Antennetuner

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Automatisk ATU for radioamatør-stasjon. Innsiden av en antennetuner, sett ovenfra.
Et grått frontpanel av en antennetuner med brytere, instrumenter og reguleringshjul og et delvis synlig innmat.
Instrument for SWR måling med kryssende visere på en antennetuner.

En antennetuner eller ATU, (antenna tuning unit) er en enhet som kan kobles mellom en radiosender eller/og radiomottaker og fødelinjen, eller mellom fødelinjen og antennen, for å bedre overføring av antennesignalet eller sendersignalet mellom radio, fødelinje og antenne. Det er viktig at alle tre enheter er tilpasset den samme impedansen siden feil impedanse skaper stående bølger og gir effekttap i overføringen.

En kommunikasjonsradio og fødekabelen har vanligvis en fast impedans som gjelder et stort spekter av frekvenser, men antenner har en bestemt impedans ved resonansfrekvensen og helt andre impedanser ved andre frekvenser. En antennetuner kan omdanne impedansen slik at kommunikasjonsradioen "opplever" at impedansen er riktig for et større frekvensspekter, slik at samme antenne kan benyttes for et spekter av frekvenser. Med slik tilpassing går noe effekt tapt fordi antenner alltid er best på den frekvensen antennen er i resonans med. Antennetunere kan være innbygget i radiostasjoner, eller i antenneforsterkere.


Bredbånds effektforsterkere[rediger | rediger kilde]

For anlegg som krever et bredt frekvensbånd, som lineære effektforsterkere for frekvenser mellom 1 og 30 MHz, kan det benyttes tunere med transformatorviklinger rundt ferritkjerner, som ofte ikke trenger ny justering ved frekvensskifte. Slike tunere kan tilpasse en antenne til en fødelinje, har små muligheter for fininnstilling og kan utvide frekvensspekteret for en vanlig automatisk antennetuner med smalere båndbredde.


In solid-state RF power-amp design, these networks are useful because MOSFETs and bipolar junction transistors are designed to operate with low resistance. Valved RF amplifiers are different, because the design load resistance of a valve is normally much greater; therefore, for certain power requirements the design of the circuit may differ.

An autotransformer has three identical windings on a ferrite core. If the right-hand side was connected to a resistive load of 10 ohms, the user can attach a source of different impedance at each of the terminals on the left side of the transformer.

1:1, 1:4 and 1:9 autotransformator

Narrowband design[rediger | rediger kilde]

Devices based on components and transmission lines can be purchased or constructed. The simplest example of a transmission-line-based system is the transformer formed by a quarter-wavelength of mismatched transmission line. If a quarter-wavelength of 75Ω coaxial cable is linked to a 50Ω load, the SWR in the 75Ω quarter wavelength of line can be calculated as 75Ω / 50Ω =  1.5; the quarter-wavelength of line transforms the mismatched impedance to 112.5Ω (75Ω x 1.5 = 112.5Ω).

The basic circuit required when lumped capacitances and inductors are used is shown below.

Diagram for et grunnleggende tilpassningstrinn.

This basic network is able to act as an impedance transformer. 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

X_L = \sqrt{\Big(R_{source}+jX_{source}\Big)\Big((R_{source}+jX_{source})-(R_{load}+jX_{load})\Big)}

and

X_C = (R_{load}+jX_{load})\sqrt{\frac{(R_{source}+jX_{source})}{(R_{load}+jX_{load})-(R_{source}+jX_{source})}}

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

For instance, if the source has a resistive impedance of 50Ω and the load has a resistive impedance of 1000Ω:

X_L = \sqrt{(50)(50-1000)} = \sqrt{(-47500)}= j\, 217.94\ Ohms

X_C = 1000 \sqrt{\frac{50}{(1000-50)}} = 1000\,\times\,0.2294\ Ohms = 229.4\ Ohms

If the frequency is 28 MHz,

As, X_C = \frac{1}{2\pi fC}

then, 2\pi fX_C = \frac{1}{C}

So, \frac{1}{2\pi fX_C} = C = 24.78\ pF

While as, X_L = 2\pi fL\!

then,  L = \frac{X_L}{2\pi f} = 1.239\ \mu H

Theory and practice[rediger | rediger kilde]

A parallel network, consisting of a resistive element (1000Ω) and a reactive element (-j 229.415Ω), will have the same impedance and power factor as a series network consisting of resistive (50Ω) and reactive elements (-j 217.94Ω).

Schematic diagrams of two matching networks with the same impedance
Two networks in a circuit; both have the same impedance

By adding another element in series (which has a reactive impedance of +j 217.94), the impedance is 50Ω (resistive).

Schematic diagrams of three matching networks, all with the same impedance
Three networks in a circuit, all with the same impedance

Types[rediger | rediger kilde]

Ultimate Transmatch[rediger | rediger kilde]

T network[rediger | rediger kilde]

Schematic diagram of the Ultimate Transmatch
T-network transmatch

The Ultimate Transmatch's name is a misnomer; a better transmatch, the SPC (series-parallel capacitor) circuit, was later designed after it. In all designs, the GND terminal is the terminal where the earth plane (ground plane) of an antenna should be wired; the ANT terminal is where the vertical element of a Marconi aerial should be attached. The configuration, although capable of matching a large impedance range, is a high-pass filter and will not attenuate spurious radiations above the cutoff frequency.

Theory and practice[rediger | rediger kilde]

If a source impedance of 200Ω and a resistive load of 1000Ω is connected (via a capacitor with an impedance of -j200Ω) to the inductor of the transmatch, vector mathematics can transform this into a parallel network consisting of a resistance of 1040 Ω and a capacitor with an admittance of 1.9231 x 10−4 (Xc = 5200Ω).

In the following calculations, all phase angles are expressed in degrees rather than radians. A resistive load (RL) of 1000Ω is in series with Xc -j 200 Ω.

Z = \sqrt{R_L^2\ +\ X_C^2} = 1020\, \Omega

Phase angle (\theta ) = tan^{-1}\ (\frac{X_C}{R_L}) = 11.31^\circ

Y = 1/Z = 9.8058 x 10−4

To convert to a parallel network

X_C^' = \frac{1}{Y sin\ \theta }

R_L' = \frac{1}{Y cos\ \theta } = 1040\ \Omega

If the reactive component is ignored, a 1040-to-200Ω transformation is needed (according to the equations above, an inductor of +j507.32Ω). If the effect of the capacitor (from the parallel network) is taken into account, an inductor of +j462.23Ω is needed. The system can then be mathematically transformed into a series network of 199.9Ω resistive and +j409.82Ω.

A capacitor (-j409.82) is needed to complete the network.

Schematic diagram of original Ultimate Transmatch circuit
Circuit as seen by user; parts impedance shown on diagram
Schematic diagram after the first of four transformations
After one transformation (unlabeled part impedance is -j 5200Ω)
Schematic diagram after two transformations
After two transformations
Schematic diagram after three transformations
After three transformations
Schematic diagram after four transformations
After four transformations

Pi network[rediger | rediger kilde]

A pi network can also be used:

Schematic diagram of pi-network antenna tuner
Similar circuit to the Ultimate Transmatch

SPC tuner[rediger | rediger kilde]

Schematic diagram of SPC antenna tuner
SPC transmatch

Connection[rediger | rediger kilde]

An ATU is typically connected between the antenna and the radio transmitter or receiver.[1] The antenna feedpoint is usually high in the air (for example, a dipole antenna) or far away (for example, an end-fed random wire antenna). An automatic antenna tuner in a weather-proof case is convenient for these installations. An ATU can also be connected between the feedline and the antenna to minimize loss, or between the feedline and the radio for convenience; however, feedline SWR must be considered in the latter configuration. With an ATU, it is possible to match a wide range of antennas[2] (including stealth antennas).[3][4]

Standing wave ratio[rediger | rediger kilde]

It is a common misconception that a high standing wave ratio (SWR) per se causes loss.[5] An antenna with a high SWR (4:1, for example), when properly configured with an ATU, may have only a small percentage of additional loss compared with an intrinsically matched antenna.[6] The ATU redirects reflected energy along the feedline and antenna path; additional losses are inherent to the feedline and antenna. SWR causes feedline losses to be multiplied; low-loss feedline has minimal loss with an ATU, but a "lossy" feedline-antenna combination with an identical SWR may have significant loss. A balanced feedline exhibits less loss than coaxial line in the presence of high SWR, so a tuner is primarily used with the former.

Without an ATU, SWR from a mismatched antenna could cause power reflection back to the transmitter; this causes heating and power loss. While solid-state power stages perform poorly with an SWR above 1.5, an antenna SWR of 2:1 means that 11 percent of transmitted power is reflected and 89 percent sent forward through the antenna.

Applications[rediger | rediger kilde]

Amateur radio[rediger | rediger kilde]

The Z-Match is a widely used ATU in amateur radio.[7][8][9]

High-power shortwave transmitters[rediger | rediger kilde]

One of the oldest applications for dedicated antenna tuners is in high-power shortwave broadcasting transmitters (50 kW and above), where antenna tuning is a must due to frequency changes. Every[trenger referanse] multiband shortwave transmitter sold since the 1950s has had an ATU in its design. Some designs used a miniature railroad system to switch capacitors (lower frequencies require larger capacitors).[trenger referanse]

Most antennas used in high-power shortwave broadcasting are HRS antennas, where the radiating elements are a fixed length and antenna resonance is optimized for one or two bands. It typically takes about one minute for a modern antenna tuning unit to switch to a one-band frequency change, and about two minutes to adapt to a two-band change. Some wideband shortwave broadcasting antennas (for example, log-periodic antennas) do not require antenna tuners.

Referanser[rediger | rediger kilde]

  1. ^ Dave Miller, "Back to Basics". QST, August 1995
  2. ^ HF Users Guide (SGC) http://www.sgcworld.com/Publications/Books/hfguidebook.pdf
  3. ^ Stealth Kit (SGC) http://www.sgcworld.com/Publications/Manuals/stealthman.pdf
  4. ^ Smart Tuners for Stealth Antennas (SGC) http://www.sgcworld.com/Publications/Books/stealthbook.pdf
  5. ^ M. Walter Maxwell, Reflections: Transmission Lines and Antennas. Newington, Connecticut: American Radio Relay League (1990) ISBN 978-0-87259-299-5
  6. ^ Jerry Hall, ed. ARRL Antenna Book. Newington, Connecticut: American Radio Relay League (1988), p. 25-18ff. ISBN 978-0-87259-206-3
  7. ^ Z-Match-Antennenkoppler für hohe Leistungen (DL3BCU) http://www.mydarc.de/dk7zb/Tuner/zmatch.htm
  8. ^ Balanced Line Tuner http://www.qrpkits.com/blt_plus.html
  9. ^ ZM-4 http://www.qrpproject.de/ZM4.html