Antennas

and Stuff

John Kernkamp – WB4YJT

John Kraus – W8JK

June 28, 1910 - July 18, 2004

• Invented the helical antenna, the corner

reflector, and the W8JK End-Fire array.

• In 1950 designed and built the “Big Ear”

radio telescope at Ohio State University,

which was used to carry out the Ohio Sky

Survey, and start the first SETI program.

• Wrote the “Bible” on antennas

Kraus’ “Antennas”

• First edition - 1950 - This book outlined classical

antennas and theory, and was referred to by

many as the Antenna Bible.

• Second edition – 1988 - A major upgrade to the

1950 edition, incorporating the latest

developments in antenna technology.

• Third edition – 2002 – Co-authored with Ronald

J. Marhefka. Many chapters written by experts

in their field. Updates include computer

modeling and terahertz waves.

• Not cookbooks for antennas – lots of math

Dipole

• Half-wavelength long

• Divided at center – feedline connection

Dipole Variations

• Folded

• Broad-Band

• Off-center fed

• Shortened

• Sloped

Antenna Critical Characteristics

• Radiation orientation

• Frequency / Resonance

• Bandwidth

• Impedance

• Matching

• Capture Area

• Efficiency

• End effects

• Nearby Parasitic objects

Radiation Orientation

The Dipole “Doughnut”

Radiation Orientation

The Dipole Reality

.5

.25

1.25

.75

Dipole Radiation Patterns

• www.ventenna.com – Just put “Ventenna”

into Google – it will be the first in the list

• Navigate to “Manuals” page – Click on

“Ham”, “More Info”, and “Manuals”

• Scroll down to “Application Notes”

• Download “Dipole Radiation Patterns” –

PDF format

Frequency / Resonance

• ARRL Formula – 468/F (MHz)

• Better formula - 5904/F (MHz)

• Provides half-wave length in inches

• Gives free-space length – which is a bit

longer than needed, so the antenna can

be adjusted (shortened) to resonance

• Easier to shorten it than lengthen it

Bandwidth

• Thicker element - wider bandwidth

• Typical wire dipole expected to be +/- 1% for 2:1 SWR Bandwidth

Impedance

• Dipole Impedance – 72 Ohms

• Connected directly to 50 Ohm feedline

gives 1.44:1 SWR

• Matching can bring SWR down to 1:1

Matching

• Gamma Match

• Hairpin Match

Capture Area - larger is better

NASA’s Goldstone 30 Meter (98.5 Ft) Antenna

Radiation Efficiency

• Current is the key

• High current areas should be low

resistance

• High current areas should be low

impedance

• High current is at the center of a dipole

End Sensitivity

• Ends are high impedance points

• Ends are VERY sensitive to nearby

objects

• Capacitance effects at ends will result in

the need to shorten the antenna to

compensate

Mutual Coupling Sensitivity

• Any metallic object within ½ wavelength

becomes part of the antenna

• Conductive objects which approximate ¼

or ½ wavelength become parisitic

resonators

• Parasitic resonators in the same

orientation as the antenna change the

directivity pattern

An antenna design where

parasitic elements are helpful

Antenna Critical Characteristics

• Radiation orientation

• Frequency / Resonance

• Bandwidth

• Impedance

• Matching

• Capture Area

• Efficiency

• End effects

• Nearby Parasitic objects

A Few More Topics

• Baluns

• RF Safety

• Discone Antennas

Baluns

• The word is a conflation of “Balanced”

and “Unbalanced”.

• Coax cable is “Unbalanced”

• A dipole is “Balanced”

• A Ground-mounted Vertical is

“Unbalanced”

Balun Purposes

• Prevents RF current on feedline

RF on the feedline will distort the

antenna’s pattern

RF on the feedline can come into the

shack and cause problems

• Matches feedline to antenna

1:1 for 50/75 Ohm antenna

4:1 for 300 Ohm antenna

6:1 for higher impedances

Types of Baluns

• Coax

• Choke

• Ferrite Bead

• Ferrite Core

• Transmission-Line Transformer

Coax Balun• A length of coax to give an electrical half-wave delay to

the drive for one element

• Provides 4:1 Impedance change

• Single-frequency

Choke Balun

• Simply a coil of the feedline coax to create an inductance which will prevent RF on the outside of the feedline

• A 1:1 Balun, broadband

Ferrite Bead Balun

• Ferrite beads create inductance on the outside of the feedline, which blocks RF

• A 1:1 Balun, broadband

• Different bead material for VHF/UHF, and for HF

Ferrite Characteristics

Ferrite Balun Kits

High Sierra, Amidon, Palomar,

Hex Kit, Wireman, Elecraft

Transformer Baluns

• C. L. Rutheroff first outlined designs for transmission line transformers in his paper "Some Broadband Transformers", published in Proceedings of the IRE, Volume 47, August 1959.

• These transformers had very wide bandwidth characteristics, some as high as 20,000:1. Frequencies ranged from a few tens of kilohertz to over one gigahertz.

Transformer Baluns

• Jerry Sevick – W2FMI (SK) built on Rutheroff’sconcepts and published “Transmission Line Transformers” in 2001.

• This book is the modern guide to transmission line transformers. It is out of print, but sometimes one shows up on e-bay.

Some Transformer Baluns

Transformer Balun

Parameters

• Power Level

Core size

Wire size

• Frequency Span

Material type

• Enclosure

Weatherproof

Connectors

RF Safety

• RF energy causes heating in water-

based objects. Too much heat can be

bad.

• Power level, proximity and frequency

are the critical factors.

• Higher frequencies are more

problematic.

• www.ventenna.com/RF-Safety

• http://hintlink.com/power_density.htm

RF Safety

RF Safety

Discone Antennas

• Very broad-band frequency response –

typically 10:1 range.

• Effectively a vertically-oriented dipole, but

somewhat smaller capture area.

• Narrow horizontal beamwidth

• Wide bandwidth allows it to effectively

transmit harmonics from poorly-functioning

or improperly-filtered transmitters.

Discone AntennasCritical dimensions

• Length of slant

dimension of cone

should be 1.4

wavelength at the

lowest frequency.

Cone angle should

be 25 degrees.

Discone AntennasCritical dimensions

• Diameter of disc

should be .8 of a

wavelength at the

lowest frequency.

Discone AntennasCritical dimensions

• Spacing between

disc and top of the

cone should be about

3% of a wavelength

at lowest frequency.

This dimension sets

the feed impedance.

Discone AntennasPerformance

Antennas

and Stuff

John Kernkamp – WB4YJT

john@ventenna.com

www.ventenna.com/VOI