Formula Used
Wavelength: wavelength equals 299.792458 divided by frequency in MHz.
Effective wavelength: wavelength multiplied by velocity factor.
Driven element: effective wavelength multiplied by the driven element factor.
Reflector: driven length multiplied by one plus the offset percent.
Director: driven length multiplied by one minus the offset percent.
Spacing: free space wavelength multiplied by the spacing factor.
Boom cut length: element spacing plus two boom end allowances.
Total conductor: rough driven length plus rough parasitic length, then waste is added.
Construction Notes for a Two Element Beam
A two element beam is a compact directional antenna. It uses a driven element and one parasitic element. The parasitic part can act as a reflector or a director. A reflector sits behind the driven element. A director sits in front of it. Each choice changes pattern, gain, and feed behavior.
Why Dimensions Matter
Small length changes can move the best match. Frequency sets the wavelength. Velocity factor adjusts that wavelength for wire or tubing. Diameter also matters. Thin wire is more sensitive. Larger tubing gives wider bandwidth and often needs less trimming. Because real installations vary, the calculator treats every result as a building estimate.
Spacing and Boom Planning
Element spacing controls coupling. Close spacing can improve front to back ratio, but it may lower feed impedance. Wider spacing can ease matching and change gain. A boom allowance helps when clamps, end caps, and mounting plates need space. The boom value is useful before buying stock material. It also helps compare roof, mast, and portable support limits.
Reflector or Director Choice
A reflector is usually longer than the driven element. It sends more energy forward from the opposite side. It is common for simple home builds. A director is usually shorter. It can give a sharp forward lobe, but matching may need more care. The best option depends on space, band, and the desired pattern.
Construction Accuracy
Measure from the electrical center of each element. Keep both halves equal unless your matching design requires an offset. Use strong clamps, square boom holes, and weatherproof hardware. Avoid nearby metal during first tests. Height above ground changes impedance and angle. Final tuning should happen in the actual operating position when possible.
Using the Results
The output lists element lengths, half lengths, spacing, boom cut length, and conductor needs. The gain and front to back figures are rough planning values. They are not a replacement for field measurements. After cutting, leave extra material for trimming. Check resonance with an analyzer. Trim both sides evenly. Record each change, because careful notes make the next antenna easier. For permanent work, choose corrosion resistant hardware, label every part, and keep a spare trim chart in your project folder.
FAQs
What is a two element beam antenna?
It is a directional antenna with one driven element and one parasitic element. The parasitic element can be a reflector or director. The design is compact and useful when space is limited.
Should I choose a reflector or a director?
Choose a reflector for a common compact beam with better rear rejection. Choose a director when you want a forward element and can spend more time on matching and testing.
Are the calculated lengths final cut lengths?
No. They are planning estimates. Cut slightly long, test resonance, then trim both sides evenly. Nearby metal, height, conductor size, and mounting hardware can change final tuning.
What spacing factor should I start with?
A reflector design often starts near 0.15 wavelength. A director design often starts near 0.12 wavelength. Use modeling or field tests when the build is critical.
Why is velocity factor included?
Velocity factor adjusts the electrical length for real conductors and materials. It helps bring the first estimate closer to practical resonance, especially with insulated wire or special tubing.
Does boom material affect the antenna?
Yes, a conductive boom can affect element behavior if it passes through elements without insulation. Use proper mounting methods and test the antenna in its real installation position.
What do the CSV and PDF buttons do?
The CSV button exports tabular results for spreadsheets. The PDF button creates a simple report for printing, sharing, or keeping with construction notes and material lists.
Can this replace antenna modeling software?
No. This tool gives fast construction estimates. For exact pattern, impedance, height effects, and matching details, use modeling software and confirm everything with field measurements.