EH Antenna
The EH antenna is a small stack of two coaxial cylindrical elements fed through a coil-and-capacitor matching network, promoted by its originator, Ted Hart (W5QJR), as a way to radiate efficiently from a structure a small fraction of a wavelength long. It's genuinely controversial: the original claims are far outside what classical antenna theory predicts for an electrically tiny radiator, and independent modeling and measurement by mainstream antenna engineers have not confirmed them. This guide builds the design as published and is upfront about that controversy rather than presenting it as settled, working-as-advertised technology.
What the design actually is
Two short cylindrical conductors are mounted coaxially (one above the other, or one inside the other in some variants) with a small gap between them, forming what's effectively a very short, capacitively-loaded dipole. Because a structure this small presents a highly reactive, very low-resistance feedpoint impedance, a matching network — typically a series coil and one or more capacitors — sits between the coax and the cylinders to bring the reactance to zero and the resistance up to something a transmitter can drive.
Small gap between cylinders, phasing/matching network at the feedpoint
Network brings a highly reactive small-antenna impedance to a driveable match
The original claims
W5QJR and later promoters of the design described the EH antenna as radiating efficiently despite its small size, in some presentations suggesting it could approach the performance of a much larger conventional antenna — a claim that, if true, would represent a significant departure from the well-established relationship between an electrically small antenna's size, bandwidth, and radiation efficiency (the Chu-Wheeler small-antenna limits that govern every genuinely tiny radiator, loaded mobile whips included).
What independent modeling and measurement have found
Antenna engineers who have modeled and measured EH-type antennas independently of the original promotional material — including detailed published analysis from respected antenna analysts such as L.B. Cebik, W4RNL — have generally not confirmed the extraordinary efficiency claims. Their findings are broadly consistent with an electrically small, capacitively-loaded radiator behaving the way small-antenna theory predicts: real but modest radiation resistance, meaningful loss in the matching network and structure, and overall efficiency well below a full-size dipole or vertical, not a way to defeat those known physical limits.
- What's real: the network can bring SWR down to a low, driveable value, and the antenna does radiate some signal.
- What's disputed: whether that radiated signal is anywhere close to as efficient as the original promotional claims suggested, versus behaving like any other small, loaded radiator.
Setting honest expectations before you build
Build this as an experimental or educational project, or for a genuinely space-constrained install where a full-size antenna isn't an option — not as a way to get full-size HF performance from a tabletop-sized structure. Expect signal reports more consistent with a small, loaded antenna (similar in spirit to a shortened mobile whip) than with a full-size dipole or vertical, and treat any claim to the contrary with the same skepticism independent testers have applied.
Installation options
- Indoor/attic install: the small physical size is genuinely useful where a full-size antenna can't be mounted at all, accepting the efficiency tradeoff.
- Balcony or small-lot stealth install: a common motivation for building one, given HOA or apartment restrictions.
- Bench/educational build: many builders construct this specifically to test the claims themselves against a reference antenna, which is a legitimate and useful way to approach a controversial design.
| Parameter | Typical starting value | Notes |
|---|---|---|
| Upper cylinder length | ~1/9 to 1/8 wavelength | Published variants differ; this is a common starting range, not a single fixed spec |
| Lower cylinder length | Same as upper (typical) | Some variants use unequal lengths — verify against your specific reference |
| Gap between cylinders | Small, a few percent of cylinder length | Sets much of the feedpoint capacitance; expect to iterate |
| Matching network | Series coil + capacitor(s), values found empirically | No simple closed-form formula — this is tuned on the bench with an analyzer, not calculated directly |
EH Antenna Starting Dimension Calculator
Materials for EH Antenna
Building the EH Antenna
The cylinders themselves are simple; the matching network is where nearly all the real build time goes, and it will take genuine iteration.
Cut both cylinders
Cut two lengths of tubing to the calculator's starting dimensions, leaving room to trim during tuning.
Mount both cylinders coaxially on a support mast
Fix both cylinders to a non-conductive mast, stacked with the small published gap between them, keeping them precisely aligned.
Build the matching network in a weatherproof enclosure
Wire the coil and capacitor(s) between the coax and the two cylinders inside a small weatherproof box mounted at the base of the structure.
Connect the analyzer and begin network tuning
Connect your analyzer at the coax input and begin adjusting the coil and capacitor values, watching both resonant frequency and resistance.
Iterate coil and capacitor values until SWR is acceptable
Expect several rounds of adjustment — there's no single formula that gets this right the first time, and small network changes can shift the match significantly.
Seal the matching network enclosure
Once tuned, seal the enclosure against moisture — small changes in component spacing or moisture ingress can shift this network's tuning noticeably given how sensitive it is.
Compare real-world performance against a reference antenna
If possible, compare signal reports against a known-good full-size dipole or vertical on the same band, so you have an honest, first-hand baseline rather than relying on SWR alone.
| Symptom | Most likely cause | Diagnosis | Fix |
|---|---|---|---|
| Can't get SWR down no matter what's adjusted | Matching network component values are far from the structure's actual reactance | Sweep a wide range of coil/capacitor combinations rather than fine-tuning around one starting point | Reset to the published starting values and re-iterate systematically |
| Low SWR, but signal reports are much weaker than a full-size antenna | Expected behavior for an electrically small radiator, per independent testing of this design | Compare against a reference full-size antenna on the same band | Not a build fault — this matches the honest, independently-tested performance picture for this design |
| Match drifts or degrades after time outdoors | Moisture or component shift inside the matching network enclosure | Inspect the enclosure seal and component connections | Re-seal the enclosure and re-tune if components have shifted |
Does the EH antenna really work as originally claimed?
The original claims of near-full-size performance from an electrically tiny structure have not been confirmed by independent antenna engineers who've modeled and measured the design. Expect real, but modest, performance consistent with a small loaded radiator, not the extraordinary claims sometimes made in promotional material.
Is it worth building at all?
As an experimental, educational, or genuinely space-constrained project, yes. As a way to get full-size antenna performance from a small footprint, the evidence doesn't support that expectation.
Why is the matching network so finicky?
An electrically tiny radiator presents a highly reactive, low-resistance feedpoint, which makes the matching network's component values sensitive to small changes — this is a known characteristic of small-antenna matching in general, not specific to a poorly built individual unit.
Can low SWR alone tell me the antenna is performing well?
No — a matching network can bring SWR down to a comfortable value on a genuinely inefficient radiator. Low SWR confirms the transmitter is happy, not that meaningful power is being radiated efficiently.
How does this compare to a small loaded mobile whip?
Both are electrically small, loaded radiators subject to the same fundamental small-antenna efficiency tradeoffs. A well-built loaded mobile whip has a long, well-documented performance track record; the EH antenna's real-world performance is less consistently documented and more disputed.
Who invented this design?
Ted Hart, W5QJR, is credited with developing and promoting the EH antenna concept.