Inverted V Antenna
This project is a half wavelength, inverted V, fan dipole antenna for the 40m and 20m Amateur Radio bands. For portable operation we needed a quick set-up antenna suited to Near Vertical Incidence Skywave (NVIS) propagation. We will be using it for our portable work and our National Parks DXpedition to VK7: 40m for local parks chasers and for running SARCNET on 7105kHz; 20m for DX to European stations interested in VK7 WWFF parks. It had to be a well-matched, resonant antenna on both bands requiring no antenna tuner. A single mast would make it easy to erect.
We designed the antenna using EZNEC Pro/2+ v 7.0, by Roy Lewallan W7EL. The diagonal coordinates were caculated from the desired wire length using an Excel spreadsheet. An iterative procedure was used, changing the wire lengths and re-entering the coordinates, as required to achieve resonance at the desired frequencies. The chosen height was 12m and the chosen base was 24m for each band.
EZNEC Wire Coordinates
EZNEC Antenna View
We assembled the antenna using 1.5 square millimetre PVC insulated copper wire in an inverted V shape, supported by an egg insulator attached to the top of our 12m carbon fibre telecopic mast. The two dipoles were oriented at right-angles, running them down as guys from the top of the mast, then extending them to the ground using nylon cord, secured to tent pegs. The wire was joined to the nylon cord using adhesive heatshrink tubing - Extreme heat helps fuse the ends together. The two dipoles were simply connected in parallel to 17m of RG-58 coax, with no balun required for this well-balanced antenna.
Inverted V 40/20m Fan Dipole Antenna
We used a NanoVNA Vector Network Analyser and the NanoVNA Server application to measure the impedance, at 7.1MHz and 14.2MHz, at the end of the feedline. After assembly, testing and tuning the 40m elements, we compared the NanoVNA and EZNEC results. We calibrated out the small difference simply by adjusting the permittivity of the isulated wire elements in EZNEC. We changed it from 2.0 to 2.1 to achieve very close results for both the 40m centre-frequency and feed-point impedance. Using this correction, the calculated results for the 20m elements were then spot-on. Our design yielded a good match on both bands with 1.028:1 SWR at 7.118MHz and 1.434:1 SWR at 14.236MHz. The antenna can be deplyed in 15 minutes and the four guys provided excellent stability even in high winds.
NanoVNA 40m SWR Plot
NanoVNA 20m SWR Plot
Inverted V Initial Log Book