Directional coupler

Introduction

I have been struggling to understand how my simple dipole antenna works (or doesn't work). The diycrap way to understand stuff is usually to read, build and measure, and then read some more. The key factor here is the measuring part, as I need to measure the standing wave ratio (SWR) on the feed line, as this is a key parameter. To measure the SWR, I need a directional coupler. And it is going to be homebrew.

Later, the coupler is going to be the basis of a SWR-meter, but for now, lets just look at the coupler.

The directional coupler design is classic and well known. Notice the input port and the output port on the upper line and the forward port and the reflected port on the bottom line. (My graphics software is Field Notes.)

Some theory

The principle of the coupler is based on two toroid transformers. The first is a current transformer and the second a voltage transformer. Each taking samples of the signal on the main line. The two transformers are equal, reducing the current and the voltage to the same level, meaning that the impedance is constant. The two transformers are connected in such a way that for a forward signal, the signal cancels out on the reflected sample port, but adds up on the forward sample port. And vice versa, a reflected signal adds up on the reflected sample port but cancels on the forwards sample port. Since we now have a sample of both the forward signal and the reflected signal, it is straightforward to calculate the SWR.

For a deeper understanding on how the coupler works, I recommend this web page, or the excellent YouTube video from W2AEW.

Construction

It is simple to construct the directional coupler. The transformers are FT50-43 toroid cores with 32 turns of 24 AWG enamel wire. The primary winding is simply a piece of RG58 through the torioid (i.e., one turn). Different designs use different toroids and number of turns. I settled down on a design found in Arduino projects for amateur radio.

I used a aluminum box and BNC connectors. I used copper clad boards as shielding here and there. I did not have any 50 ohm resistors in my junk box so I used two 100 ohm resistors in parallel. They are all 2W resistors, which is totally unnecessary and overkill.

Rudimentary testing

Testing the forward port. The output port is connected to my 50 Ohm dummy load. As signal source I used my GW Instek GFG-8255 signal generator, which unfortunately maxes at 5.5 MHz. 

8.2 Vpp on the input port resulted in about 244 mVpp on the forward port. Hence, the coupling factor is about -30dB. The signals are not in phase, but that does not matter for voltage measurements in a SWR-meter.

Testing the reflected port

8.2 Vpp on the input port results in 1.60 mVpp on the reflected port. This translates to a reflected signal of -74dB. The directivity is the reflected signal (-74dB) minus the coupling factor (-30dB) which equals -44dB.

Testing over the HF band

Later I borrowed a TTi TG2511 function generator which goes all the way up to 25 MHz. I tested with 10 Vpp on the input port and got these results:

frequency	coupling factor	return loss
1.8 MHz	-30 dB	-84 dB
3.5 MHz	-30 dB	-80 dB
7 MHz	-30 dB	-75 dB
10 MHz	-30 dB	-72 dB
14 MHz	-30 dB	-69 dB
18 MHz	-30 dB	-66 dB
21 MHz	-30 dB	-65 dB
25 MHz	-30 dB	-62 dB

The directivity is between 54 dB and 32 dB. The numbers seem reasonable, but indicates that the coupler should not be used for VHF/UHF.

Future work

The plan is to build a power meter and SWR meter using AD8307 logarithmic amplifiers and an Arduino. I will probably base the device on the design from the book Arduino projects for amateur radio.

Further reading

1. VK1HW - Building a directional coupler

2. VK1HW - Testing a directional coupler
3. Giangrandi - The tandem match