HF 4:1 Baluns

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Frequency range 1 to 50MHz
Impedances 50R unbalanced : 200R balanced
Size 55mm(H) 64mm(W) 64mm(D)
Power rating 400W
Weight 225g

These baluns were made for use with a doublet antenna. Two types, bifilar and coaxial, were investigated.

Click on a thumbnail below to see a larger picture

The schematic.

Baluns were made using bifilar wound twin mains flex, . .

. . and with RG179B/U 75 Ohm PTFE dielectric coaxial cable.

It was found, by trial and error, that 7 turns was just right for best bandwidth & lowest loss. Two stacked ferrite toroids of 4C6 material are used (Philips part no. 4322 020 97201).

Each toroid is 36 x 23 x 15mm (D x d x h), two together are good for 400W. For further reading, see the calculation in para. 3.2.2 in the Philips application note NCO8703 in the list below.

The baluns are mounted in rainproof boxes for outdoor use.

Mounted under a gutter, for easy access from a window.

SWR & Insertion Loss

The insertion loss of each type of balun was measured, on each band, using two identical baluns back to back.

Band Bifilar Coaxial
1.8 0.158 dB 0.154 dB
3.8 0.077 dB 0.070 dB
7.0 0.065 dB 0.057 dB
14.0 0.079 dB 0.066 dB
21.0 0.107 dB 0.102 dB
28.0 0.150 dB 0.121 dB

SWR of the baluns from 1 to 60 MHz, terminated in 200 Ohms. Coaxial (red trace) and bifilar (blue trace).

These measurements were made with a N2PK VNA using Exeter software.

Philips Application Notes

NCO8703: Wideband Linear Power Amplifier (1.6 - 28 MHz) for 300 W PEP with 2 MOS transistors BLF177 (pdf 165kB)

ECO6907: Design of H.F. Wideband Power Transformers (pdf 122kB)

ECO7213: Design of H.F. Wideband Power Transformers; Part II (pdf 57kB)

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