Calculation result
Choose a mode, enter values, then calculate.
dBi gain difference
Compare two antenna gains and express their difference in dB.
Step-by-step solution
- Select a calculation mode.
- Enter valid engineering values.
- Press calculate to see each step.
Calculation trace
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Gain and loss visualization
The chart updates after supported calculations.
| Time | Mode | Inputs | Primary result | Action |
|---|---|---|---|---|
| No calculations saved yet. | ||||
Formula reference
| Purpose | Formula | Important condition |
|---|---|---|
| Gain difference | ΔG = G₂ − G₁ | Both gains must use the same reference. |
| Power ratio | Ratio = 10^(dB/10) | Use for power, energy, or intensity. |
| Voltage ratio | Ratio = 10^(dB/20) | Impedances should remain equal. |
| dBi to dBd | dBd = dBi − 2.15 | dBd references a half-wave dipole. |
| EIRP | Ptx + Gantenna − Lsystem | Use logarithmic values consistently. |
| Free-space loss | 32.44 + 20log₁₀(dkm) + 20log₁₀(fMHz) | Assumes unobstructed free-space propagation. |
| Receiver sensitivity | −174 + 10log₁₀(B) + NF + SNR | Bandwidth B must use hertz. |
| Fresnel radius | √(λd₁d₂/(d₁+d₂)) | Distances must use matching units. |
How to use this calculator
- Choose the calculation mode matching your engineering task.
- Enter each value using the displayed unit selector.
- Review assumptions shown beneath specialized engineering inputs.
- Choose the required decimal precision from the sidebar.
- Press calculate and inspect the primary result first.
- Review ratios, percentages, margins, and secondary conversions.
- Check the substituted formula inside the calculation trace.
- Export results using CSV, PDF, print, or sharing.
Understanding dBi, dB, and related units
dB describes a ratio between two measured quantities. It has no independent antenna reference. dBi describes antenna gain against an ideal isotropic radiator. Therefore, direct conversion needs a defined comparison or reference.
Subtracting two dBi values produces a valid dB difference. Adding cable loss changes a system result logarithmically. This calculator keeps each reference visible during every calculation.
Why antenna references matter
An isotropic radiator spreads power equally in every direction. It is theoretical, yet useful. A half-wave dipole has a different reference pattern. Its gain equals about 2.15 dBi. Consequently, zero dBd equals about 2.15 dBi.
Power ratios and voltage ratios
Power calculations use ten times the base-ten logarithm. Voltage calculations usually use twenty times that logarithm. The voltage relationship assumes equal impedance. Different impedances require a power-based approach.
Interpreting common changes
A 3 dB increase nearly doubles received power. A 6 dB increase nearly quadruples it. A 10 dB increase multiplies power by ten. Negative values represent reductions using the same logarithmic relationships.
System calculations
Real links include transmitter power, antenna gains, and losses. Cable, connectors, obstacles, and path loss reduce received power. Link margin compares received power against receiver sensitivity. Positive margin usually indicates stronger operating reserve.
Engineering assumptions
Free-space loss assumes clear line-of-sight propagation. Fresnel calculations estimate clearance around that path. Radio horizon calculations approximate Earth curvature effects. Field-strength estimates assume far-field conditions and suitable antenna behavior.
Using exported results
CSV output supports spreadsheets and project records. PDF output creates a compact calculation report. Shared links preserve the selected mode and entered values. Local history helps compare repeated antenna scenarios.
Practical accuracy guidance
Use manufacturer cable data whenever available. Confirm antenna gains use matching references. Include connector losses and installation tolerances. Keep several decibels of fade margin. Accurate inputs create dependable radio planning results.
Field verification workflow
Begin with transmitter settings and verified antenna specifications. Record every connector, adapter, splitter, and device. Measure installed cable lengths instead of trusting design estimates. Use frequency-specific attenuation because cable loss changes across bands. Confirm both antenna polarizations match before trusting predicted margins. Model seasonal foliage when planning outdoor microwave paths. Add rain loss for higher frequencies and long distances. Include mounting tolerances, aging, and manufacturing variation carefully. Recalculate after equipment changes or site maintenance. Finally, compare predictions with field measurements and adjust assumptions.
Final reminder
Always verify units before applying logarithmic engineering formulas carefully.
Example data table
| Scenario | Inputs | Expected result | Meaning |
|---|---|---|---|
| Antenna comparison | 3 dBi and 9 dBi | 6 dB difference | Second antenna offers about four times power gain. |
| Cable reduction | 8 dBi, 2 dB loss | 6 dBi effective | Feedline loss reduces effective system gain. |
| dBi conversion | 8.15 dBi | 6 dBd | References a half-wave dipole. |
| Linear conversion | 10 dBi | 10× power ratio | Power becomes ten times the reference. |
| EIRP estimate | 20 dBm, 8 dBi, 2 dB loss | 26 dBm | Equivalent isotropic radiated power equals 0.398 watts. |
| Simple link | 30 dBm, 12 dBi, 12 dBi, 110 dB loss | −56 dBm before local losses | Receiver level remains comfortably measurable. |
Frequently asked questions
Can dBi be converted directly to dB?
Not as an absolute standalone conversion. dBi contains an isotropic reference. However, subtracting two dBi values produces their difference in dB.
Does 3 dB always double a signal?
Three decibels approximately doubles power. It does not double voltage unless impedance conditions are considered. Voltage doubling equals roughly 6.02 dB.
What is the difference between dBi and dBd?
dBi references an ideal isotropic radiator. dBd references a half-wave dipole. Add approximately 2.15 to dBd for dBi.
What does negative dB mean?
Negative dB indicates loss or reduction relative to a reference. The underlying power ratio remains positive and falls below one.
How is EIRP calculated?
Add transmitter power and antenna gain. Then subtract cable, connector, and other system losses. Keep every logarithmic value compatible.
Why does voltage use 20 log instead?
Power depends on voltage squared when impedance stays constant. That squared relationship changes the logarithmic multiplier from ten to twenty.
What is a good link margin?
Required margin depends on reliability goals and environment. Many practical links reserve extra margin for fading, weather, interference, and installation variation.
Are free-space results exact outdoors?
No. Terrain, buildings, reflections, foliage, weather, polarization, and interference can change real performance. Use free-space results as a planning baseline.