[TowerTalk] GAP Verticals with 75 Ohm CATV Cable and/or Balun

[w8ji.tom]

See my reply on the CATV line!

> Use an unun not a balun if you're simply going from 50-Ohm antenna or
> coax to 75-Ohm coax. Personalyy, why bother? 75 to 50 is a 1.5:1 VSWR.

The SWR on the 75 ohm line might be 1.5:1, but the SWR presented to a 50
ohm system at the end of the 75 ohm line can be the square of the mismatch
at the input end of the line when normalized to the 75 ohm line impedance
when the impedances repeat, and more if they do not!

Assume the GAP (an already inefficient antenna) has a 30 ohm resistive
feedpoint impedance at resonance, and the 75 ohm line is an odd 1/4 wl
long. The load presented to the rig will be about 188 ohms, or a 3.75 :1
VSWR even though the SWR on the line is only 2.5:1

Never mix random impedances in an uncontrolled situation! Plan ahead!

73 Tom

--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm