[Elecraft] dipole antenna efficiency

Fri Jan 4 11:29:10 EST 2008

We're back to wanting our RF "superconductor" again <G>.

First, to add a note to the original question, it's useful to think of the
RF field as flowing OVER the outside of the conductor, so a helix (coil) or
any form of zig-zag arrangement of wires simply look like fat conductors to
the RF field. In this case a very short, fat conductor. The normal rule of
thumb for fitting an antenna into a limited space is to avoid any bends
sharper than 90 degrees in the wires. Once the bend is greater than 90
degrees, the overall length of the wire - as "seen" by the RF field - is no
longer as long as the conductor. Instead it begins to look like a conductor
of varying thickness whose length is about equal to the gross distance from
one end of the antenna to the other, ignoring the bends or turns (as in a
coil). Current does flow through the conductor (wire) but the important
point here is that the field generated by the currents doesn't follow the
wire when it bends or turns too sharply. 

To transfer power to an antenna efficiently we need to balance the
capacitive reactance with the inductive reactance so the currents and
voltages are in phase (the condition we normally refer to as "resonance"). A
half wave wire - a "dipole" - is one in which the natural capacitive
reactance of the wire matches the inductive reactance of the wire at what we
call the 'resonant frequency'. A Marconi - 1/4 wave radiator that is
grounded at one end - achieves the same thing with the lower inductive
reactance of the 1/4 wave wire being matched by the higher capacitive
reactance between the wire and the earth or some artificial ground such as a
ground plane or counterpoise. 

A short radiator has lower capacitive reactance, so we need to add inductive
reactance to a short antenna to achieve resonance.

For an coil to show inductance it must have current flowing through it. The
more current, the more inductance for a given number of turns on the coil. A
coil also has loss resistance since we don't have the perfect
"superconductor" handy. That loss resistance is much, much greater than the
d-c resistance since RF currents flow only along the very surface of a
conductor. The ideal RF coil uses the best conductor material available and
it uses a very large conductor for maximum surface area (although it can be
a very thin tube, if desired, since no significant current flows below the
surface). That's why high-efficiency RF coils tend to be large and use large
sized conductors that are often silver-plated. 

For the highest current to flow to minimize the amount of inductance needed,
we might put the coil near a "current loop" or point of maximum current (the
center of a dipole or the ground end of a 1/4 wave "Marconi" antenna). The
problem with that is that higher current equals greater resistive losses. If
we move the coil farther out toward a voltage loop (minimum current) we need
more turns to reach resonance. More turns means more wire which means more
resistive losses. 

A popular compromise is to put the coil about half way in between the loops.
On a short "Marconi" radiator, such as a mobile "whip" used on a car, that
puts the coil in the center, halfway between the base and the tip. On a
short dipole, one needs two coils about half way between the current loop at
the center and the voltage loops (the electrostatic 'poles' in a dipole) at
the ends.

A third approach is to continuously load the radiator with a helical
conductor such as Shaun mentioned. To the RF field, it's still a short
radiator, so it needs inductive reactance to achieve resonance. That
reactance is provided by making a long coil the length of the radiator. 

In all of these approaches, the biggest problem is resistive losses in the
wire. 

If we only had that perfect RF superconductor, we could make short antennas
as efficient as full-sized antennas. But then our full sized antennas would
be more efficient too! 

Lacking our RF superconductor, some designs use "capacitive hats" at the
ends of the antenna. These are discs, or disc-shaped constructions of wire
or tubing that don't add to the physical length - presumably we can't make
the antenna any longer or we would - but they do increase the
self-capacitance of the radiator. Increasing that capacitance reduces the
inductance needed and hence the losses in the inductor needed to establish
resonance. 

Ron AC7AC

-----Original Message-----
I seem to remember a 40m beam made this way in one of the old ARRL books.

David
G3UNA
>
> While Cookie's comments are true, there is a useful type of short 
> antenna
> known as a Normal Mode Helix, which if built, tuned and matched correctly 

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