Thinking of Powering A QRP Ham Radio Station Out In The Field?


I recently received an email from a new ham who wishes to run his radio with batteries charged by solar power and wanted to know the best way to do it. First, there is no “cut and dried” answer to that. It depends upon many variables and what you want to do with your station.

 I assume he means operating QRP (low power). Even that, by definition, may mean different things to different operators. When you use QRP to run, say a CW station, you are usually limited in most respects (such as contesting) to 5 watts output power or less. Then there is operating QRP using SSB. A lot of groups specify SSB QRP to mean 10 watts (or less), although some groups hold to 5 watts output on SSB. These are all “maximums” in the basic QRP world. I have run as little as a few micro-watts to make contacts with other ham radio stations. There is even a group who measures their contacts in “miles-per-watt”. Records are kept for these contacts on each band. As I recall, two hams contacted each other on 80 meters using what could be the equivalent of 13 MILLION miles per watt based on the distance to each other and their micro-micro-watt transmit power.

 So let’s say you decide you wish to operate using 5 watts on CW. Doing the math, 5 watts means a DC current of .42 amps at 12 volts. But this is not the correct answer you are looking for. When you are operating 5 watts on the radio, this is your output power, not input or power consumed. Taking a base efficiency of about 80% for your radio, this would mean you would be consuming about 6.25 watts. This means you would be using a current of about .52 amps if your power source is supplying 12 volts. (And for your info, most QRP radios have a built-in regulator which keeps your power input steady even if your fresh battery reads 13.1 volts, etc) OK, so you are using a little over 1/2 amp……BUT…. this is only when you are actually transmitting. Your radio typically uses about 150 milliamps when you are receiving. These numbers are all “ball park”, the only way to read your actual consumption is to use an ammeter and key down your radio in CW mode (transmit).
Now comes the interesting part.
There is something called “duty cycle” in amateur radio operation. That is the time, in percent, that you are transmitting an actual signal. As an example, when you key your FM  2 meter transceiver, for the time it is keyed down (transmitting a signal) you can be thought of using 100% duty cycle. When you stop transmitting your power consumed is dramatically cut. This is because on FM, your signal is being sent out whether you speak or not. So let’s say you are checking in to an FM net. Most of your time will be spent listening, hence not using a tremendous amount of power. Your real-time power consumption probably drops to about 10% of the time unless you are a blabber-mouth like me where my transmit cycle, since I am talking quite a lot, may be upwards to 20%.
Using CW or SSB, your duty cycle is the time ONLY when you are transmitting a waveform. There are small spaces in time, between sending dits and dahs or between speaking words that there is no signal being transmitted therefore it changes your duty cycle. Depending on how fast you send CW or how fast you speak also varies the duty cycle of your transmit time. Also keep in mind that when you operate during a contest your duty cycle will change because you are transmitting more during a specific period of time. So duty cycle can be seen as the amount of time you’d be using the full amount of power from your battery.

Let’s now estimate your power consumed based upon the type of emission and your duty cycle. In, say, a non-contest or rag-chew operation your duty cycle on CW might be about 20%. This means that only 20% of the time are you using full power. Using 6.25 watts as a general standard and 20% duty cycle, your amp-hours drawn by your supply battery will be about .08 amp-hours. Let’s round this off to include the receive power requirement and we come up with something like .11 amp-hours. With the standard deep-cycle marine battery supplying about 100 amp-hours (100 amps for 1 hour), this means you can ideally run your QRP CW station almost 900 hours! This doesn’t even include the fact that batteries are rated in amps per hour and that when you draw less per hour your battery will last longer.

HB15_Solar_PanelThis also means that you can charge your deep-cycle marine battery using a blocking diode and a small solar panel (I use 15 watt solar panels from Harbor Freight).
I use deep-cycle marine batteries because they are basically self-regulating. You can buy these batteries at Wal-Mart.
My 30 years working QRP this way proves it is safe to do so. If you are thinking about using SSB QRP, you are safe if you cut the hours you can use your power system in half to about 450 hours. That’s still a lot of use for a weekend camping trip or Field Day.  If you are planning to use a small gel-cell battery, use the same formula divided by it’s advertised amp-hour rating.
I always keep a back-up battery as well, with one operating and one charging. If you do it this way, you can use smaller batteries to save cost.

All of the numbers are semi-accurate (rounded off) estimates grabbed from my head so don’t hold me accountable if, for instance, the  .52 amps is really .517, etc. This also doesn’t take into account your battery’s internal drain and the fact that you probably will be using a battery with alot less than 100 amp-hours. This is just to show you the relationship and what you need to know if you are considering using QRP in the field.

One Response

  1. EcomikeKi6moe Says:


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