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<div class=Section1>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Gentlemen;<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Here is a project proposal that I would like to present to
the group for comments and discussion. I’m hope you won’t find it
too long or boring. Thanks to all for the consideration, in advance.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'>Ken WR5H<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
<h1><b><font size=5 face=Arial><span style='font-size:16.0pt'>PROPOSAL FOR A
SMALL TRANSMITTING LOOP ANTENNA AND AUTOMATIC TUNER FOR SDR/JANUS/OZY<o:p></o:p></span></font></b></h1>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal align=center style='text-align:center'><b><font size=3
face="Times New Roman"><span style='font-size:12.0pt;font-weight:bold'>Abstract:</span></font></b><o:p></o:p></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>This proposal details a project targeted for the HPSDR group to
implement a small transmitting loop antenna. The antenna tuning capacitor of
the loop will be motor-driven and remotely-controlled. Frequency information originating
from the SDR will be sent to the controller from the Ozy over an I2C bus. The
controller will keep the antenna tuned to resonance at the operating frequency
without operator intervention. The controller could also be modified to
operate any local or remote antenna tuner capable of using a motor drive.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The intent of this proposal is to provide enough detail to describe the
electrical and mechanical features of the tuning system for presentation to the
HPSDR group and open a discussion for comments and suggestions. If there is
enough interest within the group to warrant, then a project will be initiated
on the HPSDR website. <o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Small
Transmitting <st1:place w:st="on">Loop</st1:place> Antennas<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>In these days of antenna restrictions, there has been much discussion
of small, easily transported, and discretely deployed hf antennas. One of
these is the small transmitting loop, which most generally consists of copper
tubing formed in a loop with diameters ranging from less than a foot (for vhf
use) to very large structures intended for 80 and 160 meters. The loop circumference
can be less than 1/10 wavelength and the loop is brought to resonance with a
capacitor across the endpoints of the loop. Efficiencies can run very high,
even into the 90 percent range, and the antenna seems to operate quite well
even close to the ground. The loop, however, exhibits extremely high Q, and
bandwidths can be in the tens of kilohertz and often lower yet. This
necessitates constant tuning of the antenna capacitor to resonate the desired operating
frequency. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>There is a plethora of information on small transmitting loops in the
ARRL Antenna Book as well as numerous sites on the web. A couple of the most
comprehensive are W2BRI’s site (<a
href="http://www.standpipe.com/w2bri/index.htm">http://www.standpipe.com/w2bri/index.htm</a>)
and AA5TBs site (<a href="http://www.aa5tb.com/loop.html">http://www.aa5tb.com/loop.html</a>
). The DX Zone has a list of exhaustive links as well: (<a
href="http://www.dxzone.com/catalog/Antennas/Loop/">http://www.dxzone.com/catalog/Antennas/Loop/</a>).
Be sure to note the design spreadsheet on AA5TBs site, which is an invaluable
design tool. At least one hf loop is available commercially from MFJ, although
it is expensive. This proposal will neither delve into the design of the
antenna, nor into a discussion of its relative merits, since all that
information is easily available in the above mentioned documents.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>A typical design which will be built as a prototype is described by
Robert Capon in “You Can Build: A Compact Loop Antenna for 30 through 12
Meters”. This article is available from the QST archives on the ARRL
website. It is a very simply constructed 3-foot diameter loop using 5/8-inch
copper plumbing tubing and capacitive coupling. <o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Tuning
Capacitor<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The tuning capacitor must withstand a considerable amount of high
voltage, necessitating either a big “bread slicer” transmitting cap
or perhaps a vacuum cap. Voltages of 4 or 5 kV are typical at 100 watts; and
past 15kV at 1000 watts. Tuning ranges go from 5pf up to several hundreds of
pf. For the compact loop described above, and at 100 watts, a capacitor that
tunes from 5pf up to 100 pf. would provide a tuning range roughly between 10 MHz
and 30 MHz.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Either type of capacitor lends itself to motor control and remote
operation.<o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Drive Motor<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Many different types of drive motors are available that would be
suitable for this type of application. The simplest would be a dc gear head
motor, but a stepper motor or any other type of dc motor would work just as
well. This proposal suggests incorporating a universal motor driver that could
accommodate any of these types of motors.<o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Controller<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The controller will take digital signals corresponding to the SDR DDS
frequency and position the antenna tuning capacitor so that the antenna resonates
at, or very near, to this frequency. As the frequency of the SDR is changed
over the range of the antenna, the antenna tuning motor/controller will keep
the antenna resonant. Since the antenna exhibits the same high-Q characteristic
while receiving as during transmitting, the antenna needs to be tuned for
reception as well. (This fact precludes a servo system that reads SWR for
tuning.) Note that the proposed controller will also be able to be adapted to
many types of motor-driven local or remote antenna tuners to offer the same
“always tuned” performance.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The heart of the proposed controller consists of a PIC 18F1330
processor, which are normally available from Mouser and other suppliers for
less than $5.00 ea. The 18F1330 is proposed since it has an integrated I2C
communication port, several A/D converters for position sensors or other analog
inputs, and an integrated PWM motor controller that can be programmed to drive many
different types of dc and stepper motors and lots of I/O ports. For the
implementation described, a much smaller PIC processor could be used, but would
not provide the flexibility for interface to many different motors. Note also
that the part has enough outputs to drive other devices, even perhaps a small
rotator. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The motor driving the antenna capacitor will have a position sensor
consisting of a potentiometer connected from +5 volts to ground providing a
signal proportional to its position, and thus resonant frequency, which will be
fed to the PIC A/D converter. For a single-turn capacitor, a direct-drive pot
will be implemented; for a vacuum cap, which requires up to thirty turns over
its range, a small nylon bevel gear at 2:1 or 3:1 will couple a 10-turn pot to
the drive shaft.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The Ozy FX2 code can be modified to strip off the DDS programming bits
that are sent to the DDS from the Power SDR software via the USB cable and send
this information to the PIC chip over the I2C bus. (This has been confirmed by
Phil Covington.) These bits determine the frequency that the DDS will be
programmed to as the frequency is changed by the operator. Note that if spur
reduction is set to “on” within the SDR, this frequency
doesn’t exactly follow the reception frequency, but is within a few kHz
of the actual received or transmitted frequency. This frequency deviation will
probably be within the bandwidth notch of the loop antenna and can be ignored,
but if not, then spur reduction will have to be set to “off”. This
issue can be resolved during implementation.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>A lookup table will be implemented in the PIC that lists position
sensor voltage input as a function of antenna resonant frequency. Incoming
frequency code from the Ozy will be converted to an equivalent sensor voltage
through reference to this table. This voltage, corresponding to the desired
tuning position voltage will then be compared with the voltage from the
capacitor position indicator. The difference will drive the motor switches
through the I/O pins of the PIC. The motor will be driven in the proper
direction until these two quantities equate, and the antenna is resonant at the
new desired frequency. This procedure is duplicated whenever the frequency of
the SDR is changed.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The lookup table will have to be generated manually, of course, but
shouldn’t pose problems. Any antenna analyzer or even the Ten-Tec VNA
would be useful for this task.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>Tuning time is yet to be determined, but short hops should be almost
instantaneous. This is another fertile ground for experimentation and
optimization. No particular type of motor drive is yet suggested in this
proposal, whether PWM or a simple bang-bang control, or stepper control.
Instead, the motor control decision is open for discussion and experimentation.
The basic hardware should provide enough flexibility to accommodate any type of
implementation desired. Note that the hardware is not expected to use a slot
on the Atlas backplane. The only connection to the Ozy will be over twisted
pairs. Of course, it could be designed to fit into the Atlas, but slots right
now seem to already be in such short supply that I am not suggesting it.<o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Mechanicals<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The mechanical makeup of the system should be fairly straightforward.
The cap, if a single-turn “bread-slicer”, will be directly coupled
to a one-turn pot (probably at the rear shaft) as well as directly coupled to
the drive motor (through the front shaft). Limit switches can be easily
attached to prevent damage to the cap should the motor attempt to overdrive.
The small dc gear head motors deliver an amazing amount of torque even at low
voltages. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>If using a multi-turn vacuum capacitor, 2:1 or 3:1 gearing to a 10-turn
pot will be required and these gears are easily available from several
suppliers fairly cheaply. Limit switches are problematic, however, and will
need to be investigated further to implement reliably yet cheaply. An
adjustable slip clutch might also be a possibility. Since a good vacuum
variable can cost hundreds, a moderate cost to insure against damage is well justified.
<o:p></o:p></span></font></p>
<h2><b><i><font size=4 face=Arial><span style='font-size:14.0pt'>Conclusion<o:p></o:p></span></font></i></b></h2>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>From my recent investigations, the described antenna and tuning system
looks doable as well as worth doing. Cost for a small antenna covering 10
– 30 MHz should be minimal, given finding a suitable tuning capacitor
surplus. Many are available on the web for $10 up, depending upon the amount
of power you are planning on feeding the system. Vacuum capacitors are
available as well, but are rather pricey even used. However, their performance
is unsurpassed. Surplus Pittman motors are available from about $15 each. The
controller board with the PIC complete with PWB should run less than $25. A
ten-foot length of 5/8 in. copper tubing for the element costs about $25. With
a few evenings work, the entire system could be realized very affordably. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>My approach to this project is the following:<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<ol style='margin-top:0in' start=1 type=1>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Present the concept
to the group and solicit comments and suggestions, hence, this proposal.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Complete a
preliminary design for the mechanicals and electronics<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Gather parts for a
prototype antenna and control system.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Build the antenna
without the controller and characterize performance.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Build a prototype
controller and the mechanicals. <o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Build a simple emulator
with an I2C bus to emulate DSS codes from the Ozy.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Demonstrate
operation and report to the group.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Modify the Ozy code
to provide DDS codes over the I2C bus.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Implement and demonstrate
operation and report to the group.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Layout a controller
PWB.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Create a complete
parts list and drawings for this implementation.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Publish all data to
the group.<o:p></o:p></span></font></li>
<li class=MsoNormal style='mso-list:l0 level1 lfo1'><font size=3
face="Times New Roman"><span style='font-size:12.0pt'>Investigate a group
buy for the PWB, if warranted, else publish the CAD files.<o:p></o:p></span></font></li>
</ol>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>The intention is to provide a basis for custom individual antenna
projects as well as a full set of build instructions to duplicate the
prototype.<o:p></o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'><o:p> </o:p></span></font></p>
<p class=MsoNormal><font size=3 face="Times New Roman"><span style='font-size:
12.0pt'>So I present this with the idea of opening the subject up for group
ideas and discussion and also to see if there is enough interest to warrant
developing the concept into a bona-fide HPSDR project. I don’t
anticipate that the end result of this project would ever be a turn-key
purchased antenna system, but would rather result in an available PWB for the
electronics as well as a source of construction information and code, where
everyone can experiment with their own implementations. <o:p></o:p></span></font></p>
<p class=MsoNormal><font size=2 face=Arial><span style='font-size:10.0pt;
font-family:Arial'><o:p> </o:p></span></font></p>
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