[TowerTalk] Wattmeters and 75 ohm Coax

K7GCO@aol.com K7GCO@aol.com
Sun, 19 Aug 2001 16:37:50 EDT


<< n a message dated 8/19/01 9:52:28 AM Pacific Daylight Time, 
W4EF@dellroy.com writes: << 
  Hi Bob, 
   I don't know about 75 ohm options for the Bird 43 or Coaxial Dynamics 
   wattmeters. You might want to call the factory and ask them if they can 
   modify the directional couplers for a 75 ohm characteristic impedance. They
   probably don't get much call for this as 75 ohm cable is typically used in 
CATV and video applications where RF power levels never get much beyond 100 
mW. Of course, the other question is do you really want to do that? If you 
are trying to match your antenna to 75 ohm CATV hardline  in order to 
minimize mismatch losses along 
a long cable run, then you would want to normalize your instruments to 75 
ohms.  
 
The other option would be to measure the 
  impedance at the input to the 75 ohm line with an impedance analyzer like 
the 
  MFJ-259/269 and then plug the impedance reading into the formula for VSWR 
  normalized to 75 ohms (this is a simple algebraic equation). You can also 
place
  a 50:75 ohm UNUN at the input to your 75 ohm cable. This will allow you to 
read 
  the 75 ohm VSWR/reflected power directly with the 50 ohm instrument. 
  
  On the other hand, if you only concern is the match that your amplifier 
sees, stay 
  with the 50 ohm instruments - WYSIWYG. Power output is just Pforward - 
   Preflected.   
  Mike, W4EF..............
   >>
 Very good suggestions.  There are many applications for a 75 ohm SWR bridge. 
I have a roll of 3/4'' 75 ohm hardline I'm eager to use.  As I just suggested 
in a previous post, it would be ideal to have a center conductor in the Bird 
that could be replaced with a smaller diameter center shaft for 75 ohms.  
Hopefully no other major adjustements would be needed.  I'll talk to Bird 
about that.
 
 In the older MFJ Analyzer I replaced the 50 ohm resistor in the RF 
Wheatstone Bridge with a 75 ohm resistor.  It was a big and difficult job for 
several reasons but it did work.  I plan to do it on my latest one and with a 
switch so I can either have 50 or 75 ohms.  I'll write it up when I get time. 
 k7gco 

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