The Power for your Radio

Foundations of Amateur Radio

A question that occurs more often than you might think is one related to powering your radio. It comes in a few different flavours, like: "I want to install a radio in my car, how do I power it?", or "I want to operate portable, what's the best way to power my radio?" or "What power-supply should I buy?" There are many more versions of this, but they all come down to the same underlying challenge. I spoke about sizing a battery a couple of years ago, but that's not the only consideration.

If you look at the power specifications of my Yaesu FT-857d, you'll see 13.8V DC +/- 15%, Negative ground, 1 Amp on Receive and 22 Amp on Transmit. Based on this I purchased two 26 Amp Hour batteries and a 45 Amp variable power supply.

My amateur license restricts me to 10 Watt and I tend to operate using 5 Watt.

On receive the actual draw, specified in the documentation at 1 Amp doesn't go above 0.5 Amp in typical use. Transmit, specified at 22 Amp doesn't go above 3.6 Amp at 5 Watts and at 10 Watts it's still only 4.5 Amp, so my 45 Amp power supply is slightly overkill, by a factor of 10. By the way, that's an FM carrier on 2m. Different modes and bands have different current draw.

I should make mention of the duty-cycle, that is the difference in time spent transmitting and receiving. A 100% duty-cycle means that you're transmitting all the time, 50% means half the time and 25% means that for every minute of transmission, you'll spend three minutes listening. There is more to the duty cycle, in brief, AM, FM and RTTY are 100% duty cycle modes, CW is a 40% mode and SSB has a duty cycle of 20%. So if you're listening half the time on SSB, your duty-cycle is only 10%.

At this point you should at least understand that what the manufacturer says on the box and what your radio actually does is entirely dependent on your use case. I have no doubt that there is a way I can operate my radio so it draws 22 Amp. I'm not quite sure how, but I'm sure it's possible.

Sizing aside, there are other things you need to consider. If you're in a car, do you wire the contraption directly to your car battery, or to a secondary battery? Should it be connected directly, or via the accessory switch? Should you get a DC to DC power supply, or some other technology? Also, not all cars are 12V, not all cars have their body as earth and the thicker the wire between the battery and the radio, the better.

My decision, given that I live in a country where distances are non-trivial, and in a state bigger than Texas, in fact Western Australia is bigger than Alaska, Texas and Minnesota combined, I decided that it would be prudent to make the power supply for my radio completely separate from my car. I have a toolbox in the boot, that's the trunk if your regulator is the FCC, which contains two 26 Amp Hour batteries. I take it out to charge and put it back when I need it.

Other solutions include second batteries with disconnect on low charge circuits, manual and automatic ones, direct connect to the main battery and variations on that theme.

In shacks I've seen batteries which are constantly charged connected to a radio and dedicated power supplies bordering on being a local sub-station to ensure that enough of the good stuff makes it into the radio and out to the antenna.

For portable operation I've seen Lithium in several different flavours, car start boost batteries, mobile phone USB batteries, remote control car batteries, and the like. If you have more than one, bring some red Velcro and use it to mark the flat battery.

One of the things you'll really only be able to learn after doing it is finding out what the noise level is that a power supply generates. A battery generally doesn't make noise, but the charger or up-converter might. Inverters are often a great source of HF noise, the cheaper the more noise, so test before you buy. Also, none of what I've said so far considers emergency preparedness, which is a whole other topic for another day.

As in any technical situation, in theory, practice and theory are the same. In practice they're not.

Be prepared to do some real world tests, see what your friends are doing and see what you can take-away from that.

My purchase of a laboratory variable 45 Amp power supply was excessive, but it's likely to outlast me. The two 26 Amp Hour sealed lead acid batteries are very heavy, but I avoid carrying them as much as I can and so far, seven years later, they still last most of the weekend during a contest. There's not a one-stop solution for power, just like there isn't one for picking a radio.

How do you power your radio?

I'm Onno VK6FLAB