06 July 2007

From 10m to 6m

Remember the old CB antenna that doesn't show a proper SWR anymore? Well, a fellow HAM on usenet suggested that the antenna whip would need a re-tune since the antenna was originally installed on a roof-clip rather than a magmount base. The latter introduces extra capacitance to ground, so a retune whould be needed.

Anyway, CB antennas are still available and I might buy a new one, maybe next Spring 2008. So I went for a 10-to-6m conversion that involved reducing the number of turns on the base loading coil.

I first tried shortening extra turns with a piece of wire when the antenna was installed on the car. I identified a point at about 4.5 turns from the top coil end that gives good SWR on 50-51 MHz. Then I reduced the number of turns and re-soldered the wire in place.

When does it resonate now? Will find out in 4 hours at lunch break ;-)

Post lunch break. The car was parked next to a tree with low branches, the antenna was surrounded by them. Lowest SWR (1.3:1) at 49.5 MHz, in the CW/SSB 6m sub-band SWR is below 1.7:1. Might improve in open space or shortening the whip a little bit. Will not touch the coil anymore. I'm satisfied so far.