How Do I Choose the Best Multiband Antenna?

Multi-band is a technology that uses multiple bands. For example, the dual bands in the Global System for Mobile Communications (GSM) are 900 MHz and 1800 MHz. The advantage of multi-band lies in its adaptability and scalability, the number of frequency bands can be different, and it can be dynamically adjusted to reduce the impact of mutual interference, and avoid occupying frequency bands required by other services. ^[1]

The electrical parameters of the antenna are generally related to the operating frequency. When the operating frequency deviates from the design value, the electrical parameter index will decrease. So the working frequency bandwidth depends on the allowable variation range of the antenna electrical parameters

In microwave relay communications and satellite communications, the operating frequency band is sometimes segmented, with a gap between the two bands.

When the working frequency band has only one continuous segment, it is called a single frequency band; when there are two continuous segments separated by a frequency band gap, it is called a dual frequency band; when there are more than three continuous segments separated by a frequency band gap, it is called a multi-frequency band. At present, it is increasingly required to design wide-band antennas to meet the needs of dual-band or multi-band operation. ^[2]

Software-defined radio requires the use of wideband antenna technology that can meet the requirements of low frequency (LF) and ultra high frequency (UHF), including co-site interference cancellation, interaction of multi-band and multi-mode antennas, insertion loss, feed capacity, and easy reconfiguration. Smart Antenna (SmartSoftAntenna).

In addition to widening the frequency response bandwidth of each array element during design, it also involves the geometry of the array elements and the array element spacing, and the directional resolution in the array processing and the correlation between the signals received by each array element. Related to the array element spacing. The uniform circular array can form multiple beams within 360 ° at the same time and has better bandwidth characteristics.

In order to receive multiple desired signals at different operating frequencies and in different directions at the same time, multi-frequency and multi-beam forming and multi-frequency multiple nulling techniques can be adopted, and the frequency and direction characteristics of the signals can be estimated by means of a joint frequency-direction estimation method.

In order to reduce the size and cost of the equipment, and to avoid the impact of inconsistent channels in a multi-channel situation, a single-channel array processing technology can be used as appropriate.

In addition, it is now possible to "print" antennas on a substrate in a superimposed manner. For example, Bell Aerospace is conducting a research on a flat glass antenna (VitreousAntenne) that allows the antenna array aperture to be superimposed on a substrate. The material between them is electronically transparent to other layers or transmitters. Covers from UHF to S-band. ^[3]

There are two ways to achieve multi-band operation:

In a non-adjacent channel full-channel system, in order to solve the problem that the transmission band of the line is too wide, and the high and low frequency gains are not easy to balance, the transmitted signal can be divided into several frequency bands to be amplified separately and then mixed and output. Amplifier.

Multi-band amplifiers are broadband amplifiers.

Multi-band amplifier circuit block diagram

Segment input, processing and amplification, there are FM, UHF-, UHF- and U four frequency band input ports to achieve multi-channel mixing, as shown on the right.

Multi-band amplifiers use LC filters for band-pass filtering, which can suppress signals outside the band, and the out-of-band rejection is above 25dB. The amplifier generally uses a transistor to form a common emitter amplifier circuit. The first stage is a pre-low-noise amplifier, the second stage is a low-power drive stage, and the third stage is a power amplifier stage. In order to improve the stability of the amplifier and obtain a flat frequency response, the amplifier circuits use parallel voltage negative feedback and series feedback circuits.

The technical indicators of the multi-band amplifier are as follows:

Gain: 24, 39dB

In-band flatness: -1 + 3dB

Noise figure: 7dB

Reflection loss: 7.5dB (), 10dB (, U)

Maximum output level: 110, 118 dB

Power supply voltage: ~ 220V

Because the multi-band amplifier amplifies signals of several bands with different frequencies, the amplifier of each channel can be adjusted separately to control the level of each band. At the same time, it also solves the problem of level imbalance and nonlinear distortion caused by too many channels. Obviously, this kind of amplifier can be used as an antenna amplifier or used in the front end of a small cable TV system, but cannot be used in an adjacent frequency transmission system. This is because the adjacent frequency system contains several supplemental channels, which are outside the several bands that the amplifier can amplify. ^[5]