**ANTENNA NOTES FOR A
DUMMY**
**Restricted
Space Antennas**

**by Walt Fair, Jr.,
W5ALT**
**Horizontal
Dipoles**

**Now that we've
gotten some preliminaries out of the way, we can start to look at some
actual antennas. It seems that most antenna texts start out with dipole
antennas, so that must be a pretty good starting
point.**

**What is it? A dipole antenna is simply a straight
section of wire fed with an RF signal. Normally it is fed in the center
and is resonant, as indicated in the diagram. In these notes we will
consider a dipole to be near resonant. If it's not resonant or nearly so,
we'll call it a "doublet". In the literature, there doesn't seem to be a
consistent nomenclature, though.**

**Length. A resonant dipole is very close to 1/2
wavelength long. It's not quite 1/2 wavelength because the speed of light,
"c", is a little slower in copper or aluminum than in free space. There is
also a reduction in the velocity due to stray capacitance from insulation
or corrosion on the wires. The resonant length is also affected somewhat
by the conductor diameter, with larger diameters giving somewhat shorter
antennas. For practical purposes, the length of a resonant dipole can be
estimated as 95% of the length of a half wave in free space. The formula
is then**

**L = 0.95 (0.5)
c/F**

L (m) = 142.5/F(MHz)

L (ft) =
468/F(MHz)
**Here's a
question: What is the length of a center fed resonant dipole for the 6
meter band (50.1 MHz)?**

**Current and Voltage. The antenna has nearly zero
current at the ends. It's not quite zero due to capacitive end effects,
but if it's not very close to zero, you'll see arcing! It also has a
current maximum at the center. The voltage distribution on the dipole is
nearly opposite that of the current distribution. The minimum voltage is
at the center, while the ends have a very high
voltage.**

**Impedance. Remember that impedance is defined as the
ratio of voltage to current. From Ohm's Law, Z = E/I. That means that the
impedance of the dipole is minimum at the center and maximum at the ends.
The high impedance at the ends means that it may be very hard to feed the
antenna at the ends. Since the impedance is nearly infinite, it acts like
an open circuit and little power is transferred. In order to feed a dipole
at the ends, special matching provisions will be
needed.**

**In free space,
the impedance of a center-fed half wave resonant dipole is about 72 ohms.
That is a near perfect match for 75 ohm coax and quite acceptable for 50
ohm coax, too. Unfortunately, the impedance in the real world depends on
the height above ground and the ground quality.**

** **
**Conductor Diameter. The figures above, computed
using MultiNEC, show the effect of conductor diameter on the resonant
length and impedance of a 40m dipole at 7.1 MHz. Note that the formula
says that the resonant length would be 468/7.1 = 65.9 ft. In most cases
the NEC predicted length in free space is somewhat longer, but the effect
of insulation, ground and other factors is not accounted for. It is
recommended that when you construct a dipole, cut the wires a little
longer than required, then trim the antenna to resonance. It's easier to
cut than to splice additional wire.**

**Note that for
diameters larger than about 1 in, there is no perceptable affect of wire
diameter and the antenna performs as if it were made with no losses. The
biggest difference between the zero loss and copper wire cases occurs when
the wire diameter is 0.25 in or less. Although the resonant length is a
little smaller with copper wire, the major effect is an increase in
impedance. This increase is mainly due to the wire resistance which
increases in proportion to 1/D because of the skin effect. The following
figure shows the effect on the antenna gain.**

**What can we
learn from this simple exercise? First, as far as a 40m dipole is
concerned, there's no reason to use a conductor larger than about 1 in,
but that's still too large for most installations, space limited or not.
If we take 72 ohms as the radiation resistance, then the difference in
impedance represents losses in the wire. To stay above 95% efficiency, we
want the impedance to be less than about 72/0.95 = 76 ohms. From the
graphs, that happens whenever the wire diameter is larger than about 0.03
inches, which corresponds to roughly AWG #20 wire. As long as the wire is
larger than that, the effect is not going to be
noticeable.**

**We can cross
check the results from the impedance calculation with the antenna gain
results. Note that 95% efficiency is equivalent to 10 log(0.95) = 0.22 dB
drop in gain. Since the free space gain of a lossless dipole is about 2.14
dBi, we are at 95% efficiency when the gain drops to about 1.92 dBi. That
happens with a wire diameter less than about 0.03 in, confirming the
earlier evaluation. These results can be scaled for other frequencies. For
80m, the limit will be approximately double the wire diameter or on the
order of #14 AWG wire.**

**Effect of Ground. The effect of a ground does
several major things to a dipole antenna. First, it causes the signal to
be reflected which modifies the radiation pattern. (We haven't talked
about that yet, but we will.) Second, the reflected signal influences the
antenna and changes the impedance. Third, the ground will absorb some of
the signal, decreasing the efficiency. The following figures show the
effect of ground on a 7.1 MHz dipole made from #14 copper wire with
various ground conditions.**

**First, notice
that the resonant length and impedance both vary quite a bit depending on
the ground conditions, which we normally have no control over. The
resonant length can vary from 66 to 68 ft, with the larger variations over
a perfectly conducting ground. The impedance also will vary from very low
to around 100 ohms, with the perfect ground showing the larger variations.
Therefore, in building a dipole, we shouldn't be too concerned about
exactly estimating the resonant length or the impedance. We'll always have
to adjust the length for resonance and the impedance will generally be in
an acceptable range, unless we are over a perfect
ground.**

**But what about
gain and perfomance? As the above figures show, there are also variations
in the gain and the take off angle. First, notice that at heights below
about 30 ft, the take off angle is 90 degress - straight up. That means
that most of the radiation is going vertically upward, so the antenna will
be less than optimum for DX contacts. If we want to obtain a low angle of
radiation, say 20 to 30 degrees, then we better invest in some tall
towers! Surprisingly, the poorer ground shows the lower takeoff angles at
lower heights, so that may be good.**

**Notice in the
gain graph that the curves for less than ideal ground conditions are lower
than that for a perfect ground. The model for a perfect ground has no
ground losses - all energy is reflected. The difference between the curves
shows the loss due to real world ground conditions. As can be seen, there
is at least a 1 dB loss from perfect to average ground and another 1 dB
loss from average to poor ground. Remember that each dB represents about
20% loss of the radiated power. But worse for limited space conditions is
the situation at low heights. At heights less than about 30 ft, losses can
be on the order of 4 to 6 dB. That amounts to losing about 60 - 75% of the
radiated power warming the ground, while most of the rest of the power
warms the clouds overhead!**

The answer to the 6
meter dipole length is:

L = 492/F(MHz) = 468/50.1 = 9.34 ft or 9 ft 4
in, approximately

Due to conductor size, etc., the actual length will
vary

Next -
Vertical Dipoles