Understanding the Role of the Counterpoise in 4:1 and 9:1 Antennas
Last updated: August 22, 2025.
In antennas using 4:1 or 9:1 UNUNs, confusion often surrounds the counterpoise wire. Is it a radiator? A ground? In reality, it is an electrical return path — essential for the transformer’s function, but not the primary radiator.
The Hot Wire vs. the Counterpoise
In end-fed antennas with a UNUN (random wires, EFHWs, OCFs), the long wire — 8 to 30 m — is the primary radiator. Most RF current flows here. The shorter “counterpoise” wire, often just a few meters, provides the return current path for the transformer.
It acts as a synthetic ground reference. Without it, the coax shield itself becomes the unintended return, leading to common-mode current and shack noise.
Hidden Counterpoises via Coax
In RF.Guru designs, the counterpoise is often integrated into the coax braid between the UNUN and the first choke. This hidden length provides the needed return path. A ferrite choke (placed 0.05–0.15 λ downline) isolates the counterpoise section, preventing it from radiating further down the feedline.
An external counterpoise connection remains available for users preferring visible wires or site-specific tuning.
Does It Radiate?
Yes — any conductor carrying RF radiates. But a short counterpoise carries less current and radiates minimally compared to the long hot wire. Its role is primarily electrical, not radiative. Think of it as the circuit return leg, not a second dipole element.
Mounting Considerations
Orientation affects impedance stability more than radiation:
- Vertical drop from the feedpoint
- Horizontal, opposite the hot wire
- Laid on ground or draped loosely
All work. Avoid close proximity to large metal objects to prevent impedance distortion.
External Counterpoise Options
A short external counterpoise can be added to the UNUN ground lug. In such cases, always combine with a dedicated 1:1 choke just below the UNUN. This blocks common-mode current while allowing the counterpoise to stabilize feedpoint impedance.
System Efficiency Factors
Performance depends more on the main radiator’s length, height, and environment than on counterpoise layout. A 10 m sloper radiates differently than a 10 m horizontal wire; soil proximity has more impact than counterpoise angle.
Conclusion
The counterpoise in UNUN-fed antennas is an essential return path, not a second radiator. It ensures the UNUN transforms impedance properly, reduces common-mode current, and keeps the system stable. Focus on the hot wire’s installation first — that’s where the performance comes from.
Mini-FAQ
- Does a counterpoise radiate? — Slightly, but the hot wire does nearly all the work.
- Can coax act as a counterpoise? — Yes, up to the first choke. Beyond that, currents should be blocked.
- How long should it be? — Typically a short fraction of λ; even 2–5 m stabilizes impedance.
- Is it optional? — A counterpoise or coax braid section is always needed. You can choose hidden (coax) or external wire implementations.
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