A correspondent wrote seeking explanation of W5DXP’s no-tuner tuner which purports to obtain a near match by adjusting the length of the transmission line using relays or switches of some kind.
The particular device that is of interest is one using a single double pole knife switch as a three position On-Off-On switch.
The accompanying explanations states that this “is a way to use a single DPDT knife switch to obtain one, two, or three feet of ladder-line depending on the position of the switch”.
It is certainly a simple explanation, but incomplete. The error may be small and insignificant in this scenario, but you come to understand whether it is or not significant by solving the scenario.
Lets say the switch above was in position 1 which connects both ends of the two foot loop and the though transmission line together. In the configuration pictured, the two foot loop behaves as two one foot long open circuit stubs, each in shunt with the through transmission line.
The effect depends on the impedance (ie V/I) at that point in the through line, the length and characteristic impedance of the loop, and the frequency.
Lets look at two examples, W5DXP’s dipole with 48′ (14.6m) of Wireman553 and the match device set for 1′ and assuming that the system was matched to 50Ω.
The effect of the two open circuit stub equivalent of the ‘shorted loop’ is shown by the green and magenta segments.
Now lets move the match device to half way along the main line, if it truly simply extends the main line by 1′, it should make no difference moving an extension from one end to the middle of the line.
The input VSWR is now more than 10.
Conclusion: the match device does not simply change the line length by 1 or 2 feet, the shorted sections act as stub elements in shunt with the main line, and their effect depends on the impedance (ie V/I) at that point in the through line, the length and characteristic impedance of the loop, and the frequency.