The Folded Monopole
In Figure 1, the basic configuration can be seen. This antenna was set up and then run for 1.9 MHz in the middle of the 160 meter band. The feed point can be at the bottom of either conductor as long as the other one is connecteded to a good ground system. Height was set at 61.5 feet (18.696m) and the spacing at 5 feet (1.52m). Initially, the antenna was simulated with #12 wire, and then later on with a 1.5 inch diameter (3.81cm) copper pipe. There was not much change in anything except impedance and resonance. The wire antenna showed an impedance of 143 + j2890 ohms and the pipe antenna impedance was 54.4 + j1277 ohms.
Figure 2 is the vertical radiation pattern and the angle of radiation is 24 degrees. The horizontal pattern is a perfect circle for all practical purposes, and will give good results for DX. This is a simple antenna, but it does require a good ground to be made at the end which is not fed.
Figure 3 shows the current distribution along the antenna. It is important to note this antenna is not an antenna constructed by gamma matching a tower and beam, but a separate antenna, made for a particular band. It can be scaled down to other bands and will enable operation to take place where any other type of antenna could not be used. This is not a version of the hairpin monopole, which was capable being used on more than one band. This is a monoband antenna, and the band width is limited to the band for which it is designed.
A place to start the design of a folded monopole is with the formula L = 234/F. This will give a 1/4 wavelength in feet. Modeling a 40 meter version with the dimensions of 16 feet tall and 2 feet spacing (4.864m and .608m) gives an impedance of 47.8 + j976 ohms. This was with the 1.5 inch pipe (3.81cm). If you set the spacing at .08 times the height of the antenna, then you will be in good shape as the VSWR will be manageable with a simple matching circuit of your choice. Basically, this makes for another type of stealth antenna design. A word of caution though, if you put one of these up, be sure pets, people or other creatures cannot come into contact with the antenna when you are on the air.
Originally posted on the AntennaX Online Magazine by Richard Morrow, K5CNF
Last Updated : 26th April 2024