Efficient Common-Mode Current Choke for EFRW and other portable antennas (1:1 Guanella)

I was putting together SOTA/POTA portable antenna kit for my son, utilizing EFHW and EFRW antennas and I needed a Common-Mode Current Choke - something not very large and efficient that he can eventually use from QRP to 100W on 40m to 6m.

This is just a quick, easy, 15 min. build, and the result is an excellent and pretty efficient CMC Choke (see the measurements below) for only $25, that one can place in-line with the antenna feedline at the transceiver, at the antenna feed-point or both.

There is nothing new here - this is "Classic" 1:1 Guanella choke build, but I did some measurements to put things in perspective and show what is to be expected from such choke.

Note regarding using a CMC choke with portable wire antennas - if used with End-Fed Random Wire (EFRW) the choke can be placed at the antenna feed point (the 9:1 UnUn transformer) or at the radio. When used with End-Fed Half-Wave (EFHW) the choke should be placed at the radio only as the coax is part of the antenna system.

The bill of materials includes FT-240-43 toroid core (source: Amazon, $12) and 5 feet of RG-316 coaxial cable with pre-installed Male and Female BNC connectors (source: eBay, $13). Other materials - 6 small zip-ties and a piece of wide heat-shrink tubing which I had laying around.

The coax is winded on the core in 2x 11 turns with a crossover turn as shown. The total is 23 turns through the core.

 First, I secured one end of the coax with 2 crossed zip-ties, wound the first 11 turns, added zip-ties for the cross-over turn to fix each side of the windings and finished it with another 11 turns and 2 crossed zip-ties.

The core of this size can easily take more turns - around 14 on each side (for total of 29) but there is a performance trade-off - more turns improve low frequency attenuation but due to capacitive coupling between the tightly spaced turns, this will decrease the attenuation factor for higher frequencies. 

It seems that for FT-240 core and RG-316 the "sweet spot" is around 11-12 turns on each side. If 12 turns are to be used, one needs 6 feet piece of coax. 

The turns must be fairly tight around the core itself, so they don't slide around (PTE-insulated coax is very slippery!) but not too tight to cause coax damage. Turns should be separated from each-other as much as possible.

The direction of the windings doesn't matter. The purpose of the Reisert Crossover turn is to allow the coax to leave the core from opposing sides so there is no coupling between input and output and makes it mechanically better for in-line placement but electrically both sides are the same as one continuous winding.

The cores I got from Amazon had rounded edges so there was no need to wrap the cores with tape but should the core has sharp edges, fiberglass tape must be used before winding.

The complete CMC Choke.  With 5 feet of coax there are approximately 4" left as pigtails on each side.

Measurements

I built the CMC Choke Test fixture on a piece of FR4 material. Two SMA connectors are soldered on each side of a ground plane and a short Teflon-insulated wire with alligator-clips is soldered to the center conductor of each connector.

A piece of high-density foam glued at the front edge of the board prevents the wires from getting too close to the ground plane during calibration and measurements.

I added a 50 Ohm LOAD (2x 100Ohm 1206 SMD resistors in parallel) to be used at part of the VNA's OSLT calibration procedure.

OPEN calibration is done with the clips just laying far apart from each other. SHORT is done by clipping the S11 (Ch. 0) of the nanoVNA to the ground plane.

LOAD calibration on S11 is using the built-in LOAD standard.

For the THRU calibration, used for the S21 measurements both clips are attached together.

To measure the attenuation provided by the CMC Choke, the alligator clips are attached only to the outer shielding of the coax.

The CMC choke's performance is excellent for the entire intended range - 160m to 6m!

For reference - anything below -20dB is very good and below -30db is considered excellent!

Marker 1 (80m band @3.5 MHz) -44.7dB.

Marker 2 (20m band @14.2MHz) -38.9dB

Marker 3 (10m band @28.5MHz) -30.8dB

Marker 4 (6m band @54.1 MHz) -19.5dB

Performance on the 6m band while still very good is a bit lower than the lower HF range due to the coupling between multiple turns. As I mentioned, this an expected trade-off with the lower bands.

As the VNA shows, on 20m the impedance (|Z|) is over 8KOhms and even higher on lower frequencies. Not too shabby!

In an attempt to test and improve further the performance on 6m, I added a split-core mix 31 bead with 3 turns to one of the pigtails.

The 3-turn split-core bead definitely improved the 6m band attenuation by -7dB, down to -26.5dB.
This is a "good-to-know option" - I probably will keep the bead in my kit as an "add-on" for 6m. The improvement from this additional core on the lower frequencies is just a few dB over an already excellent performance so there is no need to be a permanent addition.

S21 measurement for insertion loss. Since the two SMA-to-BNC adapters are M and F on the BNC side, just coupling them together for the THRU S21 calibration then yields a pretty accurate measurement.

The insertion loss is mainly due to the 5 ft. of small diameter coax (RG-316) and it is absolutely acceptable with less than -0.2dB on 20m and -0.3dB on 6m.

The finished chokes looking like hockey pucks.

A piece of heatshrink tubing fixes the coax turns in place and adds a layer of protection.