Construction Project – QRP 1:1 Choke

I have constructed a QRP 1:1 current choke for a new homebrew 28.4 metre ZS6BKW antenna fed with 11.1 metres of 300 ohm window ladder line. The choke is placed at the end of the 300 ohm cable as a means of reducing common mode current. This choke will be used in combination with an Icom IC-705 and an AH-705 ATU, hence the lower QRP power requirements.

You may wish to implement this project as a 50 ohm 1:1 line choke. Replace the terminal posts with an RF socket of your choice.

1:1 Choke materials

• Project case: 115 mm x 65 mm x 55 mm (4.5 inch x 2.5 inch x 2.125 inch high). I recycled this case from a 4:1 unun, now surplus to requirements.
• 4 x large ferrite beads. Purchased from Jaycar, Part No #LF1260. Inside diameter is 8 mm.
• 800 mm of RG174 50 ohm coax
• 1 x RF socket of your choice
• 2 x terminal posts
• 200 mm length of double sided foam tape
• 2 x cable ties
• piece of kitchen cutting board to support the choke assembly
• 3 x small blocks of foam to prevent the choke assembly from moving around in the case

Implementation

First – using a multimeter check the coax cable, inner conductor and braid for continuity. Ensure there are no breaks in the coax cable.

Place the four ferrite beads over the length coax. Fold the third and fourth beads back so that they are parallel to beads one and two, see picture. Thread the remaining coax through the bead assembly a further three times to complete four turns. At this point leave a little slack in the turns.

Next cut a length of double-sided tape slightly shorter than the length of two beads end-to-end. Carefully secure the foam between the ferrite bead sides and check the beads for alignment. This is to prevent the beads from vibrating or potentially breaking in the case. Next place a cable tie around a pair of beads and tighten slowly! Repeat this process for the 2nd pair of beads.

Now slowly tighten the coax cable turns evenly through each side of the choke. Check the cable ties have sufficient pressure on the beads to prevent any movement. The coax should exit at each end with four completed turns. Remove the cable tie excess.

Choke assembly

Next cut two short pieces of double-sided foam tape and secure both evenly to the underside of your section of insulated cutting board. See the picture below. Place the cutting board section into the case with the double-sided tape facing down. Align the cutting board section central in the case before mating the double-sided tape. Alternative to double-side tape may be a glue gun or silicone adhesive.

Next trim the coax pig tails for soldering to each of the RF socket and terminal posts. Once you have the coax cable trimmed and ready for soldering, cut two short lengths of double-side tape and secure each to the underside of the ferrite assembly, placed left and right of the cable ties (you can’t see them in the photo). Slowly lower the choke assembly while aligning the coax pig-tails with each socket. If you mess this up, you will have to scrape off the double-side tape and start again. An alternative to using double side tape is a hot glue gun, your choice 🙂

Before soldering the pig tails, double check the continuity of the coax inner conductor and coax shield separately. Ensure there are no shorts in the pig tail sections. Solder the inner conductor and braid to each RF connector. Again, check for continuity and electrical shorts.

The three blocks of foam are positioned one on each side of the ferrite assembly and the third on top or under the lid. The top piece of foam should be compressed when the lid is secured in place.

Terminal post VSWR scan using a 50 ohm carbon resistor, 0 to 30 MHz

Terminal post VSWR scan using a 100 ohm carbon resistor, 0 to 30m MHz

Published: 29 September 2023