For portable antennas, it is possible to choose a different mechanical design than for permanently installed antennas. The focus is on requirements such as low weight, easy disassembly, and compactness.
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TIPS for Portable Antennas
If certain parts of the antenna can be disassembled, wing nuts or plastic-head nuts are more suitable than standard nuts. Plastic-head screws are also a good choice. The advantage is tool-free assembly, and they are less likely to get lost.
Vlado OK1VPZ filled the antenna box on his antennas with PUR foam. This fills the space and prevents moisture from entering critical areas (joints, matching circuits made of coaxial cables, and connector connections).
Choosing a Yagi Antenna for Portable Use
YU7EF 6-element antenna
I use a YU7EF Yagi with six elements. The advantage is that it is a very well-designed antenna from an electrical standpoint, and given its length, it can be transported fully assembled on the roof of a car (length 244 cm).
The disadvantage, however, is lower gain (11.96 dBi) and a problem with lack of symmetry. This is partially addressed by a 3.5-turn attenuator on the coaxial feedline.
Gain can be increased by combining antennas into arrays. Two antennas provide a theoretical gain increase of 3 decibels, but require a more complex mounting, phase alignment, or a taller mast. A taller mast with a heavier load is more difficult to erect under temporary conditions.
That is why I opted for an antenna with a longer boom. I chose a DK7ZB Yagi antenna with a 28-ohm radiator. I had some materials left over from a previous project, and it turned out that the box containing the radiator included exactly this matching circuit. The gain of the 8-element DK7ZB is approximately 14.2 dBi.
Building a DK7ZB Yagi Antenna from Scraps of Old Antennas
The first step was the radiator box. This required replacing the connector from an SO-239 to an N-connector, replacing the center insulator, reconnecting the matching circuits made of parallel sections of 75-ohm coaxial cable, and adjusting the length of the radiator.
The N-connector is electrically connected to the Yagi antenna’s boom
The antenna boom can be disassembled into three sections with a maximum length of approximately 150 centimeters. This length is intentional. This size fits into a car trunk with the seats folded down. The advantage is that there is no need to remove any antenna elements during transport. A 15 x 15 millimeter duralumin boom was used, which is too weak for such an antenna without additional reinforcement. However, it is lightweight, and I already had antenna element mounts designed specifically for a boom of this size. A tension cable was added for reinforcement.
Measurement of the DK7ZB 8-element Yagi antenna
The mounts are from old antennas. They allow elements with a diameter of 6 millimeters to be installed simply by sliding them on. To ensure the elements “fit” well, I ground a recess into the boom at the location of each element.
Measuring the DK7ZB Yagi Antenna
Of course, I couldn’t resist testing the finished antenna, so I measured it with a JTGP-1033 D6 with a bridge. It appears that I managed to build the antenna with complete precision:
Measured SWR curve of the DK7ZB 8-element Yagi antenna
Summary of portable antenna construction
Since I had all the materials on hand, the cost of the antenna was €0. However, the estimated cost would be approximately €50: duralumin, electrical junction box, N-connector, PUR foam, screws, nuts, washers, element mounts, tension cable.
Construction took about 5 hours. But a more skilled builder with a better-equipped workshop could have built the antenna even faster.
It remains to be seen how the antenna will perform in practical use, since the higher gain is, of course, also due to the fact that the antenna’s radiation pattern is narrower, and the antenna will need to be aimed more precisely.
