Sending speed in Morse Code (CW) is measured in words per minute, but how long is a “word”? In the English language the simplest word is the indefinite article “A”. A contender for the longest word may be the name of a village in Wales called Llanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch. So the word PARIS has been chosen to represent the standard word length for measuring the speed of sending CW.
The word PARIS comprises a total of 50 units; one unit is the length of one dit. Those 50 units are made up of 22 mark units and 28 space units. If sent correctly a dah is 3 units, the space between each element in a character is one unit; the space between each character is 3 units, and the space between words is 7 units. The following diagram may make this is a bit clearer.
So What’s the Secret Wise Guy?
Just hang in there a cotton pickin’ minute pilgrim and we’ll get right to it. The secret is pretty darned important to those of us who operate backpack portable stations for activities such as POTA and SOTA. You see, we have to carry in a battery to our operating site. And that site might be miles down (or up) a trail. Batteries are heavy so we need to be able to optimize the amp-hour capacity of our portable power source to ensure we aren’t carrying excessive weight.
Oh, QRP? Yes, of course if you have the budget available to buy a fancy-dandy modern QRP rig with built-in batteries, automatic tuner, panadapter, coffee maker and Uncle Tom Cobley and all you’re good to go. The best of these rigs cost an arm and a leg while full-featured portable QRO rigs sell for generally more reasonable prices.
This humble pilgrim uses a Yaesu FT-891 radio for POTA activations. It is capable of 100 watts but I usually wind the power down to 5 watts unless propagation conditions are particularly bad. To power the FT-891 I carry a Bioenno 12 Amp-hour LiFePO4 battery. By the way, these batteries are quite expensive but the investment is well worth it. Bioenno is an excellent company to deal with – just advice from a satisfied customer.
How Much is Enough?
My operating style usually involves – at most – two park activations in a day. When I say two I mean two separate operating sessions, each of which may be an “x-fer” where credit for more than one park is earned for one activation session. My brain and body run out of steam long before my battery so even a modest 12AH battery may be overkill.
To calculate how much battery capacity is needed we first decide the proportion of transmit to receive time involved in our operation. There’s a bit of guesswork involved, but I figure I run about 50/50 during a CW POTA activation. Thanks to the online POTA spotting page I can spot myself using a mobile phone, call CQ a couple of times then work the usual pile-up until the hunters stop coming. A POTA exchange is about evenly distributed between transmitting and receiving, hence 50/50.
Having established the transmit/receive ratio and working with the current consumption for transmit and receive we can calculate the total amp-hours consumed during an operating session. I am going to use a figure of 1 amp during receive and 5 amps during transmit for my FT-891. At a 50/50 transmit/receive ratio the radio is drawing an average of 3 amps of current. If I operate for one hour I will deplete the battery capacity by 3 amp-hours. My 12AH Bioenno battery should, therefore, last for almost 4 hours of operating (the battery should not be completely depleted).
Like I said, my brain turns to mush before my battery runs out of juice. My fingers start to operate independently sending uncontrolled random characters. I think they call the phenomenon “age”, time to slow down.
The Big Secret Revealed – oh finally!
Let’s go back to the PARIS analysis for a moment. Transmit time isn’t actually all transmit! of the 50 units in PARIS, only 22 are key down. During 28 of the 50 units no signal is being transmitted. This means that, even during the 50% of time during which we are transmitting, we are consuming 5 amps for 44% of the time and 1 amp during the remaining 56%. This changes our 50/50 ratio to 22% key down and 78% key up!
Well that paints a different picture … Mr Spock!
Yes … captain; now our 4 hours operating time is extended to almost 6 hours before we have to return to “Starbase” for new “di-lithium crystals“.
Well, that didn’t sound too hard, what’s the deal?
Nothing is easy, and even if something appears easy, it has been over-simplified. Engineers, mathematicians, scientists and Vulcan Second Officers will soon find a way to re-complicate it with incomprehensible gobble-de-gook. And so it turns out with our nice, neat, simple analysis of battery consumption.
If a radio’s electronic keyer produced pure square-wave pulses the resulting code would sound harsh and unpleasant. So the boffins who design ham radios shape the audio pulses with smooth rise and fall times to soften the sound. Now we have to resort to calculus in order to accomplish the complex re-calculation of our key up to key down ratio to accommodate this effect.
Or, we can just be happy playing radio!