The quarter wave vertical is an effective and simple antenna for the 40m band.It can be realized as a ground plane - vertical with slanted raised radials, or with ground radials. I had the opportunity to compare both versions in previous CQWW contests and there was no noticeable difference.
Mechanical construction with telescopic poles from OK5IM
Since the height of the vertical part is approximately 10m, there is an option to use telescopic poles from Jirka OK5IM for the second version of the antenna. They have a height of 12.5m. We will not use the last two thinnest sections. We attach the radiator to the telescopic poles with plastic 'clamps' or tape.
From a mechanical point of view, it is advisable to make a waterproof box - a transition between the coaxial cable and the antenna. The radials are attached using clamps. An example of such a box can be seen in the photo.
Antenna Dimensions
The theoretical length of the radiator is 10.56m (for 7.1MHz). In practice, the conductor is several tens of centimeters shorter.This is due to:
- the radials are under the box on the ground, i.e., the total length of the radiator consists of the radiator wire + connection in the box + distance of the box from the ground with laid or buried radials.
- the influence of the surrounding environment in which the antenna is located.
- the shortening factor of the insulated conductor.
- wire thickness
These influences cause that the actual length of the radiator is about 9.7m. However, it is true that we try to place the box as low to the ground as possible.
To achieve satisfactory SWR for connection to a coaxial cable without an adaptation circuit (in which some loss always occurs willy-nilly), we choose only a smaller number of radials (8 to 12 depending on soil quality). A good length is 10m, if there is no space, we try to stretch at least a few longer radials which we supplement with a larger number of shorter radials.
Antenna efficiency and its improvement
The theoretical efficiency of the antenna is around 70%. If we increased the number of radials above one hundred (!) and used a matching circuit, we would achieve an efficiency of around 85%. We achieve improved radiation by improving the reflective surface around the antenna – by burying the radials. However, we do not connect them to the antenna!
Suppression of return currents
To limit return currents flowing along the surface of the shielding of the coaxial cable it is advisable to wind a few turns of coaxial cable as an inductor at the feed point of the antenna on a plastic bottle or other non-conductive material.
Portable vs home version of the antenna
In the portable version, one person can set up the antenna in a few minutes. For permanent installation, we can make the radiator self-supporting in the form of a truss structure placed on insulators. An example is shown in the photo.
Truss structure of the quarter wave antenna for 7MHz
Increasing gain and obtaining directional effects is possible by stacking, for example, four antennas. However, it is a more complex system to construct – four identical antennas, phase lines, relays, a switch with a direction indicator, power supply etc.
The quarter wave vertical is a simple and good antenna – give it a try!