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 shows my “go to” 1:1 Choke Balun when operating portable: It’s 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.   The yellow plastic is especially useful in a “robust” environment like the /P box as you are often (dis)connecting it, possibly even losing it in the grass (don’t ask me how I know this).

If only it bleeped when you whistled at it.

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 wound on an FT240-43 ferrite toroid.  It’s worth pointing out that, unlike the example above, there’s no strain-relief on the terminals because this choke generally doesn’t move very much – it sits behind the LDG AT200 tuner with the 300-ohm feeder entering the Shack through a hole in the wall.

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

The above arrangement is perfect if you have an air-brick/hole into the Shack near the back of your radios: The coax portion of the Choke should be as short as possible, keeping losses to a minimum.  If the balanced feeder is behaving itself – no RF surprises in the Shack (or on your transmitted audio) should be evident.

A version of this ATU Choke, featuring an FT240-31 or FT240-75 can be purchased on the Ham Goodies site.

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