From: jeroen@nospam.please   
      
   On 2/16/26 11:23, Liz Tuddenham wrote:   
   > I'm still struggling to understand the way things work at VHF (145 Mc/s   
   > to be precise).   
   >   
   > As far as I can tell, the Velocity Factor of a piece of wire in free   
   > space is 1.00  The VF of co-ax can be anywhere from 0.6 to 0.9 depending   
   > on the dielectric but it does not depend on the type of wire as long as   
   > the ratio of diameters is correct.   
   >   
   > Therefore, if a piece of bare wire is dangling in the air I should   
   > expect its VF to be very close to to 1.00, but if I dangled it inside a   
   > bit of plastic plumbing, should I expect the VF to reduce, as it is now   
   > surrounded by dielectric?   
   >   
   > Would this be a large enough effect to notice inside a pipe of 22mm   
   > diameter with a 2mm wall thickness made of PVC with some sort of filler?   
   > (I have tried microwaving an offcut and it doesn't heat up.)  Presumably   
   > any PVC insulation on the wire itself would also have an effect?   
   >   
   > None of the websites on aerials seems to allow for this but they nearly   
   > all suggest cut-and-try to get the final lengths right, which makes me   
   > think that their calculations or measurements did not take account of   
   > all the relevant factors to start with.   
   >   
      
   The part of the field inside the dielectric would be slower than   
   c, of course, but the part of the field in the air would still   
   propagate at c. You'd get some sort of superposition of fields.   
   This is where a 3D EM field solver would be instructive.   
      
   A coaxial cable is easier to work out, because all fields are   
   contained inside a homogeneous dielectric.   
      
   With a wire inside a dielectric pipe, you get refraction and   
   partial reflections. The plastic is thin though. Intuitively,   
   with the geometry you describe, I'd expect to find the VF to   
   be essentially unity. Most of the field is still in air.   
      
   Jeroen Belleman   
      
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