RSGB 2022 Convention - Input impedance of a feeder

Peter Duffett-Smith, GM3XJE
Most of us know already that the input impedance of a feeder at the transmitter end, connected to an antenna at its far end, depends on the length of the feeder, the antenna impedance, the frequency and so on. But why is this? Not being entirely sure himself, Peter wrote a software simulation application which reveals the interaction between the forward and reflected waves on a feeder. This is used to see how the resulting voltage and current at the input end defines the impedance and discover some surprising properties in the process.
Peter Duffett-Smith acquired the very old, large but defunct, family radio when he was eight, and found that he could give himself an electric shock when he disconnected a nine-volt battery from across the mains plug. From then on he was hooked! His grandfather taught him the basics and he gained his class A radio amateur licence aged seventeen. Peter completed a PhD at Cambridge and joined the Mullard Radio Astronomy Observatory at the Cavendish Laboratory under Nobel prize winners Professors Martin Ryle and Antony Hewish. He spent his working life tinkering with antennas and teaching Physics to undergraduates until retirement in 2013. Peter now lives in Scotland, continuing to tinker with antennas and delving into other interesting radio projects. He is the editor of RadCom Plus and a member of the RSGB Propagation Studies Committee.

[@WECB640]

EXCELLENT! More like this please. We often forget the physics, or worse...were never shown. 73

[@TheRSGB]

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[@WECB640]

I have been a subscriber for over 2 years. Keep up the great work. 73@@TheRSGB

[@TheRSGB]

Thank you!@@WECB640

[@bill-2018]

Yes, a good explanation of Input impedance of a feeder.
I can imagine C.B.'ers using this to justify trimming coax to the aerial and it must be a ½λ carefully trimmed to the exact length.
Then they say a jumper lead has to be multiples of 3 foot lengths. They forget it all connects together through the SWR meter as if it isolates the two. I asked a C.B.'er did the meter add to the aerial side or the transmitter side. I didn't get an answer.
In 49 years Ive never trimmed coax to a specific length except for crossed dipoles.
G4GHB.