28 August 2007

Telescopic VHF/UHF antenna

This is a 30 minutes project that I did in the pre-Internet era. I wanted an antenna for my VHF handheld that would outperform the rubber duckie.

Easy: take something metallic some 50cm long that is self-supporting vertical.

I had a telescopic antenna whose length reached 50cm. Its base diameter fit loosely into a PL-259, so I screwed a thick wire to the antenna base and soldered the other side to the connector pin. I kept the antenna away from the outer ring, and vertical, with some pieces of rubber (probably RG58 black coating) pressed in with a screwdriver.

When closed the antenna is about 12cm long, including the PL259. Of course the size of any reproduction will depend on the telescopic antenna used.

(pictures not in scale)




Connector detail, FWIW

So I have 12 to 50 cm of radiator, which equates to a 1/4 lambda between 140 and 625 MHz. So it covers both VHF and UHF (and 220 MHz) HAM bands with a simple movement, and anything in between or above. It is not a multiband antenna: you need to retune it for a band change.

This antenna has now turned into a VHF/UHF facility that permanently sits in my car, together with the magmount base. The fully extended whip did not mind travelling at 130 km/h.

As a bonus you can use a similar antenna as a signal pickup on a monitor receiver when testing homebrew transmitters.