[hpsdr] Proposal for Loop Antenna Project

[Ken Klein]

Gentlemen;

 

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.

 

Ken WR5H

 


PROPOSAL FOR A SMALL TRANSMITTING LOOP ANTENNA AND AUTOMATIC TUNER FOR
SDR/JANUS/OZY


 

Abstract:

 

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.

 

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.  


Small Transmitting Loop Antennas


 

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.  

 

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 (http://www.standpipe.com/w2bri/index.htm)
and  AA5TBs site (http://www.aa5tb.com/loop.html ).  The DX Zone has a list
of exhaustive links as well: (http://www.dxzone.com/catalog/Antennas/Loop/).
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.

 

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.  


Tuning Capacitor


 

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.

 

Either type of capacitor lends itself to motor control and remote operation.


Drive Motor


 

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.


Controller


 

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.

 

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. 

 

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.

 

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.

 

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.

 

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.

 

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.


Mechanicals


 

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.  

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.  


Conclusion


 

>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.  

 

My approach to this project is the following:

 

1.	Present the concept to the group and solicit comments and
suggestions, hence, this proposal.
2.	Complete a preliminary design for the mechanicals and electronics
3.	Gather parts for a prototype antenna and control system.
4.	Build the antenna without the controller and characterize
performance.
5.	Build a prototype controller and the mechanicals. 
6.	Build a simple emulator with an I2C bus to emulate DSS codes from
the Ozy.
7.	Demonstrate operation and report to the group.
8.	Modify the Ozy code to provide DDS codes over the I2C bus.
9.	Implement and demonstrate operation and report to the group.
10.	Layout a controller PWB.
11.	Create a complete parts list and drawings for this implementation.
12.	Publish all data to the group.
13.	Investigate a group buy for the PWB, if warranted, else publish the
CAD files.

 

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.

 

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.  

 

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