Calculator Inputs
Formula Used
Wavelength: λ = c / f
Perimeter: P = 2 × (width + height)
Area: A = width × height
Radiation resistance: Rr = 31200 × (N × A / λ²)²
Approximate inductance: L = μ0 / π × N² × [a ln(2a/r) + b ln(2b/r) + 2(√(a²+b²) − a − b)]
Tuning capacitance: C = 1 / ((2πf)² × L)
Efficiency: η = Rr / (Rr + loss resistance) × 100
Loaded Q: Q = XL / (Rr + loss resistance)
How to Use This Calculator
- Enter the operating frequency in MHz.
- Add the rectangular loop width and height in meters.
- Enter the number of turns used in the loop.
- Add conductor diameter and estimated loss resistance.
- Enter transmitter power and feed system impedance.
- Press Calculate to view the result below the header.
- Use CSV or PDF buttons to save the result.
Example Data Table
| Frequency | Width | Height | Turns | Conductor | Loss | Expected Use |
|---|---|---|---|---|---|---|
| 7.10 MHz | 1.5 m | 1.5 m | 1 | 22 mm | 0.15 Ω | Compact HF loop |
| 14.20 MHz | 1.0 m | 1.0 m | 1 | 10 mm | 0.25 Ω | Portable loop |
| 28.40 MHz | 0.6 m | 0.8 m | 1 | 8 mm | 0.20 Ω | Small tuned loop |
Understanding Rectangular Loop Antenna Design
A rectangular loop antenna is a closed conductor shaped by two vertical sides and two horizontal sides. Its behavior depends on frequency, perimeter, enclosed area, conductor size, and loss resistance. These values decide whether the loop works like a small magnetic loop or a larger resonant loop.
Why Shape Matters
The rectangle gives builders practical control. A wide loop may fit along a wall. A tall loop may fit beside a mast. The area supports magnetic coupling. The perimeter controls the electrical size. When the perimeter is much smaller than the wavelength, the loop has low radiation resistance. Loss then becomes very important.
Important Design Outputs
This calculator estimates wavelength, perimeter ratio, loop area, radiation resistance, inductance, tuning capacitance, reactance, loaded Q, bandwidth, and efficiency. These outputs help compare designs before cutting wire or tubing. They also show why thick conductors and clean joints can improve performance.
Tuning and Practical Use
A small transmitting loop usually needs a high voltage tuning capacitor. The required capacitance changes quickly with frequency and inductance. A receive loop may use lighter parts, but matching still matters. Always rate parts for expected power. Keep hands away from tuned loops during transmission.
Reading the Results
A lower perimeter ratio suggests a compact magnetic loop. A higher ratio suggests the loop is moving toward full wave behavior. High Q means narrow bandwidth and sensitive tuning. Low efficiency warns that conductor loss dominates radiation resistance. Use the example table as a starting point, then adjust dimensions for your band and available space.
Common Building Choices
Copper tube often gives better efficiency than thin wire. Larger diameter lowers resistance. It also spreads current over more surface. Multiple turns raise inductance and voltage. They can help reception, yet they may reduce usable bandwidth. Keep the loop rigid. Small movement can shift resonance. Place the feed point carefully. A coupling loop, transformer, or gamma match may be used. The best method depends on frequency, power, and receiver or transmitter impedance. Outdoor loops need weather protection. Indoor loops need distance from metal objects. Test with low power first. Then verify tuning, current, and heating during short transmissions. Record settings, so later improvements are easy to compare accurately.
FAQs
What is a rectangular loop antenna?
It is a closed antenna loop shaped as a rectangle. It can work as a small magnetic loop or as a larger resonant loop, depending on its perimeter compared with wavelength.
Why does loop area matter?
Loop area affects magnetic coupling and radiation resistance. A larger area usually improves radiation resistance for small loops, but size, tuning, and available space must also be considered.
What is a small loop antenna?
A small loop usually has a perimeter below one tenth of the wavelength. It often needs careful tuning because radiation resistance is low and losses can dominate.
Why is tuning capacitance important?
The capacitor resonates with loop inductance at the chosen frequency. Small changes in capacitance can shift resonance, especially when loaded Q is high.
Can this calculator design a transmitting loop?
It can estimate useful planning values. Real transmitting loops need safe spacing, high voltage parts, accurate matching, and final testing with suitable RF instruments.
Why is efficiency sometimes low?
Efficiency falls when loss resistance is large compared with radiation resistance. Better conductors, larger tubing, cleaner joints, and lower contact loss can improve performance.
What does loaded Q mean?
Loaded Q shows how sharply the loop is tuned. A higher value means narrower bandwidth, stronger tuning sensitivity, and greater circulating voltage.
Is the inductance result exact?
No. It is an estimate for planning. Real inductance changes with construction, nearby objects, conductor shape, joints, feed method, and measurement conditions.