Table Of ContentOscilloscope Applications for the QRP Enthusiast
By
James McClanahan, W4JBM
Fi rst Ed ition
Copyright 2015 © James McClanahan
1098765432
All rights reserved. No part of this book may be reproduced in any form or by any
electronic or mechanical means, including information storage and retrieval
systems, without written permission from the author, except in the case of a
reviewer, who may quote brief passages embodied in a review.
Trademarked names may appear throughout this book. Rather than use a
trademark symbol with every occurrence of a trademarked name, names are used
in an editorial fashion, with no intention of infringement of the respective owner's
trademark.
The information in this book is distributed on an "as is" basis, without warranty.
Although every precaution has been taken in the preparation of this work, neither
the author nor the publisher shall have any liability to any person or entity with
respect to any loss or damage caused or alleged to be caused directly or indirectly
by the information contained in this book.
"Trying to forget really doesn't work. In fact, it's pretty much the same as
remembering." -Rebecca Stead
Treasure the small moments that are destined to become part of the past. -me ...
Table of Contents
1. Introduction
2. Safety
3. Equipment Used
4. Selecting an Oscilloscope
5. Construction Techniques
6. The Test Jig
7. Measuring Transmit Output Power
8. Measuring DC Input Power
9. Transmitter Keying Characteristics
10. Transmitter Harmonic Suppression
11. Evaluating RF Filters
12. Final Thoughts
Bonus Chapter 1: RF Proofing the Power Supply
Bonus Chapter 2: Another Dummy Antenna
1. Introduction
QRP continues to be one of the more active segments of the amateur radio hobby.
Different people are drawn to QRP for different reasons. Some like the low cost of
QRP gear. (Although, having said that, you can certainly spend a fair amount of
money on top-notch QRP gear.) Others like the ability to operate small, portable
stations.
Personally, I have always found it rewarding to (literally) take a handful of parts
costing a few dollars and build equipment that allows me to communicate over
hundreds or even thousands of miles. From the number of kits, construction
articles, and discussions related to QRP projects, it seems that many share this
enthusiasm.
The question, "Should I buy an oscilloscope?" or, even more frequently, "I just
bought an oscilloscope, what can I use it for?" is one that comes up frequently.
A few decades ago, the minimalist philosophy that often accompanies QRP
construction and experimentation would have been at odds with the expense and
complexity often associated with the oscilloscope-especially a digital storage
oscilloscope. But this is a different era! Many QRP enthusiasts want to better
understand the details of how their gear works and the availability of low-cost,
high-performance oscilloscopes makes it possible to have a view of the inner
workings that was available only to those fortunate enough to have access to a
well-equipped lab not that long ago.
This booklet is focused on some basic applications of the oscilloscope with what I
would call a "minimalist" transmitter with the goal of showing how to a QRP
transmitter can be tested and characterized.
In addition to a radio and an oscilloscope, certain pieces of gear will be needed for
some of the tests. I will admit I struggled with a bit of "the chicken or the egg"
issue in whether to discuss the radio testing first so there was a better
understanding of just how the construction projects would be used or whether to
discuss the construction projects first so the testing descriptions could be more
specific. I eventually settled on the later, but I do suggest you study each project
and the way that project is used for testing together before you break out the
soldering iron.
I have included a number of pictures along with schematics. Some of the
schematics were scanned from my lab book, so they aren't as pretty as ones I
sometimes put together using schematic capture tools. My only excuse is that I
extended the minimalist approach there as well.
If you have built a QRP transmitter or two, you should have enough electronics and
project construction background for anything we will be discussing.
Also included at the end are two "bonus chapters" that QRP enthusiasts may find
interesting and useful.
Let's get started!
73 de
Jim, W4JBM
"With a soldering iron in one hand, a schematic in the other, and a puzzled look on
his face ... "
2. Safety
Safety is something we should always take into account as we build and
experiment.
The voltages and currents we work with at the QRP levels are not typically lethal,
although the wrong combination at the wrong time can potentially produce fatal
results.
But hot soldering irons, flying bits of wire as we trim leads, and blobs of molten
solder all can pose very serious safety hazards.
As you work on these projects, please be safe!
3. Equipmen,t Used
Yogi Bera one said that, "In theory there is no difference between theory and
practice. In practice there is."
We will certainly touch on some theory in portions of this book, but ultimately the
focus is meant to be practical. Because of that, most of the experiments make use
of gear that I have available to me.
The oscilloscope used is a Rigol DS1102E 100 MHz digital oscilloscope. I have
several analog scopes available, but we will talk about why the Rigol and other
digital storage oscilloscopes (DSOs) are good choices for the QRP radio
experimenter in a bit.
For the QRP transmitter, I choose the venerable Tuna Tin 2. I built this one from a
kit over a decade ago and it has seen its share of use. Actually my first "1000 m,iles
per watt" contact was made with this while listening on a restored vintage Radio
Shack Realistic DX-160 receiver. A pretty low-tech combination!
There is nothing magic about my selection of oscillloscope or transmitter. Both of
these are common, but it is likely that you can use a si~milar scope and whatever
project you might be working on and duplicate many of these results.
Just a note on some conventions I will use later on. The trace in the picture above
is from Channel 1 and is yellow. You can see that the Channel 1 Input connector is
labeled with yellow and that I have installed the yellow color code ring on the
probe. Because of that, I will use a yelllow circle with the number 1 to indicate
where Channel 1 is connected, a blue circle with the number 2 to indicate where
Channel 2 is connected, and a green circle with the letter G to indicate the ground
connection. These are shown in the legend below:
1IIIn1lp1ut
1Clhl~I]nelll '11
2 1111np,'ut
'Clhlalnl:~-_,e'l
