Calculator
Formula Used
The calculator uses a practical thin wire dipole estimate. It combines wavelength, electrical length, conductor loss, ground loss, and feed style.
λ = c / fElectrical length = L / (λ × VF)Zin = R + jXR = Rradiation + Rconductor + RgroundRs = √(π × f × μ0 / σ)SWR = (1 + |Γ|) / (1 - |Γ|)Γ = (Zin - Zo) / (Zin + Zo)
The curve values are planning estimates. Final results depend on surroundings, wire shape, balun design, and measurement conditions.
How to Use This Calculator
Enter the operating frequency and total end to end dipole length. Select the units that match your measurement. Add the wire diameter, height, conductor material, ground condition, feed style, line impedance, and power. Press calculate. Review resistance, reactance, SWR, return loss, efficiency, and resonant length. Use CSV or PDF buttons to save the same result.
Example Data Table
| Band | Frequency | Total Length | Height | Wire | Typical Result |
|---|---|---|---|---|---|
| 40 m | 7.1 MHz | 20.1 m | 10 m | 2 mm copper | Near half wave, moderate match |
| 20 m | 14.2 MHz | 10.05 m | 8 m | 2 mm copper | Close to resonant length |
| 10 m | 28.4 MHz | 5.0 m | 6 m | 1.6 mm aluminum | Trim needed at final height |
Dipole Antenna Input Impedance Guide
A dipole is simple, but its feed point is not fixed. Impedance changes when frequency, total length, wire diameter, height, soil, and feed style change. A half wave dipole in free space is often close to seventy three ohms. Real installations seldom match that number. Nearby ground shifts resistance. Wire length shifts reactance. Feed point choices can multiply the value seen by the line.
Why Input Impedance Matters
Input impedance tells the transmitter how the antenna accepts power. The resistive part represents radiation, conductor loss, and ground loss. The reactive part represents stored electric or magnetic energy. Negative reactance means the antenna is electrically short. Positive reactance means it is electrically long. A low reactance near the target feed resistance usually gives easier matching.
How This Calculator Helps
This calculator uses frequency, length, velocity factor, conductor diameter, material, height, ground quality, and feed configuration. It estimates radiation resistance from a thin wire dipole curve. It then adjusts the result for practical loss and installation choices. The output includes resistance, reactance, impedance magnitude, phase angle, SWR, return loss, mismatch loss, efficiency, and suggested resonant length.
Using The Results
Use the calculated reactance as a tuning clue. If reactance is capacitive, lengthen the antenna or add inductive matching. If it is inductive, shorten the antenna or add capacitive matching. Use SWR as a feed line clue, not as the only quality measure. An antenna can show a low SWR while wasting power in soil, wire, traps, or matching parts.
Practical Tuning Notes
Start with the estimated resonant length. Cut the wire slightly long. Measure at the installed height. Trim both arms equally. Keep the feed line away from the antenna if possible. Use a current balun to reduce common mode current. Recheck after rain, ice, or nearby object changes. Small changes can move resonance on higher bands. Treat this result as a planning estimate. Final tuning should use an antenna analyzer at the actual site.
Safety And Accuracy
Do not transmit into an unknown system at high power. Test with low power first. Check connector ratings, balun heating, and legal band limits. Use good instruments, because modeled values cannot see every nearby metal object or wet trees.
FAQs
What is dipole input impedance?
It is the complex resistance seen at the feed point. It has a resistive part and a reactive part. Both parts change with length, frequency, height, diameter, and nearby objects.
Why is a half wave dipole near 73 ohms?
A thin center fed half wave dipole in free space often has radiation resistance near 73 ohms. Real antennas differ because ground, height, wire thickness, and feed line routing change the feed point.
What does negative reactance mean?
Negative reactance means the dipole acts capacitive. It is usually electrically short for that frequency. Lengthening the antenna or using inductive matching can move it closer to resonance.
What does positive reactance mean?
Positive reactance means the dipole acts inductive. It is usually electrically long for that frequency. Shortening the antenna or using capacitive matching can reduce the reactance.
Does low SWR mean the antenna is efficient?
No. Low SWR only shows a match condition at the line. Poor ground, lossy wire, traps, or hot matching parts can still waste power as heat.
Can this replace an antenna analyzer?
No. This page gives a planning estimate. Use an antenna analyzer or vector network analyzer at the real installation for final trimming and matching.
Why does height affect impedance?
Ground reflection changes current, voltage, and radiation pattern near the wire. A dipole close to ground can show different resistance and more ground related loss.
Why include wire diameter?
Wire diameter affects bandwidth, reactance behavior, and conductor loss. Thicker conductors often tune less sharply and can make the antenna easier to match over a wider range.