Description

The Switchable Balun module has been designed as a general-purpose impedance matching and balancing device for use in low to medium power RF transmission/ antenna systems as typically found in the amateur and commercial HF radio services.

It may have specific interest to users involved in off-center dipole antenna design and situations where the coupling of balanced feed lines to coaxial cables are required.

A user slide switch selects the mode of operation. In the 1:1 mode the device essentially becomes a simple choke balun as normally used as the interface point from a balanced load to an unbalanced feed line as typically found in the feeding of a 50/75-ohm center fed dipole antenna with 50/75-ohm coaxial lines.

When the 4:1 mode is selected the impedance ratio becomes 4:1.    For example an unbalanced 50-ohm feed is transformed to a 200-ohm balanced load. This is particularly useful when transiting a coaxial line to an open wire feed.

Power Handling Capabilities

Insertion losses have been minimized by careful selection of materials used in manufacture.   The core windings use PTFE coated 22 AWG 19/0.15mm multistrand silver plated wire.    This wire is rated at 200 Deg C and 800V.

Two baluns, connected back to back and switched to the 4:1 mode become a useful test bed for evaluating their performance.   A variable RF power source is connected to one unit with a medium power non- reactive 50-ohm load terminating the second device.   Core temperature rise over a 60 second application of power was evaluated using this method.  The measured results being –

This suggests a safe operating power would be around 130W PEP or 100W CW. Key down type digital modes with high duty cycles should be de-rated to 80W.