Owen Duffy wrote:
One of the proponents posted on eham, the following solution to a loading
coil for 160m: "The VF of a 6" dia., 4 TPI coil on 160m would be about
0.02. Whatever number of degrees you want the coil to occupy, wind it
accordingly.", note the independence of coil size and location on the
monopole.

HUH???
THE LENGTH OF THE COIL IS NOT INDEPENDENT OF COIL SIZE!!!
THE LOCATION ON THE MONOPOLE IS NOT INDEPENDENT OF LOCATION!!!
The number of turns is dependent on coil size needed.
The number of degrees required is dependent upon location.
Please reread what I wrote until you understand those facts.

A greater number of degrees is obviously needed for center-
loading than for base-loading. This concept is extremely easy
to demonstrate in a an open-circuit stub.
************************************************** *************
When the Z0 at the impedance discontinuity *increases*,
electrical degrees are *lost*. When the Z0 at the impedance
discontinuity *decreases*, electrical degrees are *gained*.
************************************************** *************
For a base-loaded mobile antenna, electrical degrees are *gained*
at the coil to stinger junction.

For a center-loaded mobile antenna, electrical degrees are *lost*
at the base element to coil junction and *gained* at the coil to
stinger junction. Therefore, a center-loading coil has to be longer
than a base-loaded coil - all other dimensions being equal.

It is really simple transmission line analysis. Please perform
the following exercise to understand the concepts involved.

Example 1:

---600 ohm line---+---100 ohm line---open-circuit

The 100 ohm line is 10 degrees long. How many degrees does
the 600 ohm line have to occupy to be equivalent to a 1/4WL stub?

Example 2:

--100 ohm line--+--600 ohm line--+--100 ohm line--open-circuit

Each section of 100 ohm line is 5 degrees long, the same
10 degrees of total length as the 100 ohm line in the first
example. How many degrees does the 600 ohm line have to occupy
to be equivalent to a 1/4WL stub?

Hint: In the second example, the 600 ohm line will need to
be a lot longer because we have moved it from the base of
the stub to the center of the stub. Does this sound like
what happens when we move a coil from the base to the
center of a mobile antenna?

When one understands the above examples based on simple
transmission line stubs, one will understand what is
happening inside a loaded mobile antenna (but Richard,
this has nothing to do with radiation patterns).
--
73, Cecil http://www.w5dxp.com