From: liz@poppyrecords.invalid.invalid   
      
   Jeff Liebermann  wrote:   
      
   [...]   
   > The problem with collinear coax cable antennas is that for a given   
   > gain, the coax collinear is twice as long as a Franklin or AMOS   
   > antenna:   
      
   The reason I asked about VF was that I was going to try to build a   
   Franklin version from wire inside a pipe; but I needed to know the   
   correct lengths of the various components.   
      
   The design I based it on had the half-wave (in total) stubs wound up as   
   non-inductive and non-radiating coils between the ends of the elements.   
   To keep the size sensible, I decided to make it as three half-wave   
   elements with the middle one split in half to give a low impedance feed   
   point.  To my mind, the feed point impedance should be 3 x 50 ohms but   
   it measures about 65 ohms at the non-reactive frequency - which isn't at   
   145 Mc/s, despite checking my calculations and measurements several   
   times.   
      
   I would still prefer to use polarity reversals between the elements,   
   rather than phase change, as there are fewer frequency dependent   
   elements in the design and less to get wrong.  However, my experiments   
   so far with co-ax polarity reversals have never resulted in anything   
   useable.  The latest version (which I call the "NBG colinear") just gave   
   nonsense results on a Smith Chart and was more than 20dB down on the   
   same signal received with a sleeve dipole.   
      
   I feel I'm really failing to get to grips with any kind of colinear   
   aerial design - especially the co-ax types.   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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