Do you know the way of intercom?

Why are wireless walkie talkies all single work calls? This is to start with the basic common sense of wireless walkie talkies. Wireless communication is transmitted by electromagnetic wave, while interphone calls by its own transmitting power. Therefore, when the receiving sensitivity reaches a certain index, the greater the power communication distance is, the farther the power is. But when the power is higher, the current will increase, and the power and current will increase in a certain proportion.
   as shown in the figure:
Intercom call mode
The problem is that when the wireless walkie talkie requires small size and easy to carry, it is necessary to have reasonable power support if the communication distance is far away. If the duplex call is transmitted continuously for 4w-5w in 8 hours and the battery block volume wants to have half the palm size, then according to the above table, the battery capacity must reach 15000 Ma or more to meet the duplex call demand. Therefore, two thirds of the capacity of the back-to-back 10W radio station used by military forces is battery. Ask an adult male (security guard) who would like to work on a walkie talkie every day?
Therefore, half the volume of professional 4w-5w walkie talkie is battery. The battery capacity is between 1200mAh and 1600mAh, that is to say, the discharge of 1200Ma constant current will be discharged in 1 hour. Discharge with a constant current of 600mA and discharge the power in 2 hours. So, from the above chart, we can see that the 4W power emission current is about 1500mA, that is, the power is lost for about 50 minutes. However, it is impossible to speak continuously in the actual use process, because the radio is transmitted in the following ways: speaking, listening and standby. This allows for longer use. The calculation method is as follows: the transmission current of 4w-5w walkie talkie is 1400ma, the receiving current is 300mA, and the standby current is 10mA. Press 40 minutes to talk and use 933ma, 200mA for 40 minutes, and 67ma for 6.7 hours standby. This calculates about eight hours of work. If you use 2000mA battery, you can double use it. This is the radio instruction manual marked the sending, receiving, standby ratio 5:5:90 can be used for 8 hours. That is, the 5-5-90 working cycle as described by interphone dealers.
Secondly, frequency is the problem. One frequency point can be used for simplex calls, and two frequency points are required for duplex calls. In this way, the frequency resources are wasted first, and then a duplexer is installed as the receiving and receiving device. This duplexer must meet the bandwidth, and occupy the space of the interphone itself, and the volume of the interphone will increase accordingly. So there is the emergence of relay station, to make up for the lack of walkie talkie, to increase the communication distance of the interphone.