The Hidden Truth About the Counterpoise: It's Not What You Think!
When it comes to end-fed or off-center-fed antennas, many hams obsess over radiator length, feedpoint impedance, and the transformer. But the most misunderstood piece of the system is the counterpoise. Let’s set the record straight.
Does the Counterpoise Radiate?
In most EFHW or OCF setups, the answer is: barely. The counterpoise — often a 1–14 m wire or a portion of coax shield — is not intended to radiate. Its role is to provide a defined return current path at the feedpoint. Without it, your coax braid becomes the unintended return path and starts radiating noise back into your shack.
Why It Matters Anyway
- Provides a defined return path — prevents the feedline shield from carrying the differential RF return.
- Reduces return currents — these are often mislabeled “common-mode currents” but are really imbalance-driven return paths that open the door for common-mode pickup.
- Improves impedance consistency — makes SWR curves more predictable across bands (though it does not “fix” SWR alone).
- Shifts feedpoint resonance slightly — radiator trimming may be required once a counterpoise is added.
- Too short (<0.05 λ): insufficient return path → more shield current → noisy shack.
- Optimum (0.05–0.1 λ): provides stable return without significant extra radiation.
- Too long (>0.25 λ): risk of unintended resonances, detuning, or pattern skew.
Rule of thumb: size for your lowest band of operation. Trim radiator after adding counterpoise.
How Long Should It Be?
There’s no magic number, but 0.05–0.1 λ at the lowest operating frequency is a reliable guideline. This usually works out to between 1 and 14 m depending on your setup.
- Too long: risks resonances that fight your intended radiator.
- Too short: doesn’t provide a defined return → more coax shield radiation.
Practical Integration Tips
- Down the mast: works, but conductive masts couple and electrically shorten the counterpoise.
- Use coax shield: common in EFHWs, but always add a choke at ~0.05–0.1 λ from feedpoint.
- Opposite radiator direction: reduces coupling and keeps system cleaner.
- Longer than mast? Lay horizontal, slope, or zig-zag — just keep away from radiator.
- High power? Isolate from mast or use a dedicated radial to avoid heating and imbalance.
Final Word
The counterpoise is not optional. It doesn’t make gain, but it prevents your coax from becoming an antenna. It stabilizes the system so the radiator can do its job. Ignore it, and you’ll see the mess in your analyzer, your waterfall, and maybe even your transceiver.
Respect the counterpoise. Invisible when done right — painfully obvious when done wrong.
Mini-FAQ
- Does the counterpoise radiate? — No, it’s a return path. The radiator does the radiating.
- How long should it be? — About 0.05–0.1 λ at your lowest frequency, usually 1–14 m.
- Can I use coax shield as a counterpoise? — Yes, but you must choke it to stop unwanted shield radiation.
- What happens if I skip it? — Your coax shield becomes the counterpoise, dragging noise into your shack.
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