Simple 1:1 Choke Balun

As you’re probably aware, it’s good practice to have a Balun where BAL(anced) meets UN(balanced) feeder and the following is my latest accessory for when I’m operating “Portable“.  This is a very simple 1:1 Choke Balun made from a PL259, some RG58, an FT240-31 ferrite toroid plus a Dipole Centre from Ham Goodies.

The PL259 plugs into a radio or ATU and the balanced feeder attaches to the M4 bolts on the yellow centre.  This also saves using a boxed version which introduces another PL259/SO239 combo – the RG58 coax is kept deliberately short as this is, essentially, balanced line to the Shack.  8 turns form a resistive choke from 3.5MHz up to 30MHz on this 31-mix material.  Using 43-mix material would raise this to around 5MHz.

If QRP is more your thing, then you can down-size the ferrite and coax: FT140-43 and RG174.  The choke pictured below is good for around 400w in short bursts at HF assuming a 1:1 VSWR (which balanced line across a wide frequency range won’t offer!).  See my Coax Power Rating info here.

M0PZT 1:1 Choke Balun using FT240-31

Essentially, it’s just like any standard coaxial choke that you usually get “inside a box”, but this version makes connections a little easier, not to mention cheaper: Around £12 of parts if you omit the plastic fitting and go with a pair of standard power-supply terminals.

Another novel way of interfacing the coax and balanced-feeder is to use a plastic guy-runner.  This is what I use on my 20m Loop as the feeder enters the Shack: It’s 8-turns of RG58 would on an FT240-43 ferrite toroid.  It’s worth pointing out here that, unlike the example above, there is no strain-relief on the terminals because this choke generally doesn’t move very much!

Choke Balun using FT240-31 Ferrite and plastic guy-runner

For more of my words and pictures on Baluns – see the dedicated page here…