A-LevelPhysicsWaves9702/41 · Oct/Nov 2017 · Q48 Marks
A coaxial cable is frequently used to connect an aerial to a television receiver. Such a cable is illustrated in Fig. 4.1. [Figure 4.1]
- (a)Suggest two functions of the copper braid.[2]
- (b)Suggest two reasons why a wire pair is not usually used to connect the aerial to the receiver.[2]
- (c)The coaxial cable connecting an aerial to a receiver has length 14 m. The cable has an attenuation per unit length of 190 dB km⁻¹. Calculate the fractional loss in signal power during transmission of the signal along the cable.[4]
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Mark scheme
Marking points from the official mark scheme (Oct/Nov 2017). The mark or mark type is shown in brackets.
- [B2] acts as 'return' (conductor) for signal
- [B2] shielding from noise/crosstalk/interference
- [B2] small bandwidth
- [B2] (there is) noise/interference/crosstalk
- [B2] large attenuation/energy loss
- [B2] reflections due to poor impedance matching
- [C1] attenuation = 190 × 14 × 10⁻³ (= 2.66 dB)
- [C1] 2.66 = -10 lg (POUT/PIN)
- [C1] POUT/ PIN = 0.54
- [A1] fractional loss = 1 – (POUT/PIN) = 1 – 0.54 = 0.46
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