Ham Radio Antenna Q and Bandwidth Calculator
Calculate antenna bandwidth from Q-factor and resonant frequency.
Get -3dB bandwidth and 2:1 SWR bandwidth for HF, VHF, and UHF antenna designs.
Antenna Q-Factor and Bandwidth
Q-factor (quality factor) measures how sharply tuned an antenna is. Higher Q = narrower bandwidth (more selective, less tolerant of frequency drift). Lower Q = wider bandwidth (more forgiving, easier to use across a band).
The basic formulas: -3 dB bandwidth = Resonant frequency / Q 2:1 SWR bandwidth ≈ Resonant frequency / Q × 0.71
Typical Q-factors by antenna type:
| Antenna | Typical Q |
|---|---|
| Mobile whip (HF) | 100-500 |
| Trap dipole (HF) | 30-80 |
| Full-size half-wave dipole | 8-15 |
| Full-size quarter-wave vertical | 8-15 |
| Random-wire / G5RV | 10-25 |
| Magnetic loop | 200-1,000+ (very high Q) |
| Yagi-Uda (HF) | 12-25 |
| Yagi-Uda (VHF/UHF) | 10-20 |
| Helically-wound antenna | 80-200 |
| Loaded short antenna | 100-400 |
The bandwidth-efficiency tradeoff:
- High Q antennas: very efficient but narrow bandwidth — must retune every few hundred kHz on HF
- Low Q antennas: wide bandwidth but lower efficiency — usually full-size dimensions
- Magnetic loops are the extreme case: very small, very high Q (efficiency varies)
Examples:
Full-size dipole on 14 MHz, Q = 12:
- -3dB BW = 14000/12 = 1,167 kHz
- 2:1 SWR BW = 1167 × 0.71 = 828 kHz
- Covers entire 20m band (14.000-14.350 MHz)
Mobile whip on 14 MHz, Q = 200:
- -3dB BW = 14000/200 = 70 kHz
- 2:1 SWR BW = 70 × 0.71 = ~50 kHz
- Need to retune every ~50 kHz!
Magnetic loop on 14 MHz, Q = 400:
- -3dB BW = 14000/400 = 35 kHz
- 2:1 SWR BW = ~25 kHz
- Continuously retunable, very narrow
Practical implications:
- Mobile operators typically use auto-tuners or remote-tunable whips because their Q is so high
- Magnetic loop owners love this for selectivity (rejects out-of-band noise) but must retune for every QSO
- Field Day or contest dipoles usually low-Q to cover wider frequency ranges
- Multi-band operation with high-Q antennas requires either an antenna tuner or multiple antennas
Antenna tuner trick: A tuner DOESN’T change the antenna’s Q — it just makes the radio see 50 ohms across the antenna’s whole bandwidth. The actual radiation efficiency at far-from-resonance frequencies drops sharply.
Q vs efficiency: For a given physical antenna size, doubling Q by adding loading coils or capacitance does NOT improve efficiency — it just narrows bandwidth. True efficiency comes from:
- Antenna size (closer to resonance = higher radiation resistance)
- Conductor losses (silver-plated > copper > aluminum)
- Ground losses (radials, ground plane, height above ground)
- Loading components (high-Q capacitors, large-gauge inductor wire)
Measurement:
- Use a network analyzer or VNA (NanoVNA) to sweep
- Find resonance (lowest SWR)
- Find frequencies where SWR = 2:1
- Q ≈ Resonant freq / (BW between 2:1 SWR points / 0.71)
Example with NanoVNA: A dipole shows resonance at 14.080 MHz with 1.05:1 SWR. At 13.700 MHz: SWR = 2.0:1 At 14.460 MHz: SWR = 2.0:1
- 2:1 BW = 760 kHz
- -3dB BW ≈ 1071 kHz
- Q ≈ 14080 / 1071 ≈ 13 (typical full-size dipole)