Antennas and Propagation – 2007 – Lecture

8/14/2019 Antennas and Propagation 2007 Lecture

1/13

Antennas and Propagation 2007 Lecture 2.

Today we want to reinforce and build on many of the ideas from the first

Lecture.

We are talking about the various ways of getting high radio frequency

energy from one point to another. The basic thing about high frequencies

is that they want to escape and leak out into space. We have seen that

transmission lines are all about containing these fields within their

structure.

The sequence of transmission lines is:

Parallel wire Coaxial Waveguide Optical fibre

There are variants like stripline or microstripline transmission lines

which are made on circuit board material but they are just another variety

which we will touch on down the track.

To give an idea of the fields situation, first for the parallel wire line:

And then for the coaxial line:


2/13

Where red lines show electric field lines and green the magnetic. At

EVERY point E and H are at right angles and have the Plane Wave form:

And at ANY point if we take the ratio E/H = (volts/meter)/(amps/meter) =

and this must also equal the Characteristic Impedance that we have already

discussed.

Now our whole study of antennas is all about shaping the way an antenna

radiates controlling the pattern and doing that efficiently.

ALL em waves in space have a plane wave form. Space is then a

transmission line where we can still take the ratio of E/H and we find that

the value of the characteristic impedance = 120 = 377 ..

We revisited the idea of an accelerated current causing a radiated kink

which we met in the first lecture which has a maximum to the side and

nulls off the ends looks the same all the way round and will have some

kind of donut pattern:


3/13

ANY antenna pattern which we ever draw is ALWAYS a pattern of the

ELECTRIC field. Now it isnt easy for you to sketch this sort of 3 D

result so instead we sketch 2 D cuts of this in this case as:

You need to relate these to the 3 D picture. The E plane pattern is the

pattern of the Electric field in line with the wire. H lines always encircle

the wire which gives us the H plane but in that plane we sketch an E fieldresult!

We revisited the idea of bending up /4 lengths at the ends of a parallel wire

transmission line to form a very useful Half wave dipole:

Current at each end must always be zero (nowhere to go!) and we can

exactly fit in a half wave distribution of current as seen in the previous


4/13

sketch. We will formally analyse the performance of this particular dipole

in the next lecture.

As we shall see this is vital to everything in antennas and is related to the

frequency and the velocity of light by: f * = c.

We then revisited the idea of imaging or mirroring which can be analysed

by considering images as:

Or:

In each case the dipole is above a ground plane conductor. There can beNO actual radiation below that ground plane in either case BUT we can

imagine the image to be there for all analysis purposes.

We make use of antenna imaging ideas extensively partly because they

are a fact of life in the real world and also because they provide a free


5/13

second image. The most common usage is the negative image case as

shown in the next sketch:

Next we show the same arrangement from the side:

Envisaging the usual donut pattern as shown here it is clear that each dipole

is in the maximum signal of the other and they are not far apart at all! In

fact we will get serious mutual coupling effects as summarized in the

following sketch:

The values for Z12 are presented in the following diagrams ( we will be

coming back later to understand how these are evaluated):


6/13

You simply need to read off the (complex) value of Z12 and it is

straightforward to evaluate the mutual coupling equations there will be an

example shortly.

The mutual curves in the following simply show this same information in a

different way:


7/13

This is all background material for your first laboratory exercise. The lab

includes the use of a 900

corner reflector and as you can see from the

following this can be analysed by using 3 images:


8/13

The result of the analysis will then ive you the fields in the left hand open

900

corner reflector segment.

We can even do this with a 600

corner reflector which can be analysed with

5 images. Obviously in both of these cases there will be significant mutual

coupling effects to deal with which we will learn to cope with as we

progress.

We have met the idea of imaging over a ground plane and you should be

comfortable with the idea that with a vertical dipole above a ground plane

can be analysed by introducing a positive image as we see here:

When you look back at the antenna from a (plane!) horizon (far away!) you

see the dipole and its image as being equal and in phase so you would

expect a maximum there. This would be the case if the ground was a

PERFECT conductor but of course, nothing is a perfect conductor so in

reality there will be a null right at the ground as shown here.


9/13

We now look at a very real application which makes use of horizontal

dipoles above a ground plane we then analyse with negative images!

THE INSTRUMENT LANDING SYSTEM GLIDEPATH.

Because there are so many useful things to be learned from the way it works

we then proceeded to look at the Instrument Landing System (ILS) glidepath

which is in use at all major airports throughout the world for the final

approach phase of a landing aircraft.

We start by considering a horizontal dipole placed some 4.777above the

ground as shown in the following sketch:

We have found that a horizontal dipole produces a Negative image in the

ground mirror so if we look back at this antenna from the horizon (and far

away) we must see a complete null as the field from the driven antenna and

its image directly cancel.


10/13

We take our phase reference as the ground point O because it is the

centre of the antenna. Then as we move our observation (far field)

point upwards at a constant radius we can see from the lower part of this

sketch that the radiated ray from the upper (driven) dipole leads the point

O while the image contribution lags by exactly the same amount. The

phase changes by 3600 or 2 radians every wavelength. The sequence of

events is shown in the following sketch which shows the development as we

move up in elevation by 10

intervals.

You must see that 2/* 4.777 * * sin ( 10

) = /6 (equivalent to 300

)

For each 10

change in elevation the component phases change by 300

and

this progression is shown in the following sketches up to an elevation angle

of 80

. At the 00

angle I have shown the Reference phase as being

vertical ( that is the number of 2 (or 3600

) phase lags from O ) just for

neatness! That reference phase does not change provided we move ourobservation point upwards on an arc of constant radius.


11/13


12/13

You need to observe clearly that up to an elevation angle of 60

the phase of

the resultant is always constant and at the same phase to the left. At 60

there is a null in the pattern and then above this angle the phase instantly

flips by 1800

. Higher than 60

the pattern builds up again to another

maximum at 90

followed by a null again at 120

.

This business of the phase flipping by 1800

across a null ( AT A

CONSTANT RADIUS!) is a quite fundamental property of ALL antenna

patterns.

The simplest Glidepath system employs a second horizontal dipole which is

located a further 4.777above the first as shown in the following sketch.

Because this second dipole is twice as high as the first it will have nulls

occurring twice as fast! In other words the first null of the upper dipole

now occurs at 30 also shown in the following sketch. The glidepathsystem works at around a frequency of 330 MHz and is modulated with 90

Hz and 150 Hz audio tones as shown in the lower part of the sketch here:


13/13

In to the lower antenna is fed Carrier (at 330 MHz) plus in-phase sidebands

(CSB) at 90 and 150 Hz while in to the upper antenna is fed NO carrier but

Sidebands Only (SBO) with sidebands which have opposite phase. The

upper antenna has a null at 30

so at that angle in the sky the 90 and 150 Hz

tones will be equal. Below 30

you will see that the 90 Hz tones ADD while

the 150 Hz tones tend to CANCEL. Because of the phase flip at the 30null at an angle above 3

0the 90 Hz tone sidebands then tend to CANCEL

while the 150 Hz tones reinforce. The aircraft receives the 330 MHz signal

demodulates it and compares the size of 90 Hz and 150 Hz tones. This ends

up providing a + and (virtually linear) path of more than 0

each side of

30

which can be used by a landing aircraft. Way back in time these

demodulated tones were fed directly to the pilots earphones!!!

There is also a separate azimuth guidance path to keep the aircraft on

azimuth centerline which is provided by another antenna (located past thestop end of the runway) that system is called the Localizer and it

operates at around 110 MHz it provides azimuth signals (which are very

similar to the elevation ones we have just considered) to give a linear

azimuth path over at least + and - 20

about the runway centerline.

Copyright Godfrey Lucas

Updated 5 August 2007.

`

Documents

Antennas and Propagation – 2007 – Lecture