07 December 2013

Return Loss Bridge & Directivity Measurements

I built this Return Loss Bridge as a part of my continued quest to support an RF workbench. This is one of the instruments recommended for a basic 50 Ohm RF experimenter's workbench by VE7BPO in his RF Workshop series. Refer to the QRP Homebuilder website.

The following results were measured and are also shown below:

3060KHz   31.9dB
7020KHz   35.4 dB
14060KHz 33.0dB
21060KHz  33.0dB

I was unable to obtain measurements at 28MHz due to limitations at this frequency in my Scope. The Scope was used as the detector.


1
Heading
Return Loss Bridge Directivity Measurements
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Label
Return Loss Bridge
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Date
12/07/2013
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Acknowledgements
1.      Author : ZS6RSH.
2.      Reference: EMRFD Section 7
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Revision
Rev 1.
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Revision History
None
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Scope
Measure the directivity of my homebrew Return Loss Bridge for all HF Ham Bands
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History
Inspired by VE7BPO’s QRP Home Builder website
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Configuration
 Refer to attached schematic
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Test equipment specifications
1.      K2 Elecraft Transmitter.
2.      Commercial attenuator pad 20dB
3.      GOS 20MHz Scope
4.      3X4ft coax test leads
5.      50ohm commercial terminator
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DUT specifications
1.    Home brew Return Loss Bridge (RLB)
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Workbench process
1.      For each hamband. Setup the K2 to generate minimum power which will be close to 100mW. This is the input signal generator.
2.      Connect 30dB attenuation at the K2 Output.
3.      Connect up the RLB according to the schematic.
4.      With the RLB measurement port Open Circuit (at the end of the coax), measure the voltage on the scope.
5.      Terminate the RLB using a 50ohm terminator.
6.      Measure the voltage on the scope
7.      Do the above procedure for all hambands.

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Expected Results
The directivity should be at least 30db on all bands. The higher the better.
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Uncertainties
1.      Accuracy of the scope voltage readings. (scope needs calibration).
2.      Harmonics from the RF generator. 7pole filter ‘assumed’ good.
3.      The 50ohm scope terminator. Commercial
4.      The 50ohm bridge terminator. Commercial
5.      Loss on the coax cables. Same cables used in the same positions for all tests.
6.      Performance of the RLB at different power levels may vary. Measure this in future.
7.      Non 50 ohm RF generator. Assumed negligible with 20dB attenuator in place.
CONCLUSION: 1 and 6 could be significant and need quantifying. Assume all the other uncertainties are negligible.
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Preparation
Done
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Perform validation measurements
Test the output voltage from the K2 is as expected. Ie around 6.3V into 50ohms.
 Test the output voltage is as expected from the 20dB pad. Ie around 200mv into a 50ohm load.
Test the output voltage from the unterminated RLB is as expected. Should be ‘higher’ since the unterminated RLB is 100ohms with no scope connected. However once the scope is connected the voltage will drop ‘somewhat’.  Adjust the pad accordingly so that the unterminated RLB WITH scope detector attached is around 250mV to 300mV.
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Perform the full measurement plan
Done
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Observations
I could not measure the 10m band. At this frequency range I found that the scope was completely ineffective. Changing the Y attenuators at this frequency revealed no change in the scope measurements. Indicating that the built-in scope attenuators do not work at this frequency. Since this is a 20MHz scope this is not surprising. It brings home the need for a sensitive power meter for these measurements  in the future.
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Change Control
I started out with the idea of using 30dB attenuation. However 20dB was used to get the open circuit voltage into the 200mV – 300mV range.
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Computation
Directivity is calculated using the formula RL(dB) = 20 Log10 (Open CCT Voltage/50 Ohm terminated voltage)
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Analysis
The results all showed a better than 30dB directivity which meets the objective of this homebrew RLB.
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Conclusions
A sensitive power meter such as the EMRFD chapter 7 version using a logarithmic metering chip is desirable as a detector. This design will allow measurements between -80dBm and +13dBm.
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Documentation
Done






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