Stray Return Current on Coax: Why It Adds Loss, a Counterpoise Helps

—Even at 4:1—and Why Everyone Still Needs a Choke

Stray return current flowing on the outside of your coax shield is wasted power and an RFI headache. It happens whenever the antenna system is unbalanced or the feed point lacks a defined return. The fixes are simple in principle:

• Provide a counterpoise (best practice for all unbalanced feeds).
• Use a common-mode choke to keep the coax from becoming part of the antenna.
• Accept that the loss penalty grows with transformation ratio: 49:1 ≳ 9:1 ≳ 4:1.

Where the Loss Comes From

A coaxial line carries two distinct current modes:

• Differential current (wanted): flows between the center conductor and the inside of the shield, with fields confined inside the cable.
• Stray return current (unwanted): flows on the outer surface of the coax shield, radiating and heating things you didn’t mean to.

When the antenna or transformer doesn’t present a clear RF return, the system “finds” one—usually through your coax braid and everything it touches (desk frame, guttering, shack wiring). Those paths are lossy compared with a short, intentional counterpoise wire, so more transmit power becomes heat instead of field strength.

When the Coax Becomes the Return Path

If you don’t give the transformer a dedicated counterpoise, the braid itself becomes the counterpoise. The common-mode current then extends down the feed line until the first current minimum—often meters away from the transformer. Practically, it feels like the choke boundary “moved” down the line, and the coax becomes part of the antenna.

This “radiating section” adds ohmic loss (skin-effect heating of the braid) and environmental loss (coupling to walls, soil, or nearby structures). Keeping that section short pays off immediately.

Why Counterpoises Matter for 49:1, 9:1, and 4:1 Systems

• 49:1 EFHW / end-fed half-wave: Strongly unbalanced. A short 0.05–0.10 λ counterpoise at the transformer ground lug makes a big difference.
• 9:1 random-wire: Also unbalanced across many bands. Add one or two short counterpoise wires (staggered lengths) near the transformer.
• 4:1 off-center / “200 Ω-ish” loads: Still unbalanced at HF. A counterpoise is best, but allowing the coax to act as the return is usually acceptable with smaller loss. Just control how far that current travels with a well-placed choke.

Return-loss ladder (worst to least when skipping a counterpoise): 49:1 → 9:1 → 4:1.

Typical counterpoise lengths (0.05–0.10 λ):

• 80 m (3.6 MHz): 4.2–8.3 m
• 40 m (7.1 MHz): 2.1–4.2 m
• 20 m (14.2 MHz): 1.06–2.11 m
• 10 m (28.5 MHz): 0.53–1.05 m

Two shorter wires of different lengths are often better than one long one.

Balun vs Unun: What Actually Matters

Whether you use a 4:1 current balun or a 4:1 unun, the behavior is the same because most HF antennas are not balanced. The practical recipe:

• Provide a local counterpoise at the transformer’s ground side.
• If the coax acts as the counterpoise, place the choke to confine the radiating section.
• Expect less stray-current loss than with 9:1 or 49:1 systems, but not zero.
• The unun is generally more efficient on HF because most HF antennas are inherently unbalanced — their coupling to nearby objects and the ground constantly changes, making a balanced transformer less effective.

Choke Placement Strategy

Every feed should include a 1:1 common-mode choke (current choke) offering at least 5 kΩ impedance on your bands of interest.

• If you added a counterpoise: Place the choke directly at the transformer output (box → choke → coax). This keeps return current local and prevents the feed line from joining the antenna.
• If the coax is the counterpoise: Place the choke 0.05–0.15 λ down the line. That confines the “radiating section” to a short stub near the antenna, not into the shack.

Ferrite tips: Use #31 mix for 1.8–15 MHz, #43 for higher HF. Stack cores or add turns to raise choking impedance. Air-wound coils can work well for single-band setups; ferrite chokes perform better broadband.

Quick Reference Recipes

49:1 EFHW:

• Add a 0.05–0.10 λ counterpoise at the ground lug.
• Place a 1:1 choke 1–3 m down the coax or at the box if the counterpoise is present.

9:1 Random-Wire:

• Add one or two short counterpoise wires of staggered lengths.
• Choke at the box if counterpoise present, otherwise 0.05–0.15 λ down the line.

4:1 Off-Center / “200 Ω” Feed:

• Treat as unbalanced and ideally add a short counterpoise at the transformer ground.
• If the coax is the return, keep the first meters straight and clear of metal, then choke at ~0.05–0.10 λ.
• Expect lower return-current loss than 9:1 / 49:1 but still measurable.

How to Know You Fixed It

• SWR and radiation pattern stop changing when the coax is moved or coiled.
• Less RFI in microphones, USB, and audio paths.
• Clamp-on RF ammeter shows a sharp current drop on the coax shield after adjustment.

Key Takeaways

• Stray return current on the coax’s outside adds real loss and instability.
• All unbalanced feeds benefit from a counterpoise. For 4:1 systems, the coax can serve as one with a smaller loss penalty.
• Without a counterpoise, the “choke moves,” making the coax part of the antenna and adding ohmic and environmental losses.
• Every system needs a well-placed common-mode choke to confine the radiating section and keep RF out of the shack.

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