Sounds like a ridiculous idea doesn’t it? But why do so many hams do it anyway? Of course, I don’t mean the freezer in your kitchen, or the one in the garage, basement or wherever else you store the meat from your hunting trip last fall. You really wouldn’t want to get bits of food in your radio would you? No, we all keep another freezer out on the driveway and that’s where we keep the radios we paid hundreds, maybe thousands of dollars for. Well actually, if the XYL asks we got it from the bargain bin at a hamfest.
Winters in Ontario can get pretty darn cold. The mercury can routinely reach down into the minus teens on the Celsius scale. It can drop even lower. Temperatures into the minus twenties are quite common; even below minus thirty isn’t unheard of. And we aren’t talking about Ontario’s lovely beaches on the southern shore of Hudson’s Bay here. You know, the terminus for the Polar Bear Express. Wide, uncrowded pebbly beaches where hungry white furry bears can set up their BBQs for a tasty al fresco meal of fresh caught seal meat. No, we are talking Hogtown, that ugly blemish way down south on the shore of Lake Ontario. I’m not making it up; I looked up the official temperature records for the city.
But just for reference, the recommended temperature setting for a domestic food freezer is -18degC or below. And that’s the kind of temperature your mobile rig is going to experience sitting on your driveway overnight in January and February every year. So what is the first thing you do when once you have fired up your engine, cleaned the ice and snow off your windshield and driven off into the freezing morning? Your radio probably came on automatically; you did connect it directly to the battery, as recommended, didn’t you? So you grab the mic and throw out your callsign hoping for someone with whom to rag chew on your drive to work.
Fifty watts ought to do it. You could probably hit the repeater on low power but you don’t want to risk losing the signal when you drive through that RF hole on the way to work so you leave the rig on full power. The tiny circuits inside the rig are slowly beginning to warm as they recover from twelve hours of sitting in the cold and dark. The outside air is minus 20degC and after a whole night of cooling, that’s the temperature of the radio’s internal circuitry too. The car heater is blasting out hot air and you are feeling comfortable, but all those tiny transistors are still emerging from the deep freeze. It’s a phenomenon known as thermal inertia.
Here is what happens when a radio gets very cold. Everything contracts. Metal casings get ever so slightly smaller. Screws loosen. Circuit board traces contract too. But the miniature copper traces connecting components are extremely fragile. If they contract too much they could potentially separate from the circuit board. When the power transistors in the rig’s RF amplifier blast out those 50 watts they get hot quick. Now we’re in trouble. Hot finals and freezing cold circuit boards – what could possibly go wrong?
The engine has now reached running temperature, the vehicle’s cooling system thermostat has opened and the passenger compartment heater is blasting out hot air, making the driver and passengers nice and cosy despite the horrible conditions outside. The snow your winter boots brought into the vehicle has melted into pools of water in the mat beneath your feet. The heater fan is blowing the air inside the vehicle around and picking up moisture. The radio is only slowly beginning to warm. Mobile rigs have a lot of metal inside that acts as a heat sink for the big final RF amplifier stage MOSFET transistors.
Now warm moist air inside the vehicle is coming into contact with the still cold metal case and heatsink of the radio. Moisture from the air condenses on the cooler metal and begins to drip onto the circuitry:
“Copied most of that but you’ve got a bit of bacon frying on your signal”.
“Roger that” you reply “damn Icom engineering; should’ve kept the Yaesu”.
It wouldn’t have happened fifty years ago, of course. Well maybe there weren’t as many repeaters around back then. But, more importantly, radios were built using discrete components. You could count the number of semiconductors on your fingers and toes (but keep your socks on in winter). The CPU in my new computer has one hundred and sixty million transistors. The controller in a mobile radio probably only has a few million, but even so that’s a lot of fingers and toes to count.
In the old days, connections between components were made with wire not wafer thin, tiny copper traces bonded to a plastic board. Resistors were nice big things printed with coloured rings so you could read their value. Capacitors (or condensers as they were formerly known) even had their value printed on their side in a font big enough to read without a magnifying glass.
Anyway, long story short, when the temperature dropped to brass monkey territory everything in the radio contracted just the same. Only it didn’t make the slightest difference to the circuit. Sure, water condensed inside the old boat anchors the same way it condenses inside modern rigs. But while tiny water droplets can bridge several connections inside a modern rig, they drop harmlessly through clusters of giant resistors and capacitors inside the good’n’old-uns they used to build.
Enjoy the winter folks!