As you can see from the photos above, a test build of the antenna is very simple and easy to construct. The screw terminals on the coax feed point are approximately 3mm apart.
When testing, I calibrated the antenna analyzer with the coax connected to help cancel out any effects of the coax, the coax is approximately 8 feet long. OSL calibration with the EU1KY analyzer consists of 3 measurements on the end of the coax.
- 0 Ohms
- 50 ohms
- Shorted
Once this is complete we can now scan our frequency ranges and see the results. I first scanned the antenna from 100MHz to 200MHz and then from 400MHz to 500MHz.
I cut the VHF elements with an approximate length for resonance in around 120MHz to 130MHz, 22″, to ensure they would be longer than required for 145MHz, this allowed me to trim the elements later for exact placement in the Amateur band.
The UHF, elements were also longer at 7″.
Impedance with the dipole is a function of the distance from the metal tower or antenna mast. As you move the antenna closer to the metal structure the impedance drops until theoretically it would be 0 ohms at 0 distance.
I started with the antenna clamped to the tower at a distance of about 16″, this gave me an impedance of 66 Ohms. I moved the antenna closer to the tower until I had an impedance of close to 50ohms on each band.
The results are below and as you can see it works very well for the resonant frequencies of both the VHF and the UHF elements.
The 3/8″ tubing is large enough to provide enough bandwidth to cover both the 144MHz and 44MHz Amateur bands with a low SWR.
Now on to permanent construction and tuning.