03 April 2010

Shortening a VHF beam

In my last excursion in August 2009 I damaged the boom of my homebrew 5 elements VHF beam. The boom broke at the 2nd director hole, at 1.475m from the reflector. What now?

Months have passed and I have come across the broken boom again. I had already planned a longer beam for my portable VHF contests (7 elements, 3m long) so I will not replace the boom.

Instead I simulated what would happen if I shortened the 5el to 4el with MMANA-GAL. The result is encouraging because without changing element lengths I can:
  • install the 3rd director as 2nd director at 1.465m from the reflector
  • have about 7.7dBi gain and 15dB F/B (not the best)
  • keep a good match to the original 28 ohm
The original 2nd director remains unused, or could be used as 1st director to see if in the real world it provides any improvement (in SWR, gain, F/B, whatever).

Not bad! Well, this operation moves the center of gravity a bit behind, so I have to re-position the boom-to-mast adapter or add a little weight in front of the antenna.

I hope I can get it up and running for next Tuesday's monthly VHF activity day.

Note: the original 5 el. is DK7ZB's VHF Yagi 28 ohm with 2m boom.