Butterfly Dipole for DVB-T: Simple and economical DIY indoor antennas 3

Rolf Badenhausen

Construction

The whole butterfly dipole antenna assembly, including transformer and coupling capacitor, is only the thickness of a ballpoint pen. It can be con­structed on a single-sided piece of 1.5 mm thick epoxy resin printed cir­cuit board. The mounting plan is shown in Figure 6, and the layout, as always, is available for free down­loading from www.elektor-electron-ics.co.uk.

It would also be possible to construct the unit using thin sheets of zinc, cop­per or brass on an insulating substrate made of wood or Perspex. Figure 7 shows an example constructed on a thin curved piece of wood. The trans­former and coupling capacitor are mounted on the back, along with the connections for the 75 12 cable to the antenna connector.

Television transmitter antennas may be vertically or horizontally polarized, hence the dipole must be oriented accordingly. As always with indoor antennas, especially in the UHF range, some experimentation will be required to find the optimal position to minimize the effect of reflections. In general, however, the antenna cannot make up for poor reception, for example on the ground Door of a building. Use of an antenna amplifier seems not to be recommended. Reports on Inter­net forums suggest that signals from other frequency bands of the electro­magnetic spectrum, which may also be picked up and amplified by an active antenna, can cause problems. If noise spikes exceed the (rather low) input threshold level of the DVB-T receiver, then even the complex demodulation processing in the receiver will not be able to prevent distortion to picture or sound. This can also occur when an antenna amplifier is overdriven in an area of high signal strength.