HB3YAQLucerne
Quarter wave, radials and counterpoise
Why a single rod is not the whole antenna, and how the RF current’s return path matters.
Practice2 min readReviewed 15/10/2026
HB3YAQ · Ajdin HadzicLearning to take awayhttps://hb3yaq.ch/en/knowledge/groundplane
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WHAT YOU WILL LEARN
- Consider radiator and reference plane together.
- Use starting dimensions appropriately.
The other half of the system
- Radiator length
- 0.491 m
- Each radial · same starting length
- 0.491 m
An idealised quarter-wave monopole stands over a conducting reference plane. This affects current distribution and the radiated field. Ground planeA simple ground-plane antenna consists of a vertical radiator and a conductive counterpoise, often radials. In the quarter-wave model, the radiator is approximately a quarter wavelength long.Example & full explanation → can provide such a reference structure in a practical installation. With a handheld, the chassis, hand and surroundings participate instead. The visible rod’s length therefore describes only part of the antenna system.
At 145 MHz with a chosen starting factor of 0.95, radiator length is about 49.1 cm. The same quarter-wave dimension can be an initial geometric estimate for resonant elevated radials. A ground-level radial network is a different arrangement, however, and cannot be optimised using that single length alone.
↑ Back to contentsAngle and surroundings change the result
Radial count and slope affect feed-point ImpedanceImpedance is complex opposition to alternating current: it includes resistance and a frequency-dependent reactance. Antenna input impedance is therefore more than a DC resistance reading.Example & full explanation → and the field. A perfect infinite metal plane from a model rarely exists at the real mounting location. Mast, roof and feed line can participate. A useful result therefore needs defined geometry and measurement in the intended surroundings.
Counterpoise does not automatically mean electrical grounding or lightning protection. An RF return path serves a different purpose from protection against electrical or atmospheric events. Treat these tasks separately and do not infer electrical safety from a low SWR / standing wave ratioSWR is the ratio of maximum to minimum voltage on a line with standing waves. It is related to reflection caused by an impedance mismatch.Example & full explanation →.
↑ Back to contentsA controlled comparison
Keep radiator, cable and mounting height fixed and change only radial angle first. Compare impedance and resonance behaviour rather than just one SWR value. If moving the feed line causes large changes, investigate unwanted common-mode currents and feeding before shortening the radiator further.
↑ Back to contentsFrom reading to experimenting
Quarter-wave radiator
At 145 MHz and k = 0.95 the starting length is about 49.1 cm.
Open example with starting values →Frequency: 145 MHz · Velocity / shortening factor: 0.95Then change exactly one value. Compare the result and use the formula to explain why it changes. The model limits below the calculator also apply here.
Need a term explained?
These explanations help you keep reading.
Got the idea?
Why can the same rod antenna behave differently on a handheld and on a large metal surface?
Show answer & explanation
Reference plane, current return path and surroundings change. They belong to the antenna system and affect impedance and radiation.