constructing an RF choke

tools of the trade

In the past few months I have been doing quite a bit of reading in my spare time in Jerry Sevick’s (W2FMI) book Understanding, Building, and Using Baluns and Ununs. This text is readily available from a few sources including Amidon Corporation, from where I picked it up (as well as a few toroids). My main reason in delving into Sevick’s tome was to increase my knowledge and eventually save a little money by constructing my own, high quality baluns, ununs, and RF chokes at home.

My current antenna setup consists of a ground-wave vertical mounted on top of a split-rail fence behind my townhome.

Lack of space, trees, and presence of a HOA have kept me from erecting a large loop antenna, as I would like, or exploring other more conspicuous options. One issue with the use of the vertical has been the “RF in the shack” problem–especially on digital modes. In some instances, I would transmit a PSK31 signal on 20m and the SignaLink USB as well as the software controlling it on my computer would freeze up and require a hard restart in order to remedy the situation. I hadn’t been bitten by any RF on my microphone yet, but this was enough reason for me to explore a solution.

preparing the bifilar wires

I thought it was time to add an RF choke to my antenna system. After reading Sevick, receiving some helpful advice on the eHam.net Elmers forum, and doing some other reading on the internet, I decided to go with an FT240-43 toroid which would have adequate (i.e., greater than 5000 ohms) choking capability throughout the 10m through 40m bands, where I would usually operate. Others had suggested a -31 mix core, which would offer greater choking at lower frequencies; however, I don’t expect to operate much on 60m, 80m, and 160m at this time. The -31 mix core doesn’t provide as much choking over the 10m through 40m bands as the -43 core, so that made my choice a little easier.

In addition to the choice of the core, I also had to decide whether I would wind the toroid with coaxial cable or enameled copper wire. I made the choice to use #14 enameled copper wire because it seemed easier to work with. Using coax usually requires the use of Ty-Rap cable ties to hold the coax close to the core; the #14 wire would stay where I put it on the core.

One thing that seems left out of the description in Sevick’s text is the length of wire needed to cover a certain number of turns on a particular size core. Thankfully, Diz W8DIZ provides a link to a very helpful site with this information–toroids.info. This calculator found that I would require 26″ of wire to create 10 turns on the FT240-43 core. I didn’t want to run out of wire, so I used 33″ (don’t ask me how I chose that number) of wire instead. It turned out that 26″ (well, maybe 28″) would have been sufficient.

wound core

Sevick states that the impedance of bifilar #14 wire is around 45 ohms. In order to increase the impedance to 50 ohms, I would have to wrap one of the wires in Scotch #92 tape. After wrapping the wire in #92 Scotch I placed both wires in a Panavise to hold them together as I bound them together with Scotch #27 tape. This is an important step because it keeps the two wires from traveling away from each other as they are wound around the core.

I prepared the core by covering it in #27 Scotch tape. This helps the bifilar wire stay put on the core and also protects the wire from any sharp edges that may be present on the core, which could cause nicks or shorts.

The RF choke can be wound a few different ways. You can wind all of the turns on 1/2 of the toroid (closely spaced), wind the turns spaced evenly around the toroid, or wind the turns evenly around the toroid but utilize the “crossover technique” developed by Reisert W1JR. Winding the turns evenly spaced around the toroid leaves you with the input and output at the same point (nearly) on the core. Physically, this is not desirable. Using the crossover technique or winding the turns over 1/2 of the core allows for the input and output to be on opposite ends of the core. Sevick states that the electrical properties are similar if you choose either of these techniques. I chose the crossover technique since it seemed easier and more “economical” (acc. to Sevick).

preparing for soldering

Once the core was wound, I had to find a suitable enclosure. I decided to use a Carlon 4″x4″x2″ junction box. These are readily available at places like Lowe’s for less than $7 each. They have a nice rubber grommet at the top of the box which helps keep out moisture. I also found them very easy to drill with my Unibit stepped drill bit. I used SO-239 connectors which require a 5/8″ hole if mounted on the inside of the box, as I chose to do. After placing the SO-239 in the 5/8″ hole, I used my Dremel to drill holes for the 4-40 hardware to hold the connector in place. After placing one bolt in the first hole, I drilled the second hole. I mounted the connectors diagonally rather than square (this makes more sense if you look at the photos).

finished product

The next step was to solder the wires from the wound toroid to the SO-239 connectors and close up the box. After a few snips here and there I had a snug fit. The soldering iron did its job and I was done with my work.

I took the finished product to Steve KZ1X’s for final check-out. He used his AADE L/C IIB meter to measure the inductance at 136 uH. Using a simple formula, the calculated impedance across the intended frequencies of coverage was much greater than the 5000 Ohm goal I had at the beginning of the project. I hope to analyze it further and provide more detailed (i.e., measured instead of calculated) impedance values across the bands of interest in the future.

More photos of the project may be found in this album on Picasa.