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 very little 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.
'Explorer' Short Multiband 'End-Fed L'
Initially I wanted a simple multiband vertical antenna which could be mounted on a 9m glassfibre fishing pole, but although test figures were very good, it proved ineffective in the field. Various adaptations were made to this in my garden and all failed to perform satisfactorily.
The solution was to make a longer inverted 'L' shape antenna fed through a 9:1 un-un with a 6m counterpoise. The final working design (shown as a schematic, below) has proved quite satisfactory, with VSWR less than 3:1 on 40m. 20m. 17m, 15m, 12m and 10m. Performance is satisfactory as a portable, quick deployment antenna with a fairly small 'footprint'.
Final adjustment of the antenna, in the field, simply involves attaching or removing the erath.ground. On some bands the grounding is critical, but on others it is not a necessity.: having a good ground should be part of the setup plan. The height of the feedpoint (9:1 un-un) is not critical, but higher is better. The slope of the top section is best kept as level as possible. The counterpoise works well on the ground or less than 1m above ground. The SWR on all bands is low enough to permit matching with both a Yaesu FC-30 and a LDG AT-987Plus..
In some respects this is a short version of a W3EDP antenna which some people struggle to make workable due to high SWR and other oddities. So far, for me, it works well on 40m across all of the UK, and into most of Europe on the higher bands; however, lousy propagation in July 2014 interrupted field trials.
Field trials were done in August with the Inverted L mounted in the same orientation (north-south) as my OCFD, but at a lower height (only 8.5m at the top). So far, I tens to use it with electric fence posts holding the counterpoise about 0.5m above ground, but haven't seen significant differences if it is laid on the ground. Further experiments are needed to determine the extent to which the height of the counterpoise, above ground, afects performance.
Side by side readings with the OCFD show that this inverted L is often one S-point less sensitive - and sometimes two S-points. This may partly be due to the mixed polarization of this inverted L, but the comparison was not undertaken with the antennas at exactly the same heights, so this is probably also a factor. In some instances - that is, directions - the difference between the two antennas was less than 1 S-point, possibly due to the TX station's antenna polarization or other local factors.
Crucially, the loss of 1~2 S-points does have an impact on weak signals. The it is a different type of antenna to the OCFD: they are different antennas with different characteristics.
Changing the inverted L to a OCFD (6m+15m) configuration - and keeping the 9:1 un-un - at 8.5m above ground reduced the performance considerably. No further tests were done in that configuration because it was a big deviation from the principal requirement (single pole, small footprint with vertical component). By the way, the lengths of the vertical and horizontal/sloping portions is not especially critical, but the height of the end above ground does affect the performance somewhat.
The 'Explorer' is easy to set up and has reasonably decent performance for a 'rough-and-ready' antenna. However, it is sensitive to ground capacitance/coupling variations, namely the height of the counterpoise; also the height of the sloping part, etc. However, these are factors that may affect any portable antenna.
In reality, the use of the 9:1 un-un fed through an ATU is fundamentally no different to my 'normal' rapid-deployment portable set up. That comprises a long wire (15 or 21m) plus a long counterpoise and a SG231 Smart Tuner in inverted L configuration. Based only on received reports (that is, no comparative tests) the SG231 appears to be a better performer that the "Explorer" This suggests that another set of field trials are needed..
Side-note - the SG231+21m inverted L was used recently with the far end lashed to a large freight container (approx. 0.7m of rope). Received reports suggest that the massive capacitance at the end of the antenna was having a significant and beneficial effect on the antenna's performance. However, it may just have been a good day for 40m propagation.
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