خطوط الوصل بين الهوائي والجهاز .. كتاب التلفزيون الملون والعادي

4-4 Lines of communication between the antenna and the device: It is the line feeding the device with the signal coming from the antenna with the least possible losses that weaken the visual signal and the synchronization pulses and colors, making the image weak in features of different synchronization, in which ghosts, shadows and interference appear, and the antenna cable is required to be of good quality. Impedance compatibility of the antenna and input device. It is usually (300 ohms) in black and white and (75) ohms) in color - and it should be as short as possible because the signal decay reaches (12) (decibels per (100) feet) at a frequency of (100) megahertz). The decay increases at higher frequencies, and on wet, rainy winter days. This line must be far from multiple sources of interference. The types of connection lines are as shown in Figure (4-12):

1- A simple line with two parallel wires, between them a certain distance, and its impedance is 300 ohms. It is affected by ambient electromagnetic interference and interference.

2- A tubular line, which also contains two parallel wires. It is preferable to be used in the VHF and UHF fields, and is less affected by humidity and sea air.

3- An open tape line: it has empty places between its wires that reduce losses in wet weather - because losses increase with the presence of a plastic insulator when it is wet - especially in high frequencies.

4- A line with two parallel, veiled wires: it contains a cover of copper braids connected to the ground to protect the signal from interference - but it is high in signal decay and reaches (3) decibels) for every (100 feet) at the frequency (100 MHz), and its impedance is from (50-100). ohms).

5- Coaxial Cable: It is a single insulated internal wire wrapped around it with a copper braid and then externally insulated with plastic. It is widely used in color television. It prevents signal interference and noise, its impedance is mostly (75 ohms). It achieves compatibility between the antenna and the input of the color device (75 ohms).

4-5 Compatibility between antenna impedance and device input impedance:

The impedance of the antenna, the feed line, and the input of the device must be compatible. The impedance of each is often 75, 240, or 300 ohms. Impedance compatibility can be achieved with a compatibility transformer installed with the DePaul antenna or at the entrance of the device (Fig. (4)-13) and (4-14).