Having build the Bidirectional Coupler, it is time to check the performance of the coupler. Figure 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.
I use a RF-signal generator, but a LOW power radio will do the trick as well. We also need two 50Ω termination resistors (Dummy-loads) and a RF-Voltmeter.
First I am going to determine the coupling factor (CF). The coupling factor indicates the amount of power transferred (coupled) from the incident or input port P1 to the coupled port P3.
Connect the RF-Generator to P1 and the RF-Voltmeter to P3 and terminate the remaining ports with 50 Ω terminators. See Figure 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 this formula:
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 HIGH POWERED signal from a Transceiver, 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 the following formula:
NOTE: Make sure your Dummy-loads can handle the power.
Now swap the P1/P3 ports with the P2/P4 ports and make the same measurements again. The measured values should track very closely.
Figure 3 (red = P1/P3 and blue=P2/P4)
as can be seen at Figure 3, the balance 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 a 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 with 50Ω terminators. See Figure 4 for test setup.
We should now measure a lot lower value then during the previous test. The signal level at P4 will be reduced by the CF.
So if we measure 223.6μV (-60dBm), the directivity is the reflected (isolation) signal (-60dB) minus the coupling factor (-30dB) which equals -30dB (easy to calculate in dB).
Figure 5 shows a sweep of my unit in the directivity test setup.
As can be seen in Photo 2, I have included an additional shield between the two transformers. The shield improved the CF @ the 50-54MHz range and made the unit usable for tests in the 144-148MHz range.
Figure 6 show a sweep of the unit after the shield had been installed.
Here are the specs of my Bidirectional 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, well maybe.
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). Here is the formula to calculate the insertion loss.