HF Helical Antennas: Theory and Practice

To avoid a high SWR it should be installed a 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.

Operating Range of the Window Helical Antenna

Window helical antenna may be designed for only one operating ham band. Let's check possible frequencies of the operation of the window helical antennas installed in the concrete house at the window with dimension 140 x 210- cm.

Asymmetrical vertical helical antenna at the window with dimension 140 x 210- cm (Figure 20) has length 125- cm. So the antenna may be done for ranges from 10 meter (shortened factor equal 2) to 50 meter (shortened factor equal 10). The range is included amateurs bands 10, 12, 15, 17, 20, 30 and 40 meter.

Dipole vertical helical antenna at the window with dimension 140 x 210- cm (Figure 23) will operate in the ranges from 5 meter (shortened factor equal 2) to 25 meter (shortened factor equal 10). The range is included amateurs bands 6, 12, 15, 17 and 20 meter.

Asymmetrical horizontal helical antenna at the window with dimension 140 x 210- cm (Figure 21) has length 195- cm. So the antenna may be done for ranges from 15 meter (shortened factor equal 2) to 78 meter (shortened factor equal 10). The range is included amateurs bands 10, 12, 15, 17, 20, 30, 40 and 60 meter.

Dipole horizontal helical antenna at the window with dimension 140 x 210- cm (Figure 24) will operate in the ranges from 5 meter (shortened factor equal 2) to 30 meter (shortened factor equal 10). The range is included amateurs bands 6, 12, 15, 17, 20 and 30 meter.

Asymmetrical diagonal helical antenna at the window with dimension 140 x 210- cm (Figure 21) has length 235- cm. So the antenna may be done for ranges from 18 meter (shortened Factor equal 2) to 94 meter (shortened factor equal 10). The range is included amateurs bands 10, 12, 15, 17, 20, 30, 40, 60, and 80 meter.

Dipole diagonal helical antenna at the window with dimension 140 x 210- cm (Figure 24) will operate in the ranges from 5 meter (shortened factor equal 2) to 30 meter (shortened factor equal 10). The range is included amateurs bands 6, 12, 15, 17, 20, 30 and 40 meter.

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

If house is built of radio-transparent stuff the asymmetrical 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 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 to the operation of the helical antenna.

For example, take a look to installation of an asymmetrical helical antenna in 5 meter length installed under the ceiling. Figure 26 shows the antenna. Antenna with the length may be installed at lots houses. The antenna may be designed for range from 40 meter (shortening factor equal 2) to 200 meter (shortening factor equal 10). The range is included amateurs bands 40, 60, 80 and 160 meter.

Ground of the ceiling helical antenna should be made similar to the ground of the asymmetrical window helical antenna, described in the part Practical installation of the asymmetrical helical antenna in the window.

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. Lots of the shortened asymmetrical helical antennas that were tested by me in the window opening had 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 coaxial cable.

The high input impedance of the shortened helical antenna installed in the window opening could be explained by losses of the RF energy by nearest objects and the losses in the ground system of the antenna.

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