Multi-Band Portable Vertical Antenna (HF + VHF) Komunica HF-PRO-2-PLUS-T
A typical centre-fed λ/2 dipole antenna has an impedance of Zd = (73+j42.5)Ω. To cancel the inductive reactance, the length of the dipole needs to be reduced. It will also affect the resistive impedance but the effect is not significant as the reactance curve is much steeper in comparison with the resistive impedance in this region. When only one arm of the dipole antenna is placed perpendicularl to the ground plane, a quarter-wavelength (monopole) vertical antenna is made. In an ideal case, the impedance of the monopole antenna can be calculated as Zv = 0.5*Zd and approximately 36Ω can be expected at resonance. When the size of the λ/4 vertical antenna needs to be reduced, there are two main techniques based on inductive and capacitive loading which can compensate for the low radiation resistance and high capacitive reactance. These two techniques can also be combined. The negative effect of shortening is reducing the efficiency of the antenna as well as narrow bandwidth due to the high Q factor. Even though the loading coil placed at the base of the antenna is not the best option in terms of efficiency, it is a very convenient place for tuning. When the length of the whip is constant, for multi-band operation the inductance of the loading coil needs to be changed appropriately.
I have already presented this homemade antenna where the inductance can be changed in steps. Then the fine-tuning was performed by changing the length of the top telescopic section of the antenna. This configuration works well but in many cases is not practical especially when the antenna is placed on the roof of a car, so the antenna must be dismounted when the band needs to be changed or when the change from SSB to CW segment of a band is required. Also, the efficiency is given by the length of the antenna, so the best choice is to keep the antenna at its maximum length. Therefore, I was looking for other options and mechanical realisations and I found an antenna (Komunica HF-PRO-2-PLUS-T) with a continuously tunable coil and a telescopic whip. The antenna came with the main tunable coil, an extending coil for 80m band and a telescopic whip. The length of the antenna is only 30cm when disassembled and 2.7m when extended. So far I am very happy with it. I am not sure if the antenna is water-resistant. I think it is better to avoid such weather conditions when possible. The antenna can be tuned only by shifting the coil which makes the whole tuning process significantly easier and faster.