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Cheap VHF amplifier
How to increase the power of the device on 144 MHz? One option is LDMOS zosilňovač s 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 transformer. Practically the same type of amplifier can also be fitted with other MOSFET-mi, napr. MRF186 or MRF9120.
Documentation of VHF amplifier MRF9180
The disadvantage of ordering a kit is the absence of relevant documentation. Našťastie je možné získať potrebné informácie napríklad na webe 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 sú some components of different values and therefore I recommend making a plan on how to assemble them right at the beginning. Napríklad blokovacie kondenzátory v napájacej vetve môžu mať iné hodnoty, ale meniť hodnoty vo VF vetve neodporúčam.
Mounting of the PCB of the RF amplifier with MR9180
Osadenie plošného spoja začnite práve osadením SMD components in the power supply branch.
After mounting these components, I continued ATC kondenzátormi. Pozor je potrebné dať na správne hodnoty. Ideálne je kapacitu kondenzátorov premerať.
Cievky výstupného doplnopriepustného filtra sú vinuté na vrtáku. Najprácnejšie je zhotovenie transformátorov. Vstupný transformátor has a turns ratio of 3:1 (which gives a transformation ratio of 9:1). Winding needs to be done with care to avoid damaging the insulation on the wires.
Výstupná transformácia is formed by circuits of 25- and 50-ohm coaxial cables. In the supplied kit, the 50-ohm coaxial cable is red, the 25-ohm cables are blue. Both cables are relatively stiff and harder to strip. It requires good tools and a steady hand.
After mounting these components, follows kontrola správnosti. MRF9180 zatiaľ neosadzujeme. Plošný spoj skontrolujeme vizuálne a potom pomocou ohmmetra na výskyt neželaných skratov. Nasleduje pripojenie napájacieho zdroja a kontrola napätia na plôškach pre drain a gate MRF9180. Regulačný obvod predpätia by mal umožňovať nastaviť napätie plynulo od 0 po cca. 5V.
Now we can mount MRF9180. After loading the input and output, we can try adjusting the quiescent current. I was a bit concerned about possible oscillation, as the MOSFET has a high gain, but the amplifier seemed completely stable. Kľudový prúd je nastavený na 800mA. So zvyšujúcou teplotou som zaregistroval jeho nárast.
First testing of VHF amplifier with MRF9180
I first tested with VHF signal using VHF generator and VHF probe. It confirmed zisk cca. 17 dB. Keďže som sa obával možného zničenia MRF9180, tak na vstup zosilňovača som osadil útlmový článok.
Waking from Kenwood TR-751E (5W) dáva výstupný výkon cca. 100W. Odber pri 28V je 7,2A, teda účinnosť cca. 50%. Cooling is provided by a copper CPU cooler with a fan. Tá má reguláciu bimetalovým spínačom, ktorý preklenie predradný rezistor.
Switching and additional circuits of VHF amplifier with MRF9180
Prepínacie obvody RX/TX zabezpečujú prepnutie relé QN59925 and auxiliary relays at the input of the amplifier and bias turn-on. The circuit uses a driver ULN2003 v štandardnom zapojení.
Priamo na plošnom spoji zosilňovača je aj detector of forward and reflected power, which allows to create a meter PSV. Bola škoda túto možnosť nevyužiť.
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 speed stage of the cooling turbine during SSB operation, i.e. 50°C at idle on the copper cooler. I had a lot of material in stock, so náklady na zhotovenie odhadujem na 50 € sumárne. Konštrukcia však nie je vhodná pre začiatočníkov a dá zabrať aj skúsenému rádioamatérovi.
Istým rizikom je ale quality of MRF9180 from China. Mnohým konštruktérom zosilňovač nefungoval z dôvodu poruchy tohto obvodu alebo veľmi skoro LDMOS odišiel do kremíkového neba. Aj mne jeden kus MRF9180 prestal fungovať. Viaceré pramene odporúčajú nakupovať LDMOS-y od overených predajcov. V takom prípade je vraj spoľahlivosť násobne vyššia.