Copyright (c) Ralph Holland 1996
A Cost Effective Current-Mode 1:4 Balun
constructs a Guanella balun.
A cost effective current-mode 1:4 balun can be constructed from two lengths of coax, two ferrite rods, some electrical tape, cable ties, a length of PVC water-pipe and some connectors. This form of 1:4 current-mode balun is named after G. Guanella.
The operating principle is based on the cross-connection of two current-mode
baluns. On the low impedance (current) end the transmission lines from
each balun are connected in parallel while on the high impedance (voltage)
end the transmission lines are connected in series. Since the current is
divided equally between the two separate baluns the high impedance end
sees half the current of the low impedance end, and since the voltages
are also added in phase on the high impedance ends the device obtains a
1:4 impedance ratio. Only differential balanced currents are supported
on the inside of the coaxes while currents on the outside of the braid
are suppressed. The symmetry of the balanced load can be forced by grounding
the centre terminal on the high impedance end
(see the optional link in Figure 1).
Figure 1 - Schematic
Ideally the transmission lines should have a characteristic impedance of half the balanced load.
I have found that this balun is superior to the normally documented voltage-mode transformer or Ruthroff balun. The Guanella balun has perfect winding or transmission line symmetry with respect to the balanced load.
It is easy to wind the coax onto the ferrite rods. Experimental data supports 5 turns for coverage between 3 to 30 MHz, however if you want to operate at 1.8MHz I have found that about 10 to 15 turns are required. The turns can be held by insulation tape and by applying two cable ties on the ends of the last layer on each rod. The low impedance end is terminated at a coaxial connector by taking the two centre-conductors in parallel to the centre pin and the two braids in parallel to the ground pin. On the high impedance end the top centre-conductor and the bottom braid are connected to the load, while the top braid is connected to the bottom centre-conductor - this junction can be grounded to force symmetry in the load.
Figure 2 - Construction details for 1:4 balun
The two balun sections can be housed in PVC water pipe. Cut a section large enough to make two end pieces which can be flattened with the aid of the heat from a hair-drier or heat-gun. The circular end-sections can be cut with tin-snips. I drilled a hole for a panel-mount connector in one end and used banana connectors for the balanced feed on the other end. The end sections should be inserted inside each end of the pipe and held in place with the PVC pipe glue, but I have also found hot-melt glue adequate and easily removable. Extra protection is obtained for the ends if you leave an overhang by inserting the ends further into the pipe.