Antenna Fundamentals

Presented by:

Bob Kenyon - K8LJ

CRES Amateur Radio Association

December 6, 2012

Agenda

• Introduction and background

• Basic antenna theory

• Transmission line impacts

• Antenna modeling

• Members’ antenna questions discussion

• Conclusion and next steps discussion

TransceiverTransmission Line

Antenna

Basic Communications System

Electrical Signal

Generated

Electrical WavePropagated

Electromagnetic WaveRadiated

(Voltage) (Voltage & Current) (Voltage, Current & Magnetic Field)

Antenna Equivalent Circuit(Feedline Not Included)

RR RL RG

RadiationResistance

AntennaResistive Loss

Ground Losses

This is where we want the power to go

Usually not a problem for non-shortened

horizontal antennas, such as a full size dipole

Ant. Efficiency = RR

RR + RL + RG

X 100%

Often a big problem, especially for vertically

polarized antennas

Basic Antenna Concepts

• Antenna gain is achieved by pattern alteration (directivity )

• All antennas are directive (except an isotropic source)

• Antenna gain = antenna directivity - antenna losses

• Gain is affected by antenna design, physical realization, & environment

• For antennas near earth, the pattern (directivity , gain) is greatly affected by reflections from the earth’s surface (ground conductivity impact)

• Reflection of horizontally polarized signals is usually quite efficient

• Reflection of vertically polarized signals is often inefficient

• Theory of Reciprocity: Antennas behave the same transmitting & receiving

Current Feed vs. Voltage Feed(for a λ /2 dipole, not all antennas)

Zin is Low ~ 7 3 ohmsin Free Space

Zin is High - can range from100s to 1000s of ohms

Zin ~ RR

Zin >> RREnd Feed (Voltage Max.) = Voltage Feed

I

Center Feed (Current Max.) = Current Feed

I

V

V

Cable Attenuation - dB Per 100 Feet

Frequency (MHz)

Att

enua

tion

(dB

)

Transmission Line Modeling

• TLDetails free program

• Go to AC6LA.com

Practical Example - RG-8A Coax vs. 450 Line

Frequency(MHz)

1.8

3.8

7.1

10.1

14.1

18.1

21.4

24.9

28.4

Ant Impedance (Ohms)

4.5 - j1673

38.9 - j362

481 + j964

2584 - j3292

85.3 - j123.3

2097 + j1552

345 - j1073

202 + j367

2493 - j1375

SWRRG-8A Coax

>1000:1

63:1

49:1

134:1

5.6:1

65:1

73:1

18:1

65:1

SWR450 Line

>1000:1

19:1

6.2:1

15.2:1

5.7:1

7.3:1

9.4:1

3.9:1

7.3:1

Loss 100 ftRG-8A Coax

26.2 dB

5.7 dB

5.7 dB

10.2 dB

1.8 dB

8.7 dB

9.5 dB

4.9 dB

9.7 dB

Loss 100 ft450 Line

9.2 dB

0.5 dB

0.2 dB

0.6 dB

0.3 dB

0.4 dB

0.6 dB

0.3 dB

0.5 dB

• Assume a 100 ft long, 50 ft high, center-fed dipole

• Average ground conductivity (5mS/m), & permittivity of 13

• Antenna impedance computed using EZNEC (NEC-2 engine)

Current Feed vs. Voltage Feed(for a λ /2 dipole, not all antennas)

Zin is Low ~ 7 3 ohmsin Free Space

Zin is High - can range from100s to 1000s of ohms

Zin ~ RR

Zin >> RREnd Feed (Voltage Max.) = Voltage Feed

I

Center Feed (Current Max.) = Current Feed

I

V

V

α

α

To Distant Point

Direct Wave

Reflected Wave

180º Phase Reversal

+h

-h

d

Horizontal Antenna Above Earth

If d = n •180º (n odd)

If d = n •180º (n even)

Wave Reinforcement

Wave Cancellation

n = 0,1,2,3,4 ...

Earth’s Surface

·

·

(180º = λ/2)

Horizontal Antenna(End View)

ImageAntenna

(- 180º phase)

Antenna Modeling

Why Model Antennas?

• Computer horse-power now available, even on PCs

• Significant resource ($) & time savings

• Improve accuracy & repeatability

• Easily perform “what if” analyses

• Learn a lot about antennas quickly

• It’s fun! … (warning - can become additive)

What Can a Model Tell Us?

• Antenna physical depiction (view)

• Far Field Pattern - 2D plots (azimuth or elevation) - 3D plots (both together)

• Antenna gain at any angle

• Front-to-back, front-to-side ratios, 1/2 power beamwidth etc.

• SWR vs. frequency

• Impedance (real & imaginary) vs. frequency

• Wire currents - magnitude and phase for each segment

• Other stuff

Antenna Modeling Terms

• Wire - Basic antenna model building entity (linear, no bends)

• Segment - Sub-division of a wire

• Source - Feed point electrical specifics (Volts/Amps & Phase)

• Load - R, L, and C values alone or in any combination

• Ground Type - Free space, perfect and types of “real” ground

Wires and Segments

1 Wire11 Segments

4 Wires5 Segments Each

Dipole

Bent Element

Quad Loop

1

4

3

2

1

3 Wires2 With 2 Segments1 With 7 Segments

•N

= Wire Junction

•

1

2

3

= Source

= Wire Number

Antenna Modeling Guidelines

• A wire should have at least 9 segments per half-wavelength

(times 2 + 1 for impedance and SWR plots)

• Segment length should be > than 4 times wire diameter

• To extent possible, keep segment lengths equal

Antenna Modeling Products

Public Domain (Free)

• 4nec2 - Modeling and optimization program (Dutch) • MMANA - By JE3HHT, Makoto (Mako) Mori (MININEC)

• EZNEC Demo 5.0 - By W7EL (www.eznec.com)

Commercial • Nec-Win Plus (similar to EZNEC)

• K6STI - Various modeling & optimization programs (MININEC) • EZENEC 5.0, EZNEC + 5.0, EZNEC Pro (NEC-4)

(Sample)

Demo of EZNEC 5.0 DEMO

Available at W7EL@EZNEC.com

1/2 Wave Dipole Elevation Plots vs. Antenna Height

1/4 Wavelength(17.5 ft.)

1/2 Wavelength(35 ft.)

3/4 Wavelength52.5 ft.

1 Wavelength(70 ft.)

5/4 Wavelengths(87.5 ft.)

1 & 1/2 Wave-lengths(105 ft.)

14 Mhz. - Perfect Ground

Estimated Ground Conductivity in the U.S.

= 30 mS/meter= 0.5 mS/meter

mS = .001 siemens = .001 mho Salt water = 5000 mS/meter

Vertical Antenna Patterns

In Free Space(Applies to λ /2 Dipole Also)

Above a Perfectly Conducting Surface

• Love them

• Hate them

Baluns

Long Wires

V Beams

Rhombics

Creating a V Beam

A Rhombic – Two V Beams Back-to-Back

Voice of America Antennas Near Cincinnati

5,370 ft.

K8LJ 160 – 6 Meter Antenna

Cost

• Insulator - $ 0• Wire - $ 0.53• PL259 - $ 0.50• Coax - $ 4.00

Total - $ 5.03

Bucky

References

•M- Maxwell, M W. , Reflections: Transmission Lines and Antennas, Newington, CT: ARRL,1990.•T- The ARRL Antenna Book, Newington, CT: ARRL, 2005.• - Jeffrey S. Beasley & Gary M. Miller, Modern Electronic Communication, 9th Edition, Columbus, OH: Prentice Hall, 2008.• - Kraus, John D. Ph.D., Antennas, New York, NY: McGraw-Hill, 1950.• - The ARRL Handbook, Newington, CT: ARRL, 2002•W- Ward Silver, QST, Smith Chart Fun 1, 2 & 3, Dec. 2007, Jan. 2008, Feb. 2008