[RFI] Balanced-line antennas and couplers

[Donald Chester]

>  Edward McCann <edwmccann at yahoo.com wrote:

>  Could you address the difference between "tuned" and "untuned" feeders to which refer.

>  I have yet to read a description of this phenomena or technique that I can honestly say I fully understand, or that I can repeat to any other interested party.

"Tuned feedline" means the load on the transmission line is different from the surge impedance, so it operates with substantial standing waves and normally must be used with some kind of a tuner.  The feedline length is usually some integral multiple of a quarter wave, measured out just like you would the length of each leg of a dipole.

"Untuned feedline" means the load is matched to the surge impedance of the  transmission line so that it operates at or close to 1:1 SWR (no standing waves on the line).  It can be any length, and if the output impedance of the transmitter, the surge impedance of the transmission line, and the impedance the load presents to the transmission line all match each other, no tuner is needed.  Coax is nearly always run as an untuned, or matched line, since loss increases rapidly with increased SWR.  Open wire line can be run at high SWR, up to 10:1 or sometimes more, without serious loss, although the matching network must be re-tuned when you QSY beyond a certain limit.  Open wire line is nearly always used for a tuned feedline, although that stuff they call "window line" can be used as well, but it doesn't perform quite as well as  true OWL.

Don k4kyv