Having seen the basic principle of Monopulse Tracking we now concentrate on the special components and circuits used for the derivation of actual tracking errors.

**Antenna and Feed:**

A voltage proportional to the angular
difference

between Antenna axis and Target can be derived

by vectorial
subtraction (

**A - B**).How do we derive the error signals from these elements? We use what is called a Monopulse comparator. In one of the simplest forms transmission lines of

**l/4**and

**3*l/4**are used in a ring fashions as shown in next figure.

The working principle is straight forward:

**A**gets divided equally into two paths each with a strength

**A/2**;

One travelling to

**SUM**port and

another

**A/2**travelling to

**DIFF**( = Difference ) port .

A similar split
happens to

**B**input.
At

**SUM**port both**A/2**and**B/2**travel same path length and so they get added vectorially to**( A/2 + B/2 )**.
At the

**DIFF**port**A/2**components travels**l/4**while the**B/2**component travels**3l/4,**that is it travels**l/4**+ 2**l/4**( =**l/2 )**and so the output becomes**A/2**plus (**B/2***with a phase reversal*), so effectively you get (**A/2 - B/2**) at the output.
Althogh really the values are

**( A/2 + B/2 )**and**( A/2 - B/2 )**they are denoted as**(****A + B )**and**( A - B )**which though theoretically wrong is the convention universally used for simplicity as we are not doing a critical mathematical evaluation.
So we will use

**SUM**=**S =****(****A + B )**and**DIFF**=**D =****( A - B )**as the convention here onwards.**Two Axis Monopulse Feed:**

We have seen that

*two*elements are required to measure the angular difference for*one*axis.
Antennae
generally have two axes for Horizon to Horizon coverage hemisphere ( e.g. AZ/EL or X/Y or Ha/Dec etc. ).

It follows
that there should be

*4*elements to track in*2*axes.
In the example we assume that there are two
axes X and Y supporting the antenna and the four elements can be placed in two
fashions ..

a.

**Cross or**elements arrangement where the elements are mounted on axes and*On axis*
the
error about X axis would be given by the quantity (

and Y Error would be (

**A – B**)and Y Error would be (

**C – D**).
While another arrangement would be

b.

and Y Error would be (

**Diagonal arrangement**where the elements are placed in a 2 on each side of axes equally spaced from each axis and In this case X arror would be (**( A + B ) – ( C + D )**)and Y Error would be (

**( A + D ) – ( B + C )**) .Two arrangements are graphically shown in the figure below.

Notice that in second case the error is a
difference of two additions so it is
less prone to errors and is more sensitive.
That is the reason why it is commonly used.

How do we derive the error signals from
four elements? We use a set of Monopulse comparators.

In 1

^{st}case of Cross arrangement of elements we use a set of 3 hybrids as shown hree.

**D**

**X**and

**D**

**Y**signals but also

**S**which can be used as the input to communication data receiver.

Four hybrids are required to derive
tracking errors if we use the 2

^{nd}i.e. Diagonal arrangement of elements as shown below.
Although 4 elements are sufficient to
derive errors in two axes, several
other more complex arrangements are used
in sophisticated applications.

One of the most common is to use a 5

^{th}element at the centre of 4 elements**A**thru**D**as DATA pickoff element. This is useful in Parabolic reflector antennas since the error elements are always away from the prime focus of main reflector so receive less signal than what is available at the prime focus. More on this subject in a later post.
We have shown here the most commonly used
arrangements. The elements themselves could be Dipoles/Cross-Dipoles/Helices or
Horns. There are several complex variations in use .

A few designs also use a

*single Horn*feed fabricated such that Error outputs are derived at the output by multimode horn excitations .
There are also multielement feeds
wherein upto 12 elements are used to derive the Errors for
better sensitivity and flatter phase response over wide frequency range of
operation.

We will not go into more complex
configurations but just mention here that in reflector antennas ( e.g Parabolic
or Cassegrain ) generally a fifth element is added in the centre. This is not
for tracking but is used as an independent DATA receiving element.

Next post covers the overall tracking system including error computing processor.

Back to Main Index

## No comments:

## Post a Comment