Bill Turner wrote:

Dave wrote:
What's happening in the world of Antenna Physics to account for the change in
sensitivity as a function of position above or below the loading coil?

That behavior is perfectly normal. Length above the coil has far more
effect than length below the coil. Mobile operators have observed that
effect forever. Perhaps someone else can explain the physics involved?

A loading coil on an electrical 1/4WL mobile antenna can be
conceptually understood as an electrical 1/4WL stub. Consider
the following two electrical 1/4WL stubs.

Source-----Z01------+-------Z02----------open (example 1)

Source----Z02----+----Z01----+----Z02----open (example 2)

Assume Z01 = 4000 ohms, VF1 = 0.02; Z02 = 600 ohms, VF2 = 1.0

Although somewhat counterintuitive, the length of the piece
of Z01 line in the second example needs to be longer than
the piece of Z01 line in the first example to achieve an
electrical 1/4WL stub. Laying it out on a Smith Chart will
uncover the reasons.

In fact, if we create an electrical 1/4WL stub like this:

Source-----Z02-----+-----Z01-----open (example 3)

the sum of the electrical lengths of the two sections will be
electrically *longer than 1/4WL*. This is equivalent to putting
the loading coil at the very top of a mobile antenna with no
stinger.

Degrees of antenna are gained at a Z01--Z02 discontinuity,
i.e. at the loading coil to stinger transition point in
example 1 above.

Degrees of antenna are lost at a Z02--Z01 discontinuity,
i.e. at the base element to loading coil transition point
in example 2 above.
--
73, Cecil http://www.qsl.net/w5dxp