Retrofitting QYT 8900 Vehicles Radio

Due to I not often driving car, I can only use a handheld radio for connect communication. However, the power of the device is relatively small, even the Quansheng UV2PLUS is only 10W, making it difficult to connect to the relay when running a little farther. After searching online for a long time, I found a device that is actually 25W, but without a keyboard, I cannot set the frequency manually.
Do it yourself, get plenty of clothes and food! Thinking of the many mini car radios available now, I had the idea of transforming mini car radios into handheld radios. Through consultation, several mini car models were recommended, and ultimately a QYT8900 car model was found. The size was very small, and the width only 98mm, making it very suitable for changing to a handheld radio. Let’s take a look at the finished product first !

The overall plan is to connect a fixed battery box below the vehicle radio, install a battery pack inside, add a microphone and PTT key to the vehicle radio body, change the antenna interface to the upper end, and pretend to have corresponding mechanical switches and charging sockets.
The battery pack ultimately selected 18650 three strings with a capacity of 2.6AH. Based on the working current calculation, the standby time is about 10 hours and the continuous call time is about 30 minutes. Due to the non continuous call during connection, according to experience, the proportion of call time during connection is estimated to be about 40%. Therefore, the actual sustainable connection time is about 1 hour, which is enough for one day’s use.
The original antenna interface was removed and a bracket was made. The antenna interface was connected to the upper end using a 50-5 feeder and a pipe head, and two 90 degree adapters were connected in series before connecting to the antenna. This allows the antenna to be adjusted to a 90 degree vertical state at any time, and the car radio can be used flat when connecting to the microphone.
The transformation process is relatively simple. By studying the connection between the PTT key and the microphone in the hand microphone, a small switch is fixed inside the car radio as the speaking key. The microphone transformation is a bit complicated because the bias current of the electret microphone in the car radio hand microphone is provided by the internal circuit of the hand microphone. Therefore, a stable power supply needs to be found inside the car radio, and a resistor is connected to provide the bias current for the electret microphone, Then, it is connected to the microphone circuit through a coupling capacitor. Finally, a+5V power supply connected to a 7.5K resistor inside the vehicle radio is used to provide bias current for the microphone. A 10U ceramic layer capacitor is used as the audio coupling capacitor, and the microphone connection must be short. Shielding measures are taken to reduce RF interference. I have installed a magnetic ring to eliminate RF interference, otherwise the volume of the conversation will be very low.
Furthermore, considering that my communication frequency is not too high and the heat dissipation conditions for handheld use are relatively good, the original fan was simply cancelled. The battery box used an aluminum finished shell with a width of 65MM, a thickness of 35mm, and a length of 100MM. A 4mm thick aluminum connection plate was used to connect to the vehicle radio. The connection plate was fixed using the screw holes of the fan, and the antenna bracket was fixed using the screw holes of the shell, and the thread was expanded to a diameter of 3MM to improve the connection strength.
After renovation, the final product weighs about 1.1KG and is made of all aluminum structure, which is very sturdy and has a certain level of attack power. It can be fully used for armed struggles on a certain section of China’s border. After completion, the overall dimensions of 98MM * 35MM (excluding antenna bracket) * 230MM (excluding antenna) and 760MM (including antenna) are very pleasing in volume. After trial, the connectivity effect far exceeds that of a handheld device, with a total distance of 59+. It is relatively convenient to carry and does not rely on the power supply in the car. If connected for a long time, you can also plug in a charger and use it while charging.
Two 90 degree antenna interfaces

The power switch is installed on the battery box. Although the original machine has a power switch, there is still a standby current of more than ten milliamperes after shutting down, so we pretended to have a mechanical switch.

Antenna bracket and connecting feeder
Turn the antenna to a 90 degree vertical position, which can be laid flat for easy connection to the microphone

Compared to Regular handheld radios

Considering gain and portability, a 50cm soft antenna was ultimately used as a compromise.
