Homebrewers usually limit themselves to 5 watts or less. Frequently, there is a need to increase the power beyond those few watts. Described is a very simple, very stable amplifier that will provide 25 watts on any two HF bands. It takes a few evenings to be put together. Most of the parts are already in your junk box.
Why 25 watts?For most QRPers, 25 watts is substantial power. If you keep your antenna losses down, your effective radiated power will be on par with the 100 watt transceivers feeding modestly fed dipoles.
On the other hand, 25 watts is achieved far more simply with a single IRF510, far more cheaply than what it takes to build a push-pull amplifier that will provide just another 3db advantage at 50 watts or more.
If you want only CW operation, you could get even higher power at 24 volts. Miniboots by Wayne NB6M is just the thing for the CW-ers.
FeaturesSimplicity, low cost, easy of duplication were the main objectives of this project
- 25 Watts of Power – At 25 watts, the RF peak voltage is 50v. This is easily handled by the regular disc ceramics rated for 60v. Higher power will require expensive capacitors.
- Dual Band – The linear operates on any two HF bands. The prototype was built for 40 and 20 meters operation. A monoband linear is usually too restrictive and a multiband linear needs a complicated switching mechanism for each band. A dual-bander is easily switched between the two-bands with a DPDT relay.
- Stablity – The attenuator in the input section provides a stable load for the QRP rig that drives this linear and a stable input for the IRF510 as well. Two relays are used in a configuration that minimizes the RF feedback by grounding the feed-through coax during transmission.
- Simple Construction – The linear amplifier uses just two ordinary FT37-43 toroids and a single IRF510.
- Inexpensive – The linear was put together using the junk box. It costs Rs.200 in new parts. The power supply was an additional Rs.1200 in the local market. Ebay lists it for a little over 20 dollars
- Use coax from input and output connectors to the linear's board. Make ground connections at the connectors as well as the board.
- Keep the input and output connectors as apart as possible.
- Take care about which side of the output transformer goes where. Check the circuit for the correct phasing. It is not tricky but it is easy to overlook. The center tap goes to the IRF510, one end (any end) goes to the ground and the other is the output.
- Check and doubly check the relay connections they are tricky.
- The low pass filters are switched by the relay. Keep the often used LPF in the relay's 'off' position. That will reduce the current in the relay and prolong the relay's life.
- If you are installing the power supply inside the same cabinet (like the prototype) wrap insulation tape around the power switch and the power connections on the SMPS.
The construction details are visible in the picture below. The upper part is the input relay with the RF sensor build around it with to the top-left. The attenuator is to right of the hole in the board. The 7805 bias regulator and the preset in the center of the board, towards the left. The RFC can be clearly seen. The power supply decoupling electrolytics are just below the preset. The RL3 is to the left of the LPFs. Nylon tap washers were used for 14 Mhz LPF inductors. The relay on the bottom right is the RL2 for switching the antenna. Yellow wires carry 24volts, green wires carry the RF, red wires do the T/R switching. The IRF is bolted to the heatsink on the right. Note the nylon washer on the bolt of the IRF510. It insulates the IRF510's tab from the heatsink.
The amp with its built-in power supply is mounted inside the carcass of a car radio. It measures 2” by 6” by 7”.
Bringing it to Life
- Disconnect the RFC from the IRF510's drain so that the IRF510 won't be powered.
- DC smoke test : Power up the board. Measure the voltage on the power line. Is it 24 volts?
- Measure the voltage on the Gate of IRF510. It should be between 0 and 5 volts. Move the preset and check that the voltage varies smoothly.
- Set the bias voltage to zero.
- Power down. Keep the power off.
- Step 6: Check the continuity between the INPUT and OUTPUT connectors. In receive and the power 'OFF' position the INPUT should directly connect to the OUTPUT, bypassing the linear.
- Step 7: Connect a 10K resistor from +ve line to the base of T/R transistor.
- Step 8: Power up, the linear is now forced into transmit mode. Check the connectivity from the low pass filter to the OUTPUT. The T1 transformer will also provide a DC short circuit to the ground. Switch the LPF to the other band and check again.
- Step 9: Power down. Remove the 10 K resistor.
- Connect back the RFC to the drain of the IRF510. Connect a dummy load to the output. 20 1K resistors of 2 watts each connected in parallel are a good dummy load.
- Connect a VOM meter in milliamps range between the power supply and the linear.
- Power up,check that the RF indicator LED is completely dark. If not, the linear is in self oscillations.
- The VOM should read less than 10 mA (used by the 7805). Slowly increase the preset until the VOM reads 50 mAs more than the initial reading. This is the additional current being consumed by the IRF510. Now the IRF510 has been effectively biased for 50 mA.
- Leave the setup as is for a few minutes. The current may climb up by a few milliamps. But not further. Touch the heatsink and check that it is not getting too hot. It should be mildly warm.
- With the RF load connected, connect an RF probe or the oscilloscope to the dummy load.
- The voltage range of the scope/RF probe should be set to measure up to 100 volts.
- Momentarily press the key on the QRP rig. 2 watts output should read 50 volts of clean RF on the dummy load.
- Switch to SSB (if you have a QRP SSB rig), check the modulation on the scope. Monitoring the linear output in the nearby receiver is useless. The receiver will overload.
- Check that the heatsink is not getting too hot. It should still be okay to touch it.
- Swap the dummy load with an antenna with low SWR.
- Listen to the band to check that there is no attenuation of the incoming signals.
The construction details are visible here. The upper part is the input relay with the RF sensor build around it with to the top-left. The attenuator is to right of the hole in the board. The 7805 bias regulator and the preset in the center of the board, towards the left. The RFC can be cleary seen. The power supply decoupling electrolytics are just below the preset. The RL3 is to the left of the LPFs. Nylon tap washers were used for 14 Mhz LPF inductors. The relay on the bottom right is the RL2 for switching the antenna. Yellow wires carry 24volts, green wires carry the RF, red wires do the T/R switching. The IRF is bolted to the heatsink on the right. Note the nylon washer on the bolt of the IRF510. It insulates the IRF510's tab from the heatsink.
This linear came about as an adaptation from Rick's and Wes's works.