Short Dipole Antenna Calculator
Calculate wavelength, radiation resistance and gain of an electrically short dipole antenna.
2
Inputs
Live
Math
3
Related
Enter parameters and click Calculate to view results
Formula & Theory
Rr = 20π²(L/λ)² , L ≤ λ/10This formula is used to calculate antenna parameters for short dipole antenna calculator.
Overview
This short dipole antenna calculator computes wavelength, radiation resistance, and gain for an electrically short dipole — an antenna whose physical length is a small fraction of the operating wavelength (L ≤ λ/10). It's a standard first calculation for loop antennas, loaded whips, small loft/attic HF antennas, and any radiator that can't be built to a full or half-wavelength resonant size.
Input Guide
Enter Frequency, Antenna Length exactly in the units shown by this short dipole antenna. Check the operating band, unit prefix, and decimal position before calculating; these are the inputs used by the formula.
- Frequency — use MHz.
- Antenna Length — use cm.
Output Guide
The results describe the calculated short dipole antenna values for the inputs you entered. Check each value against the available space, selected components, feed system, and operating conditions before making a final design decision.
How This Calculator Works
The Short Dipole Antenna uses Rr = 20π²(L/λ)² , L ≤ λ/10. Supply Frequency (MHz), Antenna Length (cm) in the displayed units, then use the calculated values as the first engineering target for this dipole antennas design or analysis.
Design Notes
The defining trait of a short dipole is a very low radiation resistance — often just a few ohms or less — because Rr scales with the square of (L/λ). Halving the length relative to wavelength cuts radiation resistance to a quarter, which is why shrinking an antenna always comes at a steep efficiency cost. This low Rr sits in series with the antenna's loss resistance (conductor resistance, ground losses, loading coil losses), and since radiated power is proportional to Rr while dissipated power is proportional to loss resistance, a short dipole with even a modest 1–2 Ω of ohmic loss can waste the majority of the transmitter's power as heat rather than radiation. This is the central design challenge of every electrically small antenna: efficiency, not gain, is usually the limiting factor.
Build and Tuning Notes
A short dipole also presents a highly capacitive feedpoint reactance that grows larger as the antenna gets electrically shorter, so a practical build almost always needs a matching network or loading coil to cancel that reactance and present a usable impedance to the feedline. The 1.76 dBi gain figure shown here is the theoretical limit for an idealized short dipole with uniform current distribution — real short dipoles built with loading coils or capacitive hats redistribute the current along the element and typically achieve slightly less than this idealized value. Because the achievable bandwidth of a short dipole is inherently narrow (a consequence of its high reactance and low radiation resistance, i.e. high Q), expect to retune the matching network if you change bands or move the antenna's surroundings, and always verify the L ≤ λ/10 condition holds at your operating frequency — beyond that point, the uniform-current assumption behind this formula breaks down and a full method-of-moments or transmission-line model is needed.
Frequently Asked Questions
Why is radiation resistance so low on a short dipole?
Radiation resistance scales with the square of the antenna's electrical length (L/λ), so an antenna that's only a small fraction of a wavelength has very little "reach" into the surrounding fields and radiates weakly for a given current. This is a geometric consequence of the short dipole's current distribution, not a flaw in any particular design.
Why does a short dipole need a loading coil or matching network?
An electrically short antenna presents a large capacitive reactance at its feedpoint instead of the resonant, purely resistive impedance a full-size dipole would show. A loading coil (inductor) or matching network cancels that capacitive reactance so the transmitter sees a workable impedance, allowing efficient power transfer onto the antenna.
What efficiency can I expect from a short dipole?
Efficiency depends entirely on how the low radiation resistance compares to the total loss resistance in the system — conductor losses, loading coil losses, and ground losses all compete with radiation for the available power. It's common for compact HF antennas built well below λ/10 to see efficiencies of only 10–50%, which is why minimizing loss resistance (thick conductors, high-Q loading coils, good grounding) matters more for small antennas than for full-size ones.
What does the 1.76 dBi gain figure represent?
This is the theoretical maximum gain of an infinitesimally short dipole with an idealized triangular (uniform-taper) current distribution — the same directivity pattern shape as a full half-wave dipole, just without its higher radiation resistance. Real-world short antennas with loading coils or top-hat capacitance rarely exceed this figure and often fall slightly short of it due to non-ideal current distribution.
Alex Warren
B.Sc. in Electrical & Electronic Engineering (EEE)
Alex specialises in antenna design and wave propagation. His expertise helps ensure these calculators present practical RF concepts, useful design estimates, and clear engineering guidance for students, HAM operators, and wireless professionals.