25 watt linear for Dual bands (40/20)

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.

Features

Simplicity, 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

Design Notes 

A single ended amplifier powered by a 24V supply was chosen. Radio Amateurs often build 12V equipment so it could be powered by the car battery. However, higher voltage supplies really simplify building power amplifiers. They naturally increase the output  power, boost linearity and need lesser driving power. SMPS supplies with 24 volts output are inexpensively available. These also have another useful feature – they have current limiting. This can protect your IRF510 from blowing up. There is a small preset on the SMPS that sets the exact voltage. Crank it up to the max. Our SMPS gave 29 volts.

The IRF-510's gate looks like a large capacitor to the RF input. This makes the input reactive, and it could make your QRP rig or the linear go into self-oscillations. We handle this in two ways. First, we place a 10 ohms resistor in series with the IRF510's gate. That makes the gate look more like a resistor and less like a capacitor. Then, we add an RF attenuator with two 220 ohms resistors and a 22 ohms. This makes the input quite stable and solid. Yes, you lose a watt. But it is worth the stability. You are free to experiment with different types of attenuators or different values of the series gate resistor.

The IRF 510 is a non-linear device and its output is meant to quickly swing from off to on and back again. The on-resistance of the IRF510 is controlled by the gate voltage. Thus, we have to provide the gate with a voltage such that it is barely 'on'. The RF input will make it swing within its narrow linear range. We use a 7805 to bring down the power supply voltage to 5V, then use a 10K preset to provide the DC required to bias the IRF510. As the gate of an FET consumes no current, very little voltage is required to bias it. A 10 K resistor is used. Note that the other end of the 10 K resistor has a 0.1 bypass capacitor.

The IRF510 output is very spikey. A 220 pf, connected from the drain to the ground, smoothens these spikes. The drain is powered through an RFC made from 16 turns of 22 SWG over a single FT37-43. The other side of the RFC is connected to the power line. Electrolytics along with two 0.1uFs in parallel lower the ripple from the power supply.

With improper load, the voltage on the drain can swing much higher than 30 volts. Hence, four regular disc ceramics are bunched in a 2-by-2 configuration to provide higher voltage rating.

An output of 25 watts at 25 volts  makes for 12.5 ohms output impedance. This has to be stepped up 4 times to 50 ohms. A simple bifilar transformer made out of a single FT37-43 provides the step. Note that it doubles the voltage. Hence, the output will now swing upto 50 volts each side (100v peak to peak).

The two low pass filters are switched through the RL3 relay. Using a front-panel switch with long leads often leads to stray RF coupling that can cause self-oscillations. Don't try replacing the relay with a DPDT switch. You could try making this a single-band linear by completely eliminating the band-switch.

Transmit-Receive switching is a little more elaborate than usual. Two DPDT relays are used. One at the output  and one at the input. If you carefully follow the RL1's wiring, you will note that on transmit it grounds the cable that is used to bypass the linear during transmission. This detail is important in preventing the RF from feeding back into the QRP rig or the linear's own input. The output relay uses both the poles for higher power handling.

All the relays used were the standard 12v relays with 800 ohms coil. The 1K resistor in series allows them to work with 25-30 volts. Note that the bypass on the relays is necessary. It prevents the relay coils from picking up the RF energy.

An LED indicator is used to monitor the output. Continuously lit LED will indicates oscillations. Voice modulation on SSB will accordingly make the LED flicker. CW makes it flash in unison with your keying.

The 50 uF in the T/R switch is optimized for CW and PSK. It drops during pauses during transmission. If SSB is your prime mode, then you might want to change it to 200 uF or so.

Construction

The linear with the power supply is mounted inside a metal box of 6-1/2” by 7” by 2”, originally meant for a car radio. A heatsink salvaged from an old VHF radio set was used as a heat sink. The heat sink turned out to be slightly higher than the box. Those with a keen eye for beauty may take exception to it. The linear is entirely constructed on scrap piece of copper approximately  6 inches long. One end is the input, the other end is the output. See the pictures. The layout is not congested. The IRF 510  on the edge of the board; in the middle section. It is bolted through a mica spacer and a bolt with an insualting sleeve. These are purchased as 'T220 Heatsink kit' from the local market for a few rupees.

There are several guidelines to be followed. They are all simple and probably an overkill but it is a good idea to adhere to them :

• 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

The linears are unforgiving beast. You have to approach this in baby steps. First, doubly check everything. Especially the way the relays are wired.

1. Disconnect the RFC from the IRF510's drain so that the IRF510 won't be powered.
2. DC smoke test : Power up the board. Measure the voltage on the power line. Is it 24 volts? 
3. 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. 
4. Set the bias voltage to zero.
5. Power down. Keep the power off.
6. 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.
7. Step 7: Connect a 10K resistor from +ve line to the base of T/R transistor.
8. 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.
9. Step 9: Power down. Remove the 10 K resistor.

Now, we are ready to do our DC tests.

1. 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.
2. Connect a VOM meter in milliamps range between the power supply and the linear. 
3. Power up,check that the RF indicator LED is completely dark. If not, the linear is in self oscillations.
4. 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.
5. 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.

RF tests:

1. With the RF load connected, connect an RF probe or the oscilloscope to the dummy load.
2. The voltage range of the scope/RF probe should be set to measure up to 100 volts.
3. Momentarily press the key on the QRP rig. 2 watts output should read 50 volts of clean RF on the dummy load.
4. 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.
5. Check that the heatsink is not getting too hot. It should still be okay to touch it.
6. Swap the dummy load with an antenna with low SWR. 
7. Listen to the band to check that there is no attenuation of the incoming signals.

You're done!

Conclusion

At VU2ESE, this is now the common amplifier for all the QRP rigs. The on-the-air reports have been very encouraging. Using a two element beam, 7 countries were worked on SSB within the first hour. It broke through one particularly notorious dxpedition as well.

One may not use a linear at all times, but it is nice to be able to throw that switch when the going gets tough on the bands.

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.

Bibilography 

This linear came about as an adaptation from Rick's and Wes's works.

• Sec 2.11 Experimental Methods in RF Design, ARRL's excellent reference 
• Rick Campbell, KK7B's two part article on designing single ended linears. Part1 and Part2
• IRF-510 Datasheet
• Miniboots Wayne NB6M has a similarly configured amplifier for class-C and 12 volts. It makes for a great field-day boot!