Formula Used
Wavelength: λ = c / f
Ideal quarter wave: L = λ / 4
Radiator cut length: Lr = (λ / 4) × velocity factor × (1 − trim / 100)
Radial length: Lg = (λ / 4) × velocity factor × (1 − radial trim / 100)
Total wire: Lt = radiator length + radial length × radial count
Approximate feed resistance: R = 36.5 + (radial angle / 45) × 13.5 + ground loss
Reflection coefficient: Γ = |(R − Z0) / (R + Z0)|
SWR: SWR = (1 + Γ) / (1 − Γ)
Delivered power: Pd = P × (1 − Γ²)
These impedance and SWR formulas assume a mostly resistive antenna load.
How to Use This Calculator
Enter the operating frequency and select its unit. Use MHz for most radio work. Add a velocity factor for the conductor type. Bare wire often starts near 0.95. Enter trim values when you already know a correction from testing.
Select the number of radials and their droop angle. A 45 degree angle often helps approach a 50 ohm feed. Add estimated ground loss if the mount is poor. Enter feedline impedance and transmitter power. Press calculate to view the result above the form.
Use the CSV button for spreadsheet records. Use the PDF button for workshop notes. Cut long first, test carefully, then trim gradually.
About Quarter Wave Ground Plane Antennas
A quarter wave ground plane antenna is simple, rugged, and useful. It is common on VHF, UHF, CB, aviation, scanner, and small telemetry systems. The vertical radiator is one quarter wavelength long. The radial set acts as the missing half of the antenna image. This calculator turns frequency into practical cut lengths. It also estimates impedance, mismatch, current, voltage, and total wire.
Why Radials Matter
Radials are not only mechanical supports. They shape the feed point resistance. Flat radials often give a lower resistance near 36 ohms. Sloping radials usually raise the value closer to 50 ohms. That can reduce SWR on common coax. More radials also improve stability and reduce ground loss. Four radials are common. Six or eight can be better for outdoor service.
Design Notes
The velocity factor accounts for end effect and conductor style. A bare wire element often starts near 0.95. Tubing, insulated wire, nearby objects, and mounting hardware can shift resonance. Always cut slightly long first. Then trim in small steps after measuring SWR or return loss. The trim fields help record that final adjustment.
Power and Matching
The power fields do not replace a field strength meter. They show expected RMS voltage and current at the modeled resistance. The mismatch loss estimate assumes a mostly resistive load. Real antennas also have reactance, feedline loss, common mode current, and site effects. Use the values as planning guides, not certification data.
Practical Building Advice
Keep radial lengths equal when possible. Use weatherproof hardware. Bond the coax shield to the radial hub. Keep the vertical element straight. Mount the antenna clear of metal rails, gutters, towers, and wet branches. If the coax shield radiates, add a choke near the feed point. A few ferrite turns or a coax coil can help. Recheck the antenna after rain, wind, and final mounting height changes.
Best Use
This tool is best for fast design checks. It supports experiment notes, workshop builds, and classroom demonstrations. It is also useful when comparing bands, conductor types, radial angles, and feedline choices before cutting wire. For final tuning, measure at operating height. Small changes can move resonance, especially on compact roofs, balconies, vehicles, and portable masts.
FAQs
What is a 1/4 wave ground plane antenna?
It is a vertical antenna with one quarter wavelength radiator. Radials form the ground plane and complete the antenna system.
Why does this calculator use velocity factor?
Real conductors do not always match the ideal free space length. Velocity factor helps create a more practical starting length.
How many radials should I use?
Four radials are common for simple builds. More radials can improve stability, reduce loss, and support wider tuning.
Should the radials slope downward?
Sloping radials can raise feed resistance toward 50 ohms. This often improves matching with common coax feedline.
Is the SWR result exact?
No. It is an estimate using a resistive model. Real antennas also include reactance, mounting effects, and feedline behavior.
Should I cut the radiator exactly to the result?
Cut slightly long first. Measure the antenna after mounting. Trim in small steps until the desired resonance is reached.
Can this calculator be used for receive antennas?
Yes. It works well for scanner and receive designs. Matching is usually less critical for receive-only use.
Why add ground loss?
Ground loss represents wasted resistance in the mounting and radial system. Lower loss improves efficiency and radiated power.