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How to make a RF Attenuator with Resistors – Fixed or Variable

For those who work primarily with RF and audio, attenuator is an indispensable tool. We will show in this article a table resistors to build a fixed attenuator type “More” or “T” that are very simple and its impedance is 50 ohm. Also post a variable attenuator to 131 dB com chaves.

What is an Attenuator

An attenuator is effectively the opposite of an amplifier, basically an attenuator is an electronic device that reduces the power of a signal without distorting its waveform. An attenuator is usually used in audio or RF, but can also be used in other areas of electronics and it can be fixed or variable.

The RF Attenuator

RF attenuators are usually in a structure with coaxial connectors utilize precision and non-inductive resistors. Over SHF, ie above 3 A special structure GHz waveguide is required. There are some important features that a RF attenuator must have, as a good precision, baixo ROE (Standing wave ratio ) and low distortion.

 

Atenuadores Fixos
Fixed Attenuators. Photos: www.jyebao.com.tw

The size and shape of the attenuator depend on its ability to dissipate energy. Generally the more he attenuate a signal or the stronger the attenuated signal, plus it will dissipate energy and thus it will need a heatsink. RF attenuators are used as fillers for mitigation.

Use attenuator

The attenuator has a large multitude of uses in electronic, fixed attenuators are used in circuits to reduce voltage, dissipate power and improve impedance matching between two points.

It is used to assist in the measurement signal, the adapter or attenuator are used to reduce the amplitude of a signal to a known value, to allow measurements and protect the meter from signal levels that can damage it. The attenuators are also used to match the impedance and lower the ROE.

How to make an RF attenuator low cost

The resistor

For our homemade experiments a professional RF attenuator is not necessary, if we work with frequencies up to UHF band can use ordinary resistors, and three common enough resistors to build an attenuator. We must remember that we can not use inductive resistors, as resistors and others end, but we can use the metal film. For the transmission of RF low-power can use the common resistors 1/4 provided with good dissipation not to burn them.

The dissipation

An important point is the heat dissipation, We can use some techniques to build the sink attenuator, how to attach the resistor to an aluminum heatsink for transistor, assembling in a container with coolant, mount fans, etc.

Attenuation

Maybe you need an attenuator few dBs, and then a big attenuator, it does not matter, you can use the table below which has options 1 dB a 20 dB. But if you have doubts about dB, is an article with a conversion table dBm Watts x x Volts which will make your life easier for the calculations.

 

How to make a Fixed Attenuator RF

The most popular types of attenuators with resistors are the types T and PI. The resistance values ​​of attenuation are given in the table below. The identification of the resistor in the table refers to the number in scheme.

Atenuador tipo T
T-type attenuator

 

Atenuador tipo PI
Dimmer type PI

 

The T-type attenuator shown below bridge has four resistors in each section. It is used in variable attenuators due to the fact that only two resistors must be different.

Atenuador tipo T r
Atenuador tipo T r

The values ​​in the table below were calculated for an impedance of 50 ohms.

TABELA OF RESISTORS
Attenuation in dB R1 R2 R3 R4 R5 R6
1 2,9 433 870 5.8 6.1 410
2 5,7 215 436 11,6 12,9 193
3 8.5 142 292 17,6 20,6 121
4 11.3 105 221 23,8 29,3 85,4
5 14,0 82,2 179 30,4 38,9 64,3
6 16,6 66,9 151 37,3 48,9 50,3
7 19,1 55,8 131 44,8 61,9 40,4
8 21,5 47,3 116 52,8 75.6 33,0
9 23,8 40,6 105 61,6 90,9 27,5
10 26,0 35.1 96,2 71,2 108 23,2
11 28,0 30,6 89,2 81,7 128 19,6
12 29.9 26,8 83,5 93,2 149 16,8
13 31,7 23,6 78,8 106 173 14,4
14 33,4 20,8 74,9 120 201 12,4
15 34,9 18,4 71,6 136 231 10,8
16 36,3 16,3 68,8 154 265 9.4
17 37,6 14,4 66.5 173 304 8.2
18 38,8 12.8 64,4 195 347 7.2
19 39.9 11,4 62,6 220 396 6.3
20 40,9 10.1 61,1 248 450 5.6

Obs: One tip is to approximate the value of the resistors to the commercial standard, eg, for attenuator 1 dB R3 resistors in the table are 870 Ohms and the resistor R4 is 5,8 Ohms, In this case the values ​​can be approximated, if resistors R3, We can replace resistors 820 Ohms and the resistor R4 may be replaced by a resistor 5,6 Ohms.

To convert the table values ​​to another impedance, you must multiply by the factor Z / 50, where Z is the impedance of the system.

How to make a variable RF attenuator

In a variable attenuator, we can vary the attenuation, it is ideal for use on bench. In the diagram below we have a variable RF attenuator, Using keys to activate each attenuation value, are several steps, four 20 dB, four 10 dB and four steps that are one of 5 dB, one of 3 dB, one of 2 dB and a 1 dB. This attenuator can vary 1 dB até 131 dB supporting a working frequency up to 500Mhz.

 

atenuator de RF variavel

 

You can tailor this to your reality attenuator on the bench, in my case, I made a variable, but without the steps of 20 dB. Sources: Variable attenuator here e fixo here.

Calculator to build a RF attenuator

But if you want a very specific attenuation, use an online calculator Resistor attenuator in PI, Here is a good calculator making a detailed calculation for your attenuator.

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