The VK6LW BDB antenna
Want a wire antenna that has similar gain to a twoelement Yagi but in two directions simultaneously and fits into a footprint a half-wavelength square? Steve Ireland VK6VZ/G3ZZD says try a compact Bi-Directional Beam!
The antenna experts will tell you there is ‘no such a thing as a free lunch’ when it comes to building an antenna that will give you gain over a simple half-wave dipole.
Either you have to make the antenna much longer − such as the Extended Double Zepp (EDZ), a Vee Beam or Rhombic − or add a reflector and/or a director to the dipole. However, my good friend and well-known contester Kevin Smith
VK6LW (G4EHF), Fig. 1, has come up with a new antenna design that retains the bidirectional properties of a dipole, has the same length, but also gives you the gain of a 2-element Yagi − about 5dBd − in two directions.
Another nice feature is a 14MHz version of the VK6LW BDB antenna will fit into a 10m by 10m square! But, as the advertisements say, ‘there’s more’!
In comparison to the currently fashionable EDZ, the BDB offers a decibel or two extra gain, a broader pattern, single coaxial feed (with no matching stub required) using cheap RG6 UHF TV-type coaxial cable feeder and is less than half the overall length.
To conclude my sales pitch, VK6LW’s BDB comes as close to a free lunch (with dessert) in the antenna world as you’re likely to get!
How the BDB was Designed
Kevin VK6LW is arguably the best CW contester in Australia, consistently finishing as the #1 Australian station in the CQ WW, WPX and the Commonwealth Contest for over 30 years. We have been friends for a similar length of time. His success comes not only from being a world-class operator but also being expert at antenna design, using computer simulation to produce practical, highly effective antennas.
As the world contest scene has evolved to embrace computer-controlled stations, in particular Single Operator Two Radio (SO2R) ones, and computer-designed antennas, Kevin has embraced the bleeding (oops, leading) edge of both technologies.
One of the keys to success in international contesting is to have antennas optimised for each HF band and able to be rapidly switched between the long and short paths to the main population centres (i.e. Europe, North America and Japan/SE Asia).
As a result, Kevin has built several lowcost monoband wire antennas, which were sited away from his house, located on the top of a small hill but a major noise source.
Initially Kevin built Beverage-type antennas, which improved the signal-to-noise ratio markedly from his HF and 40m Yagi antennas mounted on two towers close by the house. However, as the signal levels on the HF bands continued to decline in line with solar activity, he switched to building remotely-sited wire antennas, which offered gain over a dipole and thus greater signal levels.
These were initially single-band EDZ antennas, superseded by two-element and then three-element wire Yagis.
With each change, an increase in signal and a relative drop in noise level occurred. But in order for the wire Yagis to be switched between long path and short path, both elements needed to be fed via 3/8th wavelength coaxial stubs and then relay-switched to change directions.
What started out as a cheap, simple exercise quickly became a costly, complex one, with 12V relay control lines, coaxial stubs and feeders running across a substantial portion of the VK6LW backyard. Simplification was urgently needed.
Kevin then went back to the EZNEC antenna modelling software and experimented with what would happen if a director is placed on either side of a driven element. Could he create a bi-directional Yagi with gain and a feed impedance suitable for RG6 coaxial cable? After several evenings of modelling, the BDB was the result.
How the BDBWorks
In a conventional two-element Yagi (i.e. driven element plus reflector or director) the spacing between elements is in the order of 0.1 to 0.125 of a wavelength, which gives about 6.26dBi (or around 5dB over a dipole). This means the elements are relatively closely coupled, so the feed imped
Fig. 1: Kevin Smith VK6LW/G4EHF.
Fig. 2: Original VK6LW 20mBDB, designed for CW section of 14MHz band.
Fig. 3: VK6LW BDB azimuth pattern
ance is close enough to 50Ω for it to be fed with commonly used RG-213 or LMR-400 coaxial cable. However, when a third element (i.e. a director in front of the driven element) is added, the driven element impedance drops so low a gamma match or similar matching system is necessary, although the antenna gain increases to 7 to 8dBd.
Now if you increase the spacing between each element to a quarter wave (0.25λ), the gain of the Yagi falls but the impedance of the driven element rises, so coaxial feed again becomes possible.
On this basis, VK6LW modelled adding two directors, spaced a quarter-wave, one on each side of the half-wave dipole driven element. This made the three-element antenna bi-directional. After varying the length of each director, Kevin found he could optimise the gain in two directions to almost 5dBd − the same as a conventional unidirectional two-element Yagi! Yes, Yagis don’t have to be uni-directional!
What was equally pleasing was that the antenna displayed a feed impedance of around 75Ω − perfect for being fed with RG6 UHF TV quad-shield cable. You can buy crimping tools for RG6 relatively cheaply, which gets around of the issue of having an aluminium braid that can’t be soldered.
Fig. 2 shows the configuration of 14MHz BDB in inverted-V configuration modelled by VK6VZ, with the apex of the elements at 12.8m and the ends at 11.3m. The antenna elements are made of 14SWG bare copper wire and hung from a polyester ‘catenary’ rope, which forms the boom of the BDB Yagi.
The beamwidth of the BDB antenna (before signals fall off by 3dB) is 57.2°, very similar to a conventional two-element Yagi,
Fig. 3. Its front-to-side ratio (rejection off the sides of the antenna) is just over 19dB. As shown in Fig. 4, the maximum gain of the BDB at this height is at 20° to the horizon − great for working long distances.
Fig. 5 shows the radiation pattern as a 3D image.
During the late Western Australian summer/autumn, we have long path propagation into the UK in the late afternoon and short path propagation during the evenings. In early March VK6LW quickly built up a prototype 14MHz BDB aimed at the UK/Western Europe and found he could easily work stations across Europe on both short and long paths using the antenna running 100W. He even worked a few Europeans on the SP while only using 5W!
While the BDB produced lower signal levels and reports than his main, conventional Spiderbeam Yagi, the latter had three elements working in one direction, was over twice as high in the air and also on a tower on a much higher part of his property. On the other hand, the BDB was far quieter than the Spiderbeam, owing to being farther away from his house, and weak signals could be heard on it that were inaudible on the noisier Spiderbeam.
Another advantage of the BDB is being made of a 5mm diameter catenary rope and wire elements it is virtually invisible. This meant some ‘outlining’ was necessary in the photo, Fig. 6, so the antenna could be seen!
VK6LW fed the BDB using RG6 coaxial cable via a simple choke balun (five turns of the cable on a small piece of 100mm ‘white’ (non-ferrous) PVC pipe). 50Ω coaxial, such as RG-213 or LMR-400, could be used as a feeder, as long as an SWR of 1.5:1-plus is acceptable.
Table 1 shows suggested dimensions for a BDB antenna for the 14, 18, 12, 24 and 28MHz bands, based on VK6LW’s work.
Fig. 4: VK6LW BDB elevation pattern
Fig. 5: VK6LW BDB three-dimensional pattern. The green spot shows the angle/point of maximum radiation
Fig. 6: The layout of VK6LW’s prototype BDB antenna. For clarity purposes, the antenna elements have been overdrawn/highlighted in green and the catenary rope and feeder in grey.
Kevin’s original BDB was designed for the CW end of 14MHz, but the dimensions in the table are for the middle of each of band (except 28MHz, where the dimensions are centred on 28.5MHz).
Note the dimensions aren’t critical, with the exception of the director lengths. A small variation will have a sharp effect on the symmetry of the bi-directional gain (i.e. there will not be equal gain/signal from both directions at 90° to the antenna).
Plastic egg insulators are used at the ends of each element, which are then supported/tensioned using fishing line or thin Dacron/polyester rope. While VK6LW’s 14MHz BDB had an apex height of 12.8m, a BDB for any of the HF bands with its apex at 10m and ends at 7.5m is going to be almost as effective but easier to put up by the average radio amateur living in suburbia.
Conclusions
The BDB is an excellent, non-critical and cheap bi-directional beam antenna, particularly for those with a tree or two in their backyard who are interested in making contacts with stations in one particular area on the far side of the world. No tower or rotator is required and it is going to give a performance noticeably superior in its principle direction(s) to a simple dipole or groundplane antenna at the same height.
It also has a definite advantage over the EDZ antenna as it can be configured in an inverted-vee format without loss in performance and has a wider beamwidth. Even at a lower height to that used by VK6LW, the BDB should be a very effective performer, owing to its inherent 5dB gain over a dipole.
If you want to use a long feeder, say
50 to 100m, Kevin suggests substituting RG11 75Ω coaxial, also used for digital TV installations. This has 1.5dB less loss than RG6 over a 100m run and about the same loss as RG-213 cable. A 100m roll of RG11 can be purchased from Farnell in the UK for under £100: https://tinyurl.com/wrwte25x
He also points out there is an opportunity for experimentation by adding parallel driven elements for 21 and 28MHz on the same feeder and then interleaving parasitic elements (directors) for these bands with the 14MHz ones.
If you are interested in contesting, DXing and antennas, VK6LW has made a series of podcasts under the banner of ‘Amateur Radio Topics’, which has some excellent information, visit: