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.)|
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.
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.
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.
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.