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Many moons (1970) ago I had a 10m Vertical Dipole Array comprised of three (3) dipoles arrange 120 ° appart. I made the antenna switchable to have some FB and a bit of gain. Since most SSB/CW/Digital contacts on VHF/UHF are done using horizontal polarisation I thought that this design philosophy could also be applied for VHF as an omnidirectional horizontal antenna. The HODA comprises of three (3) equal length dipoles, each positioned 120 ° from the next on a single mast to create a near circular radiation pattern.
The main issue will be the impedance of the antenna. However, instead of having each the dipoles connected in parallel let's connect them in serial. This should bring the impedance up to a more manageable value. Since a dipole at resonance has a Z of about 73 Ω which would make the Zserial 219 Ω. I could now use a 4:1 Balun. However, I decided to adjust the length of the elements to get an approximate 57 Ω impedance. Using NEC2D I modeled a dipole at Z=55.9 -j40.3 (10mm AL pipe). Connecting the dipoles serially makes Zserial about 171 Ω. And now, instead of a 4:1 Balun I can use a 1/4 λ transformer. Again a bit of arithmetic and transformer should have a Z of about 92.5 Ω. Nice, as RG-62 has a Z of 93 Ω (and I have quite a bit of that).
Ztr=√(Zdpa*Zo)
Ztr=√(171*50)
Ztr=92.46Ω
To cut the the coax to length we need to know the Velocity Factor (VF) of the cable, which according to the manufacturer is 0.8. But to be more precise I used my Antenna Analysr to sweep an open stub until X=0. So the measured VF at 145MHz was 0.831. And of course the same principle has been applied for the 1/2 λ RG58 feeders to the centre of the mast.
Below are a couple of photos from the prototype as build and used in the ACT (VK1).
The new unit uses a black delrin centre which a friend of mine manufactured on his lathe.
Performance is not to bad, my best condx was VK3EJ (SK) in Cobram from the ACT (VK1) with my FT-817 (approx. 8W) on 144.200 SSB (> 300km AE)!
I might add a more detailed build process together in the future.
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