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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 26 Asymmetrical Helical Antenna Installed Under the Ceiling

 

 

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 the 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.

 

Practical Installation of Dipole Helical Antenna in House Made of Radio- Transparent Stuff

 

If house is built of radio-transparent stuff the dipole helical antenna may be installed under the ceiling, parallel to the wall of the house, or diagonally on the ceiling or wall of the house. The length of the dipole helical antenna will be limited only by the size of the room. Check the wall or ceiling on to electrical wires and metal tubes. The electrical wires and metal tubes (water and ventilation) may cause serious influence on the operation of the helical antenna.

For example take a look to installation of a dipole helical antenna in 5 meter length installed under the ceiling. Figure 27 shows the antenna. Antenna with the length may be installed at lots houses. The antenna may be designed for range from 20 meter (shortening factor equal 2) to 100 meter (Shortening Factor equal 10). The range is included amateurs bands 20, 30, 60 and 80 meter.

Coaxial cable feeding the antenna is going perpendicular to the antenna as much distance as possible. RF choke should be installed on the coaxial cable near feeding terminals. It may be 10- 20 ferrite rings on the coaxial. Inductor RF- choke as well may be used. This RF choke may have 5- 20 turns of the coaxial cable. Diameter of turns may be 10- 20- cm. Balance/Unbalance transformer installed at feeding terminals of the ceiling helical antenna as well may improve the antenna operation.

 

When the helical antenna is tuned to the resonance frequency it should be measured the antenna input impedance. RF- bridge, MFJ-259 or anyone other device may do the job.

 

 

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January 30, 2021 14:53

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