Wire Antennas For Limited Space Jim Brown K9YC Santa Cruz, CA

Wire Antennas For Limited Space Jim Brown K9YC Santa Cruz, CA

Wire Antennas For Limited

Space

Jim Brown K9YC

Santa Cruz, CA http://audiosystemsgroup.com

Our Objectives

Good Antennas

Good efficiency

Good predictable patterns

Minimal noise pickup and RFI

Inexpensive to build

Wire

Insulators

Basic mechanical parts

Coax to feed them

Coax choke at feedpoint (for noise immunity)

1

Some Possibilities

Half-wave dipoles

Loading coils to make them shorter

Traps provide loading, fit multiple bands in

same space

Fan dipoles fit multiple bands in same space

Sloping dipoles (some of the length is vertical)

Inverted Vee (some of the length vertical)

Top-loaded verticals

Inverted L

Inverted Tee

End-fed wires

How About A Vertical?

A “good” vertical can beat a low dipole

Low means less than about 0.3λ

40 ft on 40M

80 ft on 80M

160 ft on 160M

“Good” means efficient

Good radial system

Low losses (full size or top loading)

In the clear

Most commercial verticals are increasingly

lossy below 30M

2

A Very Efficient 40M

Vertical Dipole

3

#12 White THHN

Center Insulator

RG6

End Insulator for a 40M Dipole

6 turns of RG6 around a “big clamp-on” is

enough for 500 watts of serious contesting

About 5,000Ω resistive impedance

Two of these 6-turn chokes are needed for

1.5kW

About 10,000Ω resistive impedance

4

5

Before you fall in love with a vertical dipole, compare it to a horizontal dipole!

Broadside to Horizontal Dipole

Horizontal

Before you fall in love with a vertical dipole, compare it to a horizontal dipole!

60 Degrees off-axis of Horizontal Dipole

6

When to Use A Vertical

Larger commercial verticals on 40M–10M

Install high, with good radial system

Efficient wires on 160-80M

Low or on ground, with good radial system

Top loaded or full quarter wave

A few verticals don’t need radials

Cushcraft R7000 is center-fed, W1JR design

To fill in nulls off ends of a high dipole

When Not to Bother With A Vertical

40M–10M when you can’t mount it high

and in the clear (high ground losses)

High means at least λ/8

When it’s physically shorter than 3λ/16

When you can’t install at least three λ/4

radials for each band you want to operate

When you can install high dipoles at right

angles

A high dipole will beat it, even loaded or bent

7

Low Dipole (0.2λ)

High Dipole (0.4λ)

8

Extrapolating to 40M

Advantage of 40M Dipole at 55 Ft

Compared to 33 Ft Vertical

Extrapolating to 20M

Advantage of 20M Dipole at 28 Ft

Compared to 17 Ft Vertical

9

Try To Fit A Resonant Dipole First

Well behaved pattern

Inherently has gain in horizontal plane

Vertical pattern depends on height

For most QTHs

Higher is better on 40, 80, 160

Height not as important on 20-10

Directivity tends to reduce noise

Easy to feed with coax

Chokes can minimize receive noise, RFI

80/40 Shortened (Loaded) Dipole

Coil Coil

Coax

Choke

Rig

10

Loading Coil for short 80/40M Dipole

Everything On the Roof Does Double Duty

#12 Copper Guy

Loads on 30M

0 Dip ole

One end o f 80/4

Load ing C oil

10 Ft TV Mast

11

10

Ft

M as t

Fe ed po int

The Loaded 80/40 Dipole

Guyed TV Mast

20/1

5/10

Fan

40M Dipole

80/40M Loaded Dipole, 90 Ft Long

12

80M Dipole

80/40M Loaded Dipole, 90 Ft Long

80/40M Loaded Dipole, 90 Ft Long

Compared to Half Wave Dipole on 40M

13

80/40M Loaded Dipole, 90 Ft Long

Compared to Full Half Wave Dipole on 40M

80/40M Loaded Dipole, 90 Ft Long

Compared to Full Half Wave Dipole on 80M

14

80/40M Loaded Dipole, 90 Ft Long

Compared to Full Half Wave Dipole on 80M

80/40M Loaded Dipole, 90 Ft Long Current

Distribution on 80M

15

80/40M Loaded Dipole, 90 Ft Long Current

Distribution on 40M

80/40M Loaded Dipole, 90 Ft Long Current

Distribution on 40M

16

80/40M Loaded Dipole, 90 Ft Long

Compared to Full Size Half Wave Dipole

40 Meters

No significant difference in gain or pattern

Slightly less SWR bandwidth

80 Meters

No significant difference in pattern

Gain about 0.8 dB lower

Much less SWR bandwidth

Greater feedline loss away from resonance

Build or Buy a Short Dipole?

Designing a Shortened Antenna CT1EOJ

QST Oct 2003

Model it in NEC

Tweak the design for multiband coverage

Buy from Barry, KU3X, Hypower Antenna

Company (QST, Internet) 2B8040L

He’s already done the design work

17

A 20/15/10 Trap Dipole Barely Fits between the two TV masts

Trap Dipoles

Traps are parallel resonant circuits

Below resonance, they look inductive

So they act as loading coils on lower bands

A 3-band trap dipole fits in less space

than a fan dipole

20/15/10 is about 26-27 ft (20M dipole = 33 ft)

Traps add some loss

Typically 1-2 dB

A lossy antenna is better than no antenna

Traps reduce the SWR bandwidth

Trim lengths carefully and use a tuner!

18

Fitting Full-Size Dipoles Into Less Space

Length of wire resonates the antenna

Very little current near the ends of a wire

Bending simply distorts the pattern a bit

(mostly fills in nulls)

Bend it at one or both ends

Has least effect on pattern or efficiency

Bend it anywhere along its length

A bit more effect on pattern (fills nulls)

Fitting Full-Size Dipoles Into Less Space

Use insulated wire

about 2% less wire than bare copper

Use two or more wires in parallel ??

Less than 1% shorter

50% better SWR bandwidth

Nice, but not worth the trouble

Use bigger wire

#10 only 0.5% shorter than #14

Stronger, but shortening doesn’t matter

Doesn’t change SWR bandwidth

19

Fitting Full-Size Dipoles Into Less Space

Hang from one end, let it slope

Keep center as high as practical

Skews pattern

Hang as inverted-V

Raises angle of radiation

Fills in nulls off the ends

Efficiency still good

Center is high, that’s where the current is!

As end(s) get closer to the earth (or

trees), a shorter wire will resonate

capacitance to earth

50 Ohm Coax or 75 Ohm Coax?

A Dipole in free space is a 72 ohm

antenna!

Proximity to earth changes the impedance

High dipoles are closer to 75 ohms

Low dipoles are closer to 50 ohms

Feedline SWR (and loss) depends on the

match between feedline and antenna

Use feedline that matches the antenna

XMTR will reduce power if mismatched

Use an antenna tuner to make the rig happy

20

Impedance of 80M Dipole vs Height

λ/4

λ/2

33 66 131

Antenna Height (Feet)

Impedance of 40M Dipole vs Height

λ/4 λ/2

17 33 66

Antenna Height (Feet)

131

21

Impedance of 20M Dipole vs Height

λ/4 λ/2

8 16 33

Antenna Height (Feet)

66

Transmission Line Loss Due to Mismatch

Transmission Line Loss When Matched

22

RG8 or RG11?

A 10M Dipole

75 ohms

50 ohms

Loss in 50 Ohm Transmitting Coax

23

3

Loss in 75 Ohm Transmitting Coax

RG8 or RG11?

A 10M Dipole

75 ohms

50 ohms

24

Build a Multiband Fan!

My First 20/15/10 Fan Dipole in Chicago

Only up 25 ft, but a lot of noise on E Coast

25

A Fan Dipole for 20/15/10

An 80/40 Fan Dipole

26

An 80/40 Fan Dipole

Ferrite Choke and Feedpoint of 80/40 Fan Dipole

27

Fan Dipoles – How They Work

Same efficiency and pattern as a single

dipole for each band

Lowest frequency element has same

SWR bandwidth as a single dipole

Higher frequency elements have reduced

SWR bandwidth (about 50%)

Length (tuning) more critical

Greater feedline loss at edges of band

20/15/10 fan looks like 50 ohms, even

when very high

20/15/10 Fan Dipole,

50 ohm coax

15M

20M

28

20/15/10 Fan Dipole on 10M, 50 ohm coax

10M Dipole, 75 ohm coax

Improvising an End Fed Wire

Think about where most current will be

Current must be zero at an open circuit

Current will be max λ/4 (and 3λ/4) from an

open circuit (low impedance, easier to match)

Could be closer if loading coils, capacitance

A high current point high and in the clear

usually makes the antenna more efficient

Current must be near zero λ/2 (and λ) from an

open circuit

High current parts of antenna radiate

Hight current points easier to match

29

Feed A Random Wire From the End

You will need an antenna tuner

Avoid half wavelengths (high Z at the

feedpoint, harder to match to XMTR)

The lower the frequency, the greater the

benefit of increased height

You do need a radial system

A Top-Loaded “Tee” Vertical

(ARRL Antenna Book)

30

A Top Loaded Vertical on 80/160

Inverted L

“Tee” – vertical

Load it against radials or a counterpoise

Use what you can install

It doesn’t need to be perfect

Longer/bigger is better

Do your best and call CQ!

A Top Loaded Vertical on 80/160

Ideally would be quarter wave vertical

70 ft on 80M

135 ft on 160M

Few of us can do that, so go as high as

you can and

bend it in one direction (inverted L)

Bend it in two directions (Tee)

so that it looks like a quarter wave to the transmitter

31

A Top Loaded Vertical on 80/160

Split the difference and load a Tee or

inverted L on both 80 and 160 (w/tuner)

90-100 ft is 3/8λ on 80M, 3/16 λ on 160M

160-170 ft is 5/8 on 80M, 5/16 on 160M

Radial Systems

Provide a return for the fields and currents

produced by an end-fed antenna

The earth is lossy, burns transmitter

power

Use enough radials so that fields and

current are in copper, not earth

A few resonant radials work if elevated

Many needed if on ground

32

Improvising Antennas

Feed it against radials or a counterpoise

A ground stake doesn’t help

More wire close to the feedpoint is better

A lot of short wires are better than a few long

ones

Symmetry much less important than quantity

Wire diameter enough it won’t break

Do the best you can and call CQ!

To learn more about radial systems, study

N6LF’s website

0

16

24

36

60

90

On Ground Radial Systems

(ARRL Antennna Book)

Number Length Loss Z

0.1λ

.125 λ

.15 λ

0.2 λ

0.25 λ

10 dB ?

3 dB

2 dB

1.5 dB

1 dB

0.5 dB

90 Ω ?

52 Ω

46 Ω

43 Ω

40 Ω

35 Ω

33

Coil

80/40 Loaded Dipole

Coil

70 ft 72Ω Twinlead

Choke

Rig

Top Loaded Vertical on 160M

Coil Coil

70 ft 72Ω Twinlead

Choke

ANT

TUNER

34

Shack

Feed poin t

Feedline

Start with 80/40

dipole fed with twinlead

Tie both sides

together at tuner

Feed as long

wire against radials

Wrought Iron Fence was Counterpoise for Vertical

(KK9H uses HVAC ducts and plumbing system!)

Shac k

35

Loading it as A Half Wave on 80M

Coil Coil

70 ft 72Ω Twinlead

Choke

ANT

TUNER

Building Wire Antennas

Use Insulated House Wire (THHN)

#10 or #12 for heavy loads, long spans

#14 for lighter antennas

#18 or even #22 for stealth!

Use thimbles where wire bends to

minimize stresses

The Wireman 800, 800A

Avoid “Flex-Weave”

I’ve used a lot of it – every antenna has

broken!

36

Building Wire Antennas

Don’t solder a connection that can flex

Soldering makes copper brittle, and it will

break!

Use Split Bolt Connectors for both

mechanical and electrical connections

McMaster-Carr 6921K56 ($1.89 each, 25 lots)

Lowes, Home Depot (about $3 each)

Tape up connections to minimize

corrosion

Split Bolt Connectors at Center of a Fan

Dipole

37

Building Wire Antennas

End insulators – use eggs

RF Connection

If you must climb to hang it, use a pulley!

Marine pulleys work well ($15 - $25)

Support rope

UV resistance, strength, big enough to pull

3/16-inch for light antennas, low tension

5/16-inch for heavy ones you need to pull

DX Engineering, Davis RF

A Good Center Insulator is Hard to Find!

38

Building Dipoles

Center Insulator

Mechanical Strength

Electrical connections

Weatherproof

Corrosion

A Good Center Insulator is Hard to Find!

(You always get the other kind)

Wireman 801 is best of a bad lot

Avoid commercial “baluns”

Wind a much better coax choke using

guidelines in my Choke Cookbook

Building Fan Dipoles

Spacers are easy to build

½-inch UV-resistant PVC conduit, cut into

15-inch lengths for 3-wire fans

9-inch lengths for 2-wire fans

Separate wires by about 7 inches

Drill holes for wire to pass through

For 20/15/10 fans

Spacer near center insulator

Spacer at end of 10M element

Spacer at end of 15M element

39

Building Fan Dipoles

For 80/40 fans

Spacer near center insulator

Spacers about 6 ft apart

Length of elements

Build according to usual formulas for the

wire you’re using, but cut a little long and trim to length after it’s been in the air

Include all wire starting from the coax

connector

Remember that insulated wire lowers the

resonant frequency about 2%

I’ve not seen interaction between elements

Getting Wires Into Trees

Climb the tree, install a pulley (Best)

It will stay up longer, easy to change antenna

Allows a counterweight for wind motion

Least fraying of support rope

Climbers can be expensive ($500/day typical)

Use a launcher

Put heavy fishing line over a branch

Pull up heavier line

Pull up the final support rope

40

Tennis Ball Launcher $110 - $350

Good for 200 ft

www.antennalaunchers.com

EZ Hang Launcher

$100 - $130 http://ezhang.net

41

A super slingshot on

an 8 ft pole

2 – 4ft sections

Sherrill Tree Service

About $160 w/line and

weights

http://sherrilltree.com

Good for 80 – 100 ft

Installing a Pulley with a Launcher

Launch heavy fishing line over a branch

Pull up heavier line, then final support rope

Make a continuous loop of heavy support

rope from top to ground

Attach pulley to the loop

Run final support rope through pulley

Pull pulley, with support rope, up to the top

Attach final support rope to antenna

Now you can use a counterweight with

minimal abrasion of support rope

42

Why Not an All Band Wire Fed with Twinlead?

Understanding Common Mode and Differential Mode Currents on Transmission Lines

43

Differential Mode Current

Transmission line carrying power from

transmitter to antenna, or from antenna to receiver

Signal is voltage between the two

conductors

Current flows out on one conductor and

returns on the other

I

I

44

Differential Mode Current

Transmission line carrying power from

transmitter to antenna, or from antenna to receiver

Signal is voltage between the two

conductors

Current flows out on one conductor and

returns on the other

Fields exist between the two conductors

No radiation from ideal line

Field of outgoing conductor cancels field of

return conductor

Common Mode Current

Equal and flowing in the same direction

on all conductors of balanced lines

Current flows lengthwise on the line

No cancellation of one current by another,

because they’re in polarity

Line acts as long wire antenna

It radiates and it receives

45

It’s an Antenna

Common Mode

Common Mode

Ham Antennas and Balance

Most ham antennas are unbalanced

by their surroundings, even when fed by a balanced source and line

46

What Makes a Circuit Balanced?

What Makes a Circuit Balanced?

The impedances of each

conductor to the reference plane are equal

Balance is not defined by

voltage or current

Imbalance impedances

cause unbalanced currents

47

Ham Antennas and Balance

Most ham antennas are unbalanced

by their surroundings, even when fed by a balanced source and line

Unequal capacitances to nearby

conductors

Unequal inductive coupling to nearby

conductors

Trees, buildings, towers, terrain

Feedline comes off at an angle

Coax is not a part of these imbalances

Common Mode

Common Mode

48

Common Mode

Common Mode

Current

49

Unbalanced Antennas and Lines

If the antenna is unbalanced

Unequal voltage and current to earth

Unequal currents on the feedline

The difference is common mode

current, and it radiates from the line

Coax did not cause the imbalance in

these antennas!

Coax simply adds to the imbalance

The Fields around Coax and

Twinlead are Very Different

50

Coax is Special

All the differential power (and

field) is confined inside the coax

All the common mode power

(and field) is outside the coax

A ferrite core surrounding coax

sees only the common mode power (and field)

Coax is Special

Skin effect splits the shield into

two conductors

Inner skin carries differential mode

current (the transmitter power)

Outer skin carries common mode

current (the current due to imbalance)

51

Twinlead Has Leakage Flux from Differential Current

This leakage flux is not confined to the

region between the conductors, but instead spills to the area immediately surrounding the conductors

Leakage flux causes very little radiation,

but it will cause heating in a lossy medium!

Like a ferrite core

How Much Leakage Flux?

Depends on mutual coupling between

conductors

Depends on conductor-to-conductor spacing

How close together can conductors be?

Coupling coefficient of 60-70% typical

30-40% leakage flux in best balanced cables

50% or more in ladder line

We’ll talk more about all this later on

52

Now We Can Talk About

Common Mode Chokes!

What’s a Common Mode Choke?

A circuit element that reduces

common mode current by adding a high impedance in series with the common mode circuit

Reduces radiation from the cable

Reduces reception by the cable

53

Some Common Mode Chokes

A coil of coax at the antenna

A string of ferrite beads around

coax (Walt Maxwell, W2DU)

Multiple turns of transmission line

through a toroid (Joe Reisert,

W1JR) or stack of toroids (W1HIS,

K9YC)

Most 1:1 “baluns” are common

mode chokes

Some Common Mode Chokes

Some 2:1, 3:1, and 4:1 “baluns”

are also common mode chokes

But the few I’ve measured aren’t

very good common mode chokes

54

Why Transmitting Chokes?

Isolate antenna from its feedline

Reduce receive noise

Keep RF out of the shack

Minimize antenna interaction

SO2R, Multi-multi

Dipole feedline and vertical antenna

55

Receive Noise

Common Mode

Current

RF in the Shack

“Strings of Beads” (W2DU, W0IYH Baluns)

56

A String of Different Beads

K9YC Chokes

(Improvements on W1JR, W2DU

Designs)

4 turns

RG8

5 turns

RG8

5 turns Big

Clamp-On

RG8X

7 turns

RG8X

57

Why Not Twinlead?

You can’t put a choke on it!

So:

More receive noise

More RF in the shack

More RFI to your neighbors

More antenna interaction

More loss when it’s wet

References

A Ham’s Guide to RFI, Ferrites, Baluns,

and Audio Interfacing Self-published tutorial (on my website)

Transmitting Chokes (Power Point pdf)

(on my website)

Applications notes, tutorials, and my

AES papers are on my website for free download http://audiosystemsgroup.com/publish

58

References

Dean Straw, ARRL Antenna Book,

ARRL, 2007

John Devolodere, Low-Band DXing,

ARRL, 2005

Dean Straw, ARRL Handbook, ARRL,

updated and published annually

Rudy Severns, N6LF

http://www.antennasbyn6lf.com/

Tim Duffy (editor)

http://www.k3lr.com

Dayton Antenna and Contesting Forums

References

Henry Ott, Noise Reduction Techniques in

Electronic Systems, Wiley Interscience, 1988

E. C. Snelling, Soft Ferrites, Properties and

Applications, CRC Press, 1969

E. C. Snelling and A. D. Giles, Ferrites for Inductors

and Transformers, Research Study Press, 1983

Fair-Rite Products Catalog This 200-page catalog is

a wealth of product data and applications guidance on practical ferrites. http://www.fair-rite.com

Ferroxcube Catalog and Applications Notes More

online from another great manufacturer of ferrites. http://www.ferroxcube.com

59

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