basic antennas part ii

basic antennas part ii
Basic Wire Antennas
Part II: Loops and Verticals
Loop Fundamentals
• A loop antenna is
composed of a single
loop of wire, greater
than a half wavelength
long.
• The loop does not have
to be any particular
shape.
• RF power can be fed
anywhere on the loop.
Loop Characteristics
• Electrical length - the overall length of the dipole in
wavelengths at the frequency of interest.
• Directivity - the ratio of the maximum radiation of an
antenna to the maximum radiation of a reference
antenna. It is often measured in dBi, dB above an
isotropic (non-directional) radiator.
• Self Impedance - the impedance at the antenna’s feed
point (not the feed point in the shack).
• Radiation Resistance - a fictitious resistance that
represents power flowing out of the antenna
• Radiation Pattern - the intensity of the radiated RF as
a function of direction.
The Rectangular Loop
• The total length is approximately
1.02 .
• The self impedance is 100 - 130 
depending on height.
• The Aspect Ratio (A/B) should be
between 0.5 and 2 in order to have
Zs ~ 120 .

• SWR bandwidth is ~ 4.5% of
design frequency.
• Directivity is ~2.7 dBi. Note that
the radiation pattern has no nulls.
Max radiation is broadside to loop
• Antenna can be matched to 50 
coax with 75   /4 matching
section.
The Delta Loop
• A three sided loop is known as a
delta loop.
• For best results, the lengths of the 3
sides should be approximately equal
• The self impedance is 90 - 110 
depending on height.
• Bandwidth ~ 4 %
• Directivity is ~2.7 dBi. Note that the
radiation pattern has no nulls. Max
radiation is broadside to loop.
• Antenna can be matched to 50 
coax with 75   /4 matching
section.
Design Table: Rectangular and Delta Loop
BAND
LENGTH OF ANTENNA
(# 14 copper wire)
160 (1.83 MHz)
80 (3.6 MHz)
75 (3.9 MHz)
40 (7.1 MHz)
30
20
17
15
12
10 (28.4 MHz)
549 ft 4 in
279 ft 2 in
257 ft 8 in
141 ft 7 in
99 ft 1 in
70 ft 9 in
55 ft 6 in
47 ft 4 in
40 ft 4 in
35 ft 5 in
LENGTH OF MATCHING
SECTION
(RG-11 75  VF = 0.66)
88 ft 8 in
45 ft 1 in
41 ft 7 in
22 ft 7 in
16 ft 1 in
11 ft 5 in
8 ft 11 in
7 ft 8 in
6 ft 6 in
5 ft 8 in
Reduced Size Loops
• Loops for the low HF bands
can be inconveniently large.
• Loading can be used to
shorten the perimeter of the
loop
• Directivity ~ 2 dBi
• SWR Bandwidth is ~ 2.5%
of design frequency
• Radiation pattern is almost
omnidirectional
• Input impedance is ~ 150 .
Can be matched with 4:1
balun
Design Table: Inductively Loaded Loop
BAND
LENGTH A
LENGTH B
160 (1.83 MHz)
80 (3.6 MHz)
75 (3.9 MHz)
40 (7.1 MHz)
60 ft 0 in
35 ft 6 in
28 ft 2 in
15 ft 5 in
90 ft 0 in
45 ft 9 in
42 ft 3 in
23 ft 2 in
LOADING
INDUCTANCE (4)
63 H
30 H
27 H
15 H
The loop is vertically oriented, with the lower wire
approximately 10 feet above ground
Harmonic Operation of Loops
• A loop antenna is also resonant at integral multiples of
its resonant frequency.
• The self impedance of a /2 dipole at these multiples of
the resonant frequency is 200 - 300 ohms.
• The directivity is lower on harmonic frequencies
• Vertically oriented loops will have high angles of
radiation on harmonic frequencies.
• Horizontally oriented loops will have lower angles of
radiation on harmonic frequencies.
Polarization of Loop Antennas
• The RF polarization of a
vertically oriented loop may be
vertical or horizontal
depending on feed position
• Horizontally polarized loops
are predominantly
horizontally polarized in all
cases.
• Vertical polarization is
preferred when antenna is low
Putting up a loop
• Vertically oriented loops
may be erected with one
or between 2 supports
• A Horizontally oriented
loop will require at least 3
supports
• When more than one
support is used, they do
not have to be exactly the
same height
Putting up a loop
• The diagram at
the lower left
shows a sloping
loop that uses
only 2 supports
• Sloping loops
radiate both
horizontally and
vertically
polarized RF
Characteristics of Vertical Antennas
• Electrical length - the overall length of the antenna in
wavelengths at the frequency of interest.
• Radiation Angle - the takeoff angle for which the
radiation is maximum.
• Self Impedance - the impedance at the antenna’s feed
point (not the feed point in the shack).
• Ground Loss Resistance - a fictitious resistance that
represents power lost in the ground system
• Reflection Losses - reduction in signal strength due to
reflection of signals from the ground. (ground is a poor
reflector for vertically polarized RF).
The Importance of the Ground
• The ground is part of the vertical antenna, not just a reflector of
RF, unless the antenna is far removed from earth (usually only
true in the VHF region)
• RF currents flow in the ground in the vicinity of a vertical
antenna. The region of high current is near the feed point for
verticals less that /4 long, and is ~ /3 out from the feed point for
a /2 vertical.
• To minimize losses, the conductivity of the ground in the high
current zones must be very high.
• Ground conductivity can be improved by using a ground radial
system, or by providing an artificial ground plane known as a
counterpoise.
• Counterpoises are most practical in the VHF range. At HF, radial
systems are generally used.
Notes on ground system construction
• Ground radials can be made of almost any type of wire
• The radials do not have to be buried; they may lay on the
ground
• The radials should extend from the feed point like spokes
of a wheel
• The length of the radials is not critical. They are not
resonant. They should be as long as possible
• For small radial systems (N < 16) the radials need only be
/8 long. For large ground systems (N > 64) the length
should be ~ /4
• Elevated counterpoise wires are usually /4 long
Radial/Counterpoise Layout
• Note: The radials used in a counterpoise are not grounded !!
Design Table: Ground Radials for  /4
Vertical Monopole
No OF
RADIALS
4
8
16
24
36
60
90
120
LENGTH OF RADIALS
(in wavelengths)
0.0625
0.08
0.10
0.125
0.15
0.2
0.25
0.40
GROUND RESISTANCE
(ohms)
28
20
16
10
7
4
1
<<1
• Radial wires may be in contact with earth or
insulated
• Wire gauge is not important; small gauge wire such
as #24 may be
• The radial system may be elevated above the earth
(this is known as a counterpoise system)
Vertical Monopole Antennas
• Length < 0.64
• Self impedance:
ZS = ZANT+RGND + R REF
• Efficiency:
 = |ZANT | /|ZS|  ranges
from < 1% to > 80%
depending on antenna
length and ground system
• Efficiency improves as
monopole gets longer and
ground losses are reduced
 /4 Vertical Monopole
• Length ~ 0.25
• Self impedance:
ZS ~ 36 - 70 
• The  /4 vertical requires a
ground system, which acts as
a return for ground currents.
The “image” of the monopole
in the ground provides the
“other half” of the antenna
• The length of the radials
depends on how many there
are
• Take off angle ~ 25 deg
Design Table:  /4 Vertical Monopole
BAND
160 (1.83 MHz)
80 (3.60 MHz)
75 (3.90 MHz)
40 (7.10 MHz)
30
20
17
15
12
10 (28.4 MHz)
LENGTH OF
MONOPOLE (#14 wire)
127 ft 10 in
65 ft 0 in
60 ft 0 in
33 ft 0 in
23 ft 1 in
16 ft 6 in
12 ft 11 in
11 ft 0 in
9 ft 5 in
8 ft 3 in
 /2 Vertical Monopole
• Length is approximately
0.48
• Self impedance ~ 2000 
• Antenna can be matched to
50 ohm coax with a tapped
tank circuit
• Take off angle ~ 15 deg
• Ground currents at base of
antenna are small; radials
are less critical for /2
vertical
Design Table: /2 Vertical
BAND
160 (1.83 MHz)
80 (3.60 MHz)
75 (3.90 MHz)
40 (7.10 MHz)
30
20
17
15
12
10 (28.4 MHz)
LENGTH OF
MONOPOLE (#14 wire)
255 ft 8 in
130 ft 0 in
120 ft 0 in
66 ft 0 in
46 ft 2 in
33 ft 0 in
25 ft 10 in
22 ft 0 in
19 ft 0 in
16 ft 6 in
Short Vertical Monopoles
• It is not possible for most
amateurs to erect a /4 or /2
vertical on 80 or 160 meters
• The monopole, like the dipole
can be shortened and resonated
with a loading coil
• The feed point impedance can
be quite low (~10  ) with a
good ground system, so an
additional matching network is
required
• Best results are obtained when
loading coil is at the center
Design Table: Short(/8 ) Vertical Monopoles
BAND
160 (1.83 MHz)
80 (3.60 MHz)
75 (3.90 MHz)
40 (7.10 MHz)
LENGTH OF
MONOPOLE (#14 wire)
67 ft 2 in
34 ft 2 in
31 ft 6 in
17 ft 4 in
For base loading an inductive reactance of j550  is req’d
For center loading and inductive reactance of j1065  is req’d
Inverted L
• The inverted L is a vertical
monopole that has been folded
so that a portion runs
horizontally
• Typically the overall length is ~
0.3125 and the vertical
portion is ~ 0.125 long
• Self impedance is ~ 50 + j200
• Series capacitor can be used to
match antenna to coax
Design Table: Inverted L
BAND
LENGTH A
LENGTH B
160 (1.83 MHz)
80 (3.6 MHz)
75 (3.9 MHz)
40 (7.1 MHz)
67 ft 2 in
34 ft 2 in
31 ft 6 in
17 ft 3 in
100 ft 9 in
51 ft 3 in
47 ft 3 in
26 ft 0 in
MATCHING
CAPACITANCE
410 pF
220 pF
200 pF
110 pF
Use of a Vertical Monopole on several
bands
• If a low angle of radiation is desired, a vertical antenna
can be used on any frequency where is is shorter than
0.64  :
• The lower frequency limit is set by the capability of the
matching network and by efficiency constraints.
• The ground system should be designed to accommodate
the lowest frequency to be used. Under normal
circumstances, this will be adequate at higher
frequencies
Loop and Vertical Antenna Materials
• Wire
– #14 Copperweld
•
•
•
•
very strong
kinks very easily; it is difficult to work with
does not stretch
subject to corrosion
– #14 stranded copper wire with vinyl insulation
•
•
•
•
moderately strong
easy to work with, does not kink
can stretch under high tension (a problem with long antennas)
does not corrode
– Monel trolling wire
• strong
• much higher resitivity than copper
• corrosion resistant
Loop and Vertical Antenna Materials
• Insulators
– ceramic
•
•
•
•
strong
resist very high voltages
not affected by sunlight
expensive
– plastic
•
•
•
•
weaker than ceramic insulators
resist moderately high voltages
can be degraded by sunlight
relatively inexpensive
Dipole Antenna Materials
• Baluns
– choke balun (several turns of coax wound into coil ~ 6 in in
dia) is usually sufficient unless impedance transformation is
required
– Powdered-iron core baluns should be used within their ratings
to avoid core saturation.
• Support ropes
– should be at least 3/16 inch diameter and UV stabilized
– UV stabilized Dacron works well in most applications
– polyolefin ropes quickly degrade in sunlight and should be
avoided
Loop/Vertical Antenna Supports
• Almost any structure can be used to support a loop or
vertical
• A loop antenna should be kept at least 12 inches away
from a conducting support and a vertical antenna
should not be run parallel to a conducting support
• If trees are used, leave some slack in the antenna so
that swaying of the branches does not snap the wire
• If a tree is used to support a vertical antenna, the wire
should not run straight down the trunk. The wire can
be run 10 - 20 degrees from vertical without problems
• The top wire of a horizontally polarized vertically
oriented loop should be at least 1/2 wavelength about
the surrounding terrain (/2 =492/f)
Other useful information
• Do not run a loop or inverted L above power lines!!!!
• When the feed line leaves the loop, it should run
perpendicular to it for at least 1/4 wavelength
• If an elevated counterpoise is used for a vertical
antenna, place it high enough that it people cannot
touch it
• If a loop antenna’s lower wire has to be close to the
ground, place it high enough that no one will tamper
with it
Antenna Comparison
ANTENNA
Pros
Cons
1λ loop
GAIN
(dBi)
2.7
Good gain
“Small Loop”
2
Smaller than equivalent 1λ
loop
Can be very large on low
HF bands
Low gain and 4 loading
coils are required
λ/4 vertical
<0
Simple to erect
λ/2 vertical
<1
More gain, less affected by
ground
Short Vertical
< -1
Shorter support needed
Radials or counterpoise
required
High support and complex
matching network required
Generally lossy ; good
ground system required
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