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[ Previously GW4ALG (QRT in February 2007) ]

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Switched Balun

 

I've been using home made 1:1 and 4:1 baluns using bifilar and trifilar windings on a toroidal core for many years.  When, in February 2024, I started looking at baluns again, I came to the conclusion that I'd been doing it all wrong!  Firstly, I'd been using '-2' material (e.g. T200-2 cores) which has a very low permeability and, therefore, doesn't work well in choke balun applications.  Secondly, I'd been using single-core designs that try to be both a balun and a matching transformer.   

My doublet is fed with balanced feeder.  With my improved understanding of the requirements of a balun, I started to understand why, when disconnecting one leg of the balanced feeder from my balun, the received signals on 40, 80 and 160m hardly changed in signal strength.  And I always wondered why the tuner in my  IC756PRO3 didn't like matching my doublet via that balun.

In early February, I removed the single trifilar-wound T200-2 core from my switchable 1:1/4:1 balun, and replaced the single core with two identically wound 1:1 current baluns, each wound with 12 turns on a FT140-43 core.   

 

Two current baluns in parallel

  

4:1 Guanella Current Balun

 

I then connected everything together using the pre-existing switch so that I could either connect the two current baluns in parallel (for an impedance ratio of 1:1), or configure it as a 4:1 balun.   Here's the circuit diagram:

The photo shows the components mounted in a plastic enclosure.  A short length of RG58 coaxial cable enters the enclosure via a sleeved grommet and is retained by applying hot melt glue.  The two cores were also held in place using hot melt glue.

 

 

Results
I am delighted with my new balun!   When selecting the 1:1 ratio, the IC756PRO3 now tunes the balun-connected doublet reliably on all bands 6m to 160m, except for 40m, where I need to select the 4:1 configuration.   

When I disconnect one leg of the balanced feeder from this new balun, the received signals on 30/40/60/80/160m drop several S points, as they should do.

Although this new balun has not been optimised for all the bands 6m through to 160m, it does work well with all my antenna matching units.    Because the IC756PRO3 seems to like this balun, it's probably good enough.