Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Manuel d'utilisateur télécharger pdf (Page 82)

100

Joseph

Carr, K4IPV

[11

ON HAM RADIO

Resurrecting

the

old fa

the

alterna

LOOKING

AROUND

NEIGH

borhood

might

lead

one to

conclude

that half

the amateur

operators

in "4-

land"

live here.

every block

there is

at least

one 40-

60 -foot

tower topped

with

a multi-

element tri -band

beam

an-

tenna or quad.

Given that such

antennas

are

expensive,

and

the mortgage

com-

pany eats

a considerable

portion

of my

budget

every month,

I am noticeably

jealous,

of course.

But

are those

beams

really

necessary?

ask a

few of

the top DXers

you

would

think

so, but

amateur

operations

do not depend

having such

anten-

na.

Some years

ago I knew

amateur

who

hailed from

Brooklyn,

and then

moved

downtown Washington,

DC.

In both

places,

he lived

in an

apartment

building.

His

antenna

was

a dipole

ex-

cited by the

30 -watt

output of

a Heathkit

-20

transmitter.

With

that little

rig

and "impossible"

antenna

he worked

-meter

DXCC

from

both

Brooklyn

and Wash

ington...which

proves

that it is

the oper-

ator

not the

equipment

that

makes

the

critical

difference.

(A good

mechanic

never

blames his

or her

tools.)

Sure,

the

little

guy has

work

a little

harder,

but

still

can get

good

results...and

become

a lot better

operator

the bargain

than

his "blowtorch"

buddy.

Recently

resurrected

the

old -fash-

ioned

half-wavelength

dipole

to supple-

ment

my four

-band

trap vertical.

Too

few

amateurs

have

proper

respect

for the

dipole,

and

that's a shame

for

it really

works.

The

dipole

(when

done

right) is

a good performing

antenna,

and it's dirt

cheap...which

makes the

cost/benefit

ratio

tremendous.

The word

dipole means

"two

pole."

The

basic

configuration

is shown

in Fig.

Two quarter-

wavelength

sections

form

the half

-wavelength

radiator.

The

transmission

line is

typically

75 -ohm

coaxial

cable (which

we'll

discuss

later),

and is

applied to the

center point

468

L"f

MHz

INSULATOR

OHM

COAX

TRANSMITTER

Fig.

1 -The

dipole

(meaning

two

pole

") consist

of two

quarter -wave-

length

sections

to form

the half -

wavelength

radiator.

The transmission

line is

typically

-ohm coaxial

cable,

which is

applied

to the

center

point between

the two

elements.

The

coax inner

conductor

connected

one

radiator.

and the

shield is

connected

to the

other radiator.

between

the two

elements.

The

coax

inner

conductor

connected

one ra-

diator,

and

the shield

is connected

to the

other radiator.

According

to best practice, the ideal

dipole should be installed several wave-

lengths

above ground, and several

wavelengths

from any conducting

ob-

ject. In the real world,

however, that's

seldom

attainable except (possibly) in

the upper

HF bands (i.e., 21 -MHz

and

up). At lower frequencies,

the ideal is

seldom

achievable.

Consider the

40-

meter

band dipole, for

example. Half a

wavelength

is around 66 -feet,

so a full

wavelength

is I32 -feet.

Several

wave-

lengths

is usually taken

to mean five

wavelengths,

the ideal 40 -meter

an-

tenna would

be 660 -feet off the

ground,

or 105 -feet higher than

the

Washington

Monument!

Obviously,

a lot of good working

di-

poles

are floating

around the

at-

mosphere

anything that

approaches

the correct

(ideal) altitude.

Fortunately,

the ideal need

not be

achieved

in order to make

a good work-

ing

dipole. If the

dipole is only

a half

wavelength

above ground

semi

-ideal

performance

is approximated...not

the

best, but

darn

good.

l0- meter, 13-

meter, or

I5 -meter

dipole can

easily

meet

the half -wavelength

high

criteria.

A 15

-meter

dipole need only

be 22 -feet

off the deck

to meet that

criteria.

But

even at less

than a half -wave-

length

high,

the dipole

performs well,

but

with

some

qualifications.

For sev-

eral years

my 40

-meter

dipole (with

which

worked

a lot of DX) was

only

15 -feet

off the

ground at the

porch

end

and 26 -feet

off the ground

at the tree

end; when

a friend

managed

to climb

couple of

trees for

fat of K4IPV

and

install

-meter

dipole at

the

-foot

level,

I was

in hog heaven.

I bet

that

most

amateur

-radio

dipoles are in-

stalled

a situation

(less

than ideal)

such

as illustrated

Fig. 2: one

end

attached

to the house

(or in

my case a

porch)

and the other

end attached

to a

tree

or another

building.

There

are

two effects

of installing

dipole

at less than

optimum

height.

First,

the pattern

is changed.

The stan-

dard

dipole pattern

(ideal)

is shown

Fig.

3A. The

"figure-8" pattern

shows

that the

dipole is

bidirectional,

and as

such, places

the

vast

majority

of the

energy in

the two main

lobes. At

non-

ideal

heights,

the ideal

pattern

degener-

ates into one with

a lot

of sidelobes (see

Fig.

3B).

But for practical

purposes

that's

rarely

a problem,

and indeed

there

are points where

you'd

make contact

in several

directions,

then

may actually

be a

benefit.

The

other

defect of less

than ideal

dipoles is

that the feedpoint

impedance

is not

the ideal 72 -ohms.

The

actual

impedance

can

vary

from about 40

almost

140

ohms depending

upon the

antenna's

height

above ground.

If the

impedance

causes a

VSWR,

will

(at most)

72/40 =

1.8:1 in one

direction

or 140:72

1.9:1 in the

other. That

VSWR

almost not worth

correcting,

but

if necessary,

a coaxial "Q-

section"

or other method

can be

used to over-

come

the

VSWR.

1 2 ... 77 78 79 80 81 82 83 84 85 86 87 ... 99 100

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