Foundations of Amateur Radio

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The Power for your Radio

Posted: 23 Jun 2018 10:00 AM PDT


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...
This posting includes a media file:
http://podcasts.itmaze.com.au/founda...teur-radio.mp3