Question 1

What is antenna resonance?

Accepted Answer

An antenna resonates when its physical length matches the wavelength (or a fraction) of the RF signal. At resonance, the antenna has minimum impedance mismatch and maximum radiation efficiency. Off-resonance operation causes poor matching and wasted power.

Question 2

What is a half-wavelength dipole?

Accepted Answer

A dipole antenna is two rod conductors of length λ/2 each, fed at the center. Half-wavelength dipoles have an impedance near 73 Ω (close to standard 50 Ω coax), making them easy to match. They radiate in a figure-8 pattern perpendicular to the axis.

Question 3

What is a quarter-wavelength monopole?

Accepted Answer

A monopole is a single conductor of length λ/4 mounted vertically above a ground plane. The ground plane acts as a mirror, creating a virtual second λ/4 element, so total effective length is λ/2. Monopoles are smaller than dipoles and common in car/base station antennas.

Question 4

What is velocity factor?

Accepted Answer

Velocity factor (VF) is the ratio of wave propagation speed in a conductor vs. free space (0.98 c for copper, 0.95 for aluminum in air). Nearby conductors or dielectrics reduce VF. Use VF to shorten antenna length for physical space constraints.

Question 5

How do I calculate antenna length?

Accepted Answer

For a dipole: Length = (300 MHz·m ÷ Frequency MHz) × VF ÷ 2. Example: 2.4 GHz dipole in free space (VF=0.98) = (300÷2400) × 0.98 ÷ 2 = 61 mm per arm (122 mm total).

Question 6

What is a Yagi antenna?

Accepted Answer

A Yagi is a directional antenna with a driven element (dipole), reflector (5% longer), and directors (5% shorter). Yagis focus radiation in one direction, providing gain (5–15 dBi typical) and directivity. Common in WiFi and amateur radio.

Question 7

What is antenna gain?

Accepted Answer

Gain (measured in dBi vs. isotropic) describes how much an antenna focuses energy in one direction vs. radiating equally in all directions. A dipole has ~2 dBi. A Yagi with 3 directors has ~8 dBi.

Question 8

What materials can I use for antenna wire?

Accepted Answer

Copper: Best (99% conductivity). Aluminum: Good (62% conductivity, lighter). Brass: Fair (28% conductivity). Stainless steel: Poor (3% conductivity, but corrosion-resistant). For efficiency, use copper or aluminum.

Question 9

How do I impedance-match an antenna?

Accepted Answer

Use a tuning network (L-network, Pi-network) or a matching transformer. For 50 Ω coax to a 73 Ω dipole, use a 1:1.5 transformer or a quarter-wave matching section. Smith Chart helps visualize matching.

Question 10

Can I shorten an antenna?

Accepted Answer

Yes, but with penalties. Use a loading coil in series to "extend" electrical length without physical length. The inductor introduces losses and reduces bandwidth. Reduced length (below λ/4) requires capacitive loading, reducing efficiency further.

Antenna Length Calculator