Friday, 17 December 2010

A centre-fed "co-axial" dipole for 4m

by Tony Hawker G4CJZ
This design has the following advantages:
Low-cost components
Easy to set-up - nothing to adjust
No metalwork required, and only four soldered joints!

The complete dipole in situ, built by M3DCR. Obviously not a 'wireless'station!

In contrast to the end-fed designs, which can be difficult to set up because of their narrow bandwidth, this alternative uses a low-impedance centre-feed, and hence offers a broader bandwidth.
In this design, the radiating half-wave section is formed from the centre conductor of a piece of co-axial cable, which is fed via two sections of its outer braiding which have been left in place.

The complete dipole

The feed is essentially a delta-match, which makes use of the intrinsic capacitance of the two co-axial stubs.
At the ends of these delta-match sections, the braiding is soldered to the centre conductor.

 centre feed

 end of delta match

The whole antenna is built inside lengths of 20mm plastic conduit, with the feed housed in an "inspection box" T-piece. This offers sufficient rigidity and weather-proofing, once the ends of the conduit are sealed with stoppers. A third section of conduit houses the feeder.

mechanical construction

The flat VSWR response is characteristic of a low-impedance centre-feed.

VSWR curve

The antenna may be clamped at the centre, either in vertical omni, or horizontal bi-directional configuration.

detail of clamp

Original design by Tony G4CJZ, pictures by John GW4TQD.

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