Actually all Doppler radar applications, including the police radar, are using the X, Ku, K, or Ka frequency bands. The motivation beyond this has its origin in the unique advantages that high frequency can provide. These advantages are:

· Antenna gain is proportional to the electrical size of the antenna. At higher frequencies, more antenna gain is therefore possible for a given antenna size, which has important consequences for implementing miniaturized microwave systems.

Microwave signals travel by line of sight and are not bent by ionosphere, as are the low frequency signals.

The effective reflection area (radar cross section) of a radar target is usually proportional to the target’s electrical size. This fact coupled with the frequency characteristics of antenna gain; generally make microwave frequencies preferred for radar systems.

The ability of such frequencies to reach remote objects, while keeping an almost narrow beam with small antenna size as compared to low frequencies.

 

However, as we go higher in frequency and consequently shorter in wavelength of microwave energy, as many problems and difficulties in the analysis and design of microwave components and systems arise. But the greatest difficulty of all is the availability of the components, as well as -if ever available- it’s extremely high cost.

As a result of the urgent need for high frequency system, and the lack of reliable microwave sources and components, we were forced to think of an alternative that implies all the above advantages. And the solution was the Ku band LNB that with few modifications turned out to be a complete radar system that transmit, receive and down converts the signal to produce the known Doppler frequency shift.

Introduction:

The low noise block down converter, known as LNB has the important function of detecting the microwave signal relayed from the feedhorn and converting it to an electrical current, amplifying this weak signal by 40 to 50 dB (a gain of 10000 to 100000) and down converting or lowering its carrier frequency to an intermediate range, this signal is subsequently relayed along cable to an indoor satellite receiver.

LNB is the first "active" electronic component in the series of steps by which a satellite signal is processed.

Block down converters use a fixed frequency local oscillator to lower the whole satellite band to an intermediate range (see Fig 1 and Table 1) this was not practical

because the frequency produced was not sufficiently stable. By contrast, DRO dielectric resonant oscillators are stable to within 10 MHz and can be stable to nearly 1 MHz over the whole satellite band.

Fig(1):Block Down conversion.

Ku-band LNBs use a low side injection, with fixed oscillator to generate either a 950 to 1450 MHz or a 950 to 1700 MHz band (see table 1). In Ku band satellite transmission, the bandwidth and frequency centers of the entire broadcast as well as individual channels vary. For example some regions assigned downlink frequencies of 11.70 to 12.2 GHz with, while others use a range of 12.25 to 12.75 GHz or 10.95 to 11.70 GHz etc…

 

Input frequency	Output frequency	LO frequency
GHz	MHz	GHz
11.70-12.20	950-1450	10.75
10.95-11.75	950-1700	10.00
12.50-12.75	1025-1275	11.475
12.25-12.75	950-1450	11.30

Table (1): Ku LNB frequency ranges

LNB for Ku-band satellite use either rectangular or circular type of input flange and waveguide, rectangular is evolving into standard while the circular is becoming less common.

Each LNB consists of two or three GaAsFET transistor stages organized in a cascaded arrangement, two or three conventional amplification stages and the down conversion circuitry. A voltage regulator is also included in circuit design, LNBs usually draw between 80 to 150 mA of current and operate at 15 to 24 Vdc.

Fig (2) is a schematic of typical LNB.

The LNB consists of:

1) Wave guide to microsrtip transition

2) Isolator

3) Power supply

4) Dielectric resonant oscillator

5) DC block

6) Connector

7) Block IF amplifier

8) Block IF filter

9) Mixer

10) Band pass filter

11) 13 15 1^st, 2^nd & 3^rd GaAsFET amplifier stages

12) 14 &16. Impedance matching circuits

After deep study of different types of Ku band LNB constructions and designs, we started the search for a suitable LNB that we can modify its construction to satisfy our needs. Finally we succeeded in finding a special type of LNBs called "Dual LNB/Feed" which is designed to simultaneously detect both polarities ‘horizontal and vertical’, this type has a circular waveguide terminated by two independent probes and two LNBs built into one housing. It is designed so that each probe/LNB detects signals of only one polarity, one for vertical and the other for horizontal polarities. Its input, output frequency ranges, as well as its local oscillators frequencies are listed in table (2).

Modification and design

 

Oscillator:

Our aim is to modify the LNB, which is merely a receiver into a complete transceiver system, to attain this we modified the LNB construction as follows; the oscillator is used as a resonator for the transmitter and as a local oscillator in the receiver.

So we divided the signal into two signals. One signal feeds an external power amplifier (20 dB gain) that in turn excites the probe of the transmitting horn antenna and the other signal is fed into the coupler-mixer as a local oscillator to produce the frequency shift ∆f in addition to harmonics of frequencies that are filtered using 3^rd order Butterworth low pass filter to extract ∆f.

It is an X-band power amplifier with gain of 20dB,its maximum input power is 0dBm, and it is designed using three GaASFET amplifier stages with external biasing circuitry.

Even though the LNB is merely a receiver, with slight modification we succeeded to transform it into a complete transceiver system in the X-band and to use it as a Doppler radar for speed detecting.