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Butterfly Dipole for DVB-T: Simple and economical DIY indoor antennas 2

Published:2011/8/23 21:59:00 Author:Amy From:SeekIC

Rolf Badenhausen

Butterfly dipole

The real antenna impedance of a butterfly dipole is of the same order of magnitude as that of a hill-wave dipole. For such a full-wave dipole sys­tem a typical impedance would be between 240 Si and 300 Q, and in prac­tice probably nearer 300 SI than 24012. In any case, matching is required to the nominal 75 Q input impedance of the antenna input of the receiver.
Figure 2. The transformer matches the antenna to a 75Ω impedance.


For a matching transformer, used as shown in Figure 2, with a transmission ratio of N, we have:

N=(zi/zo)1/2 = ni/no

where zi and zo are respectively the input and output impedances of the system and ni, and no are the numbers of turns on each side of the trans­former For a transmission ratio N = 2 with ni = n1+n2 and no = n3 we Dud that the numbers of turns n1, n2 and n3 are all equal. This makes the practical construction of this transformer consid­erably simpler: because the same volt­age appears across all the windings, and because we do not need the gal­vanic isolation between input and out­put that a full transformer would offer, we can use a single coil with a centre tap as shown in Figure 3.

Figure 3. Matching using a transformer with a single winding.

Commercial low-loss air-cored versions of this transformer have a bifilar con­struction. Figure 4 shows the arrange­ment, where eight turns of two paral­lel 0.5 mm diameter enameled copper wires (here shown in black and white for clarity) are wound on a 4-mm diameter cylindrical former, resulting in negligible coupling losses. Winding 3-4 of the air-cored transformer ends up being slightly larger than the other: these results in better matching to the actual antenna impedance encoun­tered in practice.

Figure 4. Practical construction using a bifilar air-cored coil.

As the circuit diagram of Figure 5 shows, a ceramic coupling capacitor CI is fitted. This ensures that any DC level on the antenna input of the DVB • T receiver is not shorted to ground. Generally, receivers provide a 5-V sup­ply for an optional active antenna or for a separate antenna amplifier.

Figure 5. Antenna with transformer and coupling capacitor.



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