Introduction: Building a low-cost 10GHz transmitter using a radar module has always been a project I've wanted to try (although I haven't seen many other ham radio operators experimenting with this approach in modern communications). Now that the HamZone forum is open, I hope this post will be a good starting point and contribute to its growth.
Generally, communication in the GHz band is considered a purely recreational activity for amateur radio enthusiasts. In China's allocation of radio frequencies, amateur users can use up to the 250 GHz band. Specifically, 1.2 GHzThe amateur radio bands between 10 GHz are used in experiments for EME (Earth-to-Moon), tropospheric ducting, and satellite communication. Higher frequency bands are typically used by hobbyists to build their own equipment, challenging the limits of line-of-sight radio communication distances.
From the lowest frequency band of 1.2 GHz to 248 GHz, most frequency bands lack commercially available amateur radio transceivers. If you want to experiment, you typically need to build your own equipment and coordinate with other enthusiasts. However, with the development of the microelectronics industry and advances in software-defined radio technology, self-built devices in lower frequency bands can benefit from advancements in other fields (such as drones and satellite television), resulting in better performance. This design is based on a simple 10 GHz microwave ranging radar, which enables basic communication experiments.
The core of this design is the HB100 microwave radar module. This microwave radar module primarily uses a DRO-based crystal oscillator internally, simplifying the internal structure. If we disassemble this module, we can see that its main circuitry is mainly based on this DRO crystal. The structure diagram of the HB100 is shown in Figure 1.
It can generate a 10 GHz local oscillation frequency internally using a DRO crystal. This frequency is then directly sent to the transmitting antenna (PCB antenna), and also used as a local oscillator for mixing with the incoming signal from the PCB receiving antenna, producing an output intermediate-frequency signal. In this design, we primarily utilize the DRO portion, so the receiving part is not our primary focus.
Through testing the HB100 module, we can observe that the transmitting frequency of the HB100 is primarily determined by the input power supply voltage and the inherent properties of the DRO crystal itself. The original HB100 module operated at 10.525 GHz, while the amateur 10 GHz band in China is 1010With a bandwidth of 500 MHz at 0.5 GHz, it is necessary to modify the HB100 module so that its transmission frequency falls within the amateur radio band.
By adjusting the distribution of objects around the DRO, it is possible to significantly adjust the oscillation frequency of the DRO within a certain range. For the HB100 module, the original metal shielding enclosure not only provides shielding but also fixes the oscillation frequency of the HB100. When modifying, a suitable hole needs to be drilled on the metal shield facing the DRO, and an appropriately sized screw is inserted so that the lower surface of the screw faces the upper surface of the DRO. By changing the depth at which the screw is inserted into the shield, the space above the DRO is changed, thereby adjusting the oscillation frequency of the DRO. The HB100 module with the installed adjustment screw is shown in Figure 2. With this modification, the module can operate at 10.361 GHz, placing it within the amateur band.
The oscillation frequency of the HB100 module is also affected by the power supply voltage. However, this effect is relatively small. We can use this method to FM modulate a single-frequency signal, thereby achieving FM modulation transmission. This modulation method is relatively simple, using a transistor amplifier circuit to amplify the input audio signal, and making the output voltage amplitude approximately 5V DC. The schematic diagram for this part is shown in Figure 3. The finished product is shown in Figure 4.
Through testing, this circuit can modulate the input audio and achieve FM modulation by adjusting the DRO power supply voltage. To enable reception, we also need a corresponding receiver section. Since it operates in the 10GHz band, we can use the LNB (low-noise block) from a satellite TV. We need an LNB that operates at 9.75GHz and is powered through a Bias-T. The circuit diagram for the Bias-T is shown in Figure 5.
The overall system architecture is shown in Figure 6.
Using the SDR Console on a computer, you can operate the Adalm Pluto to receive signals transmitted by the transmitter. Since the frequency of the signal we transmit is 10.361 GHz and the local oscillator frequency of the LNB is 9.75 GHz, we expect to receive signals around 10.361 - 9.75 = 611 MHz. We can first select a 6 MHz bandwidth to check the entire band, and then use BC-FM to demodulate after finding the transmitted signal. The software reception results are shown in Figure 7. (Input audio signal: 50mV, 1 kHz sine wave)
From the diagram, we can see that the stability of the transmitted signal is poor. The two main causes of this problem are the temperature stability of the DRO and the LNB. The first problem can be improved by preheating the DRO and providing a stable working environment for it. The second problem can be improved by improving the LNB's self-oscillation and introducing a GPS clock. Again, I will not repeat myself.
Because the bandwidth occupied by an FM modulation scheme is determined by the bandwidth of the modulating signal, when applied to voice communication, filtering must be performed on the input signal. This helps minimize the transmitted bandwidth. A photograph of the entire test system is shown in Figure 8.
References:
[1] PA1SDB. Available for €3 on the 3cm band with the "HB100" motion detection device. ![EB/OL]. [2017]. http://home.deds.nl/~knol/HB100/.
[2] IK1HGI. HB100 Module for ATVs [EB/OL]. [2021]. https://www.qsl.net/ik1hgi/atv/hb100.htm.
[3] PE1RKI. Dro modulator [EB/OL]. http://www.pe1rki.com/dro.html.
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