Design, manufacture and debugging of 30.275MHz FM wireless intercom

I. Main technical indicators:
1. Frequency: 30.275MHz
2. Modulation mode: FM
3 Frequency deviation: 5KHz
5. Communication mode: Same frequency simplex
6. Power supply voltage: 9.6V 10% (8 nickel-cadmium rechargeable batteries, negative pole grounded. Some models have 6 batteries)
7. Current consumption:
Squelch standby: less than 10mA
Receiving: less than 150mA
Short-range transmission:
Long-range transmission: less than 0.7A
8. Carrier frequency output power: 2w
9. Receiving sensitivity: less than 1.0uV (signal-to-noise ratio above 12dB)
1 0. Squelch sensitivity: 0.5uV
11. Intermediate frequency: 455 KHz
12. Audio undistorted power: more than 200 nlw
1 3. Volume: 125 x 55 x 30 mm
14. Weight:

2. Working Principle
The whole machine consists of two parts, the receiving part and the transmitting part. Except for the antenna and the impedance matching circuit, the other circuits of the two parts are independent of each other.

1. Receiver
The high-frequency radio signal received by the antenna is filtered out of the interference signal outside the frequency band by the low-pass filter composed of L1, L2, c1, c2, and c4, and sent to D1, D2 and L3 to form a frequency selection circuit through c6. The resonant frequency of this frequency selection circuit is 30.275MHz, which selects the carrier signal sent by the walkie-talkie and filters out other interference waves. The high-frequency signal is sent to the cascade high-frequency signal amplifier circuit composed of N1 and N2 through c7 for high-frequency amplification. This cascade high-frequency signal amplifier circuit has the advantages of high gain, stable operation, and no need to use neutralizing capacitors. N1 forms a common-emitter circuit, and N2 is connected to a common-base circuit. The common-emitter circuit has the advantage of high gain, and the common-base circuit has the characteristic of stable operation. The high-frequency signal after amplification by N1 and N2 is selected again by a double-tuned circuit composed of L4, c9, T1, and c12, and then sent to the 16-pin internal mixing stage of ICl (MC3361) through c16 for mixing.
N3, CRY1, L5 and other components form the local oscillator. L5 and the corresponding loop capacitor resonate at the third harmonic of 10.243MHz, that is, 10.24333x3=30.730MHz, which is higher than the transmission frequency 30.275MHz (the triple frequency of 10.0917, that is, 10.0917MHzx3=30.275MHz) by an intermediate frequency of 455kHz (that is, 30.730-30.275=0.455MHz). The local oscillator signal is also sent to the first foot of Icl and mixed inside Icl.
Ic1 (Mc3361) is a narrowband FM receiver dedicated integrated circuit, which contains an oscillator, a mixer, a high-gain limiting intermediate frequency amplifier, a frequency detector and an active filter, a noise trigger circuit and an audio amplifier circuit. Its limiting sensitivity is 2uV. It is the main gain stage of the whole machine, and the intermediate gain can reach 65dB.
The 455kHz intermediate frequency signal obtained by mixing inside Ic1 is output from pin 3 of Icl. The intermediate frequency signal is selected by the ceramic filter cRF1, and other harmonic components are filtered out. The selected intermediate frequency signal is input from pin 5 of Icl, and high-gain intermediate frequency amplification is performed inside Icl. Finally, the audio signal is demodulated by the frequency detector and output from pin 9 of Icl.
One of the signals output from pin 9 is de-emphasized and filtered by the de-emphasis circuit composed of c30, R13 and c32, and then sent to Ic2 for audio power amplifier through potentiometer VR1 to drive the speaker to sound. The other signal is sent to the active filter inside Icl by potentiometer VR2 for frequency selection amplification and then output from pin 11 of Icl. It is detected by D3 and D4 to control the internal noise trigger circuit, and a control level is output at pin 13 to control the conduction and cutoff of N4 and N5, so that the power supply of IC2 is controlled to achieve the purpose of noise suppression. We know that the sensitivity of FM receiver is very high. When no signal is received, the speaker will emit strong noise. Once a signal is received, its signal-to-noise ratio is very high. The main spectrum of noise is distributed between 10-25kHz, and the spectrum of audio signal is between 100-3000Hz. We can use a special filter to select this noise signal, and convert it into a DC component through detection. Then, through an electronic switch circuit, we can control a circuit to achieve the purpose of noise suppression. In this way, when the receiver does not receive a signal, the speaker will be silent to eliminate the annoying noise. Once a signal from the intercom is received, it can automatically open the amplification circuit for communication. At the same time, setting a noise suppression circuit can also achieve the purpose of power saving.
N11 forms a regulated power supply. The regulated output depends on the value of Dzl. Dzl is selected to be 6.2V, and the regulated output is about 5.6V. N11 is also a switch transistor for transceiver conversion, and N9 is the power switch tube of the transmitter part. When the sw_PTT switch is pressed, D6 is turned on, N11 is turned off, the receiver loses voltage and stops working, N9 is turned on due to bias, and the power supply is supplied to Ic3 through N9, and the transmitter front stage gets power and starts working. Therefore, this transceiver conversion circuit is also called an electronic PTT switch, which is a new circuit not found in other amateur walkie-talkies. Its advantage is that a micro switch can be used to control large currents, making the circuit work more reliably. Although N7 and N6 of the transmitter stage are also connected to the public power supply circuit, they are turned off because they cannot get base excitation in the standby state, so the transmitter part does not work when the walkie-talkie is in standby.

2. Transmitter The
transmitter part consists of a voice amplifier, a main vibration stage, a buffer amplifier stage, a push stage, and a final power amplifier stage.
The voice signal is amplified by a two-stage audio amplifier composed of N13 and N14, and the high-frequency component is filtered out by a high-frequency filter composed of c74, c71, c70, and L13 to prevent the high-frequency signal of the oscillator from interfering with the work of the voice amplifier stage. At the same time, the voice signal is pre-emphasized and sent to the variable capacitance diode Dc through c70 to achieve frequency modulation.
The main oscillation stage is composed of N15, cRY2 and peripheral components. Its oscillation frequency mainly depends on the working frequency of cRY2. In this circuit, cRY2 selects 10.0917MHz (because 10.0917x3=30.275MHz), and its triple frequency signal is selected by T5 and C64 frequency selection circuit (that is, the transmission frequency is 30.275MHz), and is coupled to the buffer amplifier stage by T5.
The carrier signal is amplified by the buffer amplifier composed of N10. T4 and the tank capacitor c61 also resonate at the third frequency (i.e., the transmitting frequency 30.275MHz) to filter out other harmonic components. N7 is the driving amplifier stage, which provides sufficient driving current for the power amplifier stage. After c55, c51, L8, the frequency is selected and matched, it is coupled to the final power amplifier stage N6 for power amplification. N7 and N6 both work in the Class C amplification state, and their operating points depend on R23 and R21 respectively. Since the second harmonic component output by the Class C amplifier is very large, the fundamental component must be selected by the Lc frequency selection circuit. The driving circuit is selected by c55, c51, and L8, and the power amplifier circuit is selected by a series resonant circuit composed of C48, C47, and L6. Finally, L1, L2, C1, C2, and C4 form a low-pass filter to select the carrier signal and perform impedance matching. The carrier current is converted into electromagnetic waves by the antenna, the transducer element, and radiated into the air.
The function of the branch of R7 and D3 in the circuit diagram is that at the moment of the receiving and transmitting conversion, due to the energy storage effect of the receiving part capacitor, the receiver is not immediately shut down, and the 13th foot of Icl fails to change from a high level to a low level immediately, and the work of Ic2 fails to stop immediately. In this way, at the moment of the receiving and transmitting conversion, a short receiving and transmitting noise will be emitted from the speaker, which makes people feel extremely uncomfortable. Therefore, when the power is switched to the transmitting circuit, this branch is added to the 12th foot of Icl through R7 and D3, so that the 13th foot of Icl immediately becomes a low level, N4 and N5 are cut off, and Ic2 stops working to eliminate the conversion noise.

3. Manufacturing process and component selection
The success or failure of walkie-talkie production depends not only on human factors such as theory, experience, accurate working frequency and correct debugging methods, but also on the quality of key components. If one of the components is of poor quality, you may not succeed even after several sleepless nights of struggle. According to the author's experience in making more than ten walkie-talkies, the sensitivity of the receiver is most closely related to N1 and N2. In addition to their high-frequency characteristics, another important parameter of N1 and N2 is their noise coefficient. The noise coefficient of ordinary cheap high-frequency tubes such as s9018 is large, making it difficult to achieve the expected sensitivity. In addition to
the N1 and N2 high-frequency triodes, the CRFl ceramic filter also has a great impact on the sensitivity of the whole machine. Genuine components should be selected, preferably a five-terminal ceramic filter, because its frequency selection characteristics are better than those of a three-terminal filter. High-frequency ceramic capacitors should be selected with low leakage and good thermal stability.
In addition to the components mentioned above, other components can be selected from ordinary components, which is completely acceptable under amateur conditions. According to the author's experience, those non-main components have a very slight impact on the sensitivity of the receiving signal.
Because ICl is a dedicated narrowband FM receiving chip, its performance is generally guaranteed. The best quality is the product of MOTOROLA, as shown in the figure. The second is the product produced by MALAYSIA, which is also good. It is worth mentioning that the author got several MC3361 chips made in China. Through comparative experiments using German signal generators (frequency is accurate to 10Hz in the 50MHz range. The output resolution can reach 0.01uv) and other instruments, the sensitivity of domestic products is basically the same as that of MOTOROLA products. Therefore, there is no need to worry about the performance parameters of ICl.

The resistor can be a general carbon film resistor, and there is no special requirement for accuracy. 1/8w and 1/16W are both acceptable.
Of course, the walkie-talkie hopes that the volume is as small as possible, and amateur production is no exception, so the components should be ultra-small components as much as possible.
The components of the transmitting part are also the key part of the success or failure of the production. Among them, the transistors of the driving stage and the power stage have the greatest impact. The author has tested several types of tubes, all of which are 2sc2078, but they are produced in different places. Earlier, I used a general tube (from the appearance, the silk screen is not very clear, and the workmanship is also poor). With a 9.6V power supply, excluding other factors, the power can not be adjusted to 2W anyway, and the emission current is only a little more than 400mA. I thought that the frequency was not adjusted to the third harmonic of 10.0917, and it might be the fourth or fifth harmonic. After checking with an oscilloscope and a frequency meter, it was inferred that the quality of the power tube was poor. I replaced it with a 2sc2078 produced by Mitsubishi. When the power was turned on, the current soared to nearly 0.8A, and the power meter measured it as 2.6w. This shows that the quality of the power tube at the end directly affects the power output, which will obviously affect the call distance of the walkie-talkie.
In addition, should the high-frequency power tube be selected as high as possible? For example, some enthusiasts are happy to use tubes such as 2SC1971, 2SC1972, etc., which are up to 175Mt{z in the VHF band. The author does not recommend the use of power triodes with too high a frequency. The first reason is that such tubes are quite expensive. The second is that the frequency is too high, the circuit is prone to self-excitation and difficult to debug. The author encountered such a problem. In a desktop computer, 2sc1969 was used. The circuit worked quite normally and the power could reach about 8w. It was used for a long time. Later, it was changed to 2sc1971, but the circuit was seriously self-excited. It took a lot of effort and many measures to solve it. The actual output power is basically the same as that of 2SC1969.
In addition to transistors, the frequency of the quartz crystal must be selected accurately. The frequency deviation will obviously affect the call distance. We will explain the debugging part. The number of turns of the high-frequency part of the coil has been marked in the circuit diagram. It can be wound with φ0.17-0.35mm enameled wire on the high-frequency core or the intermediate frequency core. The wire diameter of LI and L2 needs to be larger because they are also the transmission circuit of the carrier frequency power. The intermediate frequency discrimination coil can be replaced by a ready-made 455kHz (or 465kHz) intermediate frequency.
There are no special requirements for other components. The selection of resistors and capacitors is the same as that of the receiving part.
4. Assembly and debugging
The assembly method of the walkie-talkie is similar to that of the general radio. It should be noted that the pins of the components should be as short as possible so that they are close to the printed board, and the connection to the antenna base should also be as short as possible, otherwise the output power will also drop significantly. If the output end is far away from the antenna base, generally speaking, if it is greater than 1Omm, a 50 coaxial cable should be used for connection.
The layout of the components and the routing of the PCB are the key to amateur production. If you go around randomly, many unforeseen problems will arise during debugging, such as crosstalk, coupling, and self-excitation between stages, and these problems may not be solved by other methods. In the end, you have to go through the PCB again, reassemble, re-debug, and take many detours. The PCB routing principle of the high-frequency board is roughly as follows: each stage needs to be placed as much as possible (one area), and the circuit working at the same time, the high-power output stage should be as far away from the small signal circuit as possible. A large ground wire encirclement structure should be used between the stages, and the iron shielding cover such as the mid-circuit should be cleverly used to isolate the inter-stage circuit. If possible, the small signal (especially the receiving part) also needs to be arranged with a shielding cover and reliably grounded. One thing is very important. The intermediate frequency resonant coil T2 must be close to the 8th pin of Icl. If the wiring is too long, the sensitivity will be very low and may cause other problems such as self-excitation. For the same reason, the intermediate frequency filter CRF1 must also be close to IC1. This point must not be ignored when laying out the PCB.
The layout of PCB components should not simply pursue beauty and neatness, but should be based on the principle of reliable operation. Of course, it can not only ensure the reliable and stable operation of the circuit but also make the layout of components beautiful and beautiful. It is a master!
The following figure is the PCB wiring diagram of one of the author's walkie-talkies. Of course, it is a work that has been modified and improved N times. If you are interested, you may wish to take a look. It is for reference only. I believe it is still helpful for beginners.

After the assembly of the whole machine is completed, it should be carefully checked before it can be turned on for debugging. The DC operating points of these four circuits do not need to be adjusted. Their working conditions have been fully guaranteed during the design. You can check the voltage points once and the difference is not much. The debugging of wireless walkie-talkies generally requires the help of instruments to ensure their performance indicators. The instruments required for adjustment are generally the following:
1. 1. High frequency signal generator (such as xFG-6)
2. Oscilloscope (such as VP5204 40MHz)
3. Digital frequency meter (such as cFc-8450, 0-1000MHz)
4. Power meter (such as Gz-3)
5. DC regulated power supply (such as wYJ-30V/5A)
6. Multimeter (such as MF-47, if there is a digital meter Fluke-87 and other high-end instruments with the best)
7. Field strength meter (can be made by yourself)
Of course, there is also a sweeper. Complete instruments will bring great convenience to debugging, and at the same time can guarantee the performance indicators of the whole machine.
For general amateurs, they generally do not have the above instruments, but the frequency meter is indispensable. Here we will combine the characteristics of amateurs and professionals
to introduce the debugging method of FM wireless walkie-talkies in detail.

1. Debugging of the transmitter
The debugging order is generally: first adjust the oscillation stage, the frequency multiplication stage, the driving stage, and the final power amplifier stage. Finally, adjust the voice amplifier circuit.
Connect an ammeter (3A range) in series in the main power supply circuit, turn on the machine, and press the transmitter switch. If the current value is greater than 1.5A at this time, it means that there is a short circuit in the whole machine. The machine should be turned on again after the fault is eliminated. Although the machine can work safely under 13.6V power supply, do not use too high power supply voltage at the beginning to prevent the final power tube from burning when the circuit is detuned. Generally, 8.6V power supply voltage can complete the debugging. Note that the final power tube needs to be equipped with a large enough heat sink.
First, determine whether the main oscillation stage is oscillating. The method is to use a multimeter to measure the emitter voltage of N15, which is normally about 2V. If it is oscillating, the voltage should be as high as the base voltage, or even higher than the base voltage. This is an obvious feature of the oscillation circuit oscillation. Amateur production does not have an oscilloscope, so this principle must be mastered. It is very useful for debugging. If you have an oscilloscope, you can use it to observe the collector of N15, and you will observe the waveform shown in Figure 3. If the waveform amplitude is too small, you can adjust T5, and generally you can adjust the waveform. If you can't observe the waveform, it means that there is a fault in the circuit (generally T5 is poorly wound). You should find out the cause and eliminate the fault. When the circuit does not oscillate, it will work in a linear amplification state, and the emitter voltage will be about 0.65v lower than the base voltage.
After confirming that the circuit oscillates, you can use the frequency meter probe to connect to the collector of N15 to measure the oscillation frequency. Normally it should be 30.275MHz (or 10.917MHz, depending on the adjustment of T5 and the connection of the frequency meter. The measured frequency is 10.917MHz because the fundamental frequency of the crystal is measured, and the measured value of 30.275MHz is the third harmonic). If there is an error, you should adjust the capacity of C69 until it meets the requirements. You can also change the resistance values ​​of R32 and R33 to make adjustments in advance, but you cannot change their resistance values ​​too much, and you can only make small adjustments. The carrier frequency error is required to be no more than 1.5kHz. At this time, an oscilloscope can be used to observe the waveform of the secondary of T5. Adjust the core of T5 to make the waveform of the third harmonic clear, without burrs, and with the largest amplitude. However, the waveform must be stable as the principle. The switching power supply can start normally several times. Then observe the collector of N7 and adjust the core of T4 to make the waveform the best, with the largest amplitude, close to the sine wave, as shown in Figure 4. Connect a 50Ω dummy load to the antenna end. You can connect a low-voltage, low-power small lamp in series and observe the brightness of the lamp on the dummy load slightly greater than 50Ω to judge the output power. Adjust L8, L6, L1, and L2 respectively to maximize the output power, the sine wave amplitude, and a good waveform, as shown in Figure 5. When speaking into the microphone, the waveform does not change. At this time, the current value is about 0.75A. If there is no oscilloscope, a homemade
field strength meter (refer to Figure 6) can be used to monitor next to the antenna. Adjust L8, L6, L1, and L2 so that the field strength meter placed next to the antenna indicates the maximum.

One thing to note is that when adjusting at the resonant frequency, the current indication is the minimum. However, when adjusting T5 and T4, if the resonant point is deviated, the amplitude will decrease and the current will also decrease. This must be treated differently during adjustment. Generally, when adjusting L8 and L6, the larger the current, the better and the greater the power. When adjusting L1 and L2, the current should be appropriate. After repeated debugging, ensure that the frequency is accurate and the output power is maximum.
Sometimes, during debugging, the power cannot be adjusted, and the current value cannot reach 0.75A. At this time, the quality of the power tube and the driving tube used should be considered. Generally, a good quality authentic triode should be selected. The quality of the final power tube directly affects the power output.
When debugging, the circuit should be stable. Do not blindly pursue high power output and ignore the stability factor. At the same time, the power supply voltage should be reduced to 7V or increased to 12V, and the circuit should be able to work reliably and stably.
Debugging of voice processing circuit: In fact, as long as the components are reliable at this level, the circuit does not need to be debugged. If you want to check, you can send a 1kHz/50mV audio signal to the microphone input end, and measure it with a millivoltmeter or oscilloscope at the collector of N14. There should be an audio voltage of about 2Vp-p.

2. Debugging of receiver
Under amateur conditions, the final total adjustment of the receiver can be used as a signal source for joint debugging. Under amateur conditions, there is no frequency meter, and the signal of the transmitter is used for adjustment. If the transmission frequency differs by a few kHz, the receiver will also deviate by a few kHz, which has little impact on the production. Amateur production is completely possible, but the performance indicators are slightly lower. Generally, distance debugging does not require too much power. The signal source radiation is too strong, and it is difficult for the receiver to adjust to the best point, that is, it is not easy to adjust to the resonance point. It is appropriate to just receive the signal of the transmitter with a little noise.
First, use a multimeter to measure the emitter voltage of N11. Normally, it should be 5.6V. To be on the safe side, you can measure the working voltage of each point of the circuit. The value can be found in the circuit diagram. Generally, it should be close to the value in the circuit diagram, otherwise it means that there is still a problem in the circuit. In this way, you can have a general understanding of the working state of the circuit.
First, determine whether the local oscillation level is oscillating. The method is similar to adjusting the main oscillation level of the transmitter. Use a frequency meter to measure the collector of N3. The frequency should be 30.730MHz. If there is an error, a capacitor of several P to 20P can be connected at both ends of CRY1 to make the frequency meet the requirements. Similarly, the frequency error should not exceed 1_5KHz. If there is an oscilloscope, you can use it to observe the waveform of the secondary of T3. Adjust L5 and T3 to make the waveform the best and the amplitude the largest, reaching 80mV to 100mVp-p.
Close the noise potentiometer to make the speaker have noise, adjust the magnetic cap of T2 to make the noise in the speaker maximum, and use an oscilloscope to observe the waveforms at both ends of the speaker. Adjust T2 to make the noise waveform (the waveform should be chaotic at this time) have the maximum amplitude and symmetrical waveform.
Set the signal generator to: frequency of 30.275MHz, frequency deviation of 5kHz, modulation frequency of 1kHz, input this signal at the antenna end, gradually increase the signal level, generally more than ten uV can hear the audio sound in the speaker, adjust T2, T1, L4, L3 to make the sound maximum, gradually reduce the signal level, and then adjust the above adjustable components, if necessary, adjust L5, T3, make the audio sound maximum, the sound quality is best, generally the receiving sensitivity can be adjusted to 1.0uV, of course, this does not mean that there is no noise at all, it refers to the receiving sensitivity at a signal-to-noise ratio of 12dB.
Remove the signal source, slowly adjust the squelch potentiometer to make the noise just disappear, reduce the signal source level to 0.5uV, reconnect the signal to the antenna, the squelch gate should be opened, and the receiver should be able to receive the signal.
If the above sensitivity can be adjusted, the receiver is basically adjusted. Exchange the two machines, adjust their transmitting and receiving parts, and then conduct joint debugging.

3. Joint debugging
Put the debugged whole machine into the case, connect the battery, connect the antenna, LED and microphone. It should be stated here that the antenna plays a decisive role in the call distance. It is difficult to guarantee the performance of amateur homemade antennas without adjusting the transmitter and receiver. If conditions permit, it is best to use finished antennas first, and they must be in the 30MHz frequency band, otherwise the call distance will be difficult to meet the design requirements. Use one as a transmitter (the transmitter should remove the unstage power stage to reduce the RF power) and the other as a receiver. Increase the distance between the two machines until the signal is just received. Fine-tune the receiver's T2, T1, L4, L3, L2, and L1 to make the received signal strongest and the sound quality best. Then increase the distance between the two machines and adjust again until the communication distance between the two machines is the farthest. It should be noted that only fine-tuning can be done here, because after the above adjustments, the frequency is generally adjusted more accurately. If you make larger adjustments, sometimes the sound will only become more chaotic.
Connect the final power tube of the transmitter and the local antenna. Generally, you only need to fine-tune L1 and L2 to make the field strength meter next to the transmitter indicate the maximum. If the matching network is not adjusted properly, the transmission power will be seriously affected. That is, the transmitted electric power is large enough, but the electromagnetic wave energy radiated from the antenna is much smaller, because the carrier current is not fully transmitted to the antenna, but a part of the carrier current returns to the power amplifier stage in the form of standing waves. Obviously, the efficiency of such a circuit will be greatly reduced, and the call distance will be greatly shortened.
The simplest and most reliable method is to use a field strength meter to monitor next to the walkie-talkie. The debugging of the transmitter is based on the maximum indication of the field strength meter. Based on my own years of practical experience, do not underestimate the homemade signal field strength meter. It is very useful in the entire production process. If you think that the instrument must be a finished product, or imported high-end instruments can be used to debug the equipment, this is a common misunderstanding. You must fully believe in the simple but very realistic instruments you make. Relying on them, you can also make and debug a walkie-talkie with excellent performance. This does not violate the original intention of the hobby.

During joint debugging, the power of the transmitter should be as small as possible so that the receiver can be adjusted more accurately, and the distance debugging is gradually increased. Generally, after the transmitter is adjusted, it is not advisable to adjust the transmitter coil randomly, otherwise, the state of the transmitter may be adjusted randomly. When adjusting L1 and L2, sometimes there will be contradictions between the transmitter and the receiver. At this time, it is better to take care of the receiver.
The above debugging methods are for reference only. You may encounter many problems in actual debugging. You should constantly summarize in practice and make debugging records. Amateurs have a common problem. They may have made a lot of their own works, but in the end they have nothing in writing. If there is a problem, they don’t know how to find the cause, or there is no basis to check. Because there is no record, it is strongly recommended that amateurs develop a habit of taking notes (recording). The first is to record achievements, and the second is to facilitate their own tracing and analysis once there is a problem. As long as you carefully analyze the working principle of the circuit and master the debugging essentials, you will be able to make a wireless walkie-talkie with good performance.
4. About the call distance
Wireless walkie-talkies work in the ultra-short wave band, with high frequency and poor diffraction ability. They mainly rely on direct waves, and the propagation distance is mainly within the line of sight. Since the surface of the earth is a curved surface, assuming that there are two people with a height of 1-8 meters in a flat area of ​​the plain, then the line of sight distance at which they can see each other's heads is about 10-12 kilometers (we refer to the line of sight distance, not the real visible distance with the naked eye, similar to looking straight ahead with the help of a telescope). Since the wavelength of radio waves is very short, the wavelength of 30MHz radio waves is 10 meters. Small buildings on the ground have a significant impact on radio waves. For example, in media such as mountains, trees, buildings, and high-voltage transmission lines, electromagnetic waves must lose energy to induce currents in them. In addition, there are different degrees of interference radio waves in the air, which will reduce the call distance.
In this case, if they use a 30MHz frequency band walkie-talkie with a 0.15m normal mode spiral antenna, it is difficult to successfully communicate at a distance of 10-12 kilometers. Even within the line of sight, it is not possible to communicate clearly. The
actual call distance is as follows:
1. In cities, where there are many buildings, the electromagnetic environment is heavily polluted, and the radio wave interference is strong, the call distance can only be about 1 kilometer. Using a telescopic antenna, it can only be about 2 kilometers.
2. If you go up to the 3rd or 4th floor and use a telescopic antenna, or in the flat area mentioned above, use a telescopic antenna, the call distance can reach 3-5 kilometers.
3. One or both parties holding the walkie-talkie stand at a height of 10-20 meters, or use an outdoor antenna with a 50Q coaxial feeder, and there is no mountain blocking, which is equivalent to increasing the line of sight distance, and the call distance can reach 5-10 kilometers.
4. Therefore, it is worth exploring how users choose the call location, or one party uses an outdoor antenna to give full play to the effectiveness of the equipment in their hands and increase the call distance.
5. As far as walkie-talkies are concerned, the sensitivity of the receiver has a significant impact on the call distance, while increasing the transmission power does not increase the call distance significantly. When the transmission power increases by 10 times, the call distance may sometimes only double.
6. In addition, electromagnetic waves in the 30MHz frequency band can sometimes be reflected by the ionosphere. Therefore, at night or at the turn of spring and summer, it is not surprising that walkie-talkie signals from dozens or even hundreds of kilometers away can be received, but the signal will not be very stable.
7. In cities, various industrial electromagnetic interferences are quite serious, so 30MHZ frequency band walkie-talkies are more suitable for use in rural areas with less interference. Urban enthusiasts who have the conditions can also go to the suburbs for experiments, and the effect will be better.
8. For the convenience of carrying, walkie-talkies are generally equipped with local antennas (i.e. normal mode spiral antennas), which are short in length and low in gain. The call distance is generally not far. In order to achieve long-distance communication, conditions should be met to equip them with 0.5m-1.0m bottom-loaded pull-rod antennas. 0.5m-1.0m bottom-loaded pull-rod antennas can be made by yourself. The method can be found in the following information.

V. Homemade 30MHz walkie-talkie antenna
1. Normal mode helical antenna
The normal mode helical antenna used in walkie-talkies is generally short, with a length of only 12-16cm. It is made of an insulated frame (which can be a hollow structure with a diameter of about 11mm, such as a polypropylene plastic rod, etc.) with mutually insulated metal wires (such as enameled wire) evenly wound on the frame and then an antenna seat is added. If the length is 16cm, the number of turns of the coil is about 120T. If the length is 12cm, the number of turns of the coil is about 155T. The specific debugging method is:
connect the wound antenna (about ten more turns) to the walkie-talkie, put a field strength meter next to it, press the transmitting switch, start from the top of the antenna, and shorten the coil one circle at a time until the field strength meter indicates the maximum, which is the resonance state of the antenna. At this time, make some fine adjustments to the tightness of the antenna so that the field strength meter indicates the maximum, that is, wrap the antenna firmly with insulating tape.
There is an obvious characteristic when the antenna resonates. When a person's hand is close to the antenna and almost touches it, the wire on the top of the antenna will radiate strong sparks (if the power of the walkie-talkie is large enough, this phenomenon can be felt at about 3W), which can be clearly felt by hand. If the power of the walkie-talkie reaches 3W to 5W, the hand will have an obvious burning sensation. However, the power of the walkie-talkie is too large, and it is not recommended to use this method for testing, because it may cause harm to the human body! This must be done with caution in the experiment!

2. Bottom-loaded inductive
antenna The method of making the bottom-loaded inductive antenna is to put a hollow insulating skeleton on the antenna base, with a length of about 50mm and a diameter of 10mm, and then put a 0.5m long pull rod antenna on the top of the insulating skeleton. The skeleton is wound with 34T with a magnetic sheath of 0.5-0.7mm. The pull rod antenna is extended to determine the increase or decrease in the number of coils until the field strength meter indicates the maximum when the antenna is fully pulled out, and the coil is fixed.
The gain of the bottom pull rod antenna is relatively low when it is not fully extended. This is because the antenna is not matched at this time. However, when the antenna is fully extended, the antenna is in a resonant state, and the gain is higher than that of the normal mode spiral antenna, so the communication distance is also longer.

3. Middle loading antenna
Now there is also a middle loading antenna. The principle and production method are similar to the bottom loading antenna, except that the loading coil is placed in the middle of the antenna. Interested friends can also try it.

4. 1/4 wavelength Brown antenna (outdoor antenna)
There are many types of outdoor antennas. The 1/4 input antenna is relatively easy to make and has a higher gain. If it is well made, the gain can reach 3-5dB. The production method is: take 4 aluminum tubes with a length of 2.5m. If there is no aluminum tube, ordinary wires can be used instead, but the effect is slightly worse. One of them is placed vertically as the main oscillator, and the other three are evenly distributed at 120 degrees to each other and 60 degrees to the vertical direction. As radiating oscillators, the main oscillator and the radiating oscillator need to be insulated. The core wire of the 50-ohm coaxial cable is connected to the main oscillator, and the ground network is connected to the radiating oscillator. The height is 15m. Since the gain of the outdoor antenna is high and there are no obstacles, the call distance of the walkie-talkie will be greatly increased when equipped with an outdoor antenna. The antenna of the handheld unit is shorter, but the power of the desktop unit is larger, generally reaching 10w_15w, which can make up for the disadvantage of the low gain of the handheld unit. The power of the handheld unit is generally smaller, but the desktop unit uses an outdoor antenna with a higher gain, which can also make up for this deficiency. Therefore, when the desktop unit and the handheld unit are networked, the call quality of both parties is basically the same.

VI. About the secondary frequency conversion technology
[1]. These circuits are all single-channel walkie-talkies. The working principles of secondary frequency conversion walkie-talkies are also basically similar. For example, F36 is a secondary frequency conversion walkie-talkie circuit. Enthusiasts can easily see the similarities and differences in the circuits. The secondary frequency conversion circuit only adds an intermediate frequency amplifier circuit and a second local oscillator stage on the basis of the primary frequency conversion circuit. The other circuits are similar. The secondary
frequency conversion is designed to make the gain and sensitivity of the walkie-talkie higher and the operation more stable. We know that the gain of a single-stage amplifier circuit cannot be increased infinitely by increasing the number of amplifier stages. Because the gain of a single-stage circuit is too high, it will inevitably cause crosstalk and self-excitation problems between stages, and the operation will become unstable or even unable to work normally. Secondary frequency conversion can better solve this problem, because the amplifier circuit is divided into multiple stages, and different stages amplify signals of different frequencies. The interference between stages can be overcome by means of notching, filtering, etc., so that each stage circuit can work stably and reliably, and the total gain can be made very high. For example, in the short wave band (or VHF) 30-150MHz, the receiving circuit first performs high-frequency amplification, with the gain set to about 10-20db, and then frequency-converts to the first intermediate frequency of 10.7MHz, and then performs a first intermediate frequency amplification with a gain of about 10-15dB, and then performs secondary frequency conversion by the second frequency conversion stage to generate a 455kHz second intermediate frequency signal, and then performs high-gain intermediate amplification of about 65dB, so that the gain of the entire circuit can be made very high. The sensitivity of the intercom of the secondary frequency conversion circuit can easily reach 0.2-0.5uV. Such a high receiving sensitivity is the key to improving the call distance of the intercom.
It is worth mentioning that the secondary frequency conversion sensitivity is as high as 0.2uV, and even some professional machines (such as TK208, TK308, TK378) can reach a sensitivity of 0.16uV. With such a high sensitivity, the noise coefficient of the high-power amplifier tube is very important. Generally, crystal triodes are not used, but dual-gate field effect tubes with small noise coefficient and high impedance are used for high-frequency amplification, which can be seen in the C36 circuit diagram.
[2] The four circuit diagrams that I asked my friends to download this time are all circuits designed by the author after years of reference to other professional and amateur walkie-talkies and various radio receiving equipment, and comprehensive performance and amateur homemade factors. According to the author's experience, circuits that are too "amateur" cannot guarantee performance, the call distance is very short, and enthusiasts cannot be addicted to playing. Even if they are made, there is not much sense of accomplishment. As the saying goes, it is not fun. This is one reason. Second, if the circuit is too professional, it requires too many components, the circuit is too complicated, and the components are difficult to purchase. The equipment required may not be affordable for amateur enthusiasts. The assembly and debugging are complicated. If you have the enthusiasm to do it but do not have the material conditions to do it yourself, you will not be able to play it.
[3] For multi-channel walkie-talkies, low-end circuits use multiple groups of crystals to form multiple channels, and use mechanical or electronic switches to switch. More advanced circuits use phase-locked loops and frequency synthesis technology to achieve this, such as Mcl45152 and uPB569C and other dedicated ICs. More advanced machines use microprocessors to achieve frequency synthesis and switching, such as the microprocessor uPD7514G of the first-generation famous machine c450. The use of these circuits naturally makes the machine performance much better, but it requires amateurs to master digital circuits and even microprocessor programming and development skills in addition to mastering radio technology. This is not something that most amateurs can do.
[4] The circuits of desktop machines and handheld machines are basically the same, except that a power amplifier stage is added at the end stage. For the 30MHz frequency band, 25C1969 (TO-220) or 25C1945 transistors can be used. If debugged well, the output power can reach 10-15w. Of course, L1 and L2 need to be wound with larger wire diameters.
[5] The intermediate frequency amplifier circuits of these four circuits all use the narrowband FM dedicated receiving integrated circuit Mc3361. In fact, there are many circuits that can realize the intermediate frequency amplifier function, such as TK10487M. There are more than ten products of Motorola alone, such as MC3357, MC3359, MC3362, MC3363, MC3367, MC3371, and MC3372. MC3363 integrates almost all the circuits of secondary frequency conversion including high frequency amplifier. The author has also made two of them. The performance of this IC is very superior.

VII. Discussion on the spirit of amateurism
[1]. I personally think that these four circuits are more suitable for amateurs to make their own circuits. All components are easy to obtain, and the required instruments and equipment are not many. It is just that more instruments are convenient for debugging and easier to succeed. However, the lack of dedicated instruments and equipment does not mean that it cannot be made. When I made my first walkie-talkie, I only had a multimeter and a field strength meter that I made myself. I was able to make a walkie-talkie with good performance. The specific parameters are unknown. Judging from the call distance, in an open area, using a normal mode spiral antenna, the distance can also reach 3-4km. This shows that special equipment is not the most important and necessary, but the most important thing is one's own hobby and work enthusiasm. On the contrary, without special equipment, you may take a lot of detours, but you can also learn a lot. From another perspective, it is not a good thing. I think most of us enthusiasts do not have too many special equipment. Without instruments and equipment, will we not be interested in it? Will we not be feverish? I don't think so. There is only one purpose for saying this, to encourage us enthusiasts and enthusiasts to be proactive and not rely on instruments and equipment blindly. Without special equipment, force yourself to work hard. If there are conditions, you must go on. If there are no conditions, create conditions and go on! This is the spirit of hobbies! It is also the most valuable thing for amateurs. As long as you have this spirit, difficulties will bow to you. I believe that there must be many excellent talents among our enthusiasts, who can definitely design circuits better than these four circuits and produce walkie-talkies with better performance. This does not need further discussion.
Of course, as an amateur, the guiding role of theory should not be underestimated. A solid theoretical foundation plays a very important role in the success or failure of production. No one has ever said that amateurs do not need theoretical guidance. Similarly, an excellent enthusiast cannot produce excellent works without extraordinary perseverance, self-confidence, and enthusiasm. At least that is what I think.
[2] This article is about my experience in designing and making walkie-talkies more than ten years ago. I have made more than ten handheld machines, including the shells, and I have also made several desktop machines. I am a radio enthusiast who discusses with new and old friends. In the past ten years, I have been engaged in the development of single-chip microcomputers and CNC engineering. These radio fields are almost dusty, and I don’t have much energy to study them. Now I see new and old friends playing with these things and having a lot of fun. My hands are itching again, but due to limited energy, it seems that I don’t have much time to continue to "fever". Due to the author's limited ability and tight time schedule, errors and omissions in the article are inevitable. I hope my friends will point them out.