Rhotheta RT-1000 Manual de usuario

User Manual

RT-1000

Antenna

VHF Bearing-System

RT-1000 Antenna

RHOTHETA Page 2 of 30 User Manual

Edited by:

RHOTHETA Elektronik GmbH

Kemmelpark

Dr.-Ingeborg-Haeckel-Str. 2

82418 Murnau

Germany

Tel.: +49 8841 4879 - 0

Fax: +49 8841 4879 - 15

Internet: www.rhotheta.de

E-Mail: [email protected]

-Issue: 2016/11/04 [Rev 1.01.a]

-Document-ID: 12-9-2-0019-3-1-60

Note

The manufacturer reserves the right to make modifications at any time and without previous

information of the here described product.

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Content

General Information.........................................................................................................4

1.1

Reflections and their Influence to the Bearing Result. ...........................................................................5

1.2

Influence of the signal ground reflection to the DF accuracy.................................................................9

Antenna Construction...................................................................................................14

2.1

Side View..............................................................................................................................................14

2.2

Bottom View..........................................................................................................................................15

2.3

Component List.....................................................................................................................................16

Assembly Instructions ..................................................................................................17

North Alignment of the Direction Finder Antenna and Determining the System

Accuracy at the Installation Site .........................................................................................19

4.1

North Alignment Using a Ground Transmitter (Pre-setting) .................................................................19

4.2

Flight Checking for Exact North Alignment and Determining the System Accuracy at the Antenna

Installation Site...............................................................................................................................................21

4.2.1

Determining the Position Using a Theodolite ..........................................................................22

4.2.2

Determining the Position Using a GPS Receiver ....................................................................22

4.2.3

Simplified Method ....................................................................................................................22

4.3

Evaluation.............................................................................................................................................23

4.3.1

Evaluation of Direction Finding Signal.....................................................................................23

4.3.2

Evaluation of QDR Live Display (Green Light Dot Circle) .......................................................25

4.3.3

Evaluation of Measuring Results.............................................................................................26

4.4

Determining the North Correction.........................................................................................................28

Mechanical Dimensions................................................................................................29

Notes...............................................................................................................................30

RT-1000 Antenna

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1 General Information

DF-Systems are used to locate aircraft, ships, vehicles or persons who have a radio

transmitter. All prevalent DF-Systems determine the direction from which the signal reaches

the DF-Antenna. If this direction is the same as the direction to the aircraft, vessel or the

vehicle, everything is fine. If the radio signal arrives, the DF-Antenna not in a direct way,

because of shadowing or reflections, the direction finder shows in the direction to the

reflector, where the signal (or the majority of the signal) comes from.

This direction can have a big difference to the direction of the source of the signal, the aircraft,

the vessel or the vehicle where the transmitter is located. The operator will regard this as a

malfunction of the direction finder. In the physical way, this is not true. The problem is that the

bearing we get from the DF is not the information the operator desires.

This means for the praxis, that we have to find an antenna location, where the radio signals

always arrive the antenna in the direct way.

The achievable bearing accuracy depends largely on the physical conditions at the antenna

location.

The remote concept of the RT-1000 C DF-System separates the antenna and the high

frequency components (RF-Antenna, Receiver Unit) from the controller side (Controller) with

the signal processing. So it is easy to place the antenna somewhere in the open field where

the physical conditions are good, while the Controller is located at the tower side.

The following information should help you, to find a physical convenient antenna location.

RT-1000 Antenna

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1.1 Reflections and their Influence to the Bearing Result.

The antenna of a DF-System can be seen as a sensor, which analyses the incoming

electromagnetic wave field to find out where it comes from. As the waves on quiet water

surface also radio waves in the free (unobstructed) field dispread circular from around the

transmitter.

Sender Empfänger

Dipol Dipol

Weg

0 0 0 0 00 0 0 0 0

+ + + + +

bei v = c

Grad

Isophasen

---

v = Ausbreitungsgeschwindigkeit

c = Lichtgeschwindigkeit

= Wellenlänge

λλ

Fig. 1: Free-space propagation of radio waves

Fig. 1 gives an extremely simplified representation of even radio wave propagation in free

space. The sine wave in part a) corresponds to the instantaneous value of the electric field on

RT-1000 Antenna

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the plane path to the receiver. Part b) is the vertical projection of part a). The circles represent

the lines of equal phase relations for even waves.

If the distance between transmitter and receiver is adequate, these are practically straight

lines when they reach the receiver location. Such an idealized situation is not to be found in

built-up areas, and especially not in mountainous regions. In such areas, the propagation path

is obstructed by obstacles, mirror reflectors; diffuse reflectors with and without absorption

characteristics, diffracting edges and resonators. Reflectors and conducting rods are effective

as resonators if their size is approximately that of the wavelength to be received. Therefore,

reflections increase as wavelengths become shorter, diffractions at edges however are

reduced and so the effect of shadowing obstacles is more important.

Accordingly, the propagation characteristics of radio waves from approximately λ< 10 m

increasingly resemble those of light.

At a wavelength of 1 m to 3 m, wave propagation requires a direct path and if this is not

available, only reflected waves are received. In urban areas, these may come from several

directions simultaneously. But that is not all: the mostly horizontal or vertical plane polarized

waves propagated by the transmitter are also rotated to a certain degree due to diffuse

reflectors and diffracting edges. When the wave arrives at the receiver, it may be oblique,

elliptically, or even circular polarized. This fact becomes apparent by the often curious

antenna positions which are necessary to obtain the best reception of radio or television

waves.

These points are meant to indicate that in the VHF, UHF range, direction finding of a

stationary transmitter using a stationary direction finder in a built-up area or even inside a

building is practically impossible.

Transmitter Reflection

Fig. 2: Field of lines of equal phase relations for two coherent waves

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Conditions at airports are much more favourable. These instructions are intended to allow the

best positioning of the direction finder antenna.

Of course, airports are not without reflectors, but these do not normally cause noticeable

problems. All direction finders with field probes calculate the angle of signal incidence by

finding out the path direction (vector), at which the largest phase modification per unit of

distance is present.

In Fig. 1 this vector is vertical to the lines of equal phase relations. Fig. 2 shows the distorted

field of lines of equal phase relations for two coherent waves (reflection) from different

directions with different field strengths.

Mainly 4 parameters influence the deviation of the bearing caused by reflections:

1. Position of the DF antenna

2. Position for the Transmitter antenna

3. Position of the reflector

4. Wave length of the signal (signal frequency)

The advantage of wide base direction finders is most noticeable in static conditions. Static

conditions indicate that the position of the transmitter and direction finder as well as the

transmitter frequency does not change with time. Should one of the three named items

change (e.g. transmitter in aero-plane) the direction finder antenna and the field of lines of

equal phase relations begin to move in relation to one another. This movement accelerates in

proportion to the relationship between the reflected path distance and the direct path distance

of the radio wave (Fig. 3).

As shown in Fig. 2, this movement in the case of wide base direction finders causes slight

azimuth oscillation. In contrast, this oscillation is larger in the case of narrow base direction

finders - the series of small circles in Fig. 2. When several values are averaged out however,

both systems give the same azimuth.

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W1 W2

Reflector

Aeroplane

Direction finder

W2 W1

Reflector

Aeroplane

a) b)

Direction finder

Fig. 3: Reflected path distance an direct path distance of the radio wave

favourable: unfavourable:

W1/W2 » R1/R2 W1/W2 ≈R1/R2

With moving transmitter:

Rapid phase shifting between W and Only very slow phase shifting between W and

R signal, therefore good bearing average. R signal, so no averaging possible. The

possible displayed bearing oscillates slowly

around the rated value.

The following conclusions can be drawn:

Vertical reflector surfaces e.g. buildings, hangars, metal fences, metal masts, overhead

lines as well as bushes and trees should not be within 100 m of the direction finder

antenna if possible.

RT-1000 Antenna

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1.2 Influence of the signal ground reflection to the DF accuracy

The signal out of an aircraft transmitter will reach the DF antenna on the direct way. In

addition a part of the signal will be reflected on the ground and will reach the antenna as well.

Depending on the angle of incidence and the height at which the DF antenna is located above

the ground, the direct signal and the reflected signal have to go different distances.

Ground

h = large

h = small

unfavourable

favourable

A moving transmitter causes rapid

phase shifts between W and R signals

and therefore a lot of amplitude

fluctuations at the direction finder.

A moving transmitter causes slow

phase shifts between W and R signals

and therefore few amplitude fluctuations

at the direction finder.

Fig. 4: The phases between a direct wave W and a reflected wave R

This is the reason why there are angles of the signal incidence, where the signals are in

phase and there are angles where the signal arrive the antenna in opposite phase. On one

case the direct and the ground signal add each other, on the other case the direct signal is

compensated or reduced by the ground reflected signal. This is the reason why the vertical

antenna diagram become more zero points as higher the antenna is placed (see Fig. 5).

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Vertical diagram for h = 23 m Vertical diagram for h = 3.5 m

Fig. 5: Signal strength lobes plotted against angle of elevation

The zeros of the vertical diagram can be determined using the following formula:

tan

; f

;

299792458

= light velocity [m/s]

λ= wave lengths of the signal [m]

h = height of the antenna head above the ground [m]

n = ordinal numeral of the zero point (1. 2. 3….)

f = signal frequency [Hz]

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