[Elecraft] RF on pipes

[Ian White GM3SEK]

Geoffrey Mackenzie-Kennedy wrote:
>Evening Fred,
>
>If the pipe is OPEN at its ends, would not the current flowing on the 
>outside of the pipe "fold over" at the end of the pipe and continue to 
>flow on the inside skin? The reverse, if you like, of the situation 
>where a coax feeder is used to feed a dipole / doublet.
>

No - HF fields cannot propagate inside an open tube because it behaves 
as like a 'waveguide beyond cutoff'. Any HF EM field injected into the 
open end will only penetrate a very short distance inside before it 
decays away.

Coax is different because it contains *two* conductors: the inner skin 
of the tube and the outer skin of the centre conductor. The internal 
field is created between these two conductors and is thus able to 
propagate down the line.

So the answer to the OP's question is that tubing will only give him the 
benefit of the larger outside skin.



-- 

73 from Ian GM3SEK
http://www.ifwtech.co.uk/g3sek