RF-amplifier protection - Page 1
Forum section: electronics / beginners

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 22, 2024, 05:51:30 pm
content: Suddenly, I became an RF hobbyist. I have a 1 GHz 10 W RF-amplifier module (24VDC powered). The amplifier itself is quite typical (I guess) and consists of a pre-amplifier and an output stage (RF-FET/MOSFET?) in a small metal box. It has three connectors - input, output, and DC power supply. The most typical problem, as I know, is standing waves when somebody forgets to connect a load (antenna or dumb load).RF-amplifier usually is used with a CC-CV power supply module. Can this CC-CV power supply module help and work as more or less reliable protection for RF amplifiers? Or is it too slow?Can somebody give some keywords where to look at this topic (possible protections for this or other typical use cases)?Possibly, I missed something else about how to design the whole thing (RF-amp + antenna) a bit more reliably, kind of user-proof? (radio-engineering is not my sphere).Thank you in advance.

author: MarkT
author_rank: Frequent Contributor
timestamp: January 22, 2024, 07:37:36 pm
content: Its called VSWR, and many solid state RF power devices can burn out at full power if the VSWR isn't close enough to 1 (matched). So if you can arrange to operate at low power while tuning the load and use a VSWR meter to check for matching, you can then dial up the power to the maximum for the particular device. Maybe adding a switchable attenuator at the _input_ to the power amp would make this easy to do.You can also check the load with a VNA or antenna analyzer to make sure it has low VSWR at the frequency of interest, which can be done once - hopefully the load is stable over time.

author: jwet
author_rank: Frequent Contributor
timestamp: January 23, 2024, 03:00:54 am
content: At these power levels, with reasonably designed equipment, it shouldn't be too fragile and decent stuff should have some SWR protection at least to 2:1. Amps built for lab use have complete SWR protection for gross mismatch. You can abuse the heck out of Mini Circuits amps in this class. (ask me how I know).The worst case voltage that you can have with a completely open load is 2x of your nominal peak voltage- an SWR of infinity.The doomsday scenario is what Ham's do at legal limit power. At 1500 W into 50 ohms, you have 387 volts peak when matched and near 800 on a complete mismatch. (P=V^2/R, Vpk=1.414*Vrms)). This is a lot and break down coax, fry finals, etc.At 10W, the matched peak is 31.6V and the worst case is 63v. If you can sample and rectify your output voltage- nothing fancy, you can kill PA bias on a problem. It might make sense to trip at SWR of 2+. This would be peak volts of (1+(SWR-1/SWR+1)) or 42V (+33%). Not terrible and easy to implement. You could bring this high SWR signal fault back to your PS and kill it there or use a load switch or relay.Most QRP gear (<5-10W) doesn't bother with SWR protection. Standard 100W Ham gear does. Elekraft makes high quality HF transceivers and generously post their schematics. Their K2 is a classic 10W rig and has an optional 100W PA. The 10W version does nothing, the 100W does a bit which you can study in their schematics. A lot of solid-state equipment doesn't even see the peaks above, they're transformer coupled and generate power at lower Z's and step up at the output.You can solve a lot of problems with a mechanical interlock switch that won't let power go to AMP without a connector on the output.I did a project for biologists that used a little RF amp like this to boost a little 10 dBm transmitter to 30 dBm to transmit ISM control signals at 433 MHz. It probably wasn't strictly legal but it was a late fix when we needed more signal because of dense vegetation that I hadn't taken into account in the link budgets. It was always used in the boonies for bird tracking and for short periods. It was assembled in the field and attached to an antenna by a preoccupied graduate student. There was a big red note on it that said "don't run without antenna connected". We never cooked an amp over the couple years it ran. I bought a spare that I probably still have.Have fun.

author: Bud
author_rank: Super Contributor
timestamp: January 23, 2024, 06:38:33 am
content: Do Not connect any test/measurement equipment directly to the amplifier output , unless the equipment was specifically designed for that purpose, like VSWR meters, pass-thru power meters or dummy loads.

author: CaptDon
author_rank: Super Contributor
timestamp: January 23, 2024, 01:57:35 pm
content: The constant current function of the power supply being used to limit maximum available current is useless for V.S.W.R. protection. VSWR to a certain understanding is power rejected by the load such as 10 watts forward 2 watts back. Chances are that with an open circuit load and infinite VSWR and seeing up to 2X voltage at the output port your power supply current may indeed go down, not up. (until the output device fabric blows up and typically dead shorts)

author: radiolistener
author_rank: Super Contributor
timestamp: January 25, 2024, 11:06:11 am
content: Quote from: Vovk_Z on January 22, 2024, 05:51:30 pmNo, usually such protection requires VSWR meter circuit which measure reflection wave and automatically reduce voltage applied to amplifier output stage when VSWR rise up, it helps to avoid damage. This is how it works in professional transceivers. But in some rare cases even such protection cannot help.Another way is to use more powerful transistors at less power to have sufficient power reserve. But it will be more expensive solution because more powerful transistors much more expensive. For example at short wave frequency band you can use RD16 transistors for 5-10W power with no protection, they can survive even with open output.

author: Vovk_Z
author_rank: Super Contributor
timestamp: January 27, 2024, 09:34:15 pm
content: Thank you all. I guess I mostly found an info I was interested in.