Calculator Inputs
Enter the target frequency, feed split, wire allowances, and power details. The form uses a 3 column layout on large screens, 2 columns on smaller screens, and 1 column on mobile.
Example Data Table
These examples use the simple 468 divided by frequency rule before custom allowances.
| Frequency | Split | Total Length | Long Arm | Short Arm | Typical Use |
|---|---|---|---|---|---|
| 3.550 MHz | 66.7 / 33.3 | 131.831 ft | 87.932 ft | 43.899 ft | 80 m planning |
| 7.100 MHz | 66.7 / 33.3 | 65.915 ft | 43.966 ft | 21.949 ft | 40 m planning |
| 14.200 MHz | 66.7 / 33.3 | 32.958 ft | 21.983 ft | 10.975 ft | 20 m planning |
Formula Used
Total half-wave length: L = C / f
Corrected total: Lc = L × VF × (1 + (sag% + trim%) / 100)
Long arm: Long = Lc × long split + end allowance
Short arm: Short = Lc × short split + end allowance
Wavelength: λft = 984 / fMHz
Harmonic frequency: fn = n × f
Power after loss: Pout = Pin × 10^(-loss dB / 10)
Voltage and current: Vrms = √(P × R), Irms = √(P / R)
The result is a planning value. Real resonance changes with height, insulation, soil, nearby objects, balun type, and feedline routing.
How to Use This Calculator
- Enter the target frequency in megahertz.
- Keep 468 as a common starting constant, or enter your own value.
- Select the long arm percentage. A common OCF split is near 66.7 percent.
- Add velocity factor, sag allowance, trim reserve, and end allowance.
- Enter height, output unit, harmonic count, impedance, power, and feedline loss.
- Press calculate. The result appears above the form and below the header.
- Use CSV or PDF export to save the design sheet.
- Cut long, test safely, and trim slowly with an analyzer.
OCF Dipole Antenna Planning Guide
What the Design Means
An off center fed dipole places the feed point away from the middle of a half wave wire. This shift creates unequal arms. It can make the antenna useful on several related bands when the feed system is matched correctly.
Length and Split
The starting length comes from the familiar half wave dipole rule. Divide the chosen constant by frequency in megahertz. Then apply velocity factor, sag reserve, and trimming allowance. The calculator separates the total length into long and short arms by the selected feed point ratio.
Matching and Height
Common ratios include 66.7 percent and 33.3 percent. Other builders use 60 and 40, 70 and 30, or custom splits. The best choice depends on bands, height, wire insulation, nearby objects, and the tuner or matching transformer. A 4 to 1 current balun is often used, but real feed point impedance changes with location and installation.
Height matters because current distribution and ground coupling alter the match. A low wire can work well for local coverage. A higher wire usually improves lower angle radiation. Keep the wire clear of metal roofs, gutters, power lines, and wet tree branches.
Testing and Records
This tool is designed for planning, not final proof. Cut long, measure safely, and trim in small steps. Check the lowest target band first because short errors matter most there. Use an antenna analyzer or low power transmitter while tuning.
The harmonic table shows simple frequency multiples. It does not promise resonance, because an OCF wire has complex behavior. Balun type, coax length, common mode current, and surroundings can move the result. Still, the table helps you inspect possible multiband coverage before cutting wire.
Use the CSV button to export numbers for notes. Use the PDF button to save a simple field sheet. Recalculate after every design change. Small frequency, split, or allowance changes can move each arm by several inches on lower bands. For safer work, plan supports before raising the antenna. Mark both arm ends, the feed point, and the service loop. Record weather, height, and analyzer readings. These notes make later repairs easier, especially after storms, seasonal pruning, or changing from bare copper to insulated wire. They also improve repeatable future builds.
FAQs
1. What is an OCF dipole?
An OCF dipole is an off center fed dipole. Its feed point is not in the exact middle. This creates unequal wire arms and can support multiband operation when matched correctly.
2. Which split ratio should I use?
A common split is near 66.7 percent and 33.3 percent. Some builders use 60/40 or 70/30. The best split depends on bands, height, balun, tuner range, and local installation effects.
3. Why is the 468 constant used?
The 468 rule gives a practical half wave dipole length in feet. It includes real world shortening compared with a free space half wavelength. It is a starting value, not a perfect final cut.
4. Should I cut the wire exactly to the result?
No. Cut slightly long, install the antenna, measure resonance, then trim gradually. Wire insulation, height, slope, ground, trees, and nearby metal can move the actual resonant point.
5. Does the harmonic table guarantee multiband operation?
No. It only shows simple frequency multiples. OCF antennas have complex impedance behavior. Use an analyzer and suitable matching system before assuming a band will tune well.
6. What balun is commonly used?
Many OCF dipoles use a 4 to 1 current balun. Some installations need other ratios or common mode choking. The right choice depends on feed point impedance and coax behavior.
7. Why include sag and trim reserve?
Sag changes geometry and effective length. Trim reserve gives extra wire for tuning. These allowances help avoid cutting too short before real installation measurements are available.
8. Can this calculator replace antenna testing?
No. It is a planning tool. Always verify with safe low power tests, an antenna analyzer, and careful trimming. Outdoor surroundings strongly affect the final match.