The antenna elements are aluminium tubes of 8 and 10 mm diameter. Thicker sections are attached to a wooden boom with steel screws. The thinner tubes slide in and out as necessary to provide different total element lengths (fixed with hose clamps).
4nec2 indicated that for 10 mm elements the antenna will have an impedance of 49 ohm (purely resistive) at 144.2 MHz. While making the antenna I kept the original elements size, even though 50% length is now 8 instead of 10 mm diameter and the hose clamps have their capacitance. The antenna was located in the living room and fed with about 10 meters of RG-58 (approximately 1.8 dB loss). The first impedance measurement revealed a resonance at 144.8 MHz and about 40 ohms impedance.
After extending the fed element by 1 cm (5 mm on each side) the resonance frequency came down to almost 144.3 MHz:
I have rerun 4nec2, this time with true element diameters. The general conclusion of the analysis was that the elements had to be extended by about 10 mm. After I applied length corrections to all elements, the impedance curves look like this:
There is a resonance at 144.4 MHz, but the impedance at this region is about 40 ohms.
The best match is offered just above 146 MHz, with resistance of about 50 ohms and reactance very close to zero. I will have to make the fed element a bit longer to see if I can get the best match region down by 2 MHz or so.
