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Return loss bridge. (part 2)


Prototype version 1.

For my first return loss bridge prototype, I quickly put together some components I had in stock.
Not with the idea to make a good device for high frequencies, but just to have something working.


Figure 1:  the first prototype (version 1) of the return loss bridge
The used ferrite cores are type BN43-5170 from Amidon.
The permeability of the "43" material is: 850.
As you see, five cores are used, the more cores you use, the lower is the minimum frequency on which the bridge can measure.
These cores have two holes, each coax cable is put through one hole.
In this prototype RG58 coax is used through the ferrite cores.
No separate REF port connector is used, the reference resistors are directly soldered into the circuit.


Figure 2:  detail of the input side.
For this test, normal carbon film resistors are used.
Because I didn't had 50 Ω resistors, I put two 100 Ω resistors parallel.
I selected the (5 % tolerance) resistors to be close to 100 Ω.
 


Figure 3:  detail of the output side.
The shielding of the two coax cables are soldered together.
The ground of the output connector is connected to the ground of the input connector, via the spectrum analyser / tracking generator.



Figure 4:  the response of this return loss bridge, from 0 - 2000 MHz.
The upper trace (yellow) is the reference trace, measured with the test port open.
In the ideal case, this reference trace should be flat on all frequencies, but in this case it drops considerably above 1000 MHz.
The lower trace is measured with the test port terminated with a 50 Ω BNC terminator plug.
The number of dB's between the two traces is the directivity of the bridge, this is the accuracy of the bridge.
The directivity determines the maximum return loss you can measure.
For instance in figure 4, the directivity is 25 dB at 200 MHz.
Above 1000 MHz, this return loss bridge seems to be completely useless.
Preferable, the directivity is at least 30 dB for the whole frequency band of interest.



Figure 5:  this picture shows the response at low frequencies (100 kHz - 10 MHz).
The low frequency - 3 dB point of this bridge is 350 kHz, this point is indicated with "marker 1" on the yellow reference trace.
The directivity is about 50 dB in this part of the RF spectrum.



Figure 6:  this is what happens when you disconnect the balance line (the second coax cable).
The bridge comes in unbalance at low frequencies, and this reduces the directivity.
Both the "REF" and "TEST" port are terminated with 50 Ω in this test.

Measurement on a FM band ground plane antenna.

Although this return loss bridge version 1 has not very high specifications, we can already do an antenna measurement with is:


Figure 7:  this is the antenna to be tested.
It is a 50 Ω FM band ground plane antenna.

The antenna is via some metre coax cable connected to the test port of the return loss bridge.
And the results are:



Figure 8:  return loss measurement on a FM ground plane antenna.
This antenna is at resonance at 103.2 MHz, on that point we read the return loss to be 32 dB (the bottom of the marker, not the centre of the marker is at the measured level)
But from figure 4 we knew that this value (32 dB) is very close to the directivity of the bridge.
This makes the actual return loss value at resonance somewhat uncertain.

You can also read the bandwidth of the antenna, between the points of 14 dB return loss (= 1.5 : 1 VSWR).
This gives a bandwidth of about 8 MHz.

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