Hi all,
The best of ham radio is that you can build and experiment.
My current portable yagi is a dualband single fed (VHF element) home built, from DK7ZB designs, 4+5 elements version. VHF has 9.25dB gain 1.03SWR, UHF 8.67dB 1.12SWR. It works like a charm.
Now that I'm on the final stages of condo permits for a tower - although it's a tower on top of the building there are still space limitations - my goal is to build a crossed polarity set of my portable antenna. Something similar to what Gulf Alpha had, but instead of 4 feed points, one for each band/polarisation, I'll have only two, combined into one impedance matched feed line to the shack.
Due to the required materials for the feed points and the elements positions I came up with 2 versions where the vertical antenna is ahead of the horizontal by 3 inches and 16 inches. My understanding is that circular polarisation would require a specific position for the vertical antenna plus the proper cable circuit for it. I'm aware that RHCP would give me a stable signal reception, rather than my setup even tilted 45 degrees instead of plain horizontal/vertical. I saw PU3GUO report on his VHF only antenna while receiving ANDE.
I've ran the model in 4nec2 for the two options, 145MHz and 435MHz, and using 5W. Here are my findings.
3 inches separation:
VHF Both antennas 1.03SWR Combined gain 9.25dB
UHF Both antennas 1.12SWR Combined gain 8.67dB
16 inches separation:
VHF Both antennas 1.03SWR Combined gain 9.24dB
UHF Horizontal antenna 1.12SWR; Vertical antenna 1.49SWR Combined gain 8.27dB
My conclusion is that advancing the vertical antenna by so many inches will result in interference from the horizontal one in such way that the gain is lower and SWR increases. Gain in UHF is lower, expected as same happens in my current portable antenna, and that's because it's being fed in the VHF element.
Has anyone built something similar that can share any comments? Would there be any other configuration possible for a single boom?
Thanks in advance.
73s de Pedro CU2ZG, HM77