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.
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1
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Heading
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Return Loss
Bridge Directivity Measurements
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2
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Label
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Return Loss
Bridge
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3
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Date
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12/07/2013
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4
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Acknowledgements
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1.
Author : ZS6RSH.
2.
Reference: EMRFD Section 7
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5
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Revision
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Rev 1.
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6
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Revision
History
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None
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7
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Scope
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Measure the
directivity of my homebrew Return Loss Bridge for all HF Ham Bands
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8
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History
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Inspired by
VE7BPO’s QRP Home Builder website
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9
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Configuration
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Refer
to attached schematic
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10
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Test
equipment specifications
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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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11
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DUT
specifications
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1. Home brew Return Loss Bridge (RLB)
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12
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Workbench
process
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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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13
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Expected
Results
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The
directivity should be at least 30db on all bands. The higher the better.
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14
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Uncertainties
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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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15
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Preparation
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Done
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16
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Perform
validation measurements
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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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17
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Perform the
full measurement plan
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Done
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18
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Observations
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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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19
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Change
Control
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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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20
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Computation
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Directivity
is calculated using the formula RL(dB) = 20 Log10 (Open CCT Voltage/50 Ohm
terminated voltage)
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21
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Analysis
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The results
all showed a better than 30dB directivity which meets the objective of this
homebrew RLB.
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22
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Conclusions
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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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23
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Documentation
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Done
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