What Is an Antenna Feed?

The antenna feed line refers to the electrical signal energy transmission line connecting the antenna and the transceiver. Common feeders include overhead lines, coaxial cables, waveguides, and so on.

The antenna feed line refers to the electrical signal energy transmission line connecting the antenna and the transceiver. Common feeders include overhead lines, coaxial cables, waveguides, and so on.

Chinese name

Antenna feeder

English name

antenna feed line

definition

Connect the electrical signal energy transmission line between the antenna and the transceiver. Common feeders include overhead lines, coaxial cables, waveguides, and so on.

Applied discipline

Communication technology (level 1 discipline), wireless transmission and access (level 2 discipline)

The above content was published by the National Science and Technology Terminology Examination Committee.

Chinese name

Antenna feeder

Foreign name

antenna feed line

Applied discipline

Communication technology, wireless transmission and access

definition

The antenna feed line refers to the electrical signal energy transmission line connecting the antenna and the transceiver.

classification

Common feeders include overhead lines, coaxial cables, waveguides, and so on.

2.1 Overhead line

Overhead open wire is the most commonly used feeder line antenna is an overhead two-wire transmission line. Its advantages are simple structure, economy, and low loss. The disadvantage is that antenna effect is easy to occur at high frequencies. Therefore, it is mostly used for long, medium and short waves, or for ultra-short wave bands with short transmission distances. In order to overcome the shortcomings of the antenna effect, a shielded double wire can also be used as the feeder. The use of dielectric-insulated double wires in low-power transmission is not only immune to external climate change, but also has the advantages of convenient installation and low price. The overhead four-wire or six-wire transmission line used by the transmitting or receiving station is a modification of the two-line transmission line. As the cross section of the wire is increased, the transmission loss is reduced accordingly. Diagonal reception of four-wire transmission lines can reduce spatial interconnection and avoid interference.

2.2 Coaxial cable

When the frequency increases, the coaxial cable can overcome the antenna effect of the transmission line. A coaxial cable consists of an inner conductor and a coaxial outer conductor. The source voltage is connected to the inner and outer conductors, respectively. The outer conductor should generally be grounded and shielded. The same axis is an asymmetric feeder. When connecting to a symmetric antenna, a corresponding converter should be connected. The inner conductor of a coaxial cable is generally a copper wire, and the outer conductor can be a braided wire or a copper-aluminum tape coil. The two are insulated with a high-frequency dielectric (such as polyethylene). There are forms such as padding, spacers, spiral strips, fish cells, etc., and they have the advantages of flexibility, easy installation, and uniform impedance, so they are widely used in places where the transmission power is not large. The inner conductor of a hard coaxial wire is a copper core, and the outer conductor is a copper tube. Filling with dry compressed air or other inert gas (such as nitrogen) can improve the ability to resist breakdown and moisture. The inner conductor is generally supported by a washer or a spiral band. Suitable for transmitting high power.

2.3 waveguide

When the frequency is further increased, the inner conductor loss of the coaxial line increases due to the skin effect; the power capacity is reduced, so the inner conductor can be eliminated and the energy can be transmitted with a hollow waveguide. Commonly used feed waveguides are rectangular and circular. The electromagnetic waves propagating in the waveguide are no longer transverse electromagnetic waves (referred to as TEM waves), but transverse or magnetic waves (referred to as TE or TM waves). Compared with coaxial feeders, waveguides have the advantages of low loss, large power capacity, and simple manufacturing; their disadvantages are that they are prone to produce unwanted wave patterns (transmission modes), are limited by critical frequencies, and require higher accuracy in processing and installation.

Main parameters of feeder

The main parameters of the feeder are characteristic impedance, standing wave ratio, transmission loss (or efficiency), breakdown voltage, power capacity, and frequency bandwidth.

Sometimes in order to save money or be limited by the antenna site, two (or more) transmitters need to share an antenna. At this time, the feeder from each transmitter should be connected with a filter before receiving the common antenna, so that the electromagnetic energy flow of this unit can pass smoothly, but the electromagnetic energy flow of other transmitters will not. Backflow. Enter the machine. When designing the filtering device, it must be ensured that all the transmitters can be matched with the antenna when they work separately.

The same receiving antenna can also be shared by two (or more) receivers. At this time, the radio wave received by the antenna is connected to each receiver separately through a broadband amplifier and a shunt coupling device (called an antenna duplexer).

In addition, several transmitters (or receivers) can also be connected to several different antennas through a switch on the switch. The requirements for these switches are simple devices, convenient operation, and low reflection.

Requirements for feeders

The requirements for the feeder are mainly the following:

(1) In principle, the feeder line should have no antenna effect: the feeder line connected to the transmitter should not radiate electromagnetic energy; the feeder line connected to the receiver should not be picked up by external electric fields.

(2) The efficiency of the feeder line for transmitting electromagnetic energy should be as high as possible under reasonable conditions, that is, the loss on the feeder line (including transmission loss and reflection loss at each interface) should be as small as possible.

(3) The standing wave on the feeder should be as small as possible, that is, the characteristic impedance of the feeder should match the input impedance of the antenna. Avoid transmission efficiency drop due to mismatch, cause excessive voltage, and cause corona or breakdown.

(4) The feeder should have sufficient bandwidth and power capacity.

(5) When connecting components on coaxial or waveguide feeders, such as impedance converters, tuning devices, power combiners (distributors), attenuators, phase shifters, filters, transfer switches, and the same antenna At the same time, it also serves as a duplexer that is added when transmitting and receiving. Both should reduce the insertion loss and reflection loss as much as possible.

With the development of radio technology, microstrip transmission lines, surface wave transmission lines, dielectric waveguides, etc. have gradually appeared.