[Antennas] RE: capacitors on dipole

[Robert Lay]

Dear Chris,

> What would happen if, on the down drop one added an inductor or a
capacitor?

Adding lumped inductance in the down drop portion is similar to adding it
anywhere in the antenna. It changes the impedance seen at the feed point in
the expected way. However, what it does or does not do for the efficiency is
the real question, and that cannot be answered simply - due to the number of
options on "where" and "how much". Suffice it to say, that it is ALWAYS a
compromise in comparison with real length of the antenna.

>
> I have been told that the inductor needs a length of wire after it but the
capcitor
> does not.

It needs the length of wire "after" it only in that the efficiency is
improved by having the inductance more toward the low impedance portion of
the antenna. The limit in that direction is the inductor's losses, which are
increasing as you have higher current at the lower impedance end of the
antenna.

The "capacitive hat" is the only capacitor here worth talking about and is a
"one sided" capacitor. the other side is the earth itself - therefore, no
possibility of a wire coming after it. You would never insert an actual
capacitor in the antenna as a series element unless you wanted to "shorten"
it electrically. That would serve little purpose and most people would
prefer to have the greater length.

>Is the linear loading both inductor and the capacitor or what?

I think you are asking about the "nature" of the antenna wire itself. Yes,
in each increment of antenna wire there is both inductance and capacitance -
much the same as in a transmission line. It is most easily envisioned in a
vertical, because the symmetry with respect to the ground plane simplifies
the equivalent circuit. Each increment of vertical antenna length has as its
equivalent circuit a small inductor in series and a small capacitor going to
earth.

>Electrical
> length may be increased by different means but what does such do to the
efficiency
> and the band width?

That is a difficult question that I can only answer in a very general way.
The easiest way to imagine the trend is to take it to the extreme - for
example a very very short antenna. Such an antenna needs an inductance and a
capacitor that are either in series resonance or a capacitor and inductor in
parallel with each other and a near parallel resonance. In each practical
circuit, the small antenna is contributing very little - most of the energy
is lost in the lumped circuit elements and very little is radiated in the
radiation resistance because the radiation resistance is so tiny in
comparison with the losses in the inductor and capacitor. So, it is
"resonant" but very inefficient.

Anything you do to a "short" antenna to improve the feed impedance by tuning
it to near resonance is effective to that end, but in the extreme is
resulting in lower and lower efficiency.

> I ran across an antenna on the net were the author of such used coax to
reduce the
> physical length.  I questioned what would happen.

I assume that you are referring to a variety of the "bazooka" antenna. Many
have argued over the relative merits of the bazooka antenna. My personal
opinion is that it has its following because it tends to be broad band, but
its efficiency is less than a simple, resonant dipole.

Bob Lay (W9DMK) in Dahlgren, VA
http://www.qsl.net/w9dmk