Generator for Field Day: Sizing, Inverter Types, Noise and Safety

The most expensive mistake at a Field Day site is often not the antenna or the radio but the generator. It is too small and trips under the first amplifier, or it is too noisy and fills the 40 metre receiver with hash, or it is placed in the wrong spot and the operators go home with a headache. A good generator for Field Day is chosen from the real load of the stations, not from the biggest number on the box, and it is installed with the same care as an antenna. This article shows how to calculate the load, why inverter generators suit radio work, how to keep the generator from wrecking your reception, how to run it for 24 hours and how to avoid the dangers that come with any engine at a campsite.

Start With the Load, Not the Generator

Add up what will really run at the same time. A 100-watt transceiver draws about 20 amperes at 13.8 volts when it transmits at full power, which is about 280 watts from the power supply. A linear switch-mode supply is 80 to 90 percent efficient, so the demand on the AC side is about 300 to 350 volt-amperes per station. A laptop adds 50 to 90 watts, a monitor, a lamp, a fan and a phone charger add another 100 or so. A station with a 100-watt radio therefore needs around 500 watts on the AC side at its peak.

An amplifier changes the picture. A 1,000-watt output amplifier needs 1,500 to 2,000 watts of input power on the peaks, and some amplifiers need a 240-volt supply. A Field Day group of three stations, each with a 100-watt radio and a computer, plus lights and a coffee machine, can reach 2,000 watts. The coffee machine alone is 800 to 1,500 watts, so many groups run the kettle on a separate small generator or on a gas stove. Make a table of the loads, add the peak values, and then add a margin of 25 percent. A generator should rarely run at more than 75 percent of its rated output for long periods, because heat and noise grow with the load.

The ratings are guidance, not a promise. A generator has a continuous rating and a higher surge rating, and the continuous one is what counts for a 24-hour event. Check also the output voltage and frequency, 230 volts and 50 hertz in Europe, because some amplifiers and switch-mode supplies do not like a poorly regulated output.

Why an Inverter Generator Is Better for Radio

A conventional open-frame generator runs the engine at a fixed speed, usually 3,000 revolutions per minute in 50 hertz countries, so that the alternator produces the correct frequency. Its voltage and frequency vary with the load, and its waveform is not always clean. An inverter generator produces alternating current at a higher frequency, rectifies it to direct current and then converts it to a clean sine wave at the output frequency. The engine speed follows the load, so the generator is quieter and burns less fuel at low loads. For a Field Day station the benefits are clear: a stable voltage for the power supplies, lower engine noise at the operating positions and a longer run time on a tank of fuel.

The price is a higher purchase cost and, in some designs, a little electrical noise from the inverter electronics. Not every inverter generator is quiet on the radio bands. The electronics switch at tens of kilohertz, and cheap units can spray harmonics into the HF bands, particularly on 160 to 40 metres. The article on linear and switching power supplies explains the same problem on a smaller scale. When you can, test the generator before the event with a receiver and an antenna, because the noise depends on the model and not on the price.

Keeping the Generator From Ruining Your Reception

There are three ways that a generator can spoil reception: ignition noise from the engine, switching noise from the inverter and conducted noise on the AC cable. Each has its own cure, and all of them work better when the generator is far from the antennas.

• Distance. Put the generator as far from the antennas and receivers as the cables allow, 15 to 30 metres where possible, downwind and behind some natural obstruction. Noise falls quickly with distance, and so does the carbon monoxide risk.
• Cables. Use a heavy extension cable, 1.5 to 2.5 mm² for 16 amperes, laid on the ground in a straight run, not in a coil. A long cable adds a few volts of drop but also places the noise source away from the radios. A coil acts as an inductor and does not help.
• Chokes. Place a common-mode choke on the AC cable at the generator and another at the radio end, made from mix 31 or 43 ferrite with several turns, as the article on ferrite mix choice describes. Chokes remove the noise that rides on the outside of the cable and that radiates from it.
• Resistor spark plugs. On older engines, a plug with a built-in resistor and a resistor cap reduce ignition noise. Most modern small engines already have them.
• Filtering at the radio. A line filter on the power supply, or a battery in front of the radio, isolates the receiver from what the AC mains carries. With a battery floating on the supply, the stations are almost free of generator noise.

The approach to tracking the noise is the one in the guide to finding and killing RF noise: turn the generator off and on, listen with a portable receiver, and note which band and which cable change the level. Find the worst path first and fix it before the event, not at 3 a.m.

The Battery Option and Hybrid Power

Many groups now use the generator not as the direct source but as a charger. The generator runs at an efficient load to charge a battery bank, and the stations run from the batteries. The generator is then switched off for hours at a time, and the noise is gone when the bands are open. A 100 Ah LiFePO4 pack, which stores about 1,280 Wh, can run two 100-watt stations for several hours on SSB, as the article on a LiFePO4 battery for ham radio shows. The generator then needs to run for an hour or two per cycle. The arrangement needs a charger with the right profile and wiring with proper fuses, and the planning is part of the broader scheme in the article on planning Field Day power.

Fuel and Run Time

An inverter generator of 2,000 watts typically runs about 8 to 10 hours at a quarter of its load on a tank of 4 litres, and less at higher loads, but this varies a lot between models, so read the figure for your own unit. For 24 hours at a moderate load you need two to four tank fills. Calculate in advance and carry more than you need in approved containers. Fuel that has sat in a can for many months, especially with ethanol in it, can gum up the carburettor, so use fresh fuel and a stabiliser for stored fuel.

Plan the refuelling so that it never happens while the engine is hot or running. Let the generator cool, switch it off, fill the tank away from open flames and from the radios, and wipe up any spilled fuel. In a long event with several operators it is worth assigning one person as the generator keeper, who checks the oil, refuels, logs the hours and watches the load.

Safety: Carbon Monoxide, Fire and Electricity

A portable engine-generator is an engine, and every engine produces carbon monoxide, a colourless and odourless gas. The poisoning is a real killer at camps. Never run a generator in a tent, a van, a garage, a shed or any enclosed or partly enclosed space, and keep it at a good distance from doors and windows, with the exhaust pointing away from where people sleep or work. A battery-powered carbon monoxide alarm near the sleeping area is cheap insurance.

Fire and electrical hazards come next. Place the generator on level ground, on a non-flammable surface, away from dry grass and with room around it for cooling. Protect it from rain with a canopy that does not close it in. Use a residual-current device, also called a GFCI, on the output that feeds the stations, particularly in damp fields, and use outdoor-rated cables in good condition. Follow the manual regarding the earthing of the generator: some models bond the neutral to the frame, others do not, and the correct arrangement depends on the type and on the local rules. An uncertain earth connection is a risk for anyone who touches the radios, and a licensed electrician or the manual can tell you what to do. The overview of RF exposure safety is a reminder that the dangers of the site include more than the transmitter.

Noise for the Neighbours and the Operators

Noise matters to the people who sit at the radios for hours. Many small inverter generators produce 50 to 60 decibels at 7 metres, a level that is tiring over a day, and a conventional generator is louder. Put the generator where it is out of earshot of the operating positions, and consider that a CW operator with headphones is less disturbed than an SSB operator who must hear faint voices. A low earth berm or a plywood screen reduces the noise, but must never block the air flow or trap the exhaust. Check the rules of the site: many parks have quiet hours or ban generators entirely, and some event sites require a spark arrestor on the exhaust.

A Checklist for the Week Before

• Add up the loads, including peaks, and confirm the generator rating with a 25 percent margin.
• Run the generator at home with a real load and listen on every band that you plan to use.
• Test the chokes and the cable layout and note any band that is still noisy.
• Check the oil, the air filter, the spark plug and the fuel, and carry spares.
• Pack a GFCI, outdoor cable, a fire extinguisher, a carbon monoxide alarm and a tarp.
• Decide who runs the generator, and write down the refuelling plan.

The Bottom Line

A generator for Field Day should be sized from the sum of real loads, usually 500 watts per 100-watt station on the AC side plus lights and extras, with a 25 percent margin. An inverter generator gives stable voltage, lower noise and better fuel economy, but test it on the bands before the event. Put it far from the antennas, use heavy cables with common-mode chokes, and consider a battery bank so the engine can be off while the bands are busy. Above all, run it outdoors, away from tents and sleeping areas, with fresh fuel, a GFCI, a carbon monoxide alarm and a plan for refuelling. Good power is invisible; the operators simply hear the band.

Frequently Asked Questions

How big a generator do I need for a 100 W station?

A 700 to 1,000 W inverter generator is enough for one station with a laptop. For two stations and lights, 1,600 to 2,000 W is a safer choice.

Are inverter generators quiet on the radio bands?

Not always. They are quieter acoustically, but some models radiate switching noise. Test your generator with a receiver on the bands you will use.

How far should the generator be from the antennas?

As far as the cables allow, preferably 15 to 30 metres, downwind. Distance reduces both radio noise and the risk from exhaust gases.

Can I run the generator in a tent or garage if the door is open?

No. Carbon monoxide can build up even with open doors. Run it outdoors, away from doors and windows, and use a carbon monoxide alarm where people sleep.

Do I need a generator if I have a big battery?

For a short event, a battery and solar may be enough. For 24 hours with several stations, a generator charging the battery bank is a common and efficient mix.

What does a common-mode choke on the AC cable do?

It blocks noise current that travels on the outside of the cable and radiates from it, which reduces the hash that reaches the antennas and receivers.