Off-Centre Fed Dipole for 80m to 6m
Based on a Carolina Windom off-centre fed dipole, with a third short (3.9m) length of wire, in parallel with the longer element, for 17m operation. Capacity hats, just 100mm diameter have been added to the ends of the antenna bringing the SWR oan all bands down compared to the 'staright' configuration.. The most significant effect of this has been to extend 80m coverage to the whole band (previously just the top 200KHz) and to reduce the SWR on 80m from 8:1 to 5:1.
It is 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 less than 5:1 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).
A Kenwood TS-570D has been added to the line-up and will 'tune' (match) using the internal auto ATU on all bands (including 80m).
Naturally, performance on 80m is limited by the relatively small length of the antenna, but on all other bands performance leaves littl to be desired.
Construction is using nylon cable ties and 18mm waterpipe as spacers. 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 capacity hats (still prototype versions) are fabricated from the ends of a large 'tin can' (canned grapefruit was the best choice) with a S-shaped hook placed in the centre.
The separation between the long element and the 17m section is 150mm to manage the interaction between the two elements. An antenna analyser is indispensible for setting up.
Testing and use:
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 RG8 into RG213 feedline. RG8 was used for the above ground section only. On-air tests show this fully cured the problem. An air core choke was found to affect the tuning on some bands and it was therefore removed. The antenna is mounted at 14m above ground, oriented N-S with a slight downwards slope towards the South.
The antenna hasa been subjectively compared to several RX antennas: a 'straight' dipole for 160m , a 21m inverted 'L' and a horizontal loop for 40m. The comparison antennas were used side by side with the OCFD using a Kenwood R5000 receiver and a TS-570D transceiver. Although direct comparisons are not really possible, all the indications are that the OCFD has no significant weaknesses in terms of directionality and sensitivity. Web-wisdom suggests that the Caronina Windom is a noisy antenna: but compared to the larger RX antennas no significant noise was noticed.
'The bottom line': This 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. Also, for GB0STG (St Georege's Day) the antenna was mounted at 80ft above ground (from a church tower) and could be 'tuned' (matched) on 160m using the TS-570D's internal ATU. However, it was not used or tested on that band.