In part one of this series (November issue) I discussed the numerous sources of interference from DC appliances and systems. Now I want to cover sources of AC interference on board boats.
        The two principal sources of AC power on board boats are AC generators and inverters. The big fact of the matter is most of our boat AC electrical systems were designed for linear loads such as motors, electric stoves, and light bulbs. Electronic equipment such as computers and many battery chargers are nonlinear loads, which draw current in pulses.
        There are NO perfect electrical systems. Having been built and wired by mere mortals, they can be riddled with little errors such as loose connections and corroded terminals. Both are great sources of interference.
        Many well-built boats manufactured before the electronics revolution have poor grounding. Most conditions that lead to noise generation in AC circuits can be cured with careful wiring practice and improved grounding. Even though your boat has been wired to the ABYC (American Boat and Yacht Council) standards, don't be lulled into thinking you could not have a problem. All it takes is one corroded terminal to generate noise.
        Let's assume you have checked all your DC circuits and loads, and now you are ready to check noise sources on the AC side. The first step is to isolate noise sources that are generated onboard versus those that are brought aboard by the shore power cable. So disconnect the shore cable at the power inlet to your boat as a first step.
        Of the two basic methods by which AC power is generated on a boat--static inverters and generators--inverters are the most common culprit of generated noise. Unplug all AC electrical devices. Believe it or not, the power switch in some consumer appliances disconnects the device circuitry from its power supply while leaving the supply connected to an Electronic Control Module (ECM). Power supply components can generate noise, so merely turning off such a device leaves a potential noise source connected to the line. VCRs are a good example of this type of device. Even though you turn the power switch off, the internal clock and programming circuitry is still working.
        Static inverters span a wide range of output power from 100 watts all the way to 2,000 watts continuous output, but they all have one point in common. They do not provide a smooth sinusoidal wave shape. This wave shape becomes more distorted when you work with smaller and lower cost units. The switching circuit not only produces a square wave, but also sharp spikes that are easily transferred to sensitive SSB receivers. On numerous occasions I have encountered small inverters used to power laptops in order to interface with SSB radios. They power the laptop to provide weatherfax or transmit e-mail via commercial or amateur radio stations. I have constructed a very simple piece of test equipment that I carry in my test bag. It consists of nothing more than a three-prong power plug and one foot of cable soldered to a 240 microfarad capacitor. This capacitor is simply a "running" capacitor used on 220-volt AC air conditioning compressor motors. I picked mine up from an air conditioner shop. This simple piece of test equipment has been directly responsible for finding just about all of the problems with small AC inverters.
        The critical point that should be understood here is that ALL static inverters, regardless of cost and power, do suffer from failure of the output filter system and occasionally so do the diodes. If you have a 2,000 watt unit which produces a noise signal on your SSB radio, and the noise is reduced when you plug this simple capacitor across the output, then you most likely have a failure of the filter system. Failure of capacitors in these inverters is a common problem.
        If you find that there is no appreciable noise from the static inverter operation, then switch the main selector to onboard power. At this point, detection of the circuit which is causing the problem is made possible by simply turning on the various circuits that supply AC power throughout the boat. In this manner you can isolate the particular circuits causing the noise problem. One cute little source of noise to be aware of is nicad battery chargers. There is a thermal over-temperature sensor that acts as a cutoff switch, cycling the primary power on and off. All cell phone charger stands are also equipped with this little gem, and it certainly shows up in the RF of a high gain SSB receiver.
        Before leaving your electrical panel, be aware that the contacts in the breaker itself may be pitted or burned. If any breaker in your electrical panel feels warm to the touch then rest assured there is resistance in the contact, and the breaker needs to be replaced. Check all your AC light switches carefully. Turn each AC light on and off several times to see if it reduces the noise picked up by the SSB. I think we are all guilty of screwing in a slightly higher wattage bulb in our light sockets. This will definitely shorten the life of the built-in switch. Stick with the manufacturer's bulb ratings. Light switch dimmers are also excellent sources of radiated noise.