From: liz@poppyrecords.invalid.invalid   
      
   Jeff Liebermann  wrote:   
      
   [...]   
   > I don't know how much de-tuning to expect, but I would guess(tm) that   
   > it's more than trivial.  Rather than calculate, it's easy to test with   
   > a 1/4 wave (19 inch) monopole antenna (coax cable with about 19 inches   
   > of the center conductor exposed.  Plug in a NanoVNA (or better VNA),   
   > set it for VSWR sweep, and you should see something like this:   
   >    
   > with the minimum at the about 145 MHz.  Take a length of PCV pipe,   
   > that is longer than the exposed center conductor, and cover the center   
   > conductor with it.  You should see the minimum frequency move   
   > downward.  Offhand, I don't know how large a frequency change the PVC   
   > pipe will cause, but I suspect it will be unacceptable.  Extra credit   
   > for trying it with different pipe diameters and materials.  If you   
   > want me to run the test, I just found my old NanoVNA and can try it if   
   > the power remains on, the battery is charged, and today's laptop   
   > repair decides to fix itself.   
      
   I've tried it with a wire dipole and a NanoVNA; one end of the dipole   
   held up by a polystyrene ball-point pen and the other end hanging free.   
   Slipping the free-hanging end into a piece of the pipe does not appear   
   to cause any de-tuning effects that are above the general 'noise' level   
   caused by doing the test indoors with relatively uncontrolled   
   conditions.   
      
   The Smith Chart display over a range from 100 Mc/s to 200 Mc/s was more   
   informative than the SWR graph.   
      
   Even if this did result in significant detuning, how would I separate   
   the VF effect from capacitive and loss effects?   
      
      
   --   
   ~ Liz Tuddenham ~   
   (Remove the ".invalid"s and add ".co.uk" to reply)   
   www.poppyrecords.co.uk   
      
   --- SoupGate-Win32 v1.05   
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