Marigold May L. Grageda, Glenda Mae V. Herrera, Erika Claudine G. Lalo, Erika Gayle E. BSECE 5-1
Vertical Polarized Quad Loop Antenna Loop antennas have been popular for more than 30 years. Resonant loop antennas have a circumference of 1 λ. The exact shape of the loop is not particularly important. In free space, the loop with the highest gain, however, is the loop with the shape that encloses the largest area for a given circumference. This is a circular loop, which is difficult to construct. Second best is the square loop (quad), and in third place comes the equilateral triangle (delta) loop. The quad antenna was originally designed in the late 1940's. Since then it has been the subject of controversy whether it performs better than a dipole. The debate continues but after some years several facts have become apparent. It was found out that the quad has a slight gain of approximately 2 dB over a dipole. It is also said to cover a wider area in the vertical plane and exhibits broadband characteristics. The square quad can be fed for either horizontal or vertical polarization merely by placing the feed point at the center of a horizontal arm or at the center of a vertical arm. At the higher frequencies in the HF range, where the quads are typically half to several wavelengths high, quad loops are usually fed to produce horizontal polarization, although there is no specific reason for this except maybe from a mechanical standpoint. Polarization by itself is of little importance at HF because it becomes random after ionospheric reflection.
Here, we propose a vertically polarized quad loop antenna. A vertically polarized quad loop can be considered as two shortened top- loaded vertical dipoles, spaced λ/4 apart. Broadside radiation from the horizontal elements of the quad is cancelled, because of the opposition of currents in the vertical legs. The wave angle in the broadside direction will be essentially the same as for either of the vertical members. The resulting radiation angle will depend on the quality of the ground up to several wavelengths away from the antenna, as is the case with all vertically polarized antennas. The quality of the reflecting ground will also influence the gain of the vertically polarized loop to a great extent. The quality of the ground is as important as it is for any other vertical antenna, meaning that vertically polarized loops close to the ground will not work well over poor soil.
The figure shows both the azimuth and elevation radiation patterns of a vertically
wave angle would be closer to 30º. The horizontal directivity is rather poor, and amounts to approximately 3.3 dB of side rejection at any wave angle. The quad antenna that we propose to design is specifically to operate in the frequency band of 140-150 MHz. It displays a bi-directional radiation pattern with maximum radiation in the direction perpendicular to the plane of the loop. By carefully following the instruction for constructing this antenna we should be able to get an SW R response of less than 1.5:1 over the entire band.
Proposed Design of Quad Loop Antenna
Construction First, prepare the plastic mount with dimensions shown in Figure 1
Next, prepare the metallic mast adaptor. As shown in the following illustration, the distance between one pair of 3/16" holes at the extreme ends is equal to the distance between the threaded ends of the U-bolt used (see Figure 2).
Figure 2
Mast adaptor dimensions.
Join the two plates together using four 3/16" x 1" stove bolts made of rust resistant materials (e.g. brass or stainless steel). Do not forget to include a lockwasher in each bolt (see Figure 3).
Bend the aluminum tube into a square loop with equal sides using a suitable tube bender (see Figure 4). Cut away the excess tube.
Figure 4 Forming the tube into a square loop
Flatten a small portion at both ends of the tube and drill a hole (1/8" diameter) in each flattened end (see Figure 5).
Figure 5
Flattening and drilling the ends of the tube.
Drill additional holes (1/8" diameter) in the tube as shown in the following illustration (Figure 6). The holes must be drilled through and through. Be careful in drilling the holes to avoid deforming the tube.
Figure 6
Drilling additional holes in the tube.
Insert two stove bolts (1/8" x 3/8") through the holes at both ends of the loop and attach the necessary hardware as shown in Figure 7.
Attach the prepared loop into the plastic mounting plate by bolting it through the 1/8" diameter holes as shown in the following illustration. Use 1/8" x 1" stove bolts (brass or stainless steel). See Figure 8.
Figure 8 Fixing the loop on the plastic mount.
Prepare one end of the coax cable by separating the inner conductor from the copper braid. Solder the two conductors to the two eye terminals in the loop. The braid is costumarily connected to the lower terminal (see Figure 9).
Figure 9 Connecting the coaxial cable to the loop element.
Clamp the coaxial cable to the plastic mounting plate (see Figure 10).
