Calculator
Example Data Table
| Case | Channel | Frequency | Wave Type | Velocity Factor | End Effect | Estimated Length |
|---|---|---|---|---|---|---|
| Mobile whip center band | 19 | 27.185 MHz | Quarter wave | 0.95 | 5% | 8.163 ft / 97.96 in |
| Lower channel check | 1 | 26.965 MHz | Quarter wave | 0.95 | 5% | 8.230 ft / 98.76 in |
| Upper channel check | 40 | 27.405 MHz | Quarter wave | 0.95 | 5% | 8.098 ft / 97.17 in |
| Base station trial | 20 | 27.205 MHz | Half wave | 0.98 | 3% | 17.184 ft / 206.21 in |
Formula Used
Free space wavelength: λ = c / f
Wave section: Section = λ × wave ratio
Velocity adjusted length: L1 = Section × velocity factor
End effect correction: L2 = L1 × (1 − end effect / 100)
Loading correction: L3 = L2 × (1 + loading correction / 100)
Final radiator length: L = L3 + mount lead allowance
Retune estimate: New length = current length × current resonance / target frequency
How to Use This Calculator
- Select a CB channel or enter a manual frequency.
- Choose quarter wave, half wave, full wave, or custom ratio.
- Enter velocity factor for your antenna style.
- Add end effect and loading correction values.
- Add mount lead allowance when the feed connection adds length.
- Choose your preferred output unit.
- Press calculate and review the result above the form.
- Download the CSV or PDF file after calculation.
About CB Antenna Length
A CB antenna works best when its radiator is close to a resonant fraction of the wavelength. The calculator starts with the selected channel or frequency. It then finds the free space wavelength. After that, it applies the chosen wave ratio, velocity factor, end effect, loading correction, and mount allowance. The result is a practical starting length, not a final field setting.
Why Length Matters
CB radios use the 27 MHz band. Small changes in length can move the lowest SWR point across channels. A long antenna often favors the lower channels. A short antenna often favors the higher channels. This is why trimming should be slow. Cut only small amounts. Recheck SWR after each change. A whip that is too short is harder to fix than one that is slightly long.
Advanced Adjustments
Velocity factor allows for conductor shape, insulation, coils, and nearby materials. End effect reduces the ideal electrical length because current and voltage do not stop perfectly at the tip. Loading correction lets you model base loaded, center loaded, or shortened antennas. A positive value lengthens the estimate. A negative value shortens it. The start extra setting gives safe trimming room.
Real World Tuning
Use channel 20 as a center check for regular CB operation. Then compare channel 1 and channel 40. If channel 1 reads better than channel 40, the antenna is usually too long. If channel 40 reads better, the antenna is usually too short. Mount bonding, coax routing, ground plane size, and nearby metal can change the result. Always tune the antenna in its final location.
Practical Safety
Never transmit near power lines. Keep the antenna clear of people while testing. Use low power during checks when possible. Record each trim step. Save the result table as a file for later comparison. Treat the calculator as an engineering guide. Final tuning should always be verified with an SWR meter or antenna analyzer.
Field Notes
Inspect connectors before any reading. Loose hardware can mimic a wrong antenna length. Keep doors closed during vehicle tests. Stand away from the whip. Use the same meter range each time. Write down weather, mounting point, and coax changes. These notes help repeat successful tuning later.
FAQs
What is the standard CB frequency range?
Standard CB channels run around 27 MHz. This calculator includes the common 40 channel frequency list and also supports manual frequency entry.
Why is quarter wave common for CB antennas?
A quarter wave antenna is practical for vehicles. It gives useful performance while keeping the physical whip shorter than half wave or full wave designs.
What does velocity factor mean?
Velocity factor adjusts the length for real antenna materials. Coils, insulation, wire shape, and nearby mounting surfaces can change the needed physical length.
What is end effect?
End effect accounts for electrical behavior near the antenna tip. The practical antenna is often shorter than the simple free space calculation.
Should I cut the antenna to the exact result?
No. Start slightly long when possible. Trim slowly and measure SWR after each small change. A short antenna is harder to repair.
How do I know if my antenna is too long?
If channel 1 has lower SWR than channel 40, the antenna is usually too long. Shorten it carefully in small steps.
How do I know if my antenna is too short?
If channel 40 has lower SWR than channel 1, the antenna is usually too short. Lengthening may need an adjustable tip or replacement whip.
Can this calculator replace an SWR meter?
No. It gives a strong starting estimate. Final tuning should be checked with an SWR meter or antenna analyzer at the final mount location.