A centre-fed "co-axial" dipole for 4m
by Tony Hawker G4CJZ
This design has the following advantages:
Easy to set-up - nothing to adjust
No metalwork required, and only four soldered joints!
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 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.
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.
The flat VSWR response is characteristic of a low-impedance centre-feed.
The antenna may be clamped at the centre, either in vertical omni, or horizontal bi-directional configuration.
Original design by Tony G4CJZ, pictures by John GW4TQD, model built by Dave M3DCR, HTML by Ross G0WJR.