Calculator Form
Example Data Table
| Use Case | Distance | Frequency | Tx Power | Antenna Gains | Approx FSPL |
|---|---|---|---|---|---|
| Wi-Fi Point Link | 1 km | 2400 MHz | 20 dBm | 6 dBi / 6 dBi | 100.04 dB |
| Microwave Backhaul | 15 km | 5800 MHz | 27 dBm | 24 dBi / 24 dBi | 131.24 dB |
| Satellite Estimate | 35786 km | 12000 MHz | 40 dBm | 35 dBi / 45 dBi | 205.10 dB |
Formula Used
The main formula uses distance in kilometers and frequency in megahertz.
FSPL dB = 32.44 + 20 log10(distance km) + 20 log10(frequency MHz)
The wavelength formula is simple. Wavelength equals light speed divided by frequency in hertz.
Received Power dBm = Tx Power + Tx Gain + Rx Gain - FSPL - Total Losses
Link Margin dB = Received Power - Receiver Sensitivity
Margin After Fade = Link Margin - Required Fade Margin
How To Use This Calculator
Enter the path distance first. Select the matching distance unit. Then enter the operating frequency and its unit.
Add transmit power. You may enter it as dBm, watts, or milliwatts. Add antenna gains in dBi.
Enter cable losses and other losses as positive dB values. Add receiver sensitivity and required fade margin.
Press the calculate button. The result will appear above the form and below the page header.
Use the CSV button for spreadsheet storage. Use the PDF button for a simple report download.
Free Space Path Loss Planning Guide
What This Tool Measures
Free space path loss describes signal spreading in open space. It grows when distance increases. It also grows when frequency increases. This calculator helps estimate that basic loss before field testing. It is useful for radio, Wi-Fi, microwave, satellite, telemetry, and general wireless planning.
Why Distance Matters
Radio energy spreads as it travels. A longer path spreads the same energy over a larger area. That makes the received signal weaker. Doubling distance adds about 6 dB of loss. This rule helps users understand quick changes. It is not a replacement for a full survey.
Why Frequency Matters
Higher frequency signals have shorter wavelengths. In the free space equation, higher frequency also raises path loss. This does not mean low frequency is always better. Antenna size, bandwidth, noise, and rules also matter. The calculator gives a clear first estimate.
Advanced Link Budget Inputs
The form includes transmit power, antenna gains, cable losses, receiver sensitivity, and fade margin. These items create a practical link budget. Antenna gain helps the link. Cable and extra losses hurt it. Receiver sensitivity sets the minimum usable signal level.
Understanding Received Power
Received power shows the expected signal at the receiver input. A stronger value is better. For example, minus 55 dBm is stronger than minus 85 dBm. Compare this result with receiver sensitivity. The difference is the raw link margin.
Using Fade Margin
Fade margin is safety space. Weather, alignment, multipath, foliage, buildings, and hardware changes can reduce signal. A positive margin after fade is usually preferred. A small positive margin may still be risky. A negative result means the link needs improvement.
Improving A Weak Link
You can improve a weak link in several ways. Reduce distance when possible. Use better antennas. Lower cable loss. Increase transmit power within legal limits. Choose a better mounting location. Reduce extra losses. A site survey can confirm the estimate.
Best Use
Use this calculator during early design. Compare several frequencies and distances. Save results with CSV or PDF downloads. Keep notes about antennas and cable lengths. Then verify the final link with real measurements. Good planning reduces installation surprises.
FAQs
What is free space path loss?
It is the signal loss caused by radio energy spreading through open space. It depends mainly on distance and frequency. It does not include walls, rain, trees, cable loss, or antenna alignment problems.
Does higher frequency increase path loss?
Yes. In the standard equation, higher frequency increases the calculated loss. Antenna gain, bandwidth, and system design may still make a higher frequency link useful.
Which distance unit should I use?
Use any listed unit that matches your data. The calculator converts meters, feet, miles, nautical miles, and kilometers into kilometers for the main formula.
What is a good link margin?
A larger positive margin is better. Many designs target at least 10 dB to 20 dB, depending on reliability needs, climate, terrain, and service importance.
What does receiver sensitivity mean?
Receiver sensitivity is the weakest signal that the receiver can use properly. If received power is below this value, the link may fail or perform poorly.
Why add cable loss?
Cables and connectors reduce signal strength. Transmit cable loss lowers radiated power. Receive cable loss reduces signal before it reaches the receiver input.
Can this replace field testing?
No. It gives a useful planning estimate. Real sites may include obstructions, reflections, interference, polarization issues, weather effects, and installation losses.
What does EIRP mean?
EIRP means effective isotropic radiated power. It combines transmit power, antenna gain, and transmit cable loss into one radiated power estimate.