Calculator Inputs
Formula Used
Wavelength uses λ = 299.792458 / fMHz. Wire length uses L = λ × wave fraction × velocity factor × trim factor.
Transformed impedance uses Zin = Zend / ratio. Reflection coefficient uses |Γ| = |(Zin - Z0) / (Zin + Z0)|.
VSWR uses (1 + |Γ|) / (1 - |Γ|). Feed voltage uses Vrms = √(Pdelivered × Zend).
How To Use This Calculator
Enter the operating frequency first. Choose the wave fraction for the antenna style. Use half wave for many end fed half wave designs.
Set velocity factor for the wire. Add a small trim allowance. Enter expected end impedance and transformer ratio. Then press calculate.
Cut the wire slightly long. Raise the antenna in its real position. Measure resonance with an analyzer. Trim in small steps.
Example Data Table
| Band | Frequency MHz | Wave fraction | Velocity factor | Typical ratio |
|---|---|---|---|---|
| 40 meter | 7.15 | 0.5 | 0.95 | 49:1 |
| 20 meter | 14.2 | 0.5 | 0.95 | 49:1 |
| 10 meter | 28.4 | 0.5 | 0.95 | 49:1 |
| Random wire | 10.125 | 0.625 | 0.95 | 9:1 |
End Feed Antenna Design Guide
Basic Design Idea
An end feed antenna is popular because setup is simple. Only one end connects near the operating position. The wire can run as a sloper. It can also run horizontal, vertical, or inverted L. The design still needs careful electrical planning. A half wave end fed wire has high end impedance. That point may reach thousands of ohms. A transformer reduces that value near fifty ohms. Common ratios include 49 to 1 and 64 to 1. A random wire design often uses lower ratios. The correct choice depends on length and grounding.
Length And Wavelength
Antenna length starts with wavelength. Wavelength equals wave speed divided by frequency. Radio work commonly uses frequency in megahertz. The calculator converts that value to meters. It then applies the selected wave fraction. Velocity factor adjusts for insulation and nearby materials. A bare wire often sits near one. Covered wire may be lower. Trimming allowance makes the first cut slightly long. That gives room for field tuning. Never cut the exact final length first. Soil, height, trees, roofs, and bends shift resonance. Small changes can move the best SWR point.
Matching And Power
End fed antennas can show very high voltage. The end of a half wave wire is a voltage point. That is why insulation spacing matters. The calculator estimates RMS and peak voltage. It also estimates current at the feed end. These values help choose toroids and capacitors. Transformer ratio changes the impedance seen by the radio. A poor ratio can create high standing waves. Mismatch loss shows power not delivered well. Core loss also matters at higher power. Efficiency input lets you compare practical designs. Use conservative values for small cores. Use larger cores for digital modes. Digital duty cycles heat matching parts quickly. Check toroid temperature during first tests.
Counterpoise And Installation
An end feed system still needs a return path. That path may be a counterpoise wire. It may also be coax shield current. A planned counterpoise gives better control. The calculator estimates counterpoise length by wavelength fraction. Five percent of wavelength is a common starting point. Longer wires may lower noise. They can also alter tuning. Keep feed hardware away from metal gutters. Keep the far end high when possible. Use strain relief at both ends. Weatherproof the transformer box carefully. Add a choke near the radio if needed. This can reduce common mode current. Final tuning should use an analyzer outside. Measure after raising the antenna. Then trim slowly and record each change. Good notes make later band changes easier.
Safety Notes
Keep transmitting wires away from people and pets. High RF voltage can burn skin. Use proper grounding for lightning protection. Disconnect equipment during storms. Respect legal power limits. Start with low power. Confirm SWR before full operation. Retune after rain or major wind. Safe testing protects gear and operators every time.
FAQs
What is an end feed antenna?
It is a wire antenna fed from one end. The feed point is often convenient, but impedance can be high. Matching hardware usually helps connect it to a fifty ohm radio system.
Why does a half wave end fed wire need a transformer?
A half wave wire has high impedance at the end. A transformer lowers that impedance toward the radio system value. This improves matching and reduces excessive standing waves.
What transformer ratio should I try first?
Many half wave designs start with 49:1. Some installations prefer 64:1. Random wire layouts often use 9:1. The best ratio depends on height, length, grounding, and band choice.
What does velocity factor mean?
Velocity factor adjusts wavelength for real wire conditions. Insulation, nearby objects, and mounting height can change the apparent electrical length. It helps estimate a better first cut.
Should I cut the wire exactly to the result?
No. Cut slightly long first. Raise the antenna in its final position. Measure resonance, then trim slowly. Field conditions often shift the final length.
Does an end feed antenna need a counterpoise?
Most end feed systems need some return path. A counterpoise gives that path intentionally. Without it, the coax shield may become part of the antenna.
Why is feed point voltage important?
End fed half wave designs can create high RF voltage. High voltage can arc through weak insulation. It can also heat parts and cause unsafe touch points.
Can this calculator tune every band perfectly?
No calculator can predict every installation perfectly. Nearby metal, height, soil, bends, and coax routing matter. Use the result as a strong starting point.
What does mismatch loss show?
Mismatch loss estimates power not delivered because impedance differs from the radio system. Low mismatch loss usually means better transfer and easier tuner operation.
When should I use the coil estimate?
Use it when the installed wire must be shorter than the target length. The estimate is rough. Real coil placement and antenna current distribution can change the needed value.
Is this safe for high power transmitters?
High power needs stronger insulation, larger cores, better spacing, and careful grounding. Start with low power. Watch for heating, arcing, and unwanted RF current.