TTS Systems Switchable 1:1 – 4:1 Balun Module
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 –
|200W||CW||7.4 Deg C|
|150W||CW||4.0 Deg C|
|100W||CW||3.3 Deg C|
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.
The balun is supplied as a ‘board level’ device, as a result there is no direct protection from weather. Connection to the load is via two on-board screw terminals and a BNC jack allows connection to a coaxial line. A third screw terminal which allows connection to the RF ground is located between the two-line terminals.
There are three 3mm mounting holes provided on the board that could be used to fix the board to some surface. A word of caution — there can be some quite high-level RF voltages present on this board so the appropriate precautions need to be taken.
Board dimensions are 103mm long x 37mm wide. Total height is 20mm. The BNC jack protrudes 20mm from the end of the board.
The finished board has been coated with a protective PCB lacquer, however the mode switch was masked off during the lacquer application leaving the switch exposed to the elements.
|Impedance Ratio||1:1 or 4:1|
|VSWR 1:1||less than 1.5:1 from 1.5 to 55 MHz|
|VSWR 4:1||less than 1.3:1 from 1.5 to 55 MHz|
|Power Capability||130W PEP Nominal|
|Insertion Loss||less than 0.4dB.|