Enter the frequency (MHz) into the Half Square Antenna Calculator to estimate the free-space wavelength (λ) and the 1/2 λ and 1/4 λ lengths commonly used when laying out a half-square antenna (top section ≈ 1/2 λ, each vertical section ≈ 1/4 λ).

Half Square Antenna Calculator

Enter any one value to calculate the missing variables

Related Calculators

Half Square Antenna Formula

The half-square antenna calculator estimates the free-space dimensions used to lay out a half-square antenna from its operating frequency. In a typical layout, the antenna is treated as one horizontal half-wave section with two vertical quarter-wave sections. These values are best used as starting dimensions before final trimming and tuning.

\lambda_{\mathrm{m}} = \frac{300}{F_{\mathrm{MHz}}}
\lambda_{\mathrm{in}} = \frac{11811}{F_{\mathrm{MHz}}}
L_{\mathrm{half}} = \frac{\lambda}{2}
L_{\mathrm{quarter}} = \frac{\lambda}{4}
F_{\mathrm{MHz}} = \frac{300}{\lambda_{\mathrm{m}}} = \frac{11811}{\lambda_{\mathrm{in}}}

Here, frequency is entered in megahertz, and the calculator returns the full wavelength along with the half-wave and quarter-wave dimensions commonly used to size the antenna. The constant 11811 is simply the wavelength-frequency relationship expressed in inches.

What the Calculator Returns

Output Formula Use
Full wavelength 
\lambda = \frac{300}{F_{\mathrm{MHz}}}
 The total free-space wavelength at the selected frequency.
Half-wave section 
L_{\mathrm{half}} = \frac{\lambda}{2}
 A common starting value for the horizontal top section.
Quarter-wave section 
L_{\mathrm{quarter}} = \frac{\lambda}{4}
 A common starting value for each vertical leg.

How to Calculate Half Square Antenna Dimensions

  1. Enter the operating frequency into the calculator.
  2. Read the full wavelength in your preferred unit.
  3. Use the half-wave result as the initial horizontal section length.
  4. Use the quarter-wave result as the initial length for each vertical section.
  5. Install the antenna and trim gradually if final tuning is needed.

Example

At 50 MHz, the calculator produces the following free-space dimensions:

\lambda_{\mathrm{m}} = \frac{300}{50} = 6
L_{\mathrm{half}} = \frac{6}{2} = 3
L_{\mathrm{quarter}} = \frac{6}{4} = 1.5
\lambda_{\mathrm{in}} = \frac{11811}{50} = 236.22

That means a practical first-pass layout would use a 3 meter top section and two 1.5 meter vertical sections, or the equivalent values in inches, feet, or centimeters.

Reverse Calculations

If you already know one of the section lengths, you can estimate the operating frequency by solving backward:

F_{\mathrm{MHz}} = \frac{150}{L_{\mathrm{half},\mathrm{m}}}
F_{\mathrm{MHz}} = \frac{75}{L_{\mathrm{quarter},\mathrm{m}}}

This is useful when checking an existing antenna layout or comparing planned dimensions across bands.

Useful Unit Relationships

If you prefer to work in feet, the same wavelength relationships can be written this way:

\lambda_{\mathrm{ft}} = \frac{984.25}{F_{\mathrm{MHz}}}
L_{\mathrm{half},\mathrm{ft}} = \frac{492.13}{F_{\mathrm{MHz}}}
L_{\mathrm{quarter},\mathrm{ft}} = \frac{246.06}{F_{\mathrm{MHz}}}

As frequency increases, every required antenna dimension decreases in the same inverse proportion.

Practical Tuning Notes

  • The calculator gives ideal free-space values, so actual cut lengths may shift after installation.
  • Nearby ground, supports, trees, buildings, and metal objects can affect resonance.
  • Wire insulation and conductor diameter can slightly change the final physical length.
  • Keep both vertical legs equal in length to maintain a balanced layout.
  • Measure the actual wire length rather than only the spacing between support points.
  • For best results, make final adjustments with the antenna at its intended operating height.