What is Trap Antenna?

Many antennas can effectively resonate and work in more than one band, one of which is called a trap antenna.

A trap is a combination of inductance and capacitance, usually an inductor and a capacitor connected in parallel.

When the frequency of the radio wave is equal to the resonant frequency of the parallel circuit, the circuit presents a very high impedance and can be regarded as an open circuit; when the frequency of the radio wave is lower than the resonant frequency, the circuit is inductive; and when the frequency of the radio wave is higher than the resonant frequency, it is capacitive.

Another form of Trap is commonly found in commercial antennas, and its principle is slightly different from the above. The Trap of this antenna is generally not resonant on a certain amateur band, but outside the amateur band.

When the working amateur frequency is lower than the resonant frequency of the Trap, the Trap is inductive, which is equivalent to the inductance added to the antenna, and has the effect of extending the electrical length of the antenna; conversely, when the working frequency is higher than the resonant frequency of the Trap, the Trap is capacitive, like a capacitor connected in series on the antenna arm, which can shorten the electrical length of the antenna. In this way, the physical length of the antenna remains unchanged, while the electrical length changes with the working frequency, which can ensure that the antenna is always in a resonant state.

Trap has such an effect, not only used in upright antennas and horizontal dipole antennas, but also widely used in multi-band directional antennas to avoid the various problems caused by setting up a directional antenna for each band.

Trap can also be replaced by transmission line. A 1/4λ (wavelength) transmission line with one end short-circuited and an inductor and capacitor parallel circuit resonating at the corresponding frequency have exactly the same effect.

Since the inductor and capacitor in the ordinary trap need to be waterproof and high-voltage resistant, the transmission line type trap shows its advantages and is often used in commercial multi-band antennas, especially VHF multi-band directional antennas.