Understanding the Role of the Counterpoise in 4:1 and 9:1 Antennas

When building or analyzing antennas with 4:1 or 9:1 ununs, a common point of confusion is the role of the counterpoise wire—often the shorter wire connected to the unun, opposite the main "hot" radiator. While some assume the counterpoise is meant to radiate equally or act like a dipole leg, its primary function is electrical, not radiative.

Let’s take a closer look.

The Hot Wire vs. the Counterpoise

In these unun-fed antennas—commonly end-fed designs like random wires, EFHWs, or OCFs with ununs—the longer wire (often 8 to 30 meters) is where most of the RF current flows. As a result, this is the primary radiator, responsible for the majority of the signal being transmitted or received.

The shorter wire, often referred to as a counterpoise, is typically a much shorter conductor connected to the other terminal of the unun. Its job is not to radiate, but to provide a return path for RF currents, helping the unun work correctly and improving current balance. It serves as a synthetic ground reference, especially in setups without a proper ground system.

In our antenna designs, this shorter wire is integrated into the coaxial feedline's braid by design. A ferrite choke or clip is placed at a certain distance from the feedpoint to isolate the section used as a hidden counterpoise. This approach reduces visible wiring, maintains functionality, and simplifies installation for the user. However, the option to use an external counterpoise via a separate wire connection is also available for those who prefer or require it.

But Doesn’t It Radiate Too?

Yes—technically, the counterpoise does radiate some RF, but because it is short compared to the main radiator, and carries less current, its contribution to the radiation pattern is minimal.

The focus of the system is still the main wire, which has both the length and current to create efficient radiation.

Think of the counterpoise as a helper that completes the circuit, rather than a co-radiator. It improves the electrical match and helps reduce common-mode current, but it’s not responsible for the signal strength or main radiation direction.

Mounting the Counterpoise – Does It Matter?

While its position doesn’t dramatically impact radiation, how you mount the counterpoise does matter:

  • You can hang it vertically down from the unun.
  • You can run it horizontally, ideally in the opposite direction of the hot wire.
  • You can let it lay on the ground or drape it loosely.

All these options are viable. Since its role is primarily electrical, orientation affects impedance and resonance more than radiation. However, keep in mind that any conductive element near the antenna can interact with the radiated field, so mounting it neatly and away from large metal objects or structures is a good idea.

External Counterpoise Options

With our antennas, an external counterpoise is optional. Users may choose to connect a short wire to the dedicated ground connector for this purpose. This gives flexibility in setups where a small ground reference is beneficial.

If a user chooses to operate the antenna with a ground-connected wire, the UNUN 1:1 (often called line-isolator or choke) can be placed directly under the antenna, connected via a short 0.5 cm coax jumper cable. This setup minimizes losses and enhances stability.

Antenna Mounting and Radiation Efficiency

The way you mount the entire antenna system, including the height, angle, and environment, has a much greater influence on performance than the position of the counterpoise alone. For example:

  • A 10-meter hot wire sloping up from the unun will radiate differently than one mounted horizontally.
  • Ground proximity affects radiation resistance and efficiency more than the counterpoise's angle.

The bottom line: optimize the longer wire's installation first—it's your primary radiator.

Conclusion

In 4:1 and 9:1 unun-fed antennas, the counterpoise is an electrical necessity, not a co-radiator. It helps complete the antenna system, allowing the unun to perform its impedance transformation properly. It does radiate a little, but the real star of the show is the longer hot wire.

So next time you experiment with these antennas, remember: focus on the hot wire, and treat the counterpoise as a silent partner doing its job in the background.

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Written by Joeri Van Dooren, ON6URE – RF, electronics and software engineer, complex platform and antenna designer. Founder of RF.Guru. An expert in active and passive antennas, high-power RF transformers, and custom RF solutions, he has also engineered telecom and broadcast hardware, including set-top boxes, transcoders, and E1/T1 switchboards. His expertise spans high-power RF, embedded systems, digital signal processing, and complex software platforms, driving innovation in both amateur and professional communications industries.