Calculator Form
Example Data Table
| Frequency | Design | Velocity Factor | Transformer | Typical Starting Note |
|---|---|---|---|---|
| 7.100 MHz | Half Wave | 0.95 | 49:1 | Useful starting point for a long end fed wire. |
| 14.200 MHz | Half Wave | 0.95 | 49:1 | Common portable wire length range. |
| 28.400 MHz | Half Wave | 0.97 | 49:1 | Short wire, but setup height still matters. |
| 3.800 MHz | Quarter Wave | 0.95 | 9:1 | Needs stronger counterpoise planning. |
Formula Used
The calculator starts with free space wavelength:
Wavelength meters = 299.792458 / frequency MHz
The corrected radiator length is:
Radiator = wavelength × fraction × velocity factor × end correction × site correction
The end correction is 1 - end effect percent / 100.
The site correction is 1 + site adjustment percent / 100.
The suggested cut length adds the trimming allowance.
Cut length = radiator × (1 + trim allowance percent / 100)
The transformed resistance estimate is:
Feed resistance = estimated end resistance / transformer ratio
The resistance-only standing wave estimate uses:
SWR = (1 + Γ) / (1 - Γ)
Here, Γ is the absolute mismatch ratio between transformed resistance and 50 ohms.
How To Use This Calculator
- Enter the operating frequency in MHz.
- Select the end fed wire design.
- Enter a custom fraction only when custom mode is selected.
- Set velocity factor for your wire type.
- Add end effect and site correction values if known.
- Enter transformer ratio and estimated end resistance.
- Click calculate to view the result below the header.
- Use CSV or PDF buttons to save the same design.
End Fed Antenna Planning Guide
Why Length Matters
An end fed antenna works well when its wire length follows the target wavelength. Small changes can shift tuning. Height, insulation, soil, nearby metal, and bends also affect resonance. A calculator gives a strong starting point before field trimming. It does not replace an antenna analyzer. It helps reduce waste and confusion.
Choosing the Design
The half wave end fed design is common for portable and fixed stations. It usually needs a high ratio transformer, such as 49 to 1 or 64 to 1. A quarter wave end fed design behaves differently. It needs a stronger ground system or counterpoise. Random wire designs avoid exact half wave lengths on chosen bands. They often use a lower ratio transformer or tuner.
Understanding Velocity Factor
Real wire is not an ideal conductor in free space. Insulation and nearby objects slow the wave slightly. This is why the velocity factor is important. Bare wire may be close to one. Insulated wire may need a smaller value. End effect is another correction. It shortens the final calculated wire length.
Power and Matching Notes
The calculator also checks transformer impedance, estimated feed resistance, power, and voltage. These values help choose suitable cores, wire, and insulation. High impedance points can create high voltage. Keep connections protected. Keep the matching unit dry. Use strain relief so wire tension does not pull on the transformer.
Field Adjustment
Cut the wire a little long. Install it at the planned height and shape. Measure resonance with low power. Trim small pieces from the far end. Recheck after every change. If the antenna is moved, test it again. A different tree, pole, roof, or counterpoise can move the result.
Practical Use
Use this tool for design planning, comparison, and record keeping. Export results after each design session. The table helps compare common bands. The result should be treated as a build estimate. Final tuning should always happen at the real installation site.
Good records make later changes easier. Save the chosen frequency, calculated length, measured standing wave ratio, transformer ratio, and installation height. When performance changes, these notes reveal what changed. They also help compare temporary field antennas with permanent home station wires.
FAQs
What is an end fed antenna?
It is a wire antenna fed from one end. Many designs use a matching transformer because the feed point can have high impedance.
Is the calculated length final?
No. Treat it as a starting point. Installation height, nearby objects, wire insulation, and ground conditions can change the final tuning.
Why is velocity factor needed?
Signals travel slightly slower along real wire than in free space. The velocity factor corrects the ideal wavelength for real wire behavior.
What transformer ratio should I use?
Half wave end fed antennas often use 49:1 or 64:1 transformers. Shorter random wires may use other ratios with a tuner.
Why does counterpoise length matter?
The counterpoise gives return current a path. It can affect tuning, radiation pattern, noise, and matching stability.
Can I use this for portable antennas?
Yes. It is useful for planning portable wire lengths. Always test the antenna after installing it in the actual field position.
What does SWR estimate mean here?
It is a resistance-only estimate. It does not include reactance, feed line transformation, core loss, or nearby object effects.
Why cut the wire longer first?
A long wire can be trimmed shorter. A short wire usually needs added wire, which is less convenient and less predictable.