ARLA/CLUSTER: Episódio 195 da série "Foundations of Amateur Radio" - Porque será que os nossos rádios necessitam de 13,8 V.?

[João Costa > CT1FBF]

Foundations of Amateur Radio #195


*All the power in the world and not enough battery!*
The transceiver you use to get on air and make noise needs power to
operate. The traditional voltage for our amateur equipment is 13.8 Volts.
Why not 12 Volts you ask. The short answer is chemistry, but let's move on,
there is lots to cover.

Generally that 13.8 Volt is specified with a +/- symbol and some
percentage. For my radio it's 15%, which means that if I plug it into power
that's somewhere between 11.7 Volt and 15.9 Volt, I'm good to go.

Then when you look a little closer at the specification you'll see that my
radio draws 22 Amp. That's a whole chunk of juice that needs to come from a
power supply. Of course that means that you'll also need to deal with 22
Amp fuses, wire capable of dealing with 13.8 Volt at 22 Amp, and connectors
that won't melt when you do that.

If you look closer again, you might notice that 22 Amp is when you're using
the radio at maximum power, that is, 100% duty cycle and 100% power, and
only during transmit, in the case of my radio, 100 Watts for HF.

So, if I'm using a digital mode, AM or FM, at 100 Watts on HF, my radio
says it will draw 22 Amp at 13.8 Volts.

Those numbers aren't correct of you're using CW or SSB. A rough number to
work with for CW is 40%, that means if you're doing CW for a minute, that's
the equivalent of key down at a 100% for 40 seconds and key up at 0% for 60
seconds.

SSB is roughly 4 times as efficient as AM, about 25% duty cycle, but
realistically it's more like 20%, since your power consumption depends on
how much you're yelling into the microphone. If you take long breaths, 0%
power, whistle into the microphone, 100% of SSB, or 25% of overall power.

Now all this gets even more interesting if you consider that you're not
just transmitting all the time. If you're only transmitting half the time,
you need to take your power consumption down another 50%, so SSB might be
10%, CW only 20% and the digital modes 50%, from the perspective of the
power supply.

So you want to go portable and need batteries. Batteries don't come in 13.8
Volt versions. So 12 Volts. Get the number of amp hour and you're good to
go right?

Nope.

Your battery doesn't just run at 12 Volts and then all of a sudden stop, it
runs down, you've seen it in a torch or a Walkman when the tape got slower
and slower. A 26 Ah battery should give you 26 Amp for an hour at 12 Volts,
but if you actually do that, you'll need to buy a new battery, because
you'll have destroyed the one you just exhausted.

All of this then starts a conversation about chargers, which incidentally
might put out 14.4 Volts. You might turn to solar panels, which at peak
power operate at something like 18 Volts, then you stumble into the world
of PWM vs MPPT solar converters or charges. Then there's the joys of over
and under current, battery discharge rates, continuous versus intermittent
charging, different battery types, battery safety, storage, weight,
out-gassing and more fun than you'll want to know about on your morning
commute.

And I haven't even talked about battery isolation, HF interference from
chargers and inverters, the differences between powering your radio
straight from a battery or via a DC to DC converter, using 240 Volts, or if
you're in the USA 120 Volts in the field, generators, compatibility with
others and how much all this might cost and if you need to invest in lotto
tickets to pay for this experience.

One tool I stumbled across in my travels is the Four State QRP Group
website which has the W1PNS / WA0ITP / AB8XA Battery Life Estimator, which
in a single web page gives you the ability to say what mode you'll be
using, for how long with what battery size and how much radio draw and
it'll tell you how much more battery you'll need to get the job done. Very
handy for a contest that you're hoping to operate portable from a battery.

This all to say that power is a very deep rabbit hole and it will take you
some time to figure out where your use pattern puts your requirements and
budget.

Here be dragons.

I'm *Onno VK6FLAB*
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