From: liz@poppyrecords.invalid.invalid   
      
   Jeff Liebermann  wrote:   
      
   > On Wed, 18 Feb 2026 13:13:23 +0000, liz@poppyrecords.invalid.invalid   
   > (Liz Tuddenham) wrote:   
   >   
   > >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.   
   >   
   > I suggest you try one of the various NEC antenna modeling programs. In   
   > general, if it doesn't work on paper, it's not going to work when you   
   > build the antenna.   
      
   It might be useful for testing out designs before they are built but I   
   want to learn how they work and, although I thought I had a 'feel' for   
   the basics, the results I am getting bear no relationship to the theory   
   as I currently understand it.  My understanding is obviously faulty but   
   I can't see where.   
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
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