[Elecraft] dipole antenna efficiency

[John Magliacane]

When it comes to antenna efficiency, it is important to understand
that when RF energy is applied to any antenna, three things will
invariably happen:

a) The antenna will convert a portion of the applied AC voltage
and current into electromagnetic energy and radiate it into space.

b) The antenna will convert a portion of the applied AC voltage
and current into heat energy as a result of resistive losses in the
antenna structure.

c) The antenna will reflect a portion of the applied AC voltage and
current back to the transmitter as a result of an impedance mismatch
between the antenna and that of the source.

Unless steel or nichrome wire is used, or electrically poor
connections exist in the antenna structure, losses due to (b)
will be low.

Effects of (c) can be reduced or eliminated by using intelligent,
low-loss impedance matching techniques and low-loss feedline.
(Technically, reflected power isn't a "loss" per se, since
energy isn't dissipated when a reflection occurs.)

That leaves us with (a), the desired outcome of applying RF energy to
an antenna.  Since losses due to (b) and (c) are typically low and/or
easily corrected, it is very difficult NOT to achieve high antenna
system efficiency.

Shortening the physical length of an antenna below that of a
half-wavelength DOES NOT reduce its efficiency provided the
necessary efforts to minimize resistive losses in the antenna
structure and the impedance matching networks are made.

That last statement is so important and so often misunderstood,
it bears repeating:

Shortening the physical length of an antenna below that of a
half-wavelength DOES NOT reduce its efficiency provided the
necessary efforts to minimize resistive losses in the antenna
structure and the impedance matching networks are made.

We could make an 80-meter dipole just one foot long and realize
high efficiency if the proper low-loss impedance matching techniques
are employed.

If we were to apply 100 watts to such an antenna, and we get zero
watts reflected back, and the antenna and matching networks remain
cool, then 100 watts of RF energy is being radiated from that one
foot dipole -- the same as if a full-sized dipole were used.

The penalties for using physically shortened antennas are:

(a) Decreased operating bandwidth
(b) Decreased directivity
(c) Somewhat difficult impedance matching

If we're wiling to adjust our impedance matching networks when we
QSY, then (a) isn't much of a problem since our signals are seldom
more than a few kHz wide, anyway.

Dipoles have 2.14 dB "gain" over isotropic radiators.  As we make
our dipole shorter and shorter (and keep resonating and impedance
matching it in the process), its directivity (b) approaches that
of an isotropic radiator.  If our radiating structure and impedance
matching networks are lossless, an extremely short dipole may be as
much as 2 dB weaker than the signal produced by a half-wave dipole
in the broadside direction.  That's less than half an S-unit!

So, the key to success lies in (c), the impedance matching network.
It needs to have extremely low loss, be capable of matching a very
wide range of impedances, and (unless you're rock-bound) be agile
so the antenna can be operated across a broad range of frequencies.
Ideally, it should be placed at the antenna feedpoint, itself.


73, de John, KD2BD


Visit John on the Web at:

	http://kd2bd.ham.org/
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