ANTENTOP- 01- 2015 # 019

Two Broadband Symmetrical Transformers for HF and VHF Bands

By: Alex Karakaptan, UY5ON, ES4UY, Kharkov,

Ukraine

Below there are described two broadband transformers with transformation ratio 1:5 and 1:10. First transformer could match 50 Ohm to 250 Ohm the second one could match 50 Ohm to 500 Ohm. The transformers may be used to feed different types of hi- ohmic antennas, for example, G5RV. The transformers provide symmetrical output. The transformers provide SWR 1.15 AT 144 MHz, 1.1 at 70- MHz, 1.0 at 50- MHz, 1.1 at 3.5- MHz and 1.15 at 1.9- MHz. The transformers do not overheated at 500- Watts power going through.

Figure 1 shows design of the transformer with ratio 1: 5 (50/250 Ohm). Main core of the transformer is a ferrite tube taken from an RFI filter that was placed on control wires of old CRT monitor. Such tube may have OD 18- 20- mm, ID 8- 9- mm, and length 25- 28- mm. Permeability of the core is near 800- 1000.

Figure 1 Design of the Transformer with Ratio 1: 5 (50/250 Ohm)

Length ferrite core for the transformer should be 37... 44- mm. So, two ferrite rings with equal to the tube OD and ID placed from the both sides of the core. First winding of the transformer (k1k2 at Figure 1) consist of two copper tubes (pos.1) that are inserted into the ferrite cores. The tubes should have a minimal gap between the cores. The tubes are shorted by a jumper (pos.4). The jumper made of a copper strip. The jumper is soldered (pos.5) to the copper tubes. Second winding (k3k4 + k5k6) made by teflon wire in diameter 1.5- 2.0 mm (15- 12 AWG).

Figure 2 shows design of the transformer with ratio 1: 10 (50/500 Ohm). The transformer has design similar to the transformer with ratio 1: 5 (50/250 Ohm). Difference is only in the second winding that design is cleared from Figure 2.

Figure 2 Design of the Transformer with Ratio 1: 10 (50/500 Ohm)

Both transformers were tested at real antennas. They worked perfect at least much better the usual transformer made on a ferrite ring. The transformers have equal frequency parameters from 1.9 to 144 MHz that could not provide the usual transformers on ferrite ring.

Credit Line: Radio Hobby # 6, 2014

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