Antenna tuner

# Antenna tuner

Discussion

Encyclopedia
An antenna tuner, transmatch or antenna tuning unit (ATU) is a device connected between a radio transmitter or receiver and its antenna to improve the efficiency of the power transfer between them by matching
Impedance matching
In electronics, impedance matching is the practice of designing the input impedance of an electrical load to maximize the power transfer and/or minimize reflections from the load....

the impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

of the equipment to the antenna. An antenna tuner matches a transceiver with a fixed impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

(typically 50 ohm
Ohm
The ohm is the SI unit of electrical resistance, named after German physicist Georg Simon Ohm.- Definition :The ohm is defined as a resistance between two points of a conductor when a constant potential difference of 1 volt, applied to these points, produces in the conductor a current of 1 ampere,...

s for modern transceivers) to a load (feed line and antenna
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...

) impedance which is unknown, complex or otherwise does not match. An ATU allows the use of one antenna for a broad range of frequencies. An antenna plus matcher is never as efficient as a naturally resonant antenna due to additional induced losses on the feed line due to the SWR
Standing wave ratio
In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....

(multiple reflections), and losses in the ATU itself, although issues of pattern and capture area may outweigh this in practice. An ATU is actually an antenna matching unit, as it is unable to change the resonant frequency of the aerial. Note that similar matching networks are used in other types of equipment, such as linear amplifier
Linear amplifier
A linear amplifier is an electronic circuit whose output is proportional to its input, but capable of delivering more power into a load. The term usually refers to a type of radio-frequency power amplifier, some of which have output power measured in kilowatts, and are used in amateur radio...

s to transform impedances. See impedance bridging
Impedance bridging
In electronics, especially audio and sound recording, a high impedance bridging, voltage bridging, or simply bridging connection is one which maximizes transfer of a voltage signal to the load...

for details.

## Basic principle of wide-band designs

For systems which need to operate over a wide frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...

range such as a power amplifier operating over the range 1 MHz to 30 MHz in some solid state designs a series of wide band transformers wound on ferrite
Ferrite (magnet)
Ferrites are chemical compounds consisting of ceramic materials with iron oxide as their principal component. Many of them are magnetic materials and they are used to make permanent magnets, ferrite cores for transformers, and in various other applications.Many ferrites are spinels with the...

cores can be used. This design has the advantage of not requiring any tuning when the operating frequency is changed. This type of design can also be used to match an antenna to a transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

, it has the advantage of not requiring any tuning but it has the disadvantage of not being capable of fine adjustment. These networks can be used to extend the useful range of a conventional narrow band ATU.

In solid state RF power amp design these networks are useful because MOSFET
MOSFET
The metal–oxide–semiconductor field-effect transistor is a transistor used for amplifying or switching electronic signals. The basic principle of this kind of transistor was first patented by Julius Edgar Lilienfeld in 1925...

s and bipolar
Bipolar
-Medicine:* Bipolar cell* Bipolar cell of the retina* Bipolar disorder** Bipolar I disorder** Bipolar II disorder** Bipolar NOS* Bipolar spectrum-Astronomy:* Bipolar nebula, a two-lobed, axially symmetric nebula...

transistors are designed to operate with low resistance loads. Valved RF amplifiers are very different because the load resistance which a valve is designed to operate with is normally much greater, hence for power designs the circuit designs are often very different.

Below is shown for example an autotransformer
Autotransformer
An autotransformer is an electrical transformer with only one winding. The auto prefix refers to the single coil acting on itself rather than any automatic mechanism. In an autotransformer portions of the same winding act as both the primary and secondary. The winding has at least three taps where...

which has three identical windings on a ferrite core. If the right hand side is connected to a resistive load of 10 ohms, then at each of the left hand terminals a source of different impedance can be attached.

## Basic principle of narrow-band designs

Devices based upon both lumped components and transmission line
Transmission line
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that its wave nature must be taken into account...

s can be purchased or constructed. The most simple example of a transmission line based system is the transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

formed by a quarter wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...

of mismatched transmission line. For example if a quarter wavelength of 75 Ω coaxial cable is linked to a 50 Ω load then the SWR
Standing wave ratio
In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....

in the 75 Ω quarter wavelength of line can be calculated to be 75 Ω / 50 Ω = 1.5, the quarterwavelength of line transforms this mismatched impedance to 112.5 Ω (75 Ω x 1.5 = 112.5 Ω). For more details please see the Smith chart
Smith chart
The Smith chart, invented by Phillip H. Smith , is a graphical aid or nomogram designed for electrical and electronics engineers specializing in radio frequency engineering to assist in solving problems with transmission lines and matching circuits...

.

The basic circuit required when lumped capacitances and inductors are used is shown below.
This basic network is able to act as an impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...

transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

. If the output has an impedance consisting of Rload and jXload, while the input is to be attached to a source which has an impedance of Rsource and jXsource.

Then

and

In this example circuit XL and XS can be swapped. All the ATU circuits below serve to create this network which exists between the systems with different impedances.

For instance if the source has a resistive impedance of 50 ohms while the load has a resistive impedance of 1000 ohms then.

If the frequency is 28 MHz then

As,

then,

So,

While as,

then,

### How it works

A parallel network consisting of a resistive element (1000 Ω) and a reactive element (-j 229.415 Ω) will have the same impedance and power factor as a series network consisting of resistive element (50 Ω) and a reactive element (-j 217.94 Ω). Below are
By adding another element in series which has a reactive impedance of +j 217.94, the impedance is 50 ohms (resistive).

#### Classic circuit

The Ultimate Transmatch is a good circuit with an unfortunate name, because a better transmatch called the SPC (series parallel capacitor) circuit was designed after it. In all three designs, the GND terminal is the terminal where the earth plane (ground plane
Ground plane
In electrical engineering, a ground plane is an electrically conductive surface.-Radio antenna theory :In telecommunication, a ground plane structure or relationship exists between the antenna and another object, where the only structure of the object is a structure which permits the antenna to...

) of an antenna should be wired while the ANT terminal is where the vertical element of a Marconi
Marconi Company
The Marconi Company Ltd. was founded by Guglielmo Marconi in 1897 as The Wireless Telegraph & Signal Company...

aerial should be attached. The configuration, although capable of matching a large impedance ratio, is a high-pass filter
High-pass filter
A high-pass filter is a device that passes high frequencies and attenuates frequencies lower than its cutoff frequency. A high-pass filter is usually modeled as a linear time-invariant system...

and will not attenuate spurious radiations above the cutoff frequency
Cutoff frequency
In physics and electrical engineering, a cutoff frequency, corner frequency, or break frequency is a boundary in a system's frequency response at which energy flowing through the system begins to be reduced rather than passing through.Typically in electronic systems such as filters and...

.

#### How it works

For example, imagine source impedance of 200 ohms and a resistive load of 1000 Ω which is connected via a capacitor with an impedance of -j 200 Ω to the inductor of the transmatch. Through vector mathematics it is possible to transform this into a parallel network consisting of a resistance of 1040 Ω and a capacitor of admittance 1.9231 x 10−4 (Xc = 5200 Ω).

In the following calculations all phase angle
Phase angle
In the context of vectors and phasors, the term phase angle refers to the angular component of the polar coordinate representation. The notation A\ang \!\ \theta,   for a vector with magnitude A and phase angle θ, is called angle notation.In the context of periodic phenomena, such as a wave,...

s are expressed in degree
Degree (angle)
A degree , usually denoted by ° , is a measurement of plane angle, representing 1⁄360 of a full rotation; one degree is equivalent to π/180 radians...

s, rather than in radians. Start with the resistive load (RL) of 1000 Ω which is in series with Xc -j 200 Ω.

Phase angle

Y = 1/Z = 9.8058 x 10−4

To convert to a parallel network

If for a moment the reactive part is ignored, then a 1040 to 200 Ω transformation is needed. According to the equations above an inductor of +j 507.32 Ω is needed. If the effect of the capacitor (from the parallel network) is taken into account then an inductor of +j 462.23 Ω is needed. The system can then be transformed by the mathematics into a series network of 199.9 Ω resistive and +j 409.82 Ω. The mathematics required for this step are very closely related to the above mathematics.

To finish off the ATU a capacitor (-j 409.82) is needed to finish off the network.

#### Pi network

This pi network can be used; it has the same function as the Ultimate Transmatch.

## Connecting an ATU

Typically the ATU is connected between the antenna and the radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...

transmitter
Transmitter
In electronics and telecommunications a transmitter or radio transmitter is an electronic device which, with the aid of an antenna, produces radio waves. The transmitter itself generates a radio frequency alternating current, which is applied to the antenna. When excited by this alternating...

A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...

. More specifically, the ATU can be connected between the feedline and the antenna to minimize loss, or between the feedline and the radio for convenience. However, SWR in the feedline must be considered in this second configuration.

It is a common misconception that high standing wave ratio
Standing wave ratio
In telecommunications, standing wave ratio is the ratio of the amplitude of a partial standing wave at an antinode to the amplitude at an adjacent node , in an electrical transmission line....

(SWR) in and of itself causes loss. This is not the case. An antenna with a high SWR (e.g. 4:1) when properly configured with an ATU could have only a few percent additional loss compared to an intrinsically matched antenna. The ATU essentially redirects the reflected energy back along the feedline and antenna path. The additional losses come from the inherent losses within the feedline and antenna itself. SWR causes feed line losses to be multiplied. Low loss feedline would have minimal loss when tuned with an ATU whereas a "lossy" feedline/antenna combination of the same SWR could have significant loss.

Without the ATU, SWR from a mismatched antenna could cause reflections of power back into the transmitter, which will cause heating in the transmitter and significant power loss.

While modern solid state power stages do not like an SWR above 1.5, in reality an antenna VSWR of 2 means 11 percent of power is reflected and 89 watts sent forward through the antenna. Tube amplifiers using a Collins filter could easily remedy this.

## Sample application: multiband shortwave transmitter

One of the longest lasting applications for dedicated antenna tuners is in high power shortwave broadcasting transmitters (50 kW+), where antenna tuning is a must due to frequent waveband changes. Every multiband shortwave transmitter sold since the 1950s has had one as part of its design. Most notably, some designs used a miniature railroad system to swap out capacitors as the longer wavelengths demanded larger capacitors.

Most antennas used in high power shortwave broadcasting are HRS type antennas
HRS type antennas
HRS type antennas are more or less the standard antenna used for long distance high power shortwave broadcasting .- History of HRS design :...

, where the radiating elements are of a fixed length and the resonance of the antenna is optimized for only one or two bands. It typically takes about one minute for a modern antenna tuning unit to adapt to a frequency change in a broadcast band, and about two minutes to adapt to a change of two bands plus or minus. Some wideband shortwave broadcasting antennas don't need antenna tuning units; namely the Log Periodic type.

The American Radio Relay League is the largest membership association of amateur radio enthusiasts in the USA. ARRL is a non-profit organization, and was founded in May 1914 by Hiram Percy Maxim of Hartford, Connecticut...

• Electrical lengthening
Electrical lengthening
Electrical lengthening is the modification of an aerial which is shorter than a whole-number multiple of a quarter of the radiated wavelength, by means of a suitable electronic device, without changing the physical length of the aerial, in such a way that it corresponds electrically to the next...