K6RCS · Station engineering
Common-mode chokes: measured Mix 31 data
A practical field digest of Jim Brown K9YC's measured choke work for HF feed lines. This page keeps the construction data, measured resistance targets and engineering explanation on the website, where it can be detailed without turning the printed Team Stone Doctrine into a ferrite catalog.
Why RS matters more than a pretty impedance number
Common-mode current flows on the outside of a coax shield, or as unequal current on a two-wire line. Once that happens, the feed line becomes part of the antenna system: it can radiate transmitted RF, receive local noise and alter the antenna pattern. A choke inserts impedance into that common-mode circuit.
The useful distinction in Brown's work is between resistance RS and reactance XS. Reactance can be inductive or capacitive and can partially cancel the reactance already present in the feed-line common-mode circuit. Resistance does not play that cancellation game; it adds to the common-mode impedance and dissipates the unwanted current. For many ordinary feed-point applications below roughly 600 W, Brown uses about 5 kΩ RS as a practical starting target. More difficult imbalance or higher power can justify substantially more.
Balun, matching device and choke are not synonyms
A matching transformer or a manufacturer's balun may be doing an impedance or balanced-to-unbalanced job without providing enough common-mode suppression. Brown's practical recommendation is to keep the manufacturer's matching hardware unless there is a known defect, then add a dedicated common-mode choke between that device and the feed line when common-mode control is needed.
Placement matters. At a dipole feed point the choke keeps the coax from joining the antenna. Additional chokes farther down a feed line can break a long common-mode conductor into less troublesome sections. On beam antennas, do not lash the choke and its leads tightly along a conductive boom; added capacitance can move the choke's effective range downward.
Measured cookbook
Single-core Fair-Rite #31 winding data
These are the strongest single-band choices at or near the top of Brown's published lists. The point is not that one turn count is sacred; it is that the choke is a measured RF component, not decorative ferrite jewelry.
4-inch outside-diameter #31 toroid
| Band | RG400 | #12 Teflon pair | #12 NM / THHN pair |
|---|---|---|---|
| 160 m | 23 turns · 17 kΩ | 22–23 turns · 15 kΩ | 21–23 turns · 12.5 kΩ |
| 80 m | 18–20 turns · 11 kΩ | 16–18 turns · 7.5 kΩ | 15–16 turns · 6.7 kΩ |
| 40 m | 14 or 16 turns · 7.5 kΩ | 13–14 turns · 5.7 kΩ | 12–14 turns · 5 kΩ |
| 30 m | 13–14 turns · 6.5 kΩ | 13–14 turns · 5 kΩ | — |
| 20 m | 12 turns · 6 kΩ | — | — |
2.4-inch outside-diameter #31 toroid
| Band | RG400 | #12 Teflon pair | #12 NM / THHN pair |
|---|---|---|---|
| 160 m | 18 turns · 10 kΩ | 18 turns · 9.5 kΩ | 18 turns · 9.5 kΩ |
| 80 m | 16 turns · 8 kΩ | 15–16 turns · 6.5 kΩ | 15 turns · 7 kΩ |
| 40 m | 14 turns · 6.2 kΩ | 15 turns · 6.5 kΩ | 14 turns · 6 kΩ |
| 30 m | 14 turns · 6.5 kΩ | 14 turns · 6 kΩ | 13–14 turns · 5.5 kΩ |
| 20 m | 13 turns · 5.4 kΩ | 13 turns · 5.5 kΩ | 12–13 turns · 5 kΩ |
| 15 m | 11–12 turns · 4.8 kΩ | 11–12 turns · 4.7 kΩ | 11 turns · 5 kΩ |
| 10 m | 10 turns · 4.4 kΩ | 10 turns · 4.3 kΩ | 10–11 turns · 4.2 kΩ |
Useful multi-band combinations
| Core / winding | Published useful span | Representative RS | Field use |
|---|---|---|---|
| 4-inch #31 · 16 turns RG400 | 160–40 m | 5.5 kΩ @160; 8.5 kΩ @80; 7.5 kΩ @40 | Strong three-band feed-point choice. |
| 4-inch #31 · 17 turns RG400 | 160–40 m | 7 kΩ @160; 9.5 kΩ @80; 6 kΩ @40 | More low-band resistance while retaining 40 m. |
| 2.4-inch #31 · 14 turns RG400 | 80–20 m | about 6 kΩ through 80–30; 5 kΩ @20 | Compact general HF choke. |
| 2.4-inch #31 · 13 turns RG400 | 80–20 m | about 5 kΩ across all four bands | Compact broad coverage at the 5 kΩ target. |
| Two 2.4-inch #31 · 12-turn RG400 chokes in series | 80–10 m | at least 8 kΩ 80–15; about 6 kΩ @10 | Broad higher-HF coverage with increased RS. |
| 2.4-inch #31 · 14 + 17 turn RG400 chokes in series | 160–20 m | >8 kΩ @160; ~12 kΩ @80; 8 kΩ @40; 7 kΩ @30; 5 kΩ @20 | Very broad low/mid-HF coverage. |
Transmission line used as the winding
The line wound through the core is still transmission line. Brown measured the following nominal properties around 5–10 MHz. They help explain why different winding materials do not behave identically even with the same turn count.
| Line | Z0 @ 5 MHz | VF @ 5 MHz | Loss @ 10 MHz |
|---|---|---|---|
| #12 THHN solid pair | 91.2 Ω | 0.725 | 1.2 dB / 100 ft |
| #10 THHN stranded pair | 92.4 Ω | 0.73 | 1.5 dB / 100 ft |
| #12 Teflon Ag/Cu pair | 96.6 Ω | 0.833 | 0.76 dB / 100 ft |
| #10 NM pair | 86 Ω | 0.725 | 1.5 dB / 100 ft |
| #12 NM pair | 91 Ω | 0.73 | 1.2 dB / 100 ft |
| #12 enameled pair | 43.4 Ω | 0.77 | 2.45 dB / 100 ft |
| #10 enameled pair | 41.3 Ω | 0.66 | 2.36 dB / 100 ft |
| RG400 | 50.8 Ω | 0.69 | 1.22 dB / 100 ft |
Brown ultimately recommends against enameled-wire pairs for these chokes: proximity-effect loss is higher, and the enamel can be damaged against the ferrite during winding. RG400 is attractive when the feed system is near 50 Ω; the Teflon pair has low measured loss; NM/THHN pairs can be practical alternatives for appropriate applications.
Construction rules that actually change the RF result
Wind turns in sequence and keep the geometry repeatable. Brown's measurements use turns tight to the core with adjacent turns touching at the inside diameter. For paired lines, keep the conductors paired without twisting and preserve polarity from end to end. Lead length matters to measurement and installation. The choke is not a bag of ferrite through which the cable has wandered accidentally.
Respect the minimum bend radius of the line. On arrays, remember that the cable used to wind a choke adds electrical length to that feed path. If phase matters, the choke belongs in the feed-system model rather than being added afterward as a decorative moral improvement.
Heat and power handling
There are two heat sources: ordinary differential-mode loss in the winding and dissipation caused by common-mode current. Raising RS reduces common-mode current, which is one reason a high-resistance choke is preferable to a marginal one. Ferrite properties also change with temperature.
Do not casually seal a transmitting choke into a thermally dead box. Brown specifically warns that enclosure can greatly reduce heat transfer; if an enclosure is necessary, ventilation and nonconductive construction need deliberate engineering. Weatherproof the connections without cooking the core. Duty cycle matters, particularly for long transmissions and high-duty digital modes.
How this fits the K6RCS station
At the Home QTH, the question is not simply whether an antenna has a balun or whether the transmitter sees a low SWR. The useful questions are whether the feed line is carrying common-mode current, whether local noise is coupling through that path, what RS the choke provides on the bands actually being used, and whether the installation remains mechanically and thermally sane. The 40-meter dipole repair is a natural place to treat the choke, strain relief, weatherproofing and feed-line routing as one system rather than four unrelated chores.
Source shelf
This page is a field digest, not a substitute for Brown's full measurement work. The original paper includes the complete winding lists, series combinations, measurement methodology, tolerance discussion, thermal analysis, construction photographs and bibliography.