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Projects Common-mode choke

RF / Feed line Built

1:1 common-mode choke
on an FT240 core

An attic loop fed through an unbalanced tuner puts RF on the outside of the coax, and from there onto everything else in the house. Twelve turns of RG-58 on a ferrite ring, in a weatherproof box, mounted right at the tuner. Nothing clever, just wound properly and put in the right place.

Range
80 through 10 m
Power
120 W PEP
Core
FT240-31 or FT240-43, 2.4 in OD
Winding
12 turns RG-58, crossed over
Connectors
Two N-type female
Feeding
Attic loop through an Icom AH-4
RF hot

The AH-4 chassis carries RF. Its ground terminal connects to the loop end, which makes the case live at RF while you are transmitting. Isolate it from joists and sheathing with dry plastic standoffs or hang it on ties, and do not handle it with the transmitter keyed.

What it is made of

Bill of materials

Everything but the core
is hardware store stock

QtyComponentSpecificationPurpose
1Ferrite toroidFT240-31 or FT240-43, 2.4 in outer diameterThe choke itself
3 to 4 ftCoaxial cableGood flexible RG-58, or RG-400 where it runs hotThe winding
1EnclosureWeatherproof PVC or polycarbonate junction box, 4 x 4 x 2 in or 4 x 4 x 4 inHouses and protects the assembly
2N-type female jacks4-hole flange panel mountInput and output
8Machine screws and nutsStainlessMounting the connectors
2Grounding lugsFits under a connector mounting nutShield termination
A/RZip tiesHeavy dutyAnchoring the core
3 to 4Ferrite beadsMix 31 or Mix 43, snap-onOn the AH-4 control cable

The part that matters

Winding the core

Twelve turns
Crossed over at the halfway point

The cross-over is the whole trick. Winding straight round the ring leaves both tails together at one point, which couples input to output and undoes the work. Winding half the turns one way, crossing over, and winding the other half back gives you the same twelve turns with the tails at opposite ends of the core.

1:1 COMMON MODE CHOKE WINDING DETAIL DRAWN K4DIA CORE FT240-31 / -43 TURNS 12 SHEET 1 OF 3 1 CROSS OVER AFTER TURN 6 TAIL TO J1 TAIL TO J2 TURNS 1 - 6 TURNS 7 - 12 2.4 IN OD NOTES: 1. Thread the coax through the centre once. That pass is turn 1. 2. Wind five more turns tightly, filling one half of the core. 3. After turn 6, carry the coax over the top and back through the centre from the other side. 4. Wind six more turns the other way on the empty half. 5. Twelve turns total. Both tails leave on opposite sides. 6. Leave an 8 in pigtail at the start.
Threading the coax through the middle once already counts as turn one. Leave an 8 in pigtail before you start so there is something to work with at the connector end.

Into the box

Assembly

1:1 COMMON MODE CHOKE ENCLOSURE ASSEMBLY DRAWN K4DIA BOX 4 x 4 IN SCALE NONE SHEET 2 OF 3 4 IN J1 J2 1 2 3 4 5 ITEM DESCRIPTION 1 Weatherproof PVC or polycarbonate box, 4 x 4 x 2 in or 4 x 4 x 4 in 2 FT240-31 or FT240-43 toroid, 12 turns RG-58 3 N-type female, 4-hole flange panel mount, 2 off 4 Grounding lug under one mounting nut at each connector 5 Zip tie through two 1/8 in holes in the backplate NOTES: 1. Centre conductor solders straight into the connector centre cup. Keep the exposed run short. 2. Shield braid twists into one lead and goes to the grounding lug on the connector chassis screw. 3. Strip 1/2 in of jacket, then 1/4 in of the inner insulation. 4. Trim the tails so they reach the pins without tension.
The core is tied down because a toroid free to swing will eventually work a solder joint loose.

Drilling

  1. Mark the centres for the two jacks on opposite walls.
  2. Drill the barrel clearance holes and the four screw holes for each.
  3. Drill two 1/8 in holes through the backplate for the anchor tie.

Mounting

  1. Fit both connectors with stainless hardware.
  2. Put a grounding lug under one mounting nut at each connector, inside the box.
  3. Trim the coax tails so they reach the pins without tension.

Soldering

  1. Strip 1/2 in of outer jacket from each tail.
  2. Unbraid the shield and twist it into a single lead.
  3. Strip 1/4 in of the inner insulation.
  4. Centre conductor into the connector centre cup, exposed run as short as you can make it.
  5. Shield lead to the grounding lug.
  6. Zip tie the core down through the backplate holes.

Where it goes

Installed position

At the tuner
Not at the radio

A choke works where the common-mode current is, which is at the feedpoint. Two to four feet from the AH-4 input. Putting it down at the shack end leaves the whole run between tuner and radio radiating.

1:1 COMMON MODE CHOKE INSTALLED POSITION DRAWN K4DIA SCALE NONE REV A SHEET 3 OF 3 ATTIC SHACK Attic loop unbalanced feed Icom AH-4 chassis is RF hot 1:1 choke this build 2 TO 4 FT MAX Coax to the radio 50 ohm coax 3 to 4 Mix 31 / 43 beads on the AH-4 control cable, fitted immediately before it enters the tuner NOTES: 1. Isolate the AH-4 chassis from joists and sheathing with dry plastic standoffs, or hang it on ties. 2. Its ground terminal connects to the loop end, so the chassis carries RF. 3. Mount the choke in the attic, close to the AH-4 input, not down at the radio end.
The beads on the control cable are doing a separate job: keeping stray RF from riding the four control wires back into the tuner and the radio.

Worth knowing

Limits and checks

  • Rated here for 120 W PEP across 80 through 10 m. Duty cycle matters: a long digital transmission heats a core that an SSB conversation never would.
  • Mix 31 and Mix 43 behave differently across the band. Both are named in the parts list because either will work over this range, not because they are interchangeable everywhere.
  • A choke does not fix a bad ground or a badly placed antenna. It stops common-mode current riding the outside of the coax; that is all it does.
  • RG-400 is the substitution for a hot attic, where RG-58 jacket can soften.

Built for one specific station: an attic loop fed by an Icom AH-4 into an unbalanced tuner port. The winding and the box carry over to any similar installation, but the placement advice is written for that arrangement.