This video describes how reflections create SWR in an easy to understand way. Also an experiment is shown how these results can be measured with modest test equipment. www.w0qe.com www.w0qe.com/Si...
Thanks Larry for this graphical explanation. 73... I do have a question: Since we have reflections coming back to the generator, can we consider the generator as an instantaneous short-circuit so that current is produced in the 50-Ohm generator source-resistor; like we do for DC-sources whereby their (ideal) internal resistance is considered to be zero? Retired now so trying to get back into RF which was my childhood initial interest; so trying to learn LTSpice and SimSmith while also building receivers and transmitters.
No, the generator appears as a 50 ohm impedance. Otherwise the the reflection get re-reflected at the transmitter repeatedly. This can easily be simulated in LTSpice. Larry, W0QE
In the 150 ohm load case, "5.0V @ 100mA exits TL" and "Load sees 7.5V @ 50mA". With 2.5V refected back to the source How did we get 7.5V from 5V? Do we have a split that adds up to 10V? Where did we get 10V? 73, Kent, AC1HJ
Kent, 5V @ 100mA is equivalent to 50 ohms (5/.1). However the 150 ohm load is unable to deal with this as 5V across 150 ohms results in a current of 33mA. The voltage to current ratio for the load resistor will be 150. Consequently a wave of +2.5V @ 50mA (satisfying the 50 ohm transmission line) gets generated back into the line. This leaves 7.5V @ 50mA available for the load which is exactly what a 150 ohm resistor needs. In the steady state you can also think that the 150 ohm load reflects an impedance back to the 50 ohm source driving the transmission line. Then you can analyze the circuit as a voltage divider between the internal generator voltage and the ratio of the 50 ohm source and the reflected impedance back to the generator. This can be demonstrated easily with modest test equipment and is fun to do. Hopefully this makes sense and if not you are welcome to contact me directly. 73, Larry, W0QE
By far the best explanation I have heard. Thanks.
Glad this video was able to explain SWR in a way that helps you understand what is happening.
Thanks so much for these very educational videos.
Thanks Larry for this graphical explanation. 73...
I do have a question: Since we have reflections coming back to the generator, can we consider the generator as an instantaneous short-circuit so that current is produced in the 50-Ohm generator source-resistor; like we do for DC-sources whereby their (ideal) internal resistance is considered to be zero?
Retired now so trying to get back into RF which was my childhood initial interest; so trying to learn LTSpice and SimSmith while also building receivers and transmitters.
No, the generator appears as a 50 ohm impedance. Otherwise the the reflection get re-reflected at the transmitter repeatedly. This can easily be simulated in LTSpice.
Larry, W0QE
Excellent!
Thanks!
In the 150 ohm load case, "5.0V @ 100mA exits TL" and "Load sees 7.5V @ 50mA". With 2.5V refected back to the source
How did we get 7.5V from 5V?
Do we have a split that adds up to 10V? Where did we get 10V?
73,
Kent, AC1HJ
Kent,
5V @ 100mA is equivalent to 50 ohms (5/.1). However the 150 ohm load is unable to deal with this as 5V across 150 ohms results in a current of 33mA. The voltage to current ratio for the load resistor will be 150. Consequently a wave of +2.5V @ 50mA (satisfying the 50 ohm transmission line) gets generated back into the line. This leaves 7.5V @ 50mA available for the load which is exactly what a 150 ohm resistor needs.
In the steady state you can also think that the 150 ohm load reflects an impedance back to the 50 ohm source driving the transmission line. Then you can analyze the circuit as a voltage divider between the internal generator voltage and the ratio of the 50 ohm source and the reflected impedance back to the generator.
This can be demonstrated easily with modest test equipment and is fun to do. Hopefully this makes sense and if not you are welcome to contact me directly.
73,
Larry, W0QE
Now, that _is_ teaching! Thank you.
Thanks!
Outstanding. Thank you!
This is too much to stomach. Thank you for the effort though!