Counterpoise Design for End-Fed Antennas
Every unbalanced antenna fed through an unun — a random wire, an EFHW, or anything else fed at one end — needs a defined return path for RF current, and that path is the counterpoise. Get it wrong (too short, not connected, or acting as an unintentional antenna element itself) and you get RF in the shack and unstable SWR even when the antenna and unun are both built correctly. This guide compares resonant vs. non-resonant lengths, elevated vs. ground-laid placement, and single vs. multiple counterpoise strategies.
What a counterpoise actually does
An unun (unlike a balun) does not isolate the antenna circuit from the coax shield — it is a single tapped winding, and whatever is connected to the shield/tap point becomes the "other half" of the RF circuit, the same role the second half of a dipole or the ground system of a vertical plays. A counterpoise is a wire (or set of wires) connected to that point specifically to give RF current a defined, low-impedance path to return on, rather than being forced onto the coax shield itself, which is what produces common-mode current and RF-in-the-shack symptoms.
Counterpoise vs. radial system vs. earth ground
These three terms get used loosely but describe different things. A counterpoise is typically one or a few above-ground wires whose main job is providing a stable RF return path near the feedpoint. A radial system (buried or on-ground, usually 4-120+ wires) is built primarily for ground-mounted vertical antennas, where the goal is reducing near-field ground loss to improve low-angle radiation efficiency — a different design goal than a counterpoise's common-mode/return-path job. An actual earth ground (a ground rod) is an electrical safety connection and is a poor RF return path by itself — its impedance at HF is far too high and variable to substitute for a proper counterpoise or radial system.
Resonant vs. non-resonant length
A resonant (quarter-wave) counterpoise, cut for one specific band using the standard 234/f formula, presents the lowest impedance at that frequency and gives the cleanest common-mode suppression on that band — but it is only optimized for that one band. A non-resonant counterpoise (commonly 17-25 ft, not tied to any particular band's quarter-wave length) trades a bit of peak performance for working reasonably across every band the antenna covers, without needing to be swapped or supplemented per band. Most practical EFHW installations use a single non-resonant counterpoise for exactly this reason.
Elevated vs. ground-laid, and single vs. multiple counterpoises
An elevated counterpoise (raised a few inches to a few feet above ground and insulated from it) behaves more predictably than one laid on the ground, because it avoids the variable and lossy coupling that changing ground conditions (wet grass, snow, different soil types) introduce. A ground-laid counterpoise is simpler to deploy — useful for quick portable setups — at the cost of somewhat less predictable performance. Some builders use multiple counterpoises, each cut as a resonant quarter-wave for a different band and fanned out from the same connection point, to get near-resonant performance on several bands at once instead of settling for one non-resonant compromise wire.
- Elevated, single non-resonant: The most common practical compromise — predictable, simple, works reasonably across bands.
- Elevated, multiple resonant (fan): Best per-band performance, more setup complexity and more wire to manage.
- Ground-laid, single wire: Fastest to deploy, least predictable — fine for quick portable operation, not ideal as a permanent installation.
| Strategy | Typical Length | Bandwidth Behavior | Best Use Case |
|---|---|---|---|
| Single non-resonant | 17–25 ft, any convenient length in that range | Reasonably broad, works across multiple bands without retuning | General-purpose EFHW/random wire — the standard default |
| Single resonant (quarter-wave) | 234/f for the target band | Narrow — best performance on one band only | Single-band operation wanting the lowest possible common-mode current on that band |
| Multiple resonant (fan) | One 234/f length per intended band, fanned from a common point | Broad — near-optimal on each included band | Multiband EFHW operators willing to manage more wire for better per-band performance |
| Elevated placement | Same as the underlying length strategy above | More predictable than ground-laid, same underlying bandwidth | Field/SOTA/POTA operation on rocky, uneven, or wet ground |
| Ground-laid placement | Same as the underlying length strategy above | Less predictable, varies with ground conditions | Fast temporary deployment where elevating the wire is impractical |
| Radial field (for comparison) | 4–120+ wires, buried or on ground | Optimized for vertical radiation efficiency, not end-fed common-mode suppression | Ground-mounted vertical antennas — a different application, not a counterpoise substitute |
Materials for building and deploying a counterpoise
A single elevated, non-resonant counterpoise deployed from an unun's ground terminal, raised on insulated stakes and routed away from the feedline.
Designing a Counterpoise for an End-Fed Antenna
Work through this sequence for any unun-fed wire antenna — the same process applies whether you are setting up a permanent EFHW or a portable random wire for a single activation.
Decide resonant vs. non-resonant
For multiband or general-purpose use, start with a single non-resonant counterpoise (17-25 ft is a practical starting range). For single-band operation, or if you want to squeeze out the last bit of performance on a specific band, plan a resonant quarter-wave length or a multi-length fan instead.
Calculate and cut resonant lengths, if using them
Use L(ft) = 234 / f(MHz) for each band you want a dedicated length for, cutting each wire a few inches long so it can be trimmed to final resonance rather than cut short.
Choose elevated or ground-laid placement
Elevate the counterpoise a few inches to a few feet above ground on insulated stakes or supports whenever the site allows it — this gives more consistent, repeatable performance than letting it lie on the ground. Route it away from the feedline itself; running the counterpoise directly alongside the coax increases unwanted coupling between them.
Connect to the unun/balun ground terminal
Attach the counterpoise directly to the ground/shield terminal on the unun — the same point the coax shield connects to, per the specific unun's build guide. A soldered or crimped lug makes a more reliable long-term connection than a bare twisted wire end, especially for a permanent installation.
Verify with a NanoVNA and adjust
Check SWR at the shack with the counterpoise connected, then temporarily disconnect it and compare — a working counterpoise typically shows lower and more stable SWR, and clipping on a common-mode choke should make little additional difference once the counterpoise is doing its job. If a resonant counterpoise's length needs trimming, adjust in small increments and re-check, the same way you would trim a quarter-wave vertical radial.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| RF in the shack despite good SWR at the antenna | Counterpoise missing, too short, or not actually connected to the unun's ground terminal | Confirm physical continuity from the counterpoise to the ground terminal; try temporarily lengthening the counterpoise and re-testing | Connect or lengthen the counterpoise (17-25 ft non-resonant is a reasonable starting point), and add a shack-entry choke as a second layer |
| SWR shifts noticeably when touching the coax or walking near the counterpoise | Counterpoise is too short or poorly routed, so it is behaving like an unintentional antenna element rather than a stable return path | Compare SWR with the counterpoise in its normal position versus moved or extended | Lengthen the counterpoise, reroute it away from other conductors, or switch to a resonant length for the band in use |
| SWR is unusually sensitive to counterpoise length — small changes cause large swings | Counterpoise length happens to be near resonance at an unintended frequency, interacting with the feedline | Sweep SWR while incrementally changing counterpoise length; a sharp dip/peak pattern indicates unintended resonance | Deliberately choose a resonant length for your actual operating band, or move to a clearly non-resonant length outside that sensitive range |
| Multiple fan counterpoises perform worse than a single wire did | The individual counterpoise wires are too close together or crossing, causing them to couple with and detune each other | Inspect physical routing — wires running parallel within a foot or two of each other, or crossing at shallow angles, couple more strongly | Spread the fan wires out at wider angles from the common connection point, and avoid letting any two cross close together |
| A shack-entry choke alone doesn't fully solve RF-in-the-shack | Choke and counterpoise solve related but different parts of the same problem — a choke blocks common-mode current already present on the line, while a counterpoise reduces how much gets there in the first place | Check whether symptoms improve with the choke alone, then further improve once a proper counterpoise is added | Use both together — a correctly sized counterpoise at the feedpoint and a common-mode choke at the shack entry, not one in place of the other |
What's the difference between a counterpoise and a ground?
A counterpoise is an RF return path connected at the antenna feedpoint, sized and routed to work at HF. An earth/safety ground (a ground rod at the shack) serves electrical safety and lightning protection, not RF return current — its impedance at HF is far too high and unpredictable to substitute for a proper counterpoise.
Does counterpoise length really matter, or is "just add a wire" good enough?
It matters more than "any wire will do," but less than perfect precision. A non-resonant length in the 17-25 ft range works reasonably across most HF bands for most installations; a length that happens to land near resonance at an unintended frequency can actually behave worse than a shorter or longer wire would, so avoid guessing wildly short lengths.
Can I use multiple counterpoises at once?
Yes — a fan of several resonant quarter-wave counterpoises, one per band, connected at the same point and spread at wide angles from each other, is a well-established way to get closer to optimal performance on each band instead of accepting one non-resonant compromise length.
Should the counterpoise touch the ground or be elevated?
Elevated is generally better and more predictable — even a few inches to a couple of feet off the ground reduces the variable coupling and loss that changing ground conditions introduce. Ground-laid is acceptable for quick, temporary portable deployments where elevating the wire isn't practical.
Do vertical antennas use counterpoises the same way?
Not quite — a ground-mounted vertical typically uses a radial field (many wires, buried or on the ground) aimed at reducing near-field ground loss to improve radiation efficiency, which is a different design goal from an end-fed wire's counterpoise. An elevated vertical can use a small number of resonant elevated radials that function similarly to a counterpoise, but the terminology and design targets differ from a horizontal EFHW/random-wire counterpoise.