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[TowerTalk] Figuring degrees of a coax line

Subject: [TowerTalk] Figuring degrees of a coax line
From: Dennis OConnor <>
Date: Wed, 17 Oct 2007 05:20:57 -0700 (PDT)
List-post: <>
Yup, your method will work...  
  A bit more accurate way is to use your antenna analyzer (or an RF generator) 
as an rf voltage source and a VOM with an RF voltage probe... (you can make an 
RF probe in minutes - see the ARRL handbook)
  For those who have not done this before just remember one fact - the driven 
end of a quarter wave stub reverses the impedence seen at it's far end.. So, if 
the far end is infinite impedence (open circuit) it will be zero ohms impedence 
at the driven end...
  First cut the coax to a computed .25 wave, plus a bit, then by leaving the 
far end open circuited and frequency sweeping for the frequency that shows zero 
volts at the driven end gradually cut the coax so it is a quarter wave (zero 
volts @ 90 deg) on your chosen frequency...
  By doing it this way you eliminate the variables that the coax may not have 
the velocity factor claimed by the manufacturer, etc...
  Then measure the length and compute your inches per degree and do your final 
cut to your desired degrees...  
  The other way is the same method - but to calculate at what frequency your 71 
degree stub will be 90 degrees electrically... Then use the method above to 
directly cut the stub to the 90 degree point at that offset frequency - which 
means it will be 71 degrees at the desired frequency...
  As example, we want a 71 degree stub at 3500kc..  At what frequency will that 
stub be equal to 90 degrees?  The proportion is:
      71          90
    ------    =   ------
     3500         X
  Cross multiply 90 x 3500 =  71 times X
   or                     315000  =  71 X
  or                                X = 315000 / 71
  and                             X = 4436.6 kc
  So, cut your stub for 90 degrees at 4436.6 and it will be 71 degrees at 
  You guys can check my math for brain farts, etc...
  And, yes I know about 0.7 volts  loss across the diode and offsets and 
oscillator pulling and hysteresis, etc.. But this is ham radio not the physics 
lab and this will get him as close enough for variable frequency ham use...

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