HF Helical Antennas: Theory and Practice

If the input impedance is different at least to 20 percent from the wave impedance of the feeding coaxial cable it should be installed simple L- matching device between the antenna terminals and coaxial cable. Good choice if the electrical length of the coaxial cable is multiplied by lambda/2.

Then it should be measured antenna resonance frequency at the end of the coaxial cable. Most common method is by measuring the SWR at the coaxial. In some cases helical antenna (or simple L- matching device between the antenna terminals and coaxial cable) may need some additional tuning. Free program MMANA (References 10) has option for calculation of the L- matching circuit.

Below it is examples of the practical installation of helical antennas in different conditions.

Practical installation of the Asymmetrical Helical Antenna in the Window

Very often in concrete house the window is only one place where the helical antenna may be installed. For example let's take a look for installation of the asymmetrical helical antenna at the window with sizes 140 x 210- cm. Figure 20 shows installation of the asymmetrical vertical helical antenna at the window with sizes 140 x 210- cm.

Figure 21 shows installation of the asymmetrical horizontal helical antenna at the window with sizes 140 x 210- cm. Figure 22 shows installation of the asymmetrical diagonal helical antenna at the window with sizes 140 x 210- cm.

Distance between the top of the asymmetrical helical antenna and the side of the window frame should be at least 10- 15- cm. In this case the influence of the window frame and the concrete building environment to the top of the helical antenna would be insignificant. Feeding terminals of the helical antenna may be placed near sill (Figure 20, 22) or near window frame (Figure 21). RF choke should be installed on the coaxial cable near feeding terminals. It may be 10- 20 ferrite rings on the coaxial or just 5- 20 turns of the coaxial cable. Diameter of turns may be 10- 20- cm. RF choke especially is needed for helical antennas with high (more the 5) shortened factor. Try use coaxial cable with electrical length multiplied to lambda/2.

The ground system of an asymmetrical shortened helical antenna is important part for proper work of the antenna. Just follow recommendation below to make the effective one.

Figure 20 Installation of the Asymmetrical Vertical Helical Antenna at the Window with Sizes 140 x 210- cm

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