Common TV Antenna Types

           

                                                                                                                              

Shown above is the radiation diagram for a 9-element Yagi antenna that the author once

used to receive channel 12.  To interpret this diagram, imagine that the antenna is at the

origin.  The length of a line from the origin to any point on the surface is proportional to

the gain in that direction.

 HDTV Antennas

An antenna made for analog TV will work fine for DTV.  There is nothing different about

an antenna for DTV or HDTV.  Unscrupulous people have labeled their antennas “HDTV

Antennas” as a marketing ploy.  The honest antenna makers have had to re-label their

products likewise to avoid losing sales.

Some terms

 Gain – a measure of how much signal the antenna will collect.

Beam width – how directional an antenna is.

Bandwidth – how the gain varies with frequency.  A narrowband antenna will receive

some channels well, but other channels poorly.

 

The Dipole:

 This is the simplest TV antenna.  Variations on the dipole are the bowtie (which has wider

bandwidth), the folded-dipole (which can solve an efficiency problem) and the loop (a

variation on the folded dipole).  All four have the same gain and the same radiation field: a

torroid (doughnut shape).  The gain is generally 2.15 dBi.  “dBi” means “dB of

improvement over an isotropic radiator”, which is an antenna that radiates equally in all

directions.  This sounds like a discussion of transmitting antennas, and it could be.  An

antenna will have the same gain when receiving as when transmitting, and also the same

radiation pattern.

 

           

 

The dipole has positive gain because it does not radiate equally in all directions.   This is a

universal truth.  To get more gain, an antenna must radiate in fewer directions.  Imagine a

spherical balloon.  Now press on it from opposite sides with a finger of each hand.  Push in

until your fingers meet.  The result looks like the torroid above.  But more importantly, the

balloon expanded in the other directions.  A-hah! Gain!  That’s the way antennas work.

Keep this balloon analogy in mind.  More complicated antennas work by reducing

radiation in most directions.  They distort the balloon considerably, but the volume of the

balloon remains constant.

 

Another rating system for antennas uses dBd, which means dB of improvement over a

dipole antenna.  To convert dBd to dBi, just add 2.15.  Antenna makers specify their gains

in dB.  They actually mean dBd, but given the way they exaggerate their claims, dBi is

usually closer to the truth.

In the US, TV antennas are always horizontal.  If you rotate an antenna about the forward

axis (a line from the transmitting antenna) the signal strength will vary as the cosine of the

angle.  In other words, when the antenna elements are vertical, no signal is received

because TV signals have horizontal polarization.

 Stacked Dipoles:

 Two heads are better than one, and so it is with dipoles.  N dipoles will take in N times as

much RF power as one dipole, provided they are not too close to each other.   Thus a 4-

dipole antenna would have a gain of 8.15 dBi.  (That is 2.15 dBi doubled once (plus 3 dB)

and doubled again (plus another 3 dB).)  This assumes their positions and cable lengths are

adjusted so that their signals add in-phase.  This explanation of gain may seem at odds with

the balloon explanation, but ultimately they are equivalent.  (Adding dipoles does not

increase the volume of the balloon because phase cancellation occurs in some directions.)

Dipoles are commonly stacked horizontally (collinearly), vertically (broadside), and in

echelon (end-fire).

When dipoles are stacked horizontally, the horizontal beam width becomes very narrow. 

This is because they do not add in-phase for directions not straight ahead.  Similarly, when

stacked vertically, the vertical beam width becomes narrower.

Lets say you are 20 miles from a city and TV transmitters are scattered all over the city.   A

medium gain antenna might be too weak, but a high gain antenna would be so directional

you would need a rotor.  Solution:  A bunch of dipoles stacked vertically would give you

the gain you need.  The vertical narrowness of the resulting beam is of little importance,

but the horizontal broadness of the beam means no rotor needed.

           

Reflector Antennas:

Radio waves will reflect off of a large conducting plane as if it was a mirror.   A coarse

screen works just as as well.  Reflector antennas are very common.

 

The double bow-tie above has an average gain of 6 dBi.  With a bigger screen it would

have more.  The parabolic reflector focuses the signal onto a single dipole, but its

bandwidth is a little disappointing.  The corner reflector has a little less gain but much

greater bandwidth.  The corner reflector has roughly the gain of three dipoles.  It is a good

medium gain antenna, widely use for UHF.  If you need more than 25 dBi then the

paraboloid dish is the only practical choice, but they are huge.

 Log-Periodic Dipole Arrays (LPDA)

The LPDA has several dipoles arranged in echelon and criss-cross fed from the front.   The

name comes from the geometric growth, which is logarithmic.

 

This is a very wideband antenna with a gain of up to about 7 dBi.  For any frequency, only

about three of the elements are carrying much current.  The other elements are inactive.  As

frequency increases, the active elements “move” toward the front of the array.  Most VHF

TV antennas are LPDAs. TV LPDAs come in two types: straight and Vee. The Vee type

(LPVA) has a very slightly higher gain for channels 7-13.  But this author often favors the

straight type since it has nulls 90 to each side that can be used to cancel out interference.

 Yagi Antennas:

A Yagi antenna has several elements arranged in echelon.  They are connected together by

a long element, called the boom.  The boom carries no current.  If the boom is an insulator,

the antenna works the same.

The rear-most element is called the reflector.  The next element is called the driven

element.  All the remaining elements are called directors.  The directors are about 5%

shorter than the driven element.  The reflector is about 5% longer than the driven element. 

The driven element is usually a folded dipole or a loop.  It is the only element connected to

the cable.  Yet the other elements carry almost as much current.

 The Yagi is the most magical of all antennas.  The more directors you add, the higher the

gain becomes.  Gains above 20 dBi are possible.  But the Yagi is a narrowband antenna,

often intended for a single frequency.  As frequency increases above the design frequency,

the gain declines abruptly.  Below the design frequency, the gain falls off more gradually. 

When a Yagi is to cover a band of frequencies, it must be designed for the highest

frequency of the band.

 An antenna has an aperture area, from which it captures all incoming radiation.  The

aperture of a Yagi is round and its area is proportional to the gain.  As the leading elements

absorb power, diffraction bends the adjacent rays in toward the antenna.

    

The formula for the aperture area of any TV antenna is   A=G2/4   where is the

wavelength and G is the gain factor over an isotropic antenna (not dB).

The current in the metal rod can be thought of as a standing wave: a signal bouncing back

and forth in the rod until it dies out, meanwhile getting reinforced by incoming energy.   If

the rod is too long, the bouncing signal quickly falls behind the incoming signal.  When it

gets more than 90 degrees behind, it subtracts from the incoming signal.  A rod too short is

a similar case.

The bandwidth of a Yagi can be increased by sizing the reflector for the lowest frequency

of the band while sizing the directors for the highest.  But this decreases the best gain of

the antenna.  (It is said that the gain-bandwidth product remains the same.)  A better way to

increase the bandwidth is to replace the reflector element with a corner-reflector assembly.

                       

This boosts the performance on the lower numbered channels without hurting the high

channels.  Although the Yagi/Corner-Reflector might not be the best antenna, it is the most

common UHF TV antenna, mainly because it can be mounted on the front of a VHF

antenna without degrading the VHF antenna.

 Comparing a Yagi/Corner-Reflector to an 8-Dipole-Reflector

 

 

The graph above shows the gain functions for four TV antennas:

Plot A is the Channel Master 4228 8-Bay, a stacked dipole reflector antenna.

Plot B is the Channel Master 4248, a Yagi/Corner-Reflector. 

Plot C is the 4248 with all of its directors removed, making it a pure corner reflector

antenna.

Plot D is the 4248 with its corner reflector removed and replaced by a single reflector

element, making it a standard Yagi.  The D2 plot shows the backward gain where this

exceeds the forward gain.

The point of this graph is that a Yagi/Corner-Reflector performs like a Yagi for the high

numbered channels and a corner reflector for the low numbered channels.  For the middle

channels it outperforms the sum of the two types.

A UHF Yagi today is designed for channel 69.  If you see an old Yagi, it might be intended

for channel 82.  In the future they will be cut for channel 51.  It is not possible to tell by

looking at a Yagi which era it belongs to, so be careful.

Radiation Pattern:

 

As you can see, the 8-Bay is a very directional antenna.  If miss-aimed by 5 you can lose

1 dB of signal.  If the skyline is more than 5 above horizontal, you should tilt the antenna

up to point at the skyline. The overhead view shows nulls at 30 and 90 to both sides.  

These can be used to eliminate multi-path (ghosts) or interference.  You simply rotate the

antenna until the offending signal is in one of the nulls. 

 

                       

 

A Yagi also has some forward nulls that can be used as ghost killers.  But a Yagi/Corner-

Reflector acts more like a corner reflector for most channels, and has no nulls.  At channel

60 you can finally see the Yagi pattern start to emerge.

 we prefers the 8-Bay over the Yagi/Corner-Reflector because

It has high gain.

Its gain is evenly distributed over the channels.

It has nulls that can eliminate multi-path.

It has a rectangular aperture that permits efficient stacking when more than 8 bays

are necessary.

But the high gain means it is hard to aim.  In good-signal areas, avoid high gain

antennas.

 

 

 RADIATION INTENSITY:

The radiation intensity is total power radiated per unit solid angle and is denoted by U and

it is expressed as U= P/4π.

First figure shows radiation intensity of a source and second figure is relative radiation

intensity of that source.

POWER PATTERN

The directional property of the antenna is often described in the form of a power pattern.

The power pattern is simply the effective area normalized to be

unity at the maximum.

Fig: Power pattern for isotropic source

Power pattern and relative power patterns of a source

Figure (a) shows power pattern of a source. Figure(b) shows relative power pattern of a

same source. Both Patterns have the same shape. The relative power pattern is normalized

to a maximum of unity

The radiated energy streams from the source in radial lines.

Time rate of Energy flow/unit area is called as Poynting vector (Power

Density)

It is expressed as ……….watts / square meters.

For a Point source Pointing vector has only radial component Sr

S component in Θ and φ directions are zero. Magnitude of S = Sr

Source radiating uniformly in all directions – Isotropic Source. It is independent of Θ and

φ.

Graph of Sr at a constant radius as a function of angle is POWER PATTERN

Field pattern

A pattern showing variation of the electric field intensity at a constant radius r as a function

of angle(θ , φ) is called “field pattern”

Fig: Relation of pointing vector s and 2 electric field components of a far field

The power pattern and the field patterns are inter-related: P(θ, φ) = (1/η)*|E(θ, φ )|2 = η*|

H(θ, φ)|2

P = power

E = electrical field component vector H = magnetic field component vector η = 377 ohm

(free-space impedance)

The power pattern is the measured (calculated) and plotted received power: |P(θ, φ)| at a

constant (large) distance from the antenna

The amplitude field pattern is the measured (calculated) and plotted electric (magnetic)

field intensity, |E(θ, φ)| or |H(θ, φ)| at a constant (large) distance from the antenna s

Antenna Arrays:

Antennas with a given radiation pattern may be arranged in a pattern line, circle, plane,

etc.) To yield a different radiation pattern.

Antenna array - a configuration of multiple antennas (elements) arranged to achieve a

given radiation pattern.

Simple antennas can be combined to achieve desired directional effects. Individual

antennas are called elements and the combination is an array

Types of Arrays

Linear array - antenna elements arranged along a straight line.

Circular array - antenna elements arranged around a circular ring.

Planar array - antenna elements arranged over some planar surface (example

- rectangular array).

Conformal array - antenna elements arranged to conform two some non-planar

surface (such as an aircraft skin).

Design Principles of Arrays

There are several array design variables which can be changed to achieve the overall array

pattern design. Array Design Variables

General array shape (linear, circular, planar)

Element spacing.

Element excitation amplitude.

Element excitation phase.

Patterns of array elements.

Types of Arrays

Broadside: maximum radiation at right angles to main axis of antenna

End-fire: maximum radiation along the main axis of antenna

Phased: all elements connected to source

Parasitic: some elements not connected to source

They re-radiate power from other elements

Yagi-Uda Array

Often called Yagi array

Parasitic, end-fire, unidirectional

One driven element: dipole or folded dipole

One reflector behind driven element and slightly longer

One or more directors in front of driven element and slightly shorter

Log-Periodic Dipole Array

Multiple driven elements (dipoles) of varying lengths

Phased array

Unidirectional end-fire

Noted for wide bandwidth

Often used for TV antennas

Monopole Array

Vertical monopoles can be combined to achieve a variety of horizontal patterns

Patterns can be changed by adjusting amplitude and phase of signal applied to each

element

Not necessary to move elements

Useful for AM broadcasting

Collinear Array

All elements along same axis

Used to provide an omnidirectional horizontal pattern from a vertical antenna

Concentrates radiation in horizontal plane

Broadside Array

Bidirectional Array

Uses Dipoles fed in phase and separated by 1/2 wavelength

End-Fire Array

Similar to broadside array except dipoles are fed 180 degrees out of phase

Radiation max off the ends