Wonderful Wire Antennas

5 February 2014

A Baker’s Dozen

Wire Antennas

Forrest B. Snyder, Jr.

N4UTY

Gooch’s Paradox or

Why Antennas Radiate:

RF gotta go somewhere!

o Some antennas radiate better than

others. The explanation is Resonance.

o Resonance depends on frequency o The shortest resonant antenna is an

electrical ½ wavelength long.

o Length of a resonant ½ wave antenna

(in feet) = 468/Frequency in MHz.

The Basic Antenna:

The ½ Wave Dipole

Length = 438/frequency (Mhz)

At resonance

Current (I) is maximum at feedpoint, zero at ends

Voltage (E) is zero at feedpoint, maximum at ends

At resonance, the antenna looks like a resistor.

Feed point impedance for a resonant dipole = E/I is about 72 ohms

If transmitter output impedance = 50 ohms, and a resonant dipole

is fed with 50 ohm coax, the feedline will show a theoretical SWR

= 72/50 = 1.44/1.

Actual feed point impedance varies with

Antenna height

Ratio of element diameter to wavelength

Presence of nearby objects.

Resonant on fundamental and odd harmonics

Most resonant wire dipoles display a measured feed point impedance

of about 65 ohms – a good match for 50 ohm coax.

The Basic Vertical Antenna

The ¼ Wave Marconi

Image Antenna

Surface of the Earth

Ground Radials

Half a dipole sticking up in the air

Length (feet) = 234/f (MHz)

Ground reflection forms 2d half of

dipole– the “image antenna”

Ground return through earth increases

loss resistance – ground radials decrease loss resistance

How many radials

Ground mounted: As many as possible -

120 is about optimum – One will work

(classic 30 over 30 or 60 over 60)

Mounted above ground (Ground Plane): 3 or

four but one or two will work

Make of wire, coil it up, hang one end in

a tree for portable or emergency operation

Dipole Variation: the Folded

Dipole

short short

More broadband than a single wire; can be

operated on its fundamental and odd harmonics

Length = 438/frequency (Mhz); length between

shorts = 492 x k/frequency where k = velocity factor (typically 0.82 to 0.85)

Feedpoint impedance = 300 ohms: Make out of and

feed with cheap

300 ohm TV twin lead.

The Basic Multi-band Wire: the

Center–Fed Zepp or McCoy Dipole

Length = as long as possible – cut it to fit between your

supports or to be resonant on the lowest frequency band on which you plan to operate

Feed it with open wire feeders or ladder line

Feed point impedance will vary with frequency

The feedline will operate at a high SWR

Will require a matching network/transmatch for off-resonance

operation

Off resonance.

A short antenna looks like a capacitor and a resistor in series.

A long antenna looks like an inductor and a resistor in series.

If you have only one support

Inverted Vee

Resonant length = 490/frequency in MHZ (per 1964 ARRL

Handbook)

Feedpoint impedance about 50 ohms at resonance if legs droop 45

degrees

Feed with 50 ohm coax for resonant operation, ladder line for multi

band operation

Warning: High Voltage at ends – keep ends well out of reach.

If you have only one support

The sloped dipole

Feedpoint impedance about 50 ohms

at resonance

Slightly directive from the pole to

the antenna if the support is metal

Caution: High Voltage at the ends!

If you have limited space between supports:

B

A

The Bent Dipole

Resonant length about 490/frequency in MHZ

Feedpoint impedance should be close to 50 ohms at resonance

Feed with 50 ohm coax for resonant operation, ladder line for multi

band operation

Warning: High Voltage at ends – keep ends well out of reach.

Note: few people are eight feet tall. Try to keep ends at least 8

feet above the ground

Option: End fed or Zepp

Designed to trail out of WWI Zeppelins

Traditionally, a half wave fed at one end with a quarter wave of

open wire or ladder line

High impedance at antenna feedpoint; low impedance at end of 1/4

wave feedline

Feed with open wire or ladder line

Multiband operation possible with appropriate transmatch: per 1964

ARRL Handbook, following lengths support matching

Antenna: 135 feet, feedline 45 feet (80 thru10 meters)

Antenna: 67 feet, feedline 45 feet (40 thru 10 meters)

The Multi-wire or “Fan” Dipole

Separate resonant elements for each band

Feed with coax

Each set of parallel elements affects the resonant length of all of the others – cut to formula and prune, beginning with the lowest frequency band

Novice special (80, 40, 15): Resonant at 3.7 and 7.12 MHz. Operates 80m and 40 m on the fundamental, 15m on the 3d harmonic of 40m.

91 feet of window line or 300 ohm twin lead.

Cut one conductor 31 feet from one end, cut second conductor 31 feet from other end

Separate ladder line between cuts

Connect conductors at “square” end; hook to center insulator

Attach insulators to free end of long elements, haul up, prune as needed.

Get on the air and enjoy!

Novice special plus 20 m: Weave resonant 20 m radiator (16’ 6” or 8’ 3” either side of center) through the windows line between the 80 m and 40/15 m elements.

Novice special plus 30 m: Weave resonant 30 m radiator (43’ 4” or 22’ 10” either side of center) through the windows in the ladder line between the 80 m and 40/15 m elements.

NOTE: Use the same idea for a multi band wire vertical from ladder line, TV lead in, rotator cable, zip cord or whatever. Have fun with it!

The Three Element Ground Plane for 10, 15, and 20 meters

16’ 5”

11”

8’ 3”

Use 3 wire rotator

cable for the vertical elements.

Connect all three

elements at the bottom

Four 17’ radials (ground

plane) or more (ground mounted)

Feed with 50 ohm coax

For portable use, tie a

rope to the longest element and suspend from a tree!

The Windom: Single wire offcenter fed ½ wave

L=438/f (Mhz)

.36L

Single-wire feed against ground

“The antenna will operate satisfactorily on second

harmonic frequencies” (1964 ARRL Handbook)

Single wire feeder shows an impedence of about

600 ohms to ground at resonance

Invented by Loren Windom who also invented the

product detector

The off-center fed dipole

(1964 ARRL Handbook)

136’

44’ 4”

Feed with 300 ohm twin lead

Claimed to offer a good match on 80, 40, 20, and

10 meters

“Widely used and does work satisfactorily”

Some feed line lengths “awkward”

See QST index for in depth treatment

Theory behind Cushcraft R4, R5, R7, R8 and R9

vertical antennas

Advantages of Wire Antennas

Simple and effective – they just

WORK.

Can be made to fit almost anywhere.

Cheap – a hank of wire and three or

four insulators is all you need.

Lightweight for portable operation.

Just plain fun! Throw up a wire, get

on the air, and work the WORLD!

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