MONOPOLE ANTENNA

• Monopole antenna is a type of radio antenna form by replacing 1 half of dipole antenna with a ground plane.It also describe as non-directional antenna. Figure (b)- vertical monopole.

CONTENT

• It is half a dipole placed in half space with a perfectly conducting, infinite surface at the boundary.

• If the ground plane is large enough, the monopole behaves like a dipole, as if its reflection in the ground plane formed the missing half of the dipole.

CONTENT

• monopole will have a directive gain of 2.15dB (gain is twice (4.3dB over) that for a half-wave dipole antenna), and a lower input resistance.

• It is omnidirectional if it is a half-wavelength long. ground plane with a radius approximately the same as the length of the active element, is an effective, practical antenna.

IMPEDANCE ANTENNA

The antenna's feed-point impedance, r.P = i2r

• Impedance is periodically measured to verify the stability of the antenna and ground system.

EXAMPLE OF MONOPOLE ANTENNA

a) The whip antennai) Is an antenna with a single driven element

and a ground plane.

ii) is a stiff but flexible wire mounted, usually vertically, with one end adjacent to a ground plane.

CONTENT

iii) The whip antenna can also be called a half-dipole antenna, and as such, has a toroidal radiation pattern.

b) Radio mast (when isolated from the ground and bottom-fed)

i) Is a tall structures designed to support antennas (also known as aerials) for telecommunications and broadcasting, including televisions.

CONTENT

ii) Similar structures include electricity pylons and towers for wind turbines.

1) Base-station application-Omnidirectional application with ground plane.

CONTENT

• Monopole over ground plane, and uses a set of four wire elements to provide the ground plane.

CONTENT

A monopole antenna horizontal planepattern, base-station application. Theuniform maximum gain corresponds to theouter line on the polar plot.

2) MOBILE APPLICATIONS

i) In mobile applications, monopole antenna called whip antenna.

ii) At VHF low-band, a quarter-wave monopole can be 2.5 m (approximately 8 ft) long follow by figure (a).

CONTENT

• Figure (a) is the ability to mount the (shorter) antenna in the center of a surface that will act as an acceptable ground plane (e.g., the roof or trunk of the vehicle).

• However figure (a), an inductor (coil) at the base of a monopole adds electrical length, so the physical length of the antenna can be shorter. (only whip antenna)

CONTENT

• At UHF figure (b) , a quarter-wave whip is approximately 15 cm (6 in) long. this length is physically small, but suitable design considerations can be used to increase the gain.

CONTENT

a) At 800 MHz, a quarter-wave monopole does not perform well, so the approach of stacking two monopoles, with a phasing coil between, is used.

b) Such an antenna,illustrated in figure 10(c), looks much like a mobile cellular phone antenna and has a gain of approximately 3 dBi.

CharacteristicMonopole Antenna

doneby Norman

- THE RADIATION PATTERN IS A GRAPHICAL DEPICTION OF THE RELATIVE FIELD STRENGTH TRANSMITTED FROM OR RECEIVED BY THE ANTENNA. - ANTENNA RADIATION PATTERNS ARE TAKEN AT ONE FREQUENCY, ONE POLARIZATION, AND ONE PLANE CUT.

-THE PATTERNS ARE USUALLY PRESENTED IN POLAR OR RECTILINEAR FORM WITH A DB STRENGTH SCALE.

Radiation Patterns

- TYPICALLY USED IN AN ENVIRONMENT WHERE OTHER OBJECTS ARE PRESENT THAT MAY HAVE AN EFFECT ON THEIR PERFORMANCE. 

- HEIGHT ABOVE GROUND HAS A VERY SIGNIFICANT EFFECT ON THE RADIATION PATTERN OF SOME ANTENNA TYPES.

- AT FREQUENCIES USED IN ANTENNAS, THE GROUND BEHAVES MAINLY AS A DIELECTRIC.

- THE CONDUCTIVITY OF GROUND AT THESE FREQUENCIES IS NEGLIGIBLE. WHEN AN ELECTROMAGNETIC WAVE ARRIVES AT THE SURFACE OF AN OBJECT, TWO WAVES ARE CREATED:ONE ENTERS THE DIELECTRIC AND THE OTHER IS REFLECTED.

Ground Effects

- MOST OF THE ELECTROMAGNETIC WAVES EMITTED BY AN ANTENNA TO THE GROUND BELOW THE ANTENNA AT MODERATE (SAY < 60°) ANGLES OF INCIDENCE ENTER THE EARTH AND ARE ABSORBED (LOST).

- BUT WAVES EMITTED TO THE GROUND AT GRAZING ANGLES, FAR FROM THE ANTENNA, ARE ALMOST TOTALLY REFLECTED. AT GRAZING ANGLES, THE GROUND BEHAVES AS A MIRROR. QUALITY OF REFLECTION DEPENDS ON THE NATURE OF THE SURFACE. WHEN THE IRREGULARITIES OF THE SURFACE ARE SMALLER THAN THE WAVELENGTH REFLECTION IS GOOD.

Content

- THE ANTENNA GAIN DESCRIBES THE ANTENNA’S ABILITY TO RADIATE POWER IN A CERTAIN DIRECTION WHEN CONNECTED TO A SOURCE. USUALLY WE CALCULATE THE GAIN IN THE DIRECTION OF MAXIMUM RADIATION. G = DUM

- WHERE D IS DIRECTIVITY, Μ IS EFFICIENCY AND M IS MISMATCH LOSS.

- THE GAIN IS USUALLY EXPRESSED IN DBI OR DBD. HERE DBI MEANS THAT THE DIRECTIVITY D IS MEASURED COMPARED TO AN ISOTROPIC RADIATOR (EQUAL RADIATION IN ALL DIRECTIONS). DBD IS USEDWHEN THE DIRECTIVITY IS REFERRING TO THE DIRECTIVITY OF A DIPOLE ANTENNA.

- HOWEVER, A MONOPOLE WILL HAVE A DIRECTIVE GAIN OF 5.19DB (GAIN IS TWICE (3DB OVER) THAT FOR A HALF-WAVE DIPOLE ANTENNA), AND A LOWER INPUT RESISTANCE. 

Gain & Directivity

- THE DIRECTIVITY OF THE ANTENNA DESCRIBES THE RADIATION PATTERN. THE ANTENNA CAN “SEE” AND RADIATE BETTER IN SOME DIRECTIONS THAN OTHERS. IN A FIXED POINT-TO-POINT RADIO LINK ANTENNA DIRECTIVITY CAN BE USED TO CONCENTRATE THE RADIATION BEAM IN THE WANTED DIRECTION.

- BUT IN SYSTEMS WHERE THE TRANSMITTER AND RECEIVER PLACEMENTS ARE NOT FIXED, WE WANT THE RADIATION TO BE EQUAL IN ALL DIRECTIONS. THAT IS, WE WANT THE ANTENNA TO BE OMNI- DIRECTIONAL.

- THE DIRECTIVITY OF A MONOPOLE ANTENNA IS DIRECTLY RELATED TO THAT OF A DIPOLE ANTENNA. IF THE DIRECTIVITY OF A DIPOLE OF LENGTH 2L HAS A DIRECTIVITY OF D1 [DECIBELS], THEN THE DIRECTIVITY OF A MONOPOLE ANTENNA OF LENGTH L WILL HAVE A DIRECTIVITY OF D1+3 [DECIBELS].

- THE REASON FOR THIS IS SIMPLY BECAUSE NO RADIATION OCCURS BELOW THE GROUND PLANE; HENCE, THE ANTENNA IS EFFECTIVELY TWICE AS "DIRECTIVE".

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- A DIRECTIONAL ANTENNA CAN BE SAID TO DIRECT A BEAM OF RADIATION IN ONE OR MORE DIRECTIONS.

- THE WIDTH OF THIS BEAN IS DEFINED AS THE ANGLE BETWEEN ITS HALF-POWER POINTS.

- A HALF-WAVE DIPOLE HAS A BEAMWIDTH OF ABOUT 79º IN ONE PLANE AND 360º IN THE OTHER MANY ANTENNAS ARE FAR MORE DIRECTIONAL THAN THIS.

Beamwidth

THE END&

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