Having build the Bidirectional Coupler, it is time to check the performance of the Coupler. Pic 1 shows the basic functionality of a Bidirectional Coupler.

So, to test the Coupler and to get an understanding of the limitations of the unit, we need to measure a few things. We need the following:

• a RF-signal generator,

• two 50Ω resistors,

• and a RF-Voltmeter or a device that can measure Power such as a Power-meter or Spectrum Analyser.

To determine the coupling factor (CF). The coupling factor indicates the amount of power transferred (coupled) from the incident, the input port (P1) to the coupled port (P3). Connect the RF-Generator (Radio) to P1 and the RF-Voltmeter to P3. Terminate the remaining ports (P2 & P4) with a 50Ω terminator. See Pic 2 for the test setup.

Set the RF-Generator to 0dBm output (223.6mV) and read the result of the RF-Voltmeter. For a 30dB CF you would read about 7.1mV on your RF-Voltmeter. To calculate the CF use Formula 1.

If you have problems working with decibel's (dB's), get the miniDBcalculator from Wilfried, DL5SWB to help you work with these critters. Another way to check is to use a signal from a Transceiver, but make sure to use the least amount of power, and make sure that your dummy-loads are capable of dissipating that amount of power without disintegration. The other issue would be the POWER-METER. Can the meter read accurate enough down to Milli-Watts or lower (some can). You would then use Formula 2.

The Bi-Directional Coupler is a passive device, because of this the ports are interchangeable. What that means is we can use P1 or P2 as the input port and as such P3 and P4 can also be swapped. To check how well the Coupler works "in revers" I swap P1 and P3 with P2 and P4 and make the same measurements again. The measured values should track very (very) closely.

From Pic 4 we can see that the balance of P1/P3 path and the P2/P4 path is pretty good. Another important value of a Directional Coupler is ..., yep Directivity. Directivity is the power level difference between P3 and P4. This is a measure of how independent P3 and P4 are. Because it is impossible to build the perfect Coupler, there will always be some amount of unplanned coupling between all the signal paths. To measure directivity, connect the RF-Generator to P1 and the RF-Voltmeter to P4. Then terminate the remaining ports (P2 and P3) with a 50Ω termination. Pic: 5 shows the Directivity measurement setup.

The values that we are now measuring should be very low, i.e. if the Coupler is working well to very well the values measured should be a lot lower then compared to the values measured in the CF test.

The signal level at P4 will be reduced by the CF, so if we measure 223.6 μV (-60 dBm), the directivity is the reflected (isolation) signal (-60 dB) minus the coupling factor (-30 dB) which equals 30 dB (see how easy it is to calculate using values in dB).

Pic 6 below shows the resulting measurements from the isolation test, i.e. the port isolation which are used to CALCULATE the Directivity.

To improve isolation I've included an additional shield as can be seen in Pic 7. The shield improved the CF at the 50-54 MHz range and made the unit usable for tests in the 144 - 148 MHz range.

And at Pic 8 we see a sweep result of the unit after the shield had been installed.

Pic 8 shows the specs (measured values) of my Bi-Directional Coupler after the installation of the additional shield. Ah, I hear someone asking about insertion loss. Well, at VHF, UHF or higher frequencies, I certainly would worry about the insertion loss, however at HF. Lets have a quick look. Lets remember that 30dB is a factor of 1000. Anyway, the insertion loss is easy to calculate and depends on the coupling factor (CF). The higher the CF the smaller the insertion loss (IL). Below is the formula, Formula 4, to calculate the insertion loss.

So for a 30dB CF the insertion loss (IL)is quite low, only 0.0043dB, nothing to worry about at HF.

And at Pic 9 are the results of the above unit as build.