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HF Helical Antennas: Theory and Practice

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ANTENTOP- 01- 2020, # 024

HF Helical Antennas: Theory and Practice

 

 

 

Figure 27 Dipole Helical Antenna Installed Under the Ceiling

 

 

Ceiling helical antenna may have input impedance near 30- 60 Ohm, so, usual 50- Ohm coaxial cable may be connected to the antennas without any matching device between antenna and the cable. The high input impedance of the ceiling shortened helical antenna installed could be explained by losses of the RF energy by nearest objects.

 

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.

 

Form of the Helical Antenna

 

The ideal form for the helical antenna should be lightweight, mechanically strong, have low dielectric losses at working frequencies, and preferably have low cost.

Form for the HF shortened helical antenna, if it is possible, should have diameter of 5- 10- cm. Very often plastic water tubes are used for form of the helical antenna. The tube has almost all mentioned above points- the tubes are lightweight, mechanically strong, have low dielectric losses at working frequencies, and low cost.

 

Helical antenna with form on the water pipe may be installed at the place with help of synthetic rope. The rope going inside to the pipe, knotted at the ends and the rope fixed at the place of installation of the helical antenna. Figure 28 shows installation of the helical antenna with help of synthetic rope.

 

Shortened helical antenna with form on the water pipe may be hang up for one point if the vertical installation is required.

 

If the vertical helical antenna is small and form of the antenna has sufficient strength it, could be mounted on some heavy base and then, if it is required, fixed with guys from synthetic ropes. Figure 29 shows the vertical installation of the helical antenna.

 

 

 

 

Figure 28 shows Installation of the Helical Antenna with Help of Synthetic Rope

 

 

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January 30, 2021 14:55

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