An OCFD Explained | Description | On the Amateur Bands | For the SWL | Conclusions | AN-SOF Simulation Software

I am very pleased to be able to bring to you a review of the Vine AS-OCF-404-HP off-centre Fed Dipole (OCFD) for HF/6/4m (Fig. 1).  

This aerial is primarily intended for use on the amateur bands. It covers the 40, 20, 15, 10, 6, and 4m bands (without a matching unit), and the 30, 17, and 12m bands with one.

It uses a Balun (balancing transformer, an acronym for balanced-unbalanced) that matches not only the main element but also incorporates a 75Ω port to match a 4m element. Capable of handling up to 1kW of Peak Envelope Power PEP), it is 69 ft (21.03 m) long.

With these specifications, it should be compatible with most HF transceivers.

Fig.1: The Vine Antennas Vine 404 Aerial.

An OCFD Explained

A conventional dipole is fed at the centre; when using coaxial cable, it is essentially a single band aerial,  although it can present a good match on its third harmonic. A 1:1 Balun is generally used at the feed point to prevent radiation from coax cable if this is used as a feeder. It is generally accepted that the OCFD works by taking a half wavelength of wire and moving the feed-point away from the centre to a position where the impedance is equal, or nearly equal, on a multiple of harmonics of the fundamental frequency.
This point is found at one-third of the length of the wire.

There are variations to the design but if we make an OCFD for the 40m band using 66ft of wire and put the feed point at 33% (25ft) from one end, then we will get harmonic points on the 20, 15, 10, and 6m amateur bands (a classical ‘Windom’ type). 

The impedance at this 1/3 point will be around 200/300Ω. Rather than bringing an open-wire feeder back to a balanced AMU, a 4:1 Balun is used to give a match to 50Ω coaxial cable. Quite often, 10ft of 50Ω coax is connected to the BALUN with the other end connected to a 1:1 common-mode choke. This length of cable hangs down and provides some vertical radiation.

This arrangement, referred to as the ‘Carolina Windom’, was developed by three radio amateurs, Wilkie WY4R, Lambert WA4LVB, and Wright W4UEB. Their design improved on the original single-feed-line Windom, bringing the feed arrangements up to modern thinking to provide an aerial that worked well for local and long-distance contacts (Fig. 2).  

Fig.2: Simulated SWR plot of a 40m OCFD over ‘perfect’ ground (bottom); ‘Carolina Windom’ (middle); conventional Windom (top). 

Description

The 404 (Fig. 1) arrived from Lamco in a cardboard box and with a two-page leaflet, containing a description of the aerial and assembly instructions. The aerial consists of a weather-proof plastic box, which houses the BALUN, and the two main wire elements, pre-fitted to the box. They have large plastic insulators connected to each end. The strain on these wires is taken by carabiner clips attached to an eye bolt on the top of the box.

Moreover, there are two pieces of what looks like hard-drawn copper for the 4m elements pre-fitted to the box, and also some plastic spacers used to suspend the 4m element from the main wires.

I was pleased to note that the main element wires were plastic-coated and not made from open Flex Weave, which I am not a great fan of. The RF connection was by an SO239 plug.

Overall, the construction quality of the 404 was good, and the assembly was straightforward enough.
I laid out the two main wires to their full length and clipped the 4m element to the spacers – and that was that. It just remained to get the whole up in the air. This was easy enough as the 404 is surprisingly light. I attached an SO239/BNC adaptor to match the feeder I was going to use. This was a run of about 25ft of RG58, fitted with BNC plugs on each end. This cable dropped vertically for about 12ft. 

I noted that the 404 has the BALUN placed at 12ft from the end, probably to improve matching on 15m.

On the Amateur Bands

Unfortunately, the months of January and February are not the best of time to experiment with aerials, especially as we had near flood conditions one minute and snow the next. Due to this, I was not able to get the 404 up at anything like a decent height. I was limited to a lightweight pole of some 20ft, from which to suspend the BALUN. The short element of the 404 was fitted to a hook I have on a soffit running under the roof.

The BALUN was supported on the pole, and the long element was run across the garden to a tree branch about 9ft off the ground. To compound problems, my local noise levels since lock-down began have risen to atrocious levels. 

Normally, this is not an issue, as I have free access to land that I can use as a test range. However, due to the lock-down at this time, I knew the landowner had restricted access to his land. Therefore, all my tests were carried out at home this time, which was a challenge. I initially made a Voltage Standing Wave Ratio (VSWR) sweep of the 404.  The results can be seen in Fig. 3. 

Fig. 3: Analyser sweeps of the 404 at 20ft (selection). 

Resonant points on HF can be seen at 6.7, 14.3, 20.5, and 28.5 MHz.

Due to the height of the aerial at the time of measurement, I concluded that this is OK; the resonant frequency should rise as the height of the aerial is increased.

Resonances at 52 and 72MHz were slightly high in frequency but still well below the accepted 2:1 SWR limit of most gear. As is stated, a tuner would be needed on 30, 17, and 12 meters.

My only radio with a built-in antenna system tuning unit is my venerable FT990. I found that it had no difficulty tuning the 404 on any of the HF bands. It even matched the 404 on 160 and 80m. 

I also used an FT857D with a Yaesu FC707 ASTU and experienced no difficulty in matching the 404 on 80m and above. I made a fair number of contacts with Europe and western Russia on 20m, using between 50 to 100W on both my FT990 and FT857D. Out of necessity, the stations worked were strong signals – otherwise, I would never have heard them above the noise. 

Therefore, during the daytime, the 404 gave excellent results with UK and EU contacts on 40m, and I managed to work 9K60OD Kuwait, which was especially pleasing. I even had a couple of ‘Inter-G’ contacts (working stations within the British Isles) on 80m. With a matching unit, the 404 could also be pressed into service on this band.

However, I made no contacts above 20m. If there were any stations on, I could not hear them under the noise. I put out a few calls on 6m FM but received no reply. My only radio for 4m is a reprogrammed ex PMR box fitted to my Land Rover Defender which is out of action at the moment. 

I suffered some RF feedback on 40 and 20m, and when using powers above 50W. This announced itself by a ‘clicking’ noise in a pair of PC speakers that were turned off, and also by my unmodulated signal floating out from the FT990’s SP6 external speaker.

The same was true with the 857 because this was also connected to the SP6. I added a common mode choke to the feeder, a couple of feet before it entered the FC707 tuner, and this cured the problem. The choke was wound using five turns of RG174 over an FT140-43 core. It was mounted in a waterproof box (Fig. 5). 

Fig. 5: A common-mode choke.

For the SWL

The 404 (Fig. 6) is designed to have its harmonic spots on the amateur bands, and this is where it will work best. To use the 404 on my transmitting set-up, I had to remove my doublet to get it out of the way in case they should inter-react. Because of this, I was unable, at present, to make a direct one-for-one comparison.
I did, however, have a short SWL end-fed set up from a design under evaluation, and I was able to undertake some reception comparisons with this, although by no means in terms of ‘apples-for-apples’.
I found that, on the HF bands, the 404 doubles up as a very good receiving aerial, returning consistently better results than the short end-fed it was being compared to.

Fig. 6: The 404 re-suspended without support, for photos (selection). 

Below around 2Mhz results were similar. On the MW band signals on the end-fed were 2 to 4 S-points better. On HF the overall noise levels on the 404 were a good couple of S-points lower. 

Conclusions

Despite the restricted conditions during my testing, the 404 came out of it very well. It ideally needs to be mounted higher than I was able to manage. The aerial is well constructed. I found it simple to assemble and put up, taking about 30 minutes from start to operating.

Being lightweight, it does not need excessively elaborate support and, if needed to do so, it would be possible to ‘bend’ it into a small garden, with minor performance penalties to be excepted.

To take the strain off of the dipole elements – especially if a heavy feeder is used (RG213) – the BALUN case needs to be suspended from an insulated pole; insulated because the feeder can form part of the radiating mechanism. As it stood, the SWR returned from the 404 here was not good enough for use without a tuner on many of the bands. However, I do not regard this as an issue with the design. 

At the height I had erected the aerial, the proximity of the short end to my house, and the fact that it sloped towards the ground at the far end, would de-tune most aerials, being more noticeable the lower the frequency in use. This will always be the case, especially with ‘pre tuned’ HF aerials in different environments. 

Users may find that a choke fitted on the feeder is beneficial, for the prevention of RF on the coax shield getting back into the shack. I was unable to test with more than 100W but see no reason to doubt the claimed 1kW PEP capacity of the aerial.

In short, the 404 enabled me to make many contacts. For licensed amateurs, it forms a useful multi-band radiator on all bands from 40 to 4m capable of providing long-distance, as well as local contacts.
It also doubles up as a respectable HF aerial for the SWL whose main interest is in the HF amateur bands, but who also needs something for general-purpose listening as well.

The Vine Antennas AS-OCF-404-HP presently costs £159.95.

My sincere thanks to the team at Lamco for the loan of the review model.

AN-SOF Simulation Software

Just as I was finishing this month’s copy, I had a notification of yet another update for the AN-SOF aerial simulation software suite. This latest update adds significant functionality because DXF files can now be imported into the simulator. This format (DXF) is used by packages such as AutoCad and other CAD software. Therefore, this should make it easier to add complex structures into AN-SOF.

I have not yet had a chance to fully try the new version, but it should make adding structures like buildings, masts and vehicles much easier (Fig. 7). 

Fig. 7: AN-SOF sample DXF file.

In this context, the improved computation of antennas over real ground, and an addition of a radial wire ground screen builder, are some of the features, which have been promised for the immediate future.