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.
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.
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.
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.
Print this Page | Comments | Reading(7217)
Author:Ecco Reading(29654)
Author:Ecco Reading(3440)
Author:Ecco Reading(3159)
Author:Ecco Reading(3631)
Author:Ecco Reading(5202)
Author:Ecco Reading(3229)
Author:Ecco Reading(3370)
Author:Ecco Reading(3508)
Author:Ecco Reading(3917)
Author:Ecco Reading(3681)
Author:Ecco Reading(3652)
Author:Ecco Reading(3692)
Author:Ecco Reading(6010)
Author:Ecco Reading(3670)
Author:Ecco Reading(4459)