1 Optimal Radiation Length of Antennas#
1.1 Vertical Monopole Antenna#
For an ideal upright antenna, its optimal radiation efficiency occurs at a length of approximately $$
L=\frac{\lambda}{4}
which means the antenna length is best when it is $\frac{1}{4}$ of the electromagnetic wave wavelength.
## Half-Wave Dipole Antenna
L=\frac{\lambda}{2}
## Full-Wave Antenna
L=\lambda
## Supplement
A grounded vertical antenna, when its length is $\frac{\lambda}{4}$, has the maximum voltage at the top of the antenna, zero current, and maximum base current, making it the most ideal radiation point.
# 2 Antenna Loading
By loading the antenna, the electrical length of the antenna is extended, allowing for the use of a physically shorter antenna. Essentially, this adjusts the resonant conditions of the antenna.
## 2.1 Inductive Loading
- Used for antennas shorter than $\frac{\lambda}{4}$ to compensate for their capacitive characteristics.
- An inductor is inserted at an appropriate position on the antenna, usually near the bottom or middle.
- Antennas shorter than $\frac{\lambda}{4}$ exhibit capacitive behavior (negative reactance), and adding inductance (positive reactance) compensates for this, allowing the antenna to resonate.
## 2.2 Capacitive Loading
- Used to increase the equivalent length of the antenna.
- A capacitive structure is added at the top of the antenna.
- Increases the voltage at the top of the antenna, enhances radiation efficiency, reduces the antenna's reactance to ground, and improves the stretching effect of current distribution.
| Item | Inductive Loading | Capacitive Loading |
|---|---|---|
| Reason | Compensate for the capacitance of short antennas | Enhance end voltage, improve efficiency |
| Position | Series at the bottom/middle | Parallel at the top/load "capacitive hat" |
| Effect | Achieve resonance | Electrically extend length |
| Common Applications | Shortwave whip antennas | Mobile stations, vehicle antennas, etc. |
## Impact of Loading Position
- Bottom loading: Little effect on current, requires a large inductance value, lower efficiency.
- Middle loading: Better efficiency, complex structure.
- Top loading: Best radiation efficiency, but complex structure and high wind resistance.