Front-to-Back Ratio Antenna Calculator
Calculate front-to-back ratio for directional antennas. Understand how well your antenna rejects rear signals compared to forward direction.
Signal Measurements
Signal strength in the forward direction
Signal strength in the rear direction
Front-to-Back Ratio
Enter front and back signal powers to calculate the antenna's front-to-back ratio
What Is Front-to-Back Ratio?
The front-to-back ratio (often called F/B ratio) measures how much better an antenna performs in its forward direction compared to its rear direction. It's like asking: "How much stronger is the signal in front of the antenna versus behind it?"
For directional antennas like Yagis, beams, or panels, this ratio tells you how well the antenna focuses its energy forward while rejecting signals from behind. A high F/B ratio means the antenna is very directional and won't pick up much interference from the rear.
Ham radio operators, wireless technicians, and RF engineers all care about F/B ratio because it directly affects signal quality and noise rejection.
Factors That Affect F/B Ratio
Several design and environmental factors influence an antenna's front-to-back ratio:
- Antenna Design: Yagi antennas typically have better F/B ratios than simple dipoles
- Element Spacing: Proper spacing between antenna elements improves directionality
- Frequency: Higher frequencies generally allow for better F/B performance
- Ground Reflection: Nearby ground or reflective surfaces can affect the pattern
- Installation Height: Higher antennas often have cleaner patterns
- Construction Quality: Precise element lengths and spacing matter
The calculator helps you measure the actual F/B performance of your antenna, regardless of these factors.
Remember, F/B ratio is measured, not just calculated from design. Real-world conditions always play a role.
Front-to-Back Ratio Formula
F/B Ratio Calculation Formula
F/B Ratio (dB) = 10 × log₁₀ (Front Power ÷ Back Power)
This formula expresses the power difference between front and back directions in decibels:
- • Front Power: Signal strength in the forward direction
- • Back Power: Signal strength in the rear direction
- • log₁₀: Base-10 logarithm (converts ratio to dB scale)
- • dB scale: Makes large ratios easier to work with
Example: Front = 100W, Back = 1W
Ratio = 100W ÷ 1W = 100
F/B = 10 × log₁₀(100) = 20 dB
Note: Power measurements must be in the same units (Watts, milliwatts, etc.) for accurate results. The calculator handles unit conversion automatically.
F/B Ratio Performance Examples
Typical Antenna Performance
| Front Power (W) | Back Power (W) | F/B Ratio (dB) | Performance |
|---|---|---|---|
| 100 | 1 | 20 | Excellent |
| 100 | 5 | 13 | Good |
| 100 | 20 | 7 | Fair |
| 100 | 50 | 3 | Poor |
Higher dB values indicate better directional performance. Commercial Yagi antennas typically achieve 15-25 dB F/B ratios.
Antenna F/B Ratio FAQs
Is higher F/B ratio always better?
Generally yes, but extremely high ratios (>30 dB) might indicate the antenna is too narrow for some applications. 15-25 dB is usually ideal for most uses.
Does height affect F/B ratio?
Yes, higher installation reduces ground reflection effects and can improve F/B performance by providing cleaner radiation patterns.
Can F/B ratio be measured or only calculated?
Both! You can calculate it from power measurements as shown here, or measure it directly using antenna test ranges or field strength meters.
What affects F/B ratio measurements?
Nearby objects, ground conditions, and measurement setup all influence results. Professional measurements use controlled environments.