On Nov 9, 3:02*pm, Owen Duffy wrote:
Why do you call it a 1/8 wave loading coil? It wouldn't be along the
lines of the flawed "loading coil replaces the missing degrees" concept
would it?

Of course, the loading coil doesn't replace all of the missing
degrees, but it does replace some of the missing degrees. The
following inductance calculator will give the Z0 and axial propagation
factor of a coil from which the VF of the coil can be calculated. When
one knows the Z0 and VF of the coil, it can simply be treated as a
transmission line.

http://hamwaves.com/antennas/inductance.html

It is obvious that the loading coil at the bottom of a 5/8WL antenna
somehow causes the antenna to be electrically 3/4WL (270 degrees) long
because that's the only way the reflected wave can arrive back at the
feedpoint in phase with the forward wave in order to give a resistive
feedpoint impedance. So we need to answer the question of exactly
where those delays and phase shifts occur. Here's a conceptual model
of the 5/8WL base-loaded antenna. (The 50 ohm tapped point on the coil
has been ignored to simplify the problem.)

FP-//////////-----------5/8WL-------------------

(1) The coil occupies a certain number of degrees of the antenna. (2)
Since the Z0 of the coil and the Z0 of the whip are different, there
is a phase shift at the junction of the coil and the whip that can be
easily calculated. (3) The 5/8WL whip obviously occupies 225 degrees
of the antenna. All we have to do is figure out what the phase shift
is at the coil/whip junction and how many degrees the coil occupies.

degrees of coil = 270 - 225 - coil/whip phase shift

There's no magical faster-than-light propagation through the coil as
predicted by the lumped-circuit model. Coils are known to cause a
delay and the Hamwaves inductance calculator provides us an easy way
of calculating that delay through the loading coil. I can provide an
example if necessary.
--
73, Cecil, w5dxp.com