basic antenna

The basic antenna

The most basic antenna is called “a quarter wave vertical”, it is a quarter wavelength long and is a vertical radiator. Typical examples of this type would be seen installed on motor vehicles for two way communications. Technically the most basic antenna is an “isotropic radiator”. This is a mythical antenna which radiates in all directions as does the light from a lamp bulb. It is the standard against which we sometimes compare other antennas.

This type of antenna relies upon an “artificial ground” of either drooping radials or a car body to act as ground. Sometimes the antenna is worked against an actual ground – see later.

Antenna Polarisation

Depending upon how the antenna is orientated physically determines it’s polarisation. An antenna erected vertically is said to be “vertically polarised” while an antenna erected horizontally is said (not so surprising) to be “horizontally polarised”. Other specialised antennas exist with “cross polarisation”, having both vertical and horizontal components and we can have “circular polarisation”.

Note that when a signal is transmitted at one polarisation but received at a different polarisation there exists a great many decibels of loss.

This is quite significant and is often taken advantage of when TV channels and other services are allocated. If there is a chance of co-channel interference then the license will stipulate a different polarisation. Have you ever noticed vertical and horizontal TV antennas in some areas. Now you know why.

Antenna Impedance

Technically, antenna impedance is the ratio at any given point in the antenna of voltage to current at that point. Depending upon height above ground, the influence of surrounding objects and other factors, our quarter wave antenna with a near perfect ground exhibits a nominal input impedance of around 36 ohms. A half wave dipole antenna is nominally 75 ohms while a half wave folded dipole antenna is nominally 300 ohms. The two previous examples indicate why we have 75 ohm coaxial cable and 300 ohm ribbon line for TV antennas.

A quarter wave antenna with drooping quarter wave radials exhibits a nominal 50 ohms impedance, one reason for the existence of 50 ohm coaxial cable.

The quarter wave vertical antenna

The quarter wave vertical antenna is usually the simplest to construct and erect although I know a great many people who would dispute that statement. In this context I am speaking of people (the majority) who have limited space to erect an antenna.

The reason there has been emphasis on TV antennas is simply because nearly everyone can look at examples in their own locality for comparison. At TV frequencies the physical dimensions are such I can offer practical examples with photographs.

The same basic principles apply at HF and LF although physical sizes tend to be totally impractical.

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COMMENT Uncategorized

  1. edwin s. fuentes

    i want to design an antenna that can give reception specially my home were place in the remote area,can you provide me so that i can design for y own?

  2. Antenna designer

    Thank You very much this is all very Informative

  3. Admin

    Basic Antenna Theory
    Every structure carrying RF current generates an electromagnetic field and can radiate RF power to some
    extent. Likewise, an external RF field can introduce currents in the structure. This means that theoretically,
    any metallic structure can be used as an antenna. However, some structures are more efficient in radiating
    and receiving RF power than others. The following examples explain these concepts.
    Transmission lines (striplines, coaxial lines, etc.) are designed to transport RF power with as little radiation
    loss as possible because these structures are designed to contain the electromagnetic fields. To obtain any
    appreciable radiation from such a structure requires excessively high RF currents which causes low
    efficiency due to high losses. Likewise, the ability to introduce RF currents into the structure is of
    importance as described by the feed point impedance. If the feed point impedance is very high, low, and/or
    highly complex, it will be difficult to introduce RF current with good efficiency.
    The antenna structure should be a reasonable size compared to the wavelength of the RF field. A natural
    size is a half wavelength. A half wavelength corresponds to approximately 6 cm (in air) in the 2.4 GHz
    ISM band. This size is effective because when it is fed with RF power at the center point, the structure is
    resonant at the half wave frequency. Reducing the size below the natural resonant length can cause low
    efficiency. Not all structures make an efficient antenna.
    Numerous structures have been devised that provide good efficiency and a good impedance match, but
    most of these are derived from a few basic structures. A short description of these basic antennas and some
    advice on how to implement these with success is provided later in this note.
    It is beyond the scope of this note to include complicated formulas concerning antenna theory. This note
    is intended to provide basic information about how antennas work which should allow users to achieve
    reasonable performance with a minimum number of attempts.
    If users are interested in performing complex calculations and antenna simulations, they should consult the
    abundant and widely available literature concerning antenna theory and design. Simply copying an
    existing design does not necessarily ensure reasonable performance. A lot of external factors affect
    antenna tuning, gain, radiation patterns, etc. An antenna tuned for one set of environmental factors (PCB,
    different cases, etc.) may not perform at all if put into a new environment, and may require a lot of tuning
    to achieve even reasonable performance.

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