# Selecting Directional Coupler for VSWR Measurement

Assume directional coupler is coupling with 16dB attenuation when signal is traveling from input to output. And directivity is let's say 20dB.

If I understand correctly, when the RF signal is traveling from input to output the coupled port signal will be 16dB less than the input signal. And if any signal is reflected back from the output port to input direction then this reflected signal will be coupled with 16+20 = 36dB attenuation. (learned from this video)

If I use a directional coupler to measure an antenna's performance then signal generator will be connected to output port and the antenna will be connected to input. So the reflected signal from the antenna will be coupled with 16dB less to coupling port. And the main signal we sent from output to input will be coupled with 36dB attenuation.

My question is: should I simply select a coupler with high directivity and low coupling attenuation? Because in my logic the directivity should be high enough so the main signal we sent traveling from output to input should not be coupled to couple port so much (we are not interested in it). And only the reflected signal should be coupled with as little attenuation as possible. Is this the correct approach?

• If you are comparing the specs on some couplers that you've already identified, please supply that information so we can focus our thinking. – Brian K1LI Apr 7 '19 at 23:47
• I gave the numbers for the sake of example to make it more understandable. But also i have find two couplers ZADC-10-63-S+ and ZADC-15-252+ but their directivity is a little bit low i guess. I will use at 5.8ghz, 2.4ghz, 433mhz and 900mhz. – nandflash1 Apr 8 '19 at 6:14

Yes, you understand it correctly.

Directivity is the most important thing, as it limits how low a VSWR you can measure. In your example, 20 dB directivity, will show a constant ~20 dB return loss, or a VSWR of about 1.2, even with a perfect 50 Ohm load.

Coupling ratio is not as important. You can always adjust the input power to keep the detector in its sweet spot.

Watch out, when measuring reflected power, that your readings aren't dominated by external transmitters. Any signal out there will be received, coupled and detected like reflected power. It helps to use a larger test signal, and to have a frequency selective detector.

• Thank you. I want to ask one more question. Can i design my own coupler. There are chips that are cheap like a few dolars with input output couple and termination pins (HHM2510B1 as an example). So i was simply thinking to place them on a pcb and draw 50 ohm microstrip copper traces to them with sma connector. Ready modules are expensive. – nandflash1 Apr 8 '19 at 6:27
• Yes, why not. The one you quote is only for 2.4 and 5.8 GHz. And if you want good results, you'll have to do an above-hobby-grade job of impedance matching the pads, traces and SMA launchers. Even with good simulation software, it takes several boards to get it right. It helps to build dummy boards with 10 pairs of SMAs connected by (slightly different) traces, test this carefully with a VNA and repeat. Same for the chip landing pads. So if TDK sell a demo board, rather get that! – tomnexus Apr 8 '19 at 7:38
• I will try to simulate with CST Studio (need to learn it first though) to correct the impedance disturbance created by the sma connector rectangular pad. One engineer does it with creating gap under ground layer so it couples to 3rd layer of pcb. I will try that. Thank you for your time. – nandflash1 Apr 8 '19 at 7:53