Calculator Inputs
Example Data Table
| Design Band | Frequency | Long Side | Total Length Estimate | Common Balun |
|---|---|---|---|---|
| 80 m | 3.550 MHz | 66.7% | About 39.72 m / 130.31 ft | 4:1 |
| 40 m | 7.100 MHz | 66.7% | About 19.86 m / 65.15 ft | 4:1 |
| 20 m | 14.200 MHz | 66.7% | About 9.93 m / 32.58 ft | 4:1 |
| 10 m | 28.400 MHz | 66.7% | About 4.97 m / 16.29 ft | 4:1 |
Formula Used
The calculator uses practical antenna design formulas. These formulas provide first-cut dimensions before outdoor tuning.
- Total tuned length:
L = (K / F) × VF - Initial cut length:
Cut = L × (1 + allowance / 100) - Long arm:
Long = Cut × offset / 100 - Short arm:
Short = Cut - Long - Wavelength:
λ = 300 / F - Estimated coax load:
Zcoax = Zfeed / balun ratio - Estimated SWR:
SWR = max(Zcoax / 50, 50 / Zcoax)
Here, K is the length constant in feet per MHz, F is the design frequency,
and VF is the velocity factor. The impedance model is approximate because real antennas
change with installation conditions.
How To Use This Calculator
- Enter the target design frequency in MHz.
- Set the wire velocity factor. Use 0.95 for many insulated wires.
- Choose the long side percentage. A common value is 66.7%.
- Add a trimming allowance, so the wire starts slightly long.
- Enter the planned feed height and wire diameter.
- Leave impedance blank for an estimate, or enter analyzer data.
- Select automatic balun choice, or choose your own ratio.
- Press calculate, then trim the antenna after real measurements.
Understanding an Off Center Fed Dipole
An off center fed dipole is a half wave wire antenna. It is fed away from the middle. This feed position changes the impedance and harmonic behavior. Many builders choose a one third to two thirds split. That layout can support several amateur bands when matched well. The antenna still needs careful trimming in the field.
Why Offset Matters
A center fed dipole has nearly equal current on both arms. An off center design moves the feedpoint toward one end. This raises the feed impedance. It also changes which harmonics are easier to match. A common 66.7 percent long side often pairs with a 4 to 1 current balun. Other offsets may need different matching. Height, nearby metal, soil, and wire diameter also change the final reading.
Planning Before Cutting
This calculator gives a practical first cut length. It starts with the chosen design frequency. Then it applies the velocity factor and trimming allowance. The long and short arms are split from the total wire length. Cut the wire slightly long. Raise it in the final shape. Then trim both arms in small equal steps. Always measure with an antenna analyzer at operating height.
Using Results Wisely
The harmonic table helps you see likely operating bands. It does not guarantee a perfect match. Multiband off center antennas often need a tuner. A good current balun helps reduce feedline radiation. Common mode current can still appear, especially with poor grounding or low height. Add a choke near the feedpoint when needed.
Field Tips
Keep the feedline at a right angle from the wire when possible. Avoid running coax parallel to either arm. Support the wire without sharp bends. Use strain relief at the feedpoint. Weatherproof all outside joints. Record final dimensions after tuning. That record helps when you rebuild, compare bands, or change the installation later.
Safety and Limits
Use safe supports and insulated hardware. Keep the antenna away from power lines. Higher power increases feedpoint voltage. Touch nothing while transmitting. Local rules may limit band use. Treat this tool as a design aid. Final tuning should use real measurements and careful station operating logs.
FAQs
What is an off center fed dipole?
It is a dipole antenna fed away from the center. The unequal arms create a different feed impedance and can help the antenna work on several harmonic bands.
What offset percentage should I use?
A common starting point is 66.7 percent on the long side and 33.3 percent on the short side. Other splits can work, but matching may change.
Why does the calculator add trimming allowance?
Wire antennas are usually cut slightly long. You can then trim small amounts after raising the antenna and checking resonance with an analyzer.
Is the impedance result exact?
No. It is an estimate. Real impedance depends on height, ground, nearby objects, wire size, slope, feedline routing, and the selected operating band.
Which balun is commonly used?
Many 66.7 percent off center fed dipoles use a 4:1 current balun. Different offsets or measured impedances may need another ratio.
Do I still need an antenna tuner?
Often yes. Off center fed dipoles can cover multiple bands, but not every band will present a perfect 50 ohm match.
Why is feedline routing important?
Feedline can radiate when common mode current is present. Keep coax away from the wire, use a good current balun, and add a choke when needed.
Can I use insulated wire?
Yes. Insulated wire often needs a velocity factor below one. Start slightly long, then tune the antenna in its final location.