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Cheap VHF amplifier
How to increase the device's performance to 144 MHz? One option is an LDMOS amplifier with MRF9180. These amplifiers are available as kits from various, mostly Chinese, sellers on platforms such as Ebay, Amazon, or Aliexpress. Their price is up to €40. However, such a kit requires additional components such as a power supply, heatsink, RX/TX switching system, and mechanical solution, or even a protection module. The total price will be higher. However, you can get a quality 144 MHz amplifier with high gain.
The choice of MRF9180 was also given by the fact that I have a stabilized power supply 28V/9A with a toroidal transformerPractically the same type of amplifier can also be equipped with other MOSFETs, e.g. MRF186 or MRF9120.
Documentation of VHF amplifier MRF9180
The disadvantage of ordering a kit is the absence of relevant documentation. Fortunately, it is possible to obtain the necessary information, for example, on the website GM4JTJ.
The kit arrived to me approximately three weeks after ordering. The PCB is of high quality and the components were sorted. MRF9180 and ATC capacitors were used, which the seller warned in advance. That's why I measured the MOSFET in advance, but fortunately it was functional.
Compared to the assembly diagram of components and description GM4JTJ There are some components with different values, so I recommend making a plan for how to install them right from the start. For example, the blocking capacitors in the power supply branch may have different values, but I do not recommend changing the values in the RF branch.
Mounting of the PCB of the RF amplifier with MR9180
Start the assembly of the printed circuit board by installing the SMD components in the power supply branch.
After installing these components, I continued with the ATC capacitors. It is important to pay attention to the correct values. It is ideal to measure the capacitance of the capacitors.
The output full-pass filter coils are wound on a drill. The most laborious part is making the transformers. The input transformer has a turns ratio of 3:1 (this gives a transformation ratio of 9:1). It is necessary to wind it with care so as not to rub the insulation on the wires.
The output transformation is made up of circuits made of 25- and 50-ohm coaxial cable. In the supplied kit, the 50-ohm coaxial cable is red, the 25-ohm cables are blue. Both cables are relatively hard and more difficult to strip. It requires good tools and a steady hand.
After installing these components, we check the correctness. We are not installing the MRF9180 yet. We check the printed circuit board visually and then using an ohmmeter for the presence of unwanted short circuits. This is followed by connecting the power supply and checking the voltage on the drain and gate pads of the MRF9180. The bias voltage regulator circuit should allow you to adjust the voltage smoothly from 0 to approx. 5V.
Now we can install the MRF9180. After loading the input and output, it is possible to try setting the quiescent current. I was a little worried about possible oscillation, since the MOSFET has a high gain, but the amplifier seemed completely stable. The quiescent current is set to 800mA. I registered its increase with increasing temperature.
First testing of VHF amplifier with MRF9180
I did the first test with an RF signal using an RF generator and a RF probe. This confirmed a gain of approx. 17 dB. Since I was worried about possible destruction of the MRF9180, I installed an attenuator at the input of the amplifier.
Waking from Kenwood TR-751E (5W) gives an output power of approx. 100W. Consumption at 28V is 7.2A, i.e. efficiency of approx. 50%. Cooling is provided by a copper CPU cooler with a turbine. This is regulated by a bimetallic switch that bridges the series resistor.
Switching and additional circuits of VHF amplifier with MRF9180
The RX/TX switching circuits provide switching of the QN59925 relay and auxiliary relays at the amplifier input and switching on of the bias voltage. The circuit uses a ULN2003 driver in standard wiring.
There is also a direct and reflected power detector directly on the amplifier's printed circuit board, which allows you to create a PSV meter. It would have been a shame not to use this option.
Mechanical solution of VHF amplifier with MRF9180
The amplifier is built into a case from an old measuring device. The front and rear panel are made of copper clad laminate and a printed label on A4 adhesive foil. It is sprayed with several thin layers of clear lacquer for improved durability.
Evaluation, advantages and disadvantages of VHF amplifier with MRF9180
The amplifier behaves very well in practice. I did not notice the activation of the second stage of the cooling turbine speed during SSB operation, i.e. 50°C on the sensor on the copper radiator. I had a lot of material in stock, so I estimate the cost of construction at 50 € in total. However, the design is not suitable for beginners and will take a lot of work even for an experienced radio amateur.
However, there is a certain risk in the quality of the MRF9180 from China. Many designers have had their amplifiers fail due to a malfunction of this circuit or very soon the LDMOS went to silicon heaven. I also had one MRF9180 that stopped working. Several sources recommend buying LDMOS from verified sellers. In that case, the reliability is supposedly many times higher.