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Quarter-wave radiator

Find starting dimensions for a radiator and radials.

HB3YAQ · Ajdin HadzicLearning to take awayhttps://hb3yaq.ch/en/tools/quarter-wave

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01 · INPUTS

Results update as you type. Starting values are examples.

Frequency (MHz)145 MHzVelocity / shortening factor0.95

02 · YOUR RESULT

Starting radiator length: 0.491 m
Starting length per radial: 0.491 m

Rounded model values. Assumptions and limits are explained below.

Radiator length: 0.491 m
Each radial · same starting length: 0.491 m

Original schematic, not to scale. Dimensions follow your inputs above. Starting lengths for tuning the actual installation, not a complete construction plan. The radiator connects to the centre conductor, the radials to the shield. Radials form the counterpoise. Angle and surroundings affect impedance; four radials are shown as an example.

What changes with a different input?Only this value changes: Frequency. All other inputs stay the same.

Starting radiator length (m)
Frequency	Result
72.5 MHz	0.9821 m
145 MHz	0.491 m
217.5 MHz	0.3274 m

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Try the dipole too →

THE CALCULATION

Formula & context

L = c · k / (4f)

An idealised monopole radiator is about a quarter wavelength long. A conducting reference plane or radials provide the return path for antenna current.

A worked example

At 145 MHz and k = 0.95 the starting length is about 49.1 cm.

UNDERSTANDING THE MODEL

Assumptions & limits

Radial lengths are geometric starting values too. Account for radial count, angle, ground and feeding when tuning.

Quarter wave, radials and counterpoiseUnderstand the reasoning →

From reading to experimenting