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Description: The Bird Model 43 Thruline Wattmeter is a very common test tool among radio technicians worldwide. The dummy slug can be affordably machined into a very useful tool for testing desense, monitoring waveforms, and feeding other equipment. This product does not disturb the original intent of the dummy slug; keeping out the dust and shunting the meter movement in travel. It allows coupling into the system without needing to install an Iso Tee or other such tool which can change line lengths or impedance. This allows you to optimise your set-up, knowing the achievement will stay after the device is removed from the circuit.
This unit is machined with a lathe and end mill to produce a professional job. The sampler uses a 47 to 51 ohm carbon resistor connected to a BNC connector to provide the coupling. Its usable range is 50 MHz to over 3 GHz.
This PDF document contains VNA graphs showing the coupling losses
of the sampler when rotated for maximum and minimum coupling.
NOTES: The following notes have been offered
by Allan Crites - WA9ZZU
When using an iso-tee in this instant application it is highly desireable to use a 50 ohm termination at the iso-tee to properly terminate the signal source. Using a modified and terminated Bird Wattmeter slug for signal sampling is a highly desireable means for observing off the air signals, for observing desense, and transmitter output. However, there is the possibility of the transmitter output entering the signal generator and creating intermod if used for the purposes of making a desense measurement and should be seriously considered. I recommend the use of a 10 dB attenuator on the BNC output to the signal generator to lessen the chance of intermod. While rotating the slug reduces any transmitter signal it also reduces the coupling that would be required for the signal generator to overcome. Another way to reduce the coupling would be to not insert the slug as far into the Wattmeter, thus having the same effect/benefit as the iso-tee isolation with the added characteristic of a 50 ohm termination on the BNC connector. This tool has the added benefit of directional magnetic coupling whereas the iso-tee is capacitive and does not. AC
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