Transcript Antennas demystified

Antennas Demystified
Scott Honaker
N7WLO
Importance of Antennas
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Antennas are as important as the radio
A $5000 TV with rabbit ears will have a lousy
picture
Antennas are cheaper than amplifiers
Antennas are reciprocal – they hear as well as they
talk
Choosing Antennas
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Frequency – Dictates size
Mounting location – Base or mobile
Omni or directional – Coverage or gain
Polarization – Horizontal, vertical, circular
Resonant or non-resonant – Tuner required?
Power available
Feedline length and type
Cost
dBi vs. dBd
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dBi - Gain vs. Isotropic Resonator
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dBd - Gain vs. Reference Dipole
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Isotropic Resonator is infinitely small antenna with
no feedline in free space radiating equally well in all
directions (spherical pattern)
Gain referenced to a “real” dipole antenna with a
donut-like pattern
dBd = dBi + 2.15 dB
Gain/Loss Calculations
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ERP (Effective Radiated Power) is the real
number to consider
Gain uses a Log-10 scale
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3dB = 2-fold improvement
6dB = 4-fold improvement
10dB = 10-fold improvement
20dB = 100-fold improvement
ERP=Power x (Gain - Feedline Loss)
Radiation Patterns
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Visual representation of gain,
beamwidth, F/B ratio and F/S
ratio in one plane
E-Plane is crosssection that includes
driven element
H-Plane is
perpendicular to
driven element
Dipole Patterns
Yagi Patterns
E-Plane
H-Plane
Polarization
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SSB/CW is generally horizontal
FM is generally vertical
Satellites can be circular - RHCP, LHCP
Polarization loss can be significant at
VHF/UHF and microwaves
Bounced signals can change polarization
Verticals are more susceptible to QRM
Antenna Design Considerations
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Gain, SWR, Bandwidth, Front/Back ratio are
related and optimum values are not achieved
simultaneously for each
Does antenna have power going in desired
direction? Gain/Beamwidth
SWR Power Losses
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All power fed into the line, minus the line
attenuation, is absorbed into the load (antenna)
regardless of the mismatch at the antenna terminals
Line attenuation (loss) is the key factor in
determining losses due to mismatched antennas
(high SWR)
SWR Loss Examples
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SWR losses are added to line
attenuation for total loss
values
100’ RG-58 @ 20 meters, 50’
RG-8x @ 2 meters,
50’ Belden 9913 @ 70cm
have nearly identical
attenuation of 1.5dB
SWR SWR Losses
1.0:1
0dB
1.5:1
0dB
2.0:1
0.2dB or 5%
3.0:1
0.6dB or 13%
5.0:1
1.5dB or 29%
10:1
3.0dB or 50%
Loading
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Inductive loads – base, center, top
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Screwdriver antennas (adjustable loading)
Hamstick-style antennas
Hustler center-loaded
whips
Rubber HT antennas
Capacitance “Hats”
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Texas Bugcatcher
Cushcraft MA5B
Ground Plane Verticals
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¼ wave is omnidirectional with unity (0dBd) gain
when provided a proper ground plane
½ wave is unity gain with no ground plane and 3dBd
with ground plane
5/8 wave is 3.5dBd gain with nice omni pattern and
low radiation angle
Longer antennas have more omni patterns with
asymmetric ground planes (vehicles) and lower
radiation angles (see below)
¼ wave
½ wave
5/8 wave
Ground Planes
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“Perfect” ground plane from 120 evenly spaced
radials at least ½ wave in length
Wire mesh or wire from #12 to #28, above or a
few inches below the ground work fine
Elevated feeds (1/8λ or more above ground) can
use four ¼-wave radials
Vehicles provide poor ground planes at HF but
elevating the feedpoint reduces loss
Imperfect Ground Planes
Number of radials
16
24
36
60 90
120
Length of radials in
wavelengths
Total wire installed in
wavelengths
Power loss relative to
“perfect” ground plane
Feedpoint impedance
in ohms
0.1 0.125 0.15 0.2 0.25 0.4
1.6 3
5.4
12 22.5 48
3
2
1.5
1
52
46
43
40 37
0.5 n/a
35
Other Verticals
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Discone
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Wide usable frequency range
SWR ~2:1 for fundamental
through second harmonic
SWR ~3:1 for remainder of
coverage
Omnidirectional – Unity gain
Inverted-L
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2-3 dBd gain with vertical and
horizontal components
Requires ground plane
Balanced Feed Designs
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Dipole
Simple and effective
 Vertical or horizontal polarization
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Loop
Full wave has 3dBd gain
 Circular, Quad (square) or Delta (triangular) design
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E and H-plane patterns vary with height above
ground
Dipole Types
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Sloper
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Inverted-V
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Has 3dB to 6dB of directivity
toward slope
Single high mount, internal angle
should be >90 degrees
Bent
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Good attic antenna
Keep center section straight
Remainder of element can bend
or curve to fit
Dipole Types – Cont.
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Folded
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Caged
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High impedance needs
open wire feed
Same overall size as ½ wave
dipole but contains 1 wave of wire for nearly 3 dBd gain
Standard dipole with each leg made up of multiple wires
around spacers forming a wire tube
Larger effective element diameter increases bandwidth
Extended Double Zepp
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Two 0.64λ elements provide 3dBd gain
Multiband Dipoles
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Multiple
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Multiple dipoles/loops at a single feed
Trap
Traps are tuned circuits used to generate multiple
resonances on a single wire
 Traps cause loss and decrease bandwidth
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G5RV
Non-resonant – tuner required
 Radiation patterns vary with frequency
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Off-Center Fed Dipoles
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Feedline attached 1/3 the length from the end
Same ½ wave overall size
 Resonates at even harmonics, so 1 antenna can be
used on 80m, 40m and 20m
 6th harmonic (15m) has too high impedance
 Asymmetric impedance may cause current “in the
shack”
 Requires 4:1 or 6:1 current-type balun to match
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Other Multibanders
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Random wire
Can be any length of wire
 Requires tuner
 Works against earth ground
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Windom
“T” shape single wire feed attached 14% off center
 Works against earth ground
 “RF in the shack” is a potential problem
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Wire Arrays
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Half Square
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Bi-square
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Vertical polarization with bidirectional 5.8 dBd gain
Sterba Curtain
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Horizontal polarization with ~3.5dBd gain
Bobtail Curtain
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Vertical polarization with up to 3.8dBd gain
Horizontal polarization from multiple phased loops
Lazy “H” – Four element broadside array
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Greater than 6dBd gain possible
Yagis
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½ wave dipole driven element
Reflectors are 5% larger
Directors are 5% smaller
Number of elements and boom length
determine gain
SWR, bandwidth, gain, boom length and
front/back ratios all have to be considered
Typical Yagi Gains
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10m yagi with
SWR <2:1 and
Front/Back
>20dB
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Numbers are
rounded to
nearest 0.5 dB
Elements Gain dBi Gain dBd
3
7.5
5.5
4
8.5
6.5
5
10
8
6
11.5
9.5
7
12.5
10.5
8
13.5
11.5
Hybrid Yagis
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Quad
1λ loop driven element, reflector and directors
 Up to 3dBd gain over standard yagi
 Wider bandwidth than standard yagi
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Quagi
Loop reflector and driven element
 Simpler to feed and match at UHF
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Looper
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Entirely loop (generally circular) elements
Log Periodic
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Constant characteristics over
wide band (2:1)
Several varieties but hams
generally use dipole array
(LPDA)
All elements are driven
Gain similar to 3 element
yagi – 7dBi, 5dBd
Size similar to 3 element
yagi at lowest frequency
Reflecting Antennas
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Corner reflector
Practical size at 222 MHz and up
 Simple to construct, broadbanded, gains 10-15dBd
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Pyramidal Horn
Practical at 902 MHz and up
 Sides of horn are fed for up to 15 dBi, 13dBd gain
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Parabolic dish
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Gain is a function of reflector diameter, surface
accuracy and illumination
Parabolic Dish Gain
MHz
2’
420
4’
6’
10’
20’
30’
6.0dBi 12.0
15.5
20.0
26.0
29.5
902
12.5
18.5
22.0
26.5
32.5
36.0
1215
15.0
21.0
24.5
29.0
35.0
38.5
2300
20.5
26.5
30.0
34.5
40.5
44.0
3300
24.0
30.0
33.5
37.5
41.5
47.5
5650
28.5
34.5
38.0
42.5
46.0
52.0
10Ghz 33.5
39.5
43.0
47.5
51.0
57.0