To severely abuse a biquad, mount it at the focal of a satellite dish (for perhaps TV). Biquads are polar perpendicular to their long axis so in my photos the polarisation is vertical. They aren't as polar as a dipole but they still have around 50% more gain when oriented with respect to source polarisation.
Polarisation is important with any antenna, and biquads are a bit special. At the crossing point, the conductors do not contact (hence enameled wire). The double biquad is as the biquad but with another square on the end and the two long edges are a half-wave. The feed point MUST be perpendicular to the plane of the antenna, otherwise the coax braid will really screw things up. Gain is about 6 dBi and it has two lobes, front and rear. It compares very favourably to commercial Yagis. I use such an antenna for UHF (675MHz centre) DVB-T transmissions. Build as described above but omit the reflector. The distance between the antenna elements and the plate must be as near as damn it lambda/8 (half a quarter wave), this causes constructive interference between the direct and reflected waveforms and adds about 80% to the forward gain of the antenna.Ī much simpler and very easily rigged antenna is an unreflected biquad. Also ensure a good connection between the plate and coaxial ground/braid. Build too big rather than too small for the reflector plate. In the real world, however, the radiation pattern does not extend more than about a quarter wave around the antenna's sides. The reflector, ideally, is infinite in size. A common mistake is to connect side by side, resulting in a folded dipole which has piss-poor performance. The coax feed connects, in these images, above and below. This is about 31mm for the 2.4GHz 802.11b/g band.
Each square edge is an exact quarter wave of the centre frequency you're interested in. The red sheet is electricians' insulating tape, used to ensure insulation between the feed points.ĭimensions are pretty exact: Tolerance is about 5% and bandwidth about 10%, though it's ~25% to unity gain. The wire is enameled copper transformer wire For a double or a quad, this is necessary, but any old wire can be used for a single biquad. I used a bolt, sanded so as to be solderable on a beer can taped to a cardboard backing, as seen here: A quad biquad (an octquad?) would be around 1 dBi higher still.Ĭonstruction is very simple. The double biquad detailed a bit lower is about 2-3 dBi higher. The gain figure on one of these things is pretty impressive, you're looking at 8-10 dBi. Lo and behold, the humble yet mighty biquad.
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