What is preventing you from homebrewing an HF linear amplifier?
I am having an FT-710 radio with peak envelope power (PEP) on single side band (SSB) of 100 W. My license class permits me to use 400 W on HF bands. So I am falling short by 300 W! Even with the existing 100 W power, I can improve my on air performance by improving my antenna setup. Going for a hexbeam with a rotator can certainly improve my performance on higher bands, though not on 40m. But I have limitations in mounting antennas at my tiny city home. I think I have hit a wall as far as improving antenna setup is concerned. If it was for single band operation, there are some ways of improving my antenna setup. But that will take me off other bands which I do enjoy working, all the way from 80 m to 10 m on HF, including WARC bands of 30, 17 and 12 m.
Had on air discussions with many friends who are using linear amplifiers and a few who have homebrewed linear amplifiers. Read a little bit on home brewing linear amplifiers online as well. Some of my friends suggested going for a vacuum tube linear amplifier. One of them is working with plate voltage in the range of 2.5 to 3 KV. I am not very comfortable working with such high voltages. Of course, I had homebrewed 3 x 807 TX long back. But that would have had a plate voltage only of about 850 V. Many told me that vacuum tube TX is more tolerant to mismatches in antenna system while solid state devices are very sensitive. I had direct experience of that in my era of homebrewing. The BD 139 finals in my VU2VWN QRP TX would blow off very often and I used to replace them. But they were quite cheap and easily available. On the contrary, the modern LDMOS RF power transistors may cost around $300!
LDMOS stands for Laterally Diffused Metal Oxide Semiconductor and is a type of MOSFET or Metal Oxide Semiconductor Field Effect Transistor. As they are quite sensitive to loading conditions, special control circuitry have to be incorporated to prevent inadvertent damage and loss of an expensive MOSFET. Input to the linear amplifier should not go above the specified value. Another issue is choosing the wrong band and causing an antenna mismatch with high SWR or Standing Wave Ratio. High standing wave ratio means a lot of reflected power and potential damage to the final RF transistor. That means another control circuitry should be there to sense SWR and quickly cut off the LDMOS circuit before it gets damaged.
Another complexity in dealing with linear amplifiers is the need for low pass filters to get your output free of splatter and harmonics. Even though the driving output from my commercial radio might be free of harmonics, they can be created in the linear amplifier. Multiple low pass filters will be needed for multi-band operation, making the homebrewing of linear amplifier more complex. Band pass filters will need to be switched automatically when different bands are chosen by the operator, using relays capable of handling high frequency signals.
Another issue for me will be antenna tuners. My antennas are seldom perfectly matched and I resort to internal antenna tuner within my FT-710 radio when there is a mismatch up to SWR 3:1. When the mismatch is higher, but lower than 10:1, I can use my ATU-100 EXT automatic antenna tuner. But none of these will cater to the higher power output of a linear amplifier. That means I will have to get an antenna tuner, either manual or automatic, with higher power rating, again increasing the budget allocation significantly.
Providing power supply to the linear amplifier is another challenge. The switch mode power supply (SMPS) which I am using for my FT-710 is unlikely to support the linear amplifier. That means either I have to homebrew another linear power supply of high power output or go for one more SMPS with higher power rating. SMPSs are notorious for radiofrequency interference in the shack and elsewhere. Fortunately the current SMPS of my radio does not produce much RFI for me, though there are several tiny SMPS around at home including mobile phone chargers which do give RFI in my radio. When I go for a new higher power rating SMPS for the linear amplifier, that may not quite free of RFI as the chances are higher with a higher power output. Even though all these are deterrents to homebrewing an HF linear amplifier, I do keep it strongly on my wish list and keep learning about the various aspects until I am confident enough to go ahead! After all, homebrewing is the most thrilling aspect of amateur radio.