High Power Linear Amplifier Practicalities February 5th, 2014
There is a growing trend for hams to purchase high power linear amplifiers, even though we are still stuck with a relatively low power limit here in the UK. A high power linear amplifier throws up a few potential problems for users. As soon as you move into this area of operation, you need to take a few precautions to avoid some pitfalls.
The most obvious one is connectors and cable. Never underestimate the much higher level of engineering, insulation and installation care that is needed to avoid welding connectors and cable together. And never underestimate how serious an RF burn can be if your equipment is not properly earthed and operated at near zero RF chassis voltage potential.These are all safety aspects. However, there is an interesting operational aspect that we have just started to notice.
A 600W amplifier probably runs at an input of around 1kW. That’s an AC mains drain of say 4-5 Amps. Quite easily handled by most domestic installations. BUT, step up to one of the currently available 2kW output amplifiers and you are in totally different territory. An OM-Power 2kW amplifier would draw at least 3kW of power from the AC mains. So there is around 1kW of power to dissipate (via an efficient fan) in the form of heat. That’s a lot of AC current to consider!
Your average domestic fan heater or electric radiator is quite happy at the end of an AC cable run because, if the current causes a voltage drop, it just delivers less heat. Nobody notices because there is nothing to indicate any drop. But in the case of a 2kW output amplifier, that voltage drop can be very significant to operation. It not only prevents the amplifier delivering its full power output, but can also throws up warning signals on the LCD display because the amplifier senses something is wrong.
This is something we are seeing now on a more regular basis. Some hams realise what is happening but others are jumping to the conclusion that there is a fault in their amplifier. So the rule here is to make sure that your amplifier has a short cable run directly to a properly installed wall socket, and not to a trailing cable extension lead or to a shack outside with a less than adequate cable run. Remember a 2kW amplifier will be drawing close to 13 Amps, the domestic maximum per socket. Apply ohms law and see how much voltage drop would occur on even a couple of ohms resistance.
To not follow these simple rule is to expect problems – unless you coast along at 400W output that is! Peter G3OJV.