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Tuesday, October 16, 2012

QRP Operation


QRP equipment can be very simple, but the fact that we are working with lower power circuits means that experimentation and inventiveness are possible for all of us. The QRP equipment industry is thriving, with several kit and accessory manufacturers enthusiastically supported by the market and new gear coming out constantly.

Quality? Can't beat it. Put your $100 home-built radio next to a $3000 Ginzu box and the QRP rig will clearly win more often than not. Surprises hell out of guys who have just mortgaged their XYL to buy the latest all-singing-and-dancing bells and whistles transceiver. But it shouldn't be a surprise, because the biggest part of what you are paying for in the "big rigs" is circuitry to make up for a front end like a barn door. Your typical QRP transceiver is thoroughly optimized for operation on a single band (or a few bands), CW only, and with very efficient signal processing from front-end to headphones. A good rule of thumb, which is proven by QRPers every day, is "if you can hear him you can work him." And we have better "ears" than many of those guys who can boil water on their linears.

I said CW only, there, didn't I? That's because CW is at the heart of QRP- it's what makes QRP possible as a hobby, and QRP becomes a very good justification for CW. The reason goes back to the math, but perhaps not quite as directly. Let me state this as a fact- all else being equal (operator skill included), CW has an 18dB advantage over SSB. If you read the power ratio stuff earlier, you can see that 18dB is a HUGE difference. That's about the best I can do in terms of math, because ultimately you are comparing apples with oranges, but perhaps I can at least explain the sense of it. A CW signal is either there or it is not, and that's something the ear and brain can detect and work with very easily. SSB transmits the human voice, which consists of a wide range of frequencies and a wide range of amplitude or volume. The result is that the power used to transmit SSB is spread out over a "bandwidth" of a couple KHz. All of the power in a CW signal is concentrated in a couple of Hertz. And in SSB, the peak power is used only on voice peaks, which are a very small percentage of the transmitted signal. So the 18dB figure is justifiable, if not exactly measurable (others will quote higher or lower figures, but it's all relative). In practice, especially when conditions are marginal, a 5W CW signal will work better than a 100W SSB signal. And again, you don't have to take my word for it. If you've worked much DX you know that often CW QSO's are possible long before SSB "comes in" and sometimes SSB never quite makes it. Even if you are not a DXer, you should be able to prove this to yourself very easily in half an hour on the air. If you are a real Doubting Thomas, do a real test. Get on the air with a buddy using SSB and reduce power until you can no longer copy each other. Switch to CW at the same power level, and amaze yourself at how much farther you can reduce your output and still communicate.

As you might suspect, there is not a lot of SSB QRP activity, but there is some, especially since we are experiencing good propagation on 10 and 15M, where there is next to no noise and less power is needed. In fact, a large number of long-time QRP CW operators are turning to QRP SSB as the "next challenge."

Read more http://www.mtechnologies.com/pubs/qrp.htm

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