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Directional Coupler


Basically directional coupler is a 4 ports device (port1 - input, port2 -transmitted, port3 - coupled, port4 -isolated), which has many uses in HF technik (forward and reflected power measurement, automatic level control, etc.). 

Three ports directional coupler (without isolated port) is RF passive device used to couple a part of the traveling power (from port1 to port2)  out to the other (coupled) output port (port3). I needed several tens of decibels to attenuate  abt. 100W to the low measurable power with my mW-meter. Typically used transformation ratio 100:1 or 1000:1 was selected. It means 20dB or 30dB of attenuation. The 50 Ohms load at the coupled output is transformed as 0.5 or 0.05 Ohms serial resistance in the main direction (from port1 to port2). This means that unused coupled port must be allways terminated with 50 Ohms.

Toroidal cores from 43 material was used for transformers. For 20 dB transformer 3 pieces of 10mm (out dia) due to higher power handling and for 30dB one piece of 43006301 core. The 20 dB transformer has 10 turns of 0.4mm enamel, 30 dB transformer has 31 turns of the same wire diameter  (SQRT 100 = 10 and SQRT 1000 = 31.6).

I removed the upper insulation and braid from piece of RG316 coax cable. Inner conductor with insulation will be used for connection between port1 and port2. Both wound toroidal cores were strung on this isolated wire. The winding outlets of both transformers were soldered to the coaxial connectors. I will use coupler as a symetrical device (input and output can be swapped), it means that order of winding outlets was not important.

Directional coupler with two coupled outputs was installed to the Hammond case 1550A.

Directional Coupler with -30 dB (left side) and -20 dB (right side) outputs. 

Let´s have a look on the coupler features. Input VSWR in the prefered frequency range 1 - 60 MHz is better than 1,3. It is good result. VSWR curve gradually increases with the frequency. It is  because simple  wire is used for  BNC connectors connection (its impedance increases with frequency) and because 50 Ohms terminations from coupled outputs they are transformed as serial resistors 0,5 and 0,05 Ohms. We can see the situation better from the next picture.

The real component of the impedance increased by 3 Ohms at 60 MHz. The imaginary component has inductive character and linearly increases with frequency. Character of the input impedance corresponds to the physical properties of the wire connection between BNC connetors. Better VSWR will be achieved in the case if shorter box will be used.

Next picture shows -20 dB output level in the frequency range 1 - 60 Mhz. Attenuation accuracy is better than 3% in the range 1 - 30 MHz and better than 5% above 30 MHz.

Next picture shows -30 dB output level in the frequency range 1 - 60 Mhz. Attenuation accuracy is better than 2% in the range 1 - 55 MHz.

What happend if one or both coupled outputs will stay unterminated during input impedance measuring? Situation is shown in the following pictures.

In the lower part of frequencies imput impedance components are significantly worse.  But above 30 MHz this coupler can be used with unterminated (one or both) coupled outputs (for example if high impedance RF voltmeter will be conected to the coupled output).

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