HF Helical Antennas: Theory and Practice

Do not use house equipment- metal heating and water pipes for grounding system of the antenna. It will cause RFI to different houses electronic equipment and bring noise to the receiver connected with the helical antenna.

For the ground system of the window helical antenna use 2- 3 counterpoises with length equal to lambda/4 for the each operating band of the antenna. The counterpoises must have strong plastic insulation. The ends of the counterpoises must be insulated. Counterpoises may be installed initially on the wall of the house and then go to into opposite sides. Counterpoises must be hidden from access of human and pet.

Device Artificial Ground is very welcome at the grounding system. Thick wire (it may be length of a coaxial cable) in 2- 5 meter length (find optimal length by experiment) should be connected to the device. Artificial Ground allows do optimal tuning the grounding system.

When the helical antenna is tuned to the resonance frequency it should be measured the antenna input impedance. RF-bridge, MFJ-259 or any 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 (compare to the theoretical one) of the shortened helical antenna installed at the window opening could be explained by losses of the RF energy by nearest objects and the losses in the grounding system of the antenna.

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 the helical antenna (or simple L- matching device between the antenna terminals and coaxial cable) may be need some additional tuning. Free program MMANA (References 10) has option for calculation of the L- matching circuit.

Practical Installation of the Dipole Helical Antenna in the Window

Dipole helical antenna may be installed in concrete house at (in) the window. Let's take a look for installation of the dipole helical antenna at the window with dimension 140 x 210- cm. Figure 23 shows vertical installation of the dipole helical antenna at the window with dimension 140 x 210- cm. Figure 24 shows horizontal installation of the dipole helical antenna at the window with dimension 140 x 210- cm. Figure 25 shows diagonal installation of the helical dipole antenna at the window with dimension 140 x 210- cm.

Distance between the tops of the dipole helical antenna and sides of the window frame should be at least 10- 15- cm. In this case influence of the window frame and concrete building to the top of the helical antenna would be insignificant.

Figure 23 Vertical Installation of the Dipole Helical Antenna at the Window with Dimension 140 x 210- cm

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