Antenna design frequently asked questions
Short answers to common antenna design questions. Follow the links in the glossary and contents for full articles with formulas and sources.
How long is a half-wave dipole?
About 143/f(MHz) meters in practice, a few percent shorter than the free-space half wavelength of 150/f(MHz) meters.
What is the gain of a dipole?
About 2.15 dBi, which defines 0 dBd.
What is the difference between gain and directivity?
Gain is directivity multiplied by radiation efficiency.
What VSWR is acceptable for an antenna?
2:1 (return loss about 9.5 dB, 0.51 dB mismatch loss) is a common threshold; infrastructure antennas often target 1.5:1.
What is the impedance of a quarter-wave monopole?
About 36.5 Ω on an infinite perfect ground plane.
Where does the far field begin?
Conventionally at 2D²/λ from an antenna of largest dimension D, and at least several wavelengths away.
Why do small antennas have narrow bandwidth?
Their minimum Q rises as 1/(ka)³, per the Chu limit, and bandwidth is roughly inversely proportional to Q.
Why does my PCB antenna work on the eval board but not in my product?
Ground-plane size, enclosure plastics, nearby metal and the user’s hand detune it; retune with a matching network in the final product.
What polarization does GPS use?
Right-hand circular polarization.
How much does a linear antenna lose receiving circular polarization?
3 dB in theory.
Does a higher-gain antenna increase range?
In its main beam, yes: 6 dB more gain doubles free-space range, but coverage narrows and regulations may limit EIRP.
What is the free-space path loss at 2.4 GHz over 100 m?
About 80 dB.
Do I need a balun for a dipole?
Yes, to prevent common-mode current on the coax shield.
Which free tools can simulate antennas?
NEC-2 based tools such as 4nec2 for wire antennas and openEMS for 3D structures.
What is the SAR limit in the US?
1.6 W/kg averaged over 1 g of tissue for portable devices used by the general population.