5.2.1 : Active Transponder.
Corner reflectors have the particular property of retro-reflection and hence form the standard target for calibrating monostatic radars. The RCS is large, accurately calculable and fairly independent of orientation, making alignment a relatively trivial task. However, this renders comer reflectors limited for bistatic radars: alignment is critical and RCS is large only for locations giving bistatic angles to within the reflector’s beam width of zero.
Another type of reflector is the flat plate reflector, where one directs a specular reflection from the transmitter to the receiver, and clearly allows a range of bistatic angles. The RCS at specular reflection is large, easily calculable and alignment of the plate perpendicular to the bistatic angle bisector can be fulfilled for most bistatic angles. However, for long ranges, bisectoral alignment of a large RCS, narrow beamwidth flat plate is notably time- consuming. A more suitable solution is a transponder consisting of two coupled, yet independently oriented horns. This solves the alignment problem, but may result in a small RCS. To compensate for this, an active transponder would use an amplifier. Further, with a sufficiently strong signal, Doppler behaviour may be incorporated with a low frequency modulator to emulate target motion, a feature not possible with passive units like corner reflectors. In summary, the active transponder would allow control of: • directions of incidence and scattering;
• range (controlled by position, or by delay-line);
• RCS (controlled by an amplifier with variable attenuator); • Doppler velocity (controlled by a carrier frequency modulator).
In this way (Figure 5.1), the active transponder can imitate a distant moving target of prescribed RCS, range and Doppler velocity.
Doppler shift (velocity) and rate of change of delay time (bistatic range migration) with respect to time are causally related to target motion. The ideal transponder would incorporate microprocessor control of Doppler frequency shift and delay time for consistency:
- . ^ 15-11
dr /c
This is technically sophisticated, but a simpler arrangement is to have a transponder with a Doppler LO but fixed or zero time delay opted for, and this inconsistency in target motion may be compensated for by programming the receiver to search for Doppler motion, yet employ no range migration compensation.
CftapterS ‘Bistatic ïRaxCar ‘E^eriments To receiver From transmitter T ransmitter-facing horn Receiver-facing horn ^ Delay line ^ Attenuator Modulator Amplifier Microprocessor Doppler frequency local oscillator
Figure 5.1. Schematic diagram of an active transponder.
5.2.2 : Characterisation
The constructed transponder was improvised from two identical rectangular cross-section flare horns. Aperture dimensions are 135mm by 170mm with a 400mm length to the X- band waveguide transition. To commence characterisation, consider constructing the passive transponder by connecting waveguide apertures directly via waveguide-to-cable adapters and a short length of semi-rigid cable. Neglecting plumbing and mis-alignment losses, the passive transponder RCS is estimated by receiving horn effective aperture area multiplied by transmitting horn gain, which in terms of physical dimensions reduces to:
4;r {T]abY
'passive [5.2]
For <3=0.135m, 6=0.170m, 7]=0.90 and A;=26.25mm for the TV carrier that was used in these experiments at f^ = \l,391MHz, the passive RCS is estimated to be 8.8dBm^.
The transponder was in fact used without the Doppler modulator. In the first instance, the transponder emulated a large RCS stationary target for detection characterisation. Hence, characterisation of the transponder is broken down into the following sections:
Cfiapter s (Bistatic H^acfar ‘Ejçpenments
• passive, lossless transponder RCS (above);
• plumbing losses (waveguide adapters, connectors and cables); • high gain amplifier;
• horn mis-alignment losses (boresight squints and cross-polarisations).
The chosen amplifier is a high gain wide band microwave amplifier manufactured by Avantek, model AMT-12736 (G = +50dB, band = 7GHz to 15GHz). Figure 5.2 illustrates the gain and noise figure test measurements supplied by Avantek for the unit.
51.6 51.2 50.8 50.4 m ■o Ç CO Ü 49.6 49.2 48.8 48.4 9 10 11 12 13 7 8 3.8 3.6 3.4 •5 2.8 2.6 2.4 2.2 9 10 11 12 13 7 8 Frequency/G H z Frequency/G H z
Figure 5.2. Gain and noise characteristics of the transponder amplifier.
The amplifier and cabling were enclosed within a water-tight ABS box with silica gel for humidity reduction. Space was reserved for in-line attenuators and was occupied by a IdB attenuator. This sealed unit was recharacterised using a HP8510B network analyser and the gain characteristic over the DBS TV rf band is illustrated in Figure 5.2. Gamp is marked at/c = 1 l,320MHz as +48.5dB. The feeds between horns and amplifier enclosure were is marked each WG18 to SMA-type adapters and 0.141" semi-rigid cables with SMA-type connectors. Total plumbing loss, Lpiumbing, was characterised by connecting the waveguide adapters back-to-back and placing the assembly as a device-under-test on the network analyser. Figure 5.3 illustrates plumbing losses, and is marked at/c = 11,320MHz as being -2.6dB.
CfiapteT 5 (Bistatic (Rgdar ‘Eyqjeriments
5.2.3 : Alignment
Each horn was mounted on a flat bulkhead. One horn was rigidly fixed to this while the other was free to move by -20° in azimuth and -10° in elevation. At 11,320MHz the azimuthal and elevational beam widths of the horns compute at 8.9° and 11.2° respectively. This means that having a larger beamwidth than the satellite dishes, alignment of the horns was less critical.
The transmitter-looking horn and high-gain amplifier were used to align this horn with the satellite transmitter. The high-gain amplifier was sufficiently sensitive to be able to display the rf spectrum from ASTRA directly on a spectrum analyser using the transmitter-looking flare horn for reception. The receiver-looking horn was aligned with the receiver's target satellite dish by eye with satisfactory precision. An X-band tone was radiated to the satellite dish, creating a beacon to align the dish to the transponder. Finally, the transmitter-looking hom was connected via the amplifier to the receiver-looking horn, and during this alignment procedure, the mis-alignment losses were minimised to values shown in table 5.1:
Mis alignment Quantity Loss (dB)
Transmitter-looking horn (to ASTRA) 0.0±0.3
Receiver-looking horn (to satellite dish) 0.0±0.3
Target satellite dish (to transponder) 0.2±0.2
Table 5.1. Active transponder characterised mis alignment losses.
Table 5.2 shows typical signal powers obtained by ASTRA at the transponder amplifier output after alignment, measured with a spectrum analyser:
Carrier frequency,/g/MHz Power (dBW)
11,128 -86.4±0.2
11,320 -86.8±0.1
Table 5.2. Typical carrier signal powers received by transponder from ASTRA.
Since the definition of RCS is dependent on scattered radiation power density, the RCS of the active transponder must be greater than that of the passive transponder by the overall gain of amplification. Thus
tractive “ (p assive "b C am p “ ^plumbing " ^squint [ ^ ’^ ]
In conclusion, taking full account of all gains and losses allowed the total RCS of the active transponder to be characterised as Oirans = (54.5±0.5)dBm^.