GB2238179A - Broadband balun - Google Patents

Description

A P P/8145/MCSL 1 - Broadband Balun 2:3 a:1- -7 1-3- This invention

relates to a broadband balun and in particular to a balun suitable for high power HF operation.

A balun is a network used to couple a balanced impedance, such as an aerial, to an unbalanced transmission line, such as coaxial cable. The balun serves to prevent asymmetrical loading of the balanced impedance and the induction of currents on the exterior of the unbalanced transmission line.

Most balun for high power HF work are of fixed frequency, typically in the range 2.9 to 26.1MHz and fairly narrow band width, typically about 0. 1MHz. They also have an input to output impedance ratio that is fixed by the type of circuit used. This is usually 1:1 or 1:4. For use over a broad band of frequencies, a balun must be provided with some form of tuning, In a high employing variable capacitors and switches powered transmitter of, say, 50kW and. above, these switches and capacitors require cooling, usually by water.

1 P/8145/MCSL 2 - The present invention provides a broadband balun in which the tuning components are not required, and therefore the need for cooling or control systems is removed.

According to the invention, a broadband balun comprises first and second tubular conductors arranged substantially parallel, the first tubular conductor being open at both ends and the second tubular conductor being closed at one end, each tubular conductor being grounded, the second at its closed end and the first at its end adjacent thereto, an input conductor passing through the first tube and then back through the second tube to terminate close to but spaced from the closed end thereof, and a parallel pair of output conductors, each connected to a respective one of the tubular conductors at the end thereof remote from the grounded end.

To limit fields externally of the balun, the tubular conductors may be mounted within a conductive shield such as a grounded tube having a closed end to which the grounded ends of the tubular conductors are attached.

1 A P/8145/MCSL The balun of the invention may be designed with a range of different impedance ratios and would typically provide a bandwidth of about 2-3 octaves with negligible loss. The lengths of the tubular conductors and the output conductors may be designed to be all substantially wavelength of operable.

equal to a quarter of the central the waveband over which the balun is The balun of the invention is self contained and does not have to be positioned adjacent to the transmitter but may be placed in any convenient position in the transmitter-to-aerial feeder run. This in turn further reduces costs and required space by allowing the use of coaxial feeder rather than boxed balanced feeder.

Reference is made to the drawings, in which:

Figure 1 illustrates diagrammatically the mechanical construction of a balun in accordance with one embodiment of the invention; and Figure 2 is a graph showing the expected loss and input match of the circuit of Figure 1.

r 1 F/8145/MCSL 4 - Referring to Figure 1, in practice the balun consists of two parallel tubular conductors 1 and 2, e.g. of aluminium tube, the first 1 of which is open at both ends while the second 2 is closed at one end. Each conductor 1, 2 is grounded, the second at its closed end and the first at the open end adjacent thereto. A central conductor 3, e.g. of copper rod, passes through the open tubular conductor 1 and back down the closed tubular conductor 2.. terminating adjacent to but spaced from the closed end. A pair of output conductors 4 and 5.. suitably copper tubing, extend parallel to each other and are connected to the respective ends of the two tubular conductors 1 and 2 remote from the grounded ends. The impedance 21 of the input line is 91 ohms, while that 22 of the line consisting of the tubular conductor 2 and central conductor 3 is 30 ohms. The impedance 23 between the adjacent tubular conductors 1 and 2 is 375 ohms, and that 24 between the output conductors 4 and 5 is 124 ohms. It will be appreciated that these values can be varied to vary the input or output impedance match or to vary the bandwidth of the balun.

The illustrated balun is suitable for a bandwidth P/8145/MCSL - 5 from 6MHz to 24MHz and the length of each conductor is 5 metres, being approximately a quarter of the wavelength at 15MHz. the central frequency of the operative frequency band.

Figure 2 shows insertion loss magnitude in decibels and the input VSWR against frequency for the balun illustrated in Figure 1. It will be seen that the loss in the desired bandwidth of 6 to 24MHz is very low, while the input VSWR across this range is below 1.2.

P/8145/MCSL 6 -

Claims (6)

1. A broadband balun, comprising first and second tubular conductors arranged substantially parallel, the first tubular conductor being open at both ends and the second tubular conductor being closed at one end, each tubular conductor being grounded, the second at its closed end and the first its end adjacent thereto, an input conductor passing through the first tube and back through the second tube to terminate close to but spaced from the closed end thereof, and parallel pair of output conductors each connected to a respective one of the tubular conductors at the end thereof remote from the grounded end.

2. A balun according to Claim 1, wherein the tubular conductors are mounted within a conductive shield whereby external fields maybe limited.

3. A balun according to Claim 2, wherein the conductive shield is in the form of a grounded tube having a closed end to which the grounded ends of the tubular conductors are attached.

k t i 1 1 P/8145/MCSL - 7

4. A balun according to Claim 1, 2 or 3, wherein the ends of the tubular conductors and the output conductors are all substantially equal to a quarter of the central wavelength of the waveband over which the balun is operable.

5. A broadband balun, substantially as described with reference to, or as shown in, the drawings.

Published 1991 at The Patent Office. State House. 66/71 High Holborn. LondonWClR47?. Further copies Tnay be obtained from Sales Branch. Unit

6. Nine Mile Point, Cwmfelinfach, Cross Keys, Ne-w-port. NPI 7HZ. Printed by Multiplex techniques lid, St Mary Cray, Ecnt.