I have found a velocity factor of 0.95 to be more accurate than 1.0 for copper wire (1-3 mm thickness), but due to stray capacity, trimming is usually required to optimize the length of a particular antenna setup.
jremington:
I have found a velocity factor of 0.95 to be more accurate than 1.0 for copper wire (1-3 mm thickness)
Good trick - how did you measure that as distinct from the previously explained 0.95 "end effect" of an antenna?
By "explained" do you mean the hand-waving in that link?
Grumpy_Mike:
Mark it seems like you have had little practical experience of actually tuning an antenna.What about the SWR then? Most transmitters are tuned for minimum SWR on the antenna lead.
Anyway where does this help the OP?
Yes I do have practical experience on both 2m band and with short 1/4 wave wires on
ISM bands and I can solve Maxwell's equations and I understand them intuitively. I am
right, the insulation on a whip antenna has a small effect, nothing like the velocity factor.
The effect of a dielectric on a wire antenna can be no more than making the antenna wire
thicker (think of the capacitance between the tip and the ground plane). Look up tables
of length correction for antenna thickness and you'll see your mythical 60% is very wrong.
As I said the energy is not flowing parallel to the wire like in a transmission line.
Here's another way of understanding the situation:
Consider the entire whip antenna and ground plane embedded in a block of plastic
or whatever insulation you are thinking of with a dilelectic constant of 2.7 (velocity
factor 0.6).
Clearly the magnetic properties are the same as vacuum and all the capacitances
are multplied by 2.7. Thus the overall resonance is at 0.6 times the vacuum or
air value, in other words there is a genuine velocity factor of 0.6.
Now start removing the insulator bit by bit until there's only a thin coating on the
actual whip - clearly this is 95% or so like the vacuum case and only 5% of so
like the embedded in plastic case. Thus the "velocity factor" will be about 1/20th
as strong, perhaps 0.985...