Memorex CP8 TURBO UNIVERSAL REMOTE CONTROL Manual do Utilizador descarregar pdf (Página 83)

100

ROPE

RADIATOR

468

fMHz

HOUSE

END

INSULATOR

CENTER

INSULATOR

OR BALUN

-S2

COAX

RADIATOR

-DEG.

POSSIBLE

END INSULATOR

TREE

Fig.

2 -lt is a safe bet to say that

most

amateur

-radio

dipoles are

installed

under

circumstances

that

are

less than ideal,

illustrated here, with one

end

of the

radiator

attached

to the house and the

other

end attached to

tree

or another building.

A B

Fig.

3 -There

are two

effects of

in-

stalling a dipole at less than

optimum

height.

First, the pattern is

changed.

The ideal pattern

of the dipole

"A"

shows the vast majority

the

energy

to be located in the two main lobes.

At less

than ideal heights,

that pat-

tern

degenerates into one with a lot

of sidelobes, as

shown

in "B."

Dipole Length

In free space, the length of a half -

wavelength

radiator

found from: L =

492/f, where

"L"

is the length in feet,

and

"f" is frequency in megahertz. But

close to the Earth's surface (not several

wavelengths

above it), a "ground

effect" takes over and reduces the re-

quired length by a few

percent.

The best

formula

to use

is L = 468/f. That length

is the overall length of both radiators

together -the individual elements are

exactly half that length.

But even

that

is a "first cut"

attempt,

for there are several

factors

that affect

length. Make the length a few inches

longer than that,

and initially don't

sol-

der the outer insulators. Install the an-

tenna at its regular height, and measure

the

VSWR

at several frequencies until

the frequency of minimum

VSWR

found. If it is lower

than the

design

frequency,

then

the antenna is too long;

if the frequency is higher than the

de-

sign frequency then it's too short.

Adjust the length (both elements

equally) to compensate and re-

measure.

When

the minimum

VSWR

is found at

frequency

close to

the

design

frequen-

cy, then you've made it. Tin both ends.

According to the "standard

wisdom"

about dipoles, the following

rules

apply:

Make the dipole as high off the

ground as possible.

Have the feedline go off at an angle of

to 90- degrees as possible.

Connect the center conductor

the

coax to

the end away from the house.

The

validity

the

latter is open to

some

doubt

my mind, especially

1:1 balun transformer is used

at the

feed

point between the antenna and the

transmission line. Balun manufacturers

assert that the transformer balances the

current in

the two halves, and thereby

reduces undesirable radiation from the

transmission line.

Warning!:

Under

no circumstances

install a dipole across a power line!

Every

year at

least one

amateur

or SWL

is killed

by tossing a

wire

for an antenna

across the

power

line in order to get

clearance. Some people think that be-

cause the line is insulated, it is safe -

well,

it just ain't so. Insulation gets old,

or can be damaged. It can easily cut

through when the antenna wire contacts

it...and

that can electrocute you.

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