Richard Harrison wrote:

ON4UN has a graph, Fig 9-22 on page 9-15 of "Low-Band DX-ing" which
shows current distribution of a base-loaded whip, In his example, the
whip is 45-degrees long.. The loading coil provides the extra
45-degrees required for resonance.

Those diagrams were highly misleading, and have been withdrawn from the
current 4th edition. That whole introduction to short verticals has been
completely rewritten and revised.

The problem was that the current profiles of the full-size and loaded
quarter-wave antennas were both drawn against a linear vertical scale of
*phase*. This not only misrepresented the physical lengths of the lower
and upper sections, it also misrepresented the length and effect of the
coil. This presentation is highly misleading, and the first victim was
the person who drew it, and then overlaid current profiles on it.

(I don't think this was ON4UN. When someone sets out to produce a
500-page book, they have to quote some things on trust from other
people; and it may take a few editions to iron out all the kinks. Much
more to the point, ON4UN is right at the leading edge of his subject,
and each edition contains something new and important.)

The replacement diagrams in the 4th edition are much better. They show
current profiles against *physical* height, and help bring out what's
really happening.

Current at the base of ON4UN`s whip is one amp times the cosine of
45-degrees, or 0.707 amp. The loading coil has an input of one amp.

With 1 amp into the loading coil and 0.707 amp out of the loading coil,
the coil definitely does not have the same current at both ends.

You can't quite those figures in evidence, because they were never more
than a speculation based on misunderstandings. When ON4UN came to think
about it, he quite rightly changed his mind.



This brings up another point that hasn't been mentioned so far in this
discussion: there is an important difference between purely inductive
loading, and the kind of loading you can get from any practical
inductOR.

Note the difference. An inductOR is a real-life electronic component, a
coil of wire. InductANCE is its main electrical property - but it isn't
the only one.

When a shortened antenna is loaded by pure inductance, you find the
following:

1. The magnitude and phase of the current flowing into the loading
inductance are both the same as that of the current flowing out (this is
a fundamental property of pure inductANCE). This means the current
distributions in the sections above and below the loading inductance
join up in a sharp kink.

2. What does go through a step change is the *voltage* across the
inductance. This changes in both magnitude and phase. For a typical
centre-loaded whip, the RF voltage is low in all of the bottom section,
but above the loading inductance it's very high indeed.

To repeat: this is the situation for loading with pure inductANCE.

(Sorry to keep shouting "ANCE!" and "OR!" like that, but I'll bet
someone still comes back with a reply that proves they didn't read what
I actually wrote.)

The diagram in ON4UN's 4th edition (Fig 9-44) shows these effects much
better than I can describe them in words. The diagram came from an
article by W7XC in QST for March 1990. I strongly recommend everyone to
look at these diagrams... but please don't treat them as 'bible text'.
Do your own thinking about it.

A different viewpoint on loading inductANCE is given in an article by
Boyer in 'Ham Radio'. This uses the 'antenna-transmission line analog'
theory... and comes to exactly the same conclusions about the effects of
loading inductANCE: it is simply there to resonate the capacitive
reactance that arises from having physically shortened the antenna.
(Ironically, ON4UN's worked examples and computer programs to calculate
loading inductance have always been based on this approach; the
conceptual error in early editions was only in that one diagram.)

With a firm grip on the way that pure inductANCE loads an antenna, you
are then in a good position to look at the differences that appear when
you use a practical inductOR.

With a real-life inductOR, you don't have pure inductANCE any more. It
is embedded in a component that is made from some length of wire, wound
into a coil that has a physical length and diameter, has capacitance
between its own turns, and also has capacitance to the straight sections
of antenna above and below it. The effects of the coil's inductANCE will
still be there, but you can certainly expect to see a lot of detailed
differences.

But the practical differences can't possibly be understood without that
basic understanding about inductANCE as a foundation. Without it, you're
building your house on sand.



--
73 from Ian GM3SEK 'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek