With my new DJ-G7E, I can now operate on 23cm FM much more easily than with the transverter, so I'm planning to do some mobile tests in 2010 - but my first problem was coming up with a suitable mobile aerial.
I've adapted an old mag-mount base by fitting it with an N-type socket and (a minimum length of) RG-223 cable, and my first test was using this with my "Little Big Wheel" horizontal omni.
This gave reasonable results with distant stations using beams when I was driving around the hills, but I wanted to try something with little more gain, and vertical polarisation (as used by the GB3SE repeater down in Stoke-on-Trent).
Researching into collinear stacks, I found an article Development of VHF Collinear Antennas, Matching and
Feeding Techniques by DU1ANV, so I set about building one for 23cm, using some old RG402 semi-rigid cable.
The quarter-wave sections (top radiator and lower sleeve) were cut to 55mm, and the half-wave sections to 81mm (accounting for the 0.7 velocity factor of the semi-rigid cable).
The completed antenna is very fragile, so I've fitted it inside a "B&Q radome" (20mm plastic conduit). Three foam-rubber collars are used as buffers to stop it rattling around iside the tube. And to my delight, the knurled collar of my N-type adaptor is a perfect fit inside the 20mm coupler, making a very neat base to the antenna.
So now I'm ready to go mobile...
And following my success, John MW1FGQ has made his own version of the aerial, but using one of the upper sections of a fibre-glass "roach-pole" for most of his radome:
And John has also made a biquad antenna to mount on the car (but not when actually driving!) for extra gain and directivity when operating portable from the hills:
