Optimizing End-Fed Antennas with High-CMR Chokes

Updated: December 27, 2025. (This refresh focuses on real-world common-mode behavior using impedance in ohms, not “dB” marketing numbers.)

End-Fed Antennas and the Coax Shield Problem

End-fed systems (9:1 random wires, 4:1 “end-fed-ish” wires, 49:1 EFHW, and higher ratios) are unbalanced by nature. They work, but the price of “one wire to the sky” is that the system still needs a return path. If you don’t provide one intentionally (counterpoise/radials/ground reference), the station will “invent” one: the outside of the coax shield.

Once the coax shield becomes part of the antenna system, you’ll see the classic symptoms:

• SWR that shifts when you touch the coax, change cable routing, or move gear
• RF in the shack (audio feedback, hot chassis, USB glitches, random shutdowns)
• Receive noise that follows your house (switch-mode supplies, LED drivers, solar inverters, Ethernet, etc.)

What Matters Physically: Common-Mode Impedance (Ω)

A choke is not magic. It is simply a series impedance placed in the common-mode path (the “outside of the coax” path). The unwanted current drops because it now “sees” a much higher impedance.

Useful “don’t overthink it” targets

In real HF stations, hitting a broad, mostly resistive common-mode impedance of at least ~5 kΩ across the bands you actually use is where the improvement becomes obvious. For tougher installations (high duty cycle, high power, dense RFI environment), ~10 kΩ provides real headroom.

Band region	Choking impedance target	Why it matters
160–40 m	≥ 5 kΩ (better 8–10 kΩ)	Most sensitive to house noise coupling; the coax shield easily becomes a “second receive antenna”
30–20 m	~3–5 kΩ	Still worth doing; stabilizes the system boundary and reduces mid-band weirdness
17–10 m	~2–3 kΩ	Often “works anyway,” but proper isolation keeps patterns repeatable and noise lower

Two Boundaries + One Local “Gear Firewall”

For end-fed systems, the cleanest real-world setup is: (1) define the antenna boundary, (2) define the shack boundary, and for higher power / complex stations, (3) add a local choke right at the rig/tuner/PA interface.

Boundary #1: QRO at the antenna (two chokes in series)

At the feedpoint area you want the feedline to stop behaving like a second radiator and stop importing noise. For QRO, stacking two chokes in series is a practical way to keep the impedance high across the full range.

Product	Placement	Why this combo works
10kW ICAS wideband 160–10m QRO current choke	Right at the antenna system (feedpoint boundary)	Series-stacking raises common-mode impedance and helps keep it high across a wider span. The wideband unit covers low-to-high HF, while the high-band quad-core adds extra isolation where the installation often gets “touchy” (40–10m).
5kW ICAS quad-core high bands 40–10m QRO 1:1 current balun / choke

End-fed note: if the coax is intentionally acting as part of the return, place the first choke about 0.05 λ down the coax on the lowest band of interest, then verify with a clamp-on RF ammeter. If you provide a proper counterpoise at the transformer, you can place this series stack right at the feedpoint.

Boundary #2: QRO shack entrance (RF firewall)

Even if you “solve it at the antenna,” some common-mode can still survive along the run — and the shack wiring is a giant coupling surface. This choke defines the station boundary and is often the biggest single improvement for RX noise and stability.

Use this 10kW ICAS wideband 160–10m QRO current choke at the coax entry point (wall panel / bulkhead / ground window).

Local #3: right behind the transceiver / tuner / PA

This is not a substitute for proper feedline boundaries — it’s a local gear firewall. It prevents residual common-mode energy from circulating through short jumpers, tuners, amplifiers, USB cables, audio leads, and Ethernet. For QRO stations it also helps keep “RF behavior” predictable when you change band, power, or accessories.

10kW quad-core 160–10m wideband QRO current choke placed right behind the rig/tuner or PA (on the coax jumper that leaves the last box).

100 W Stations: simple and effective “three-point” isolation

For a typical 100 W home station, the most predictable result is a three-choke chain: one at the antenna boundary, one at the shack entry, and one right at the rig.

Use 3× 100W ICAS single-core wideband 160–10m 1:1 current balun / choke placed like this:

• #1 at the antenna system: define the antenna boundary (or ~0.05 λ downline if coax is the intentional return on an end-fed)
• #2 at the shack entrance: define the station boundary and cut RX noise import
• #3 at the rig: stop local re-coupling into audio/USB/Ethernet and keep the station stable

Clip-on ferrites can be useful on individual device cables, but your feedline isolation should come from purpose-built 1:1 current chokes with enough ferrite mass and verified behavior across the HF bands you care about.

Conclusion

For end-fed antennas, the question is not “Do I need a choke?” but where do I define the boundaries of the system. A robust station uses antenna boundary + shack boundary, and for higher power / complex stations, adds a local choke right behind the rig/tuner/PA to stop re-coupling.

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