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24. How many position sensors are used by DFDR? What are they ?
25. What are the functions of the position sensors ?
EXERCISE
A. Translate Paragraph 1,2,3 and 4 of Lesson 43 into Chinese.
B. Explain the following abbreviations and acronyms.
1. ULB
2. DFDR
3. ULD
4. MCU
5. BITE
6. DFDAU
C. Answer the questions to Lesson 43.
LESSON 44
HF COMMUNICATION
VOCABULARY
1. rely on 或rely upon [ ri'lai ] 取決于； 依賴（靠）
2. propagation [ pr p' gein ] n. 傳播
3. the former [ 'f :m ] a. 前者
4. the latter [ 'læt ] ] a. 后者
5. refraction [ ri' frækn ] n. 折射
6. ionized layer [ 'ainaizd ]['lei ] 電離層
7. dependent on [ di'pendnt ] 依賴于； 視---而定
8. effective [ i 'fektiv ] a. 實際的;有效的
9. bounce [ bauns ] v. ; n. 反跳; 蹦
10. skip [ skip ] v. ; n. 跳躍, 越程
11. sensitivity [ sensi 'tiviti ] n. 靈敏度;敏感性
12. enable [ i'neibl ] vt. 使能夠;起(啟)動
13. key signal 鍵控信號
14. application [ æpli 'kein ] n. 施加; 應(使)用
15. USB = upper side band 上邊帶
16. potentiometer [ pteni' mit ] n. 電位計
17. SSB = single side band 單邊帶
18. tunable [ 'tju:nbl ] a. 可調(諧)的
19. watt [ w t ] n. 瓦特
20. PEP = peak envelope power 峰值包絡功率
21. SQL = squelch [ skwelt ] vt. ; n. 靜噪; 噪聲抑制
22. notch [ n t ] n. 槽; 凹槽
23. notch antenna 槽式天線
24. a length of [ le ] 一段; 一根(截)
25. dielectric [ daii'lektrik ] a. 不導電的; 絕緣的
26. feed line [ fi:d ] 饋線
27. metallic [ mi'tælik ] a. 金屬的
28. accessible [ æk 'sesbl ] a. 可以接近的
TEXT
[1] The HF system is a long-range, air-to-ground and air-to-air voice
communication system that operates between 2MHz and 29.999 MHz.
The HF system operation relies on ground wave propagation and sky
wave propagation. Of the two waves the former is used for short range
communication, the latter relies on refraction in an ionized layer and the
earth's surface to achieve long range communication. SKY wave
propagation distance is dependent on the angle of refraction, the varied
height of effective ionized layer, and the aircraft altitude. Areas between
ground bounces are known as " skip" . See figure 44-1.
Fig. 44-1 HF communication
[2] The HF communication system consists of a control panel, a
transceiver, an antenna coupler, and an antenna. The control panel
provides system ON/OFF control, mode selection, frequency selection
and receiver sensitivity control. The transceiver is enabled by a transmit
ting key signal from Digital Audio Control System (DACS) and transmits
a modulated RF signal to the antenna coupler. The coupler matches the
antenna impedance to the transceiver. RF signal is sent through the
coupler to the antenna for radiation. Incoming RF signal is detected by
the same antenna and sent to the coupler, the transceiver processes the
received signal and detects the audio. From the transceiver, the audio is
sent to the DACS and to the SELCAL system. See figure 44-2.
Fig. 44-2 HF COMM system
[3] Figure 44-3 shows the HF communications control panel. The control
panel contains 2 sets of selector switches which select transceiver
frequencies. The mode control switch controls power application and
mode selection, USB (upper side band) or amplitude modulation (AM).
The RF SENSE potentiometer controls the sensitivity of the receiver RF
circuit.
Fig. 44-3 HF COMM control panel
[4] The transceiver is used to communicate in the high-frequency band
on either single side band (SSB) or amplitude modulation (AM) mode of
operation. The transceiver is tunable from 2 to 29.999 MHz. Audio and
keying signals to the transceiver are from the digital audio control system,
the keying signal will modulate the radio frequency. The power output of
the transceiver is 400 watts peak envelope power (PEP) in the sideband
mode, and 125 watts in the AM mode. The front panel of the transceiver
contains the following
 microphone jack ------ for transmitting locally from the transceiver
 Headset jack ------- for listening to transceiver audio
 Three red LED (Light emitting diode) status lamps, to show the status
of the transceiver, the coupler and the control panel.
 SQL/LAMP TEST switch ---- to disable the squelch circuit and to
test the front panel LED lamp.
Fig.44-4 HF COMM transceiver
[5] Figure 44-5 shows the HF communication antenna. The purpose of
the antenna is to radiate the modulated RF signals and receive the
　
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