WHY USE THE ONE PORT RETURN LOSS MEASUREMENT INSTEAD OF THE TWO PORT TRANSMISSION LOSS MEASUREMENT

How to tune a Coaxial Filter

  

In Fig.1 is shown the connection between Spectrum Analyzer-Rhotector (VSWR Bridge ) and a Coaxial Filter:

                                                           Fig. 1. Connections between the instruments.

 

With this kind of connection you can measure the Return Loss or the Standing Wave Ratio of the coaxial filter at port F (shown in Fig. 1), Table 1 reports the relationship between the R.L and the VSWR.

Table 1. Return Loss and SWR.

 

 The Coaxial Filter is tuned by rotating the four knobs named Lin, Lacc1, Lacc2 and Lout, as in

Fig 2.

Delta Meccanica provides a graduation and a table for  the first position where is better to start the tuning.

Fig. 2. Tuning Kobs.

Delta Meccanica provides a graduation and a table for  the first position where is better to start the tuning.

Table 1 shows the settings on two commonly used models of Delta Meccanica Coaxial Filters.

                                                                                                               Table 1.

 Usually it is better to tune the coaxial filter trying to obtain the best Return Loss inside the wanted band, this is the best way to obtain also a low attenuation between the input and the output port of the filter.

In Fig. 3 is shown the plot of the measurement of a coaxial filter made with spectrum analyzer + rhotector, it is possible to see that the best Return Loss (lower line) is obtained at the side of the wanted band ( red points ) , while at the center you have a maximum that is the best compromise for the tuning and the loss ( green points )  .

If a measure like this is made only using the SWR meter of a transmitter it coud be possible to make a big mistake, because the operator could think he optimized the transmission while instead the transmitter is working at the left or right side of the band ( red and blue points ) , so during the modulation it could happen that the SWR  increases generating AM synchronous modulation .

The worst effect is if you are tuned in the red or blue points and the thermal drift go in the direction of the big slope of the transfer function ( upper trace ) .

The detuning increase losses , losses increase heat , heat increase losses and the system could go in avalanche .

Fig. 3. Results of Transmission Coefficient and Reflection coefficient as measured with the spectrum analyzer

                          Table 2. Supplementary attenuation introduced by a bad R.L. on the input port of a coaxial filter. 

From this table you can see for example that a change of Return Loss  from 20 to 15 dB (5dB)  increase the Loss from 0.04 to 0.14dB ( 0.1 dB of difference ) .

This tells us that the Return Loss is a most sensitive parameter respect to the Transmission Loss .

This is the reason why we suggest to start the measurements of this devices with the one port measurement of ReturnLoss instead of the two port Transmission Loss ( called also Transfer Function ) .

In Figure 3 , both measurements are superimposed with different scales ( 10 dB for the Return Loss and 1 dB for the Transmission Loss ) .