Off-Centre Fed Dipole for 80m to 6m
Updated 3 October 2013:
Based on a Carolina Windom off-centre fed dipole, with a third short (3.9m) length of wire, parallel with the longer element, for 17m operation.
It is now fed with RG58 into a 200W G Whip 4:1 balun at the feedpoint. I have used clip on ferrites to control common mode currents (feedback) because they do not add any inductance/loading that occurs when air-core choke coils are used.
The antenna SWR is less than 3:1 on 40, 20, 17, 15, 12, 10 and 6m bands. The SWR is 8:1 on the 80m band. This means that the antenna was easily 'tuned (ie matched) with my a new LDG AT-897Plus automatic ATU (and an Yaesu FC-30 ATU) - except for 80m where I need to use a Tokyo Hi Power HG-400L.
Construction is using nylon cable ties and 18mm waterpipe. The water pipe is drilled through to take the antenna wire which is held in place inside the pipe with a cable tie which permits a degree of slip and slide for adjustment of the spacer's.
The separation between the long element and the 17m section needs to be at least 150mm to manage interaction between the two elements. An antenna analyser is indispensible for setting up.
Performance: initially, there was some residual RF feedback which was cured with a 'clump' of eight ferrites at 3m below the balun and several other clumps of two or three along the RG58. On-air tests show this has fully cured the problem. An air core choke was found to affect the tuning on some bands and it was therefore removed.
Testing: The antenna has been used regularly with very good results. Overall it seems 'well-behaved' with no particular issues or problems.
Thanks to Andy M0FOS for his advice on this project.
OCFD Antenna Mk II
The 3-leg OCFD above has now been copied for use as a portable antenna. Performance is almost identical, so there are no surprises there. This shows (at least) that the design is pretty 'solid' and reproducible.