I realized that my impedance matching theory on an RF combiner was wrong !
In fact an RF combiner behaves more like an attenuator according to a reflection measurement that I did today.
A 3-way combiner reduces power of an input signal by a factor of 4.8 dB because it can be also considered as a 3-way splitter.
So it is just a lossy component or in other words it is just an attenuator.
To check my speculation that I posted on #4504 I measured reflection coefficients for both cases.
In the measurement I used a heliax cable, which goes from 1X2 rack to the PSL table with a length of about 10 m. Note that this is the cable that had been used as '33 MHz EOM'.
At the input of the heliax cable it was connected to a direction coupler to pick off reflections and the reflected signal was sampled in AG4395A.
The other end of the cable (output side of the cable) was basically connected to the resonant box.
Then I did a reflection measurement for both cases as drawn in this entry (see #4504).
- case 1 - the combiner was inserted at the input side of the heliax cable.
- case 2 - the combiner was directly attached to the resonant box
On the combiner, ZFSC-3-13, the port 1 and 2 were terminated with 50 Ohm, therefore the port 3 was used as an input and the source port is the output.
Here is a resultant plot of the reflection measurements.
Note that whole data are calibrated so that it gives 0 dB when the output side of the heliax is open.
There are two things we can notice from this plot:
(1) The reflection coefficient at the resonant frequencies (where notches appear) are the same for both cases.
(2) Over the measured frequency range the reflections were attenuated by a factor of about 9.6 dB , which is twice as large as the insertion loss of the combiner.
These facts basically indicates that the RF combiner behaves as a 4.8 dB attenuator.
Hence the location of the combiner doesn't change the situation in terms of RF reflections.
The input impedance of the resonant box was measured when an RF combiner was attached to the box.
Indeed the combiner makes the impedance more 50 Ohm and reduces the reflection.