Calculator Form
Example Data Table
| Band | Frequency | Included Angle | Apex Height | Velocity Factor | Typical Use |
|---|---|---|---|---|---|
| 40 m | 7.15 MHz | 120 deg | 10 m | 0.95 | Portable field antenna |
| 20 m | 14.2 MHz | 110 deg | 8 m | 0.95 | Compact backyard setup |
| 10 m | 28.4 MHz | 130 deg | 5 m | 0.97 | Light wire experiment |
Formula Used
The calculator starts with the half wave dipole length.
Total wire length = 150 ÷ frequency × velocity factor × trim factor
The value uses meters when frequency is in MHz. The trim factor is 1 - trim percent ÷ 100.
Each straight leg = total wire length ÷ 2
Cut leg length = straight leg × (1 + sag percent ÷ 100)
Horizontal half span = straight leg × sin(included angle ÷ 2)
End height = apex height - straight leg × cos(included angle ÷ 2)
The impedance estimate is approximate. It uses the included angle to estimate a practical feedpoint value.
How to Use This Calculator
- Enter the center operating frequency in MHz.
- Select meters or feet for all length outputs.
- Add the wire velocity factor from the wire type.
- Enter a trim allowance for end effects and tuning space.
- Set the included angle formed by both sloping legs.
- Enter the apex height above ground.
- Add a realistic sag allowance.
- Press the calculate button and review the result above the form.
- Use CSV or PDF export for saving the planning result.
Inverted V Antenna Planning Guide
What It Does
An inverted V antenna is a center fed half wave dipole. Its feedpoint is raised on a mast. Its two legs slope down toward supports. This shape saves space. It also lowers the feedpoint impedance. Many field stations use it because setup is fast.
Length Planning
This calculator estimates the first cut length. It uses frequency, velocity factor, and trim allowance. It then divides the total conductor into two equal legs. The tool also checks the chosen included angle. A wider angle gives a larger span. A narrow angle needs less ground space.
Height and Angle
Apex height matters. A low apex can still work, but radiation goes higher. A higher apex often improves lower angle signals. The tool reports apex height in wavelengths. This helps compare portable and permanent builds.
End Clearance
The end height result is useful. It shows whether the wire ends may be too low. Low ends can be unsafe near people, animals, and metal objects. Always keep wire clear of walkways. Use rope, insulators, and strain relief. Do not let the wire carry mast tension.
Sag and Trimming
Wire sag changes the cutting plan. Sag adds physical wire while the straight geometry stays similar. The calculator separates straight leg distance from suggested cut length. This avoids confusion during layout. Start slightly long when possible. Then trim evenly from both ends.
Impedance Notes
The impedance estimate is only a guide. Ground, wire height, angle, nearby objects, and soil all change results. A tuner may still be required. Coax loss also matters when mismatch is high. Use an antenna analyzer for final adjustment.
Practical Workflow
For best results, enter the operating center frequency. Pick the unit used on your tape measure. Add realistic sag and velocity factor values. Check the span before raising the wire. Confirm the end height after installation. Retune after rain, wind, or support changes.
Field Notes
This tool is meant for planning. It cannot replace site safety checks. It gives clear dimensions and notes before the build starts. That makes the first field layout faster and more repeatable. Record each setup in a log. Include frequency, length, angle, and measured standing wave ratio. Good notes help future adjustments. They also show how different support heights change behavior. Small changes can move resonance, especially on higher bands very quickly outdoors.
FAQs
1. What is an inverted V antenna?
It is a dipole with a high center feedpoint. Both wire legs slope downward from that point. The shape reduces required support width and often gives a feedpoint impedance near common coax values.
2. Why is the included angle important?
The included angle changes span, end height, and feedpoint impedance. A wider angle behaves more like a flat dipole. A narrower angle saves space, but it can lower impedance and end clearance.
3. Should I cut the wire exactly to the result?
Cut slightly long when possible. Real installations shift resonance because of height, nearby objects, soil, insulation, and wire diameter. Trim both legs equally after measuring with an antenna analyzer.
4. What velocity factor should I use?
Use the value for your actual wire or insulated conductor. Bare wire is often close to one. Insulated wire is usually lower. When unsure, start long and trim during final tuning.
5. Does wire sag affect antenna length?
Yes. Sag can increase physical wire length while the straight point-to-point geometry changes less. This calculator adds a sag allowance to the suggested cut length.
6. Is the impedance result exact?
No. It is only a planning estimate. Actual impedance depends on height, angle, ground, nearby structures, wire type, and feed line routing. Measure the finished antenna before regular use.
7. Can I use this calculator for any band?
Yes. Enter the center frequency in MHz. The calculator works for common amateur bands and custom frequencies. Always follow local rules for your operating frequency.
8. Why is end height important?
Low wire ends can be unsafe and can increase ground interaction. Keep wire ends away from people, vehicles, animals, and metal objects. Raise supports if the calculator shows poor clearance.