QEX №4 2014.pdf

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The national association for
ARRL
AMATEUR RADIO
225 Main Street
Newington, CT USA 06111-1494
TM
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July/August 2014
About the Cover
QEX (ISSN: 0886-8093) is published bimonthly
in January, March, May, July, September, and
November by the American Radio Relay League,
225 Main Street, Newington, CT 06111-1494.
Periodicals postage paid at Hartford, CT and at
additional mailing offices.
POSTMASTER: Send address changes to:
QEX, 225 Main St, Newington, CT 06111-1494
Issue No 285
Harold Kramer, WJ1B
Publisher
Larry Wolfgang, WR1B
Editor
Lori Weinberg, KB1EIB
Assistant Editor
Zack Lau, W1VT
Ray Mack, W5IFS
Contributing Editors
Production Department
Steve Ford, WB8IMY
Publications Manager
Michelle Bloom, WB1ENT
Production Supervisor
Sue Fagan, KB1OKW
Graphic Design Supervisor
David Pingree, N1NAS
Senior Technical Illustrator
Brian Washing
Technical Illustrator
Advertising Information Contact:
Janet L. Rocco, W1JLR
Business Services
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QEX Circulation
Offices
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Telephone: 860-594-0200
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Copyright © 2014 by the American
Radio Relay League Inc. For permission
to quote or reprint material from
QEX
or any ARRL publication, send a written
request including the issue date (or
book title), article, page numbers and a
description of where you intend to use
the reprinted material. Send the request
to the office of the Publications Manager
(permission@arrl.org).
Gary Richardson, AA7VM, designed “An RF Filter
Evaluation Tool” that will find plenty of use on your test
bench if you build or adjust RF filters. A microprocessor
controller board sends command signals to a signal
generator, which feeds the test signal through the
filter and to the input of an RF detector board. The
microprocessor board then reads the RF power
measurements from the detector and sends them to a
computer.
In This Issue
Features
3
7
16
23
27
28
41
47
An RF Filter Evaluation Tool
Gary Richardson, AA7VM
A Fully Automated Sweep Generator
Measurement System — Take 3
Dr. Sam Green, WØPCE
Android Wireless Project Control: Part 2 —
Example Application
Thomas M. Alldread, VA7TA
A Linear Scale Milliohm Meter; Another Look
Don Dorward, VA3DDN
New Book Announcement: Radio Receiver Technology
Hardware Building Blocks for High
Performance Software Defined Radios
Scotty Cowling, WA2DFI
Digital Signal Processing and GNU Radio Companion
John Petrich, W7FU and Tom McDermott, N5EG
Upcoming Conferences
Index of Advertisers
ARRL: ................................................Cover III
Array Solutions: .......................................... 48
Down East Microwave Inc:.......................... 22
Kenwood Communications: ................Cover II
M
2
:............................................................... 15
Nemal Electronics International, Inc: ...........22
Quicksilver Radio Products............... Cover IV
RF Parts:............................................... 25, 27
Tucson Amateur Packet Radio: .................. 40
QEX – July/August 2014
1
The American Radio
Relay League
The American Radio Relay League,
Inc, is a noncommercial association
of radio amateurs, organized for the
promotion of interest in Amateur Radio
communication and experimentation,
for the establishment of networks to
provide communications in the event of
disasters or other emergencies, for the advancement
of the radio art and of the public welfare, for the
representation of the radio amateur in legislative
matters, and for the maintenance of fraternalism and
a high standard of conduct.
ARRL is an incorporated association without
capital stock chartered under the laws of the state
of Connecticut, and is an exempt organization
under Section 501(c)(3) of the Internal Revenue
Code of 1986. Its affairs are governed by a Board
of Directors, whose voting members are elected
every three years by the general membership. The
officers are elected or appointed by the Directors.
The League is noncommercial, and no one who
could gain financially from the shaping of its
affairs is eligible for membership on its Board.
“Of, by, and for the radio amateur,” ARRL
numbers within its ranks the vast majority of active
amateurs in the nation and has a proud history of
achievement as the standard-bearer in amateur
affairs.
A
bona fide
interest in Amateur Radio is the only
essential qualification of membership; an Amateur
Radio license is not a prerequisite, although full
voting membership is granted only to licensed
amateurs in the US.
Membership inquiries and general corres-
pondence should be addressed to the
administrative headquarters:
ARRL
225 Main Street
Newington, CT 06111 USA
Telephone: 860-594-0200
FAX: 860-594-0259 (24-hour direct line)
Officers
President:
KAY C. CRAIGIE, N3KN
570 Brush Mountain Rd, Blacksburg, VA 24060
Chief Executive Officer:
DAVID SUMNER, K1ZZ
The purpose of
QEX
is to:
1) provide a medium for the exchange of ideas and
information among Amateur Radio experimenters,
2) document advanced technical work in the Amateur
Radio field, and
3) support efforts to advance the state of the
Amateur Radio art.
All correspondence concerning
QEX
should be
addressed to the American Radio Relay League,
225 Main Street, Newington, CT 06111 USA.
Envelopes containing manuscripts and letters for
publication in
QEX
should be marked Editor,
QEX.
Both theoretical and practical technical articles are
welcomed. Manuscripts should be submitted in word-
processor format, if possible. We can redraw any
figures as long as their content is clear.
Photos should be glossy, color or black-and-white
prints of at least the size they are to appear in
QEX
or high-resolution digital images (300 dots per
inch or higher at the printed size). Further
information for authors can be found on the Web at
www.arrl.org/qex/
or by e-mail to
qex@arrl.org.
Any opinions expressed in
QEX
are those of
the authors, not necessarily those of the Editor or the
League. While we strive to ensure all material
is technically correct, authors are expected to
defend their own assertions. Products mentioned
are included for your information only; no
endorsement is implied. Readers are cautioned to
verify the availability of products before sending
money to vendors.
Larry Wolfgang, WR1B
Empirical Outlook
Sharing Ideas
I have occasionally used this space to encourage readers to document their projects and
share their ideas by writing for
QEX.
After all, the subtitle of our magazine is
A Forum For
Communications Experimenters,
and
Forum
implies shared information. You may think that this
is a plea for you to submit an article for publication. It is certainly true that we need a lot of good
article submissions to fill the pages of
QEX.
So yes, it is.
There are other reasons to write about your projects than just publishing them in
QEX,
though. You are going to need some notes or documentation for later reference. I can almost
guarantee that some time later you will have to troubleshoot the device, or you may even want
to modify it to make use of a new IC or circuit idea. Sure some brief notes in a notebook or com-
puter file may help you remember the details, but how much easier would it be if you could read
the description you wrote for publication? With the extra care you will have to put into document-
ing the project for others, you will have a more complete record of your work.
There are other Amateur Radio operators who are also trying to do the same thing you have
been working on, or something very similar. At first, everyone has to “discover” all about the
project for themselves. That can be fun, and even provide it’s own reward, but sometimes it is
nice to know what others have been doing, rather than having to reinvent the wheel. There are
also some experimenters who aren’t engineers, and may not be able to invent the wheel, but
who would really enjoy building a wheel similar to yours, and learning how it works. With your
help, they will discover some of the same thrills you have experienced.
Here is one example that I would like to share. It’s a project I have recently become interested
in, and have begun to “play” with. We know that there are groups of hams who have taken off-
the-shelf wireless routers and installed new firmware to create Amateur Radio networks. These
have been called high speed multi-media (HSMM) mesh networks, although the more recent
term seems to be broadband-hamnet. A number of these reprogrammed routers can be
deployed to create a network in a remote area or during an emergency communications event.
There have been a few
QST
articles about the use of such networks by ARES groups or for
other applications. Some clubs have used these mesh networks to link their logging computers
at Field Day, for example. There is quite a bit of information on various websites, and the one
that I have found that seems to have the most information is
www.broadband-hamnet.org.
By searching through the documentation on this website I have been able to find the list of
routers that will work with the new firmware. Then I found a pair of LinkSysWRT54G routers that
met the criteria at a tag sale, for $3 each. More searching brought me to a set of instructions for
flashing the new firmware into these routers. Great! Now what? Slowly I am digging through the
information to learn how I might link computers, create an access point to my little broadband-
hamnet network and eventually be able to use it in some way. At this point I have one router
connected via LAN cable to my Raspberry Pi, and the other one connected via LAN cable to my
iMac. How can I connect to my network with my iPhone or my wife’s
Windows
laptop via a WiFi
link? There must be a way, I think. More searching.
My point is that there is a lot of information, but it is not necessarily the clearest documentation
for someone who knows nothing about creating and using these networks. The Technical
Editorial Staff at ARRL Headquarters has discussed our desire to have an article or series of
articles for
QST
and/or
QEX
about these routers and networks. At first we had the mistaken
impression that the routers that could be flashed with this new firmware were obsolete models
that would only be available from a junkbox or lucky find at a tag sale. Publishing an article about
something that requires you to find some obsolete hardware is not generally the best idea. We
have since learned that there is at least one model (the LinkSys WRT54GL) that is a currently
available model. We have also learned that the movers and shakers in this project have been
working on creating firmware for some other current production routers.
So there is a need for an article that gives the basic details of what to look for in a router,
including sources for new hardware, a description of how to flash the new firmware into the
router and how to configure the mesh node. There is a need for a description of how to connect
to the network with either a wired LAN connection or a WiFi connection, and how to then pass
data around the network to and from the various computers on the network. An article or series
of articles about these networks would serve to document the details in a publication for those
new to the topic. The more people who become involved in building these nodes and adding
them to the overall network, the more we can do with this technology.
I haven’t even touched on the possibilities of adding high-gain external antennas and even
power amplifiers to the routers. There are many possibilities when we continue experimenting
with the networks as Amateur Radio operators.
This is just one example. There are lots of other projects that hams are experimenting with.
Help share the wealth!
2
QEX – July/August 2014
Gary Richardson, AA7VM
PO Box 228, Marblemount, WA 98267;
aa7vm@arrl.net
An RF Filter Evaluation Tool
Here is a microprocessor controlled system to measure filter frequency response.
Unless you have a spectrum analyzer,
determining the response of an RF filter is
rather tedious — adjust the signal generator,
measure the output, record the value and
repeat until sufficient data has been collected.
Then you must convert the values to decibels
and make a plot, or enter the data in your
computer and use a program to generate the
plot. Not long ago I was occupied with this
drill, and it seemed to me that there ought
to be a way to automate the process. A little
looking around turned up a lot of information
on the subject of RF signal measurement.
1, 2,
3, 4
After several false starts I came up with
a design based on the AD8307 that seemed
promising.
A block diagram of my measurement
system is shown in Figure 1. It consists of
a computer having at least one serial port,
a signal generator that can be controlled
by commands sent to its serial port, the
filter to be evaluated (DUT), a couple of
attenuators and a bit of hardware to tie
everything together: a detector module and
a control module. The main component of
the detector module is an AD8307, which
generates a DC voltage proportional to the
1
power of the input RF signal. The control
module has two functions accomplished by
a microprocessor: relaying command strings
from the computer to the signal generator
and acquiring and sending the power
measurements to the computer.
5
Figure 2 is a photograph of the two
modules mounted in a small enclosure.
Following the advice of Hayward and
Larkin, I mounted the detector board in a
smaller enclosure made of 24-gauge copper
sheet (the top cover has been removed for
Notes appear on page 6.
Figure 2 — The photo shows the two modules of the measurement system mounted
inside a small metal enclosure. The detector board is also mounted inside a separate
enclosure. For this photo, the top cover of that enclosure has been removed.
Signal
Generator
Serial Frequency Data
–6 dB
Device
Under
Test
–6 dB
Log Power
Measurement
Serial Data
Commands And
Measurements
Control μP
Computer
QX1405-Richardson01
Figure 1 — This is a block diagram of the filter measurement system.
QEX – July/August 2014
3

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