Introduction
This section of the website departs from the prevailing "Doppler" theme of radio direction finders. It describes a classical Yagi - type DF, but one with the addition of some new technology to enhance the capabilities and "utility value" of the final instrument. I call it the YagPlot DF. Unlike other equipment on this website, I will NOT provide enough technical information to allow independant construction... this is a commercial product.
The tentative price for the DF unit itself will be US$ 280. If you can
provide your own compass module ( a PNI model V2X ) the price will be
US$ 230. It requires an external directional antenna, and a reciever that
must be slightly modified to provide an S - meter signal to the DF. It also
requires a Palm PDA for a display.
The first unit was assembled and tested in late April 2002, with good results. Inquiries can be directed to me ( Bob Simmons ) via e-mail to pelican2@silcom.com
Description
The YagPlot DF is a sophisticated, fully portable radio direction finder that generates a calibrated "polar plot" display of signal strength versus signal direction. It employs a Palm PDA display, driven by an infrared optical data link, for cordless operation. Free downloads of the Palm display program ( which includes a simulation mode ) are available near the bottom of this page.
The DF itself mounts directly on the Yagi boom, and contains an electronic compass to detect Yagi direction. In the photo above, the left BNC connector goes to the Yagi antenna, and the right BNC goes to the reciever. The infrared LED for the optical datalink is visible above the DB9 RS232 connector. Identical data is transmitted through both channels... the DB9 connector is provided in case a clear optical path is ( for any reason ) not available for the LED. ( it is also provided for future use with an IBM display program )
Although intended for use with a Yagi, there is nothing to prevent its operation with other types of directional antennas, or even omni antennas, using the "body shielding" DF method. For that matter, it can also be employed as a non-directional ( calibrated ) "field strength" meter, with both acoustic and numeric outputs.
Operation
Basic Operation is very simple. The user tunes the reciever to the desired frequency, then "taps" ( anywhere ) on the Palm ( touch - sensitive ) display screen to start the "data acquisition" cycle, and then ( slowly ) rotates the Yagi through a 360 degree circle. During this time, the DF acquires data about signal strength and Yagi direction. Radial lines on the display indicate bearings ( every ten degrees ) where data has already been accumulated. A small circle ( a "bug" ) is located just outside the azimuth scale, to indicate the present antenna direction.
After completion of the 360 degree sweep, the user again "taps" the Palm
screen ( anywhere ) and the data is analyzed, re-scaled to fill the screen,
and displayed in "polar plot" form on the Palm.
Tapping the screen once
more will clear the display, and begin another "acquisition" cycle.
Display Utilities
During the acquisition phase, a numeric readout in the top right corner of the display indicates signal strength, in decibels. An audio "beep" tone ( from the Palm PDA ) also indicates signal strength, across ( approx ) 60 decibels of range, at a rate of about two beeps per second. When no signal is present, the beep tone "idles" at 200 Hz, indicating the DF is on and serial data ( but no signal ) is being recieved. If no serial data is recieved, ( possibly due to a blocked optical path ) the display will quietly "beep" at 750 Hz, about once per second.
In addition to the Palm DISPLAY page, three polar plot "memories" are available on a second CONTROLS page. This page can be accessed by pushing either the UP or DOWN "hard" buttons on the Palm. These two buttons are located ( one above the other ) at the bottom of the Palm, in the center of a row of buttons.
To return to the DISPLAY page, push either the UP or DOWN ( hard ) buttons again...
On the CONTROLS page, there are 3 ( software generated ) "screen" buttons
to record a polar plot, and 3 more to retrieve and display a polar plot.
Another "screen" button displays the ( currently selected ) data source,
and can be used to select alternative sources. If a memory plot is selected
for the display, it will be displayed when the user returns to the DISPLAY
page.
These buttons are pretty small, and will probably require the use of the ( built - in ) Palm "stylus pen", to actuate them. This was deliberate, and is intended to prevent accidental actuation of these buttons with a ( bare ) fingertip. A small EXIT button allows the program to be terminated.
The display can be driven by the optical data input and any of the 3 "recorded" plot patterns, but it can also be driven by a direct RS232 input, which is available from a DB9 connector on the front of the DF unit. This allows "remote" DF operation, in situations where the directional antenna ( and DF ) is/are not directly adjacent to the display. ( example : vehicular "roof mounts" ) Palm PDAs come with a "HotSync" cable, ( or a docking cradle ) suitable for a direct RS232 link.
A simulation data source is included in the Palm software, which generates an idealized polar plot of a Yagi pattern. This is mostly for the benefit of people who want to download and try the Palm display program, ( free downloads near the bottom of this page ) before choosing to purchase a DF unit.
Additional Features
The DF also has internal jumpers that allow operation with S - meter signals with either a positive or a negative coefficient. For some recievers, the S - meter signal voltage will "ascend" ( i.e. move towards a "more positive" voltage ) as the signal strength increases, and this represents a "positive" S - meter coefficient. Other recievers might generate an S - meter voltage that "descends" ( i.e. moves towards a "more negative" voltage ) as the signal strength increases, and this represents a "negative" S - meter coefficient. This design can accept S - meter signals with either coefficient.
The YagPlot DF does not have a conventional power switch... the S - meter signal is constantly monitored by an internal PIC microcomputer. The PIC micro also operates the compass module, measures the PIN drive signal, contains the calibration "lookup table" and generates the RS232 and opto data outputs.
If the S - meter signal drops below 50 millivolts, the PIC interpets this to mean that the reciever has either been turned off, or the antenna coax to the reciever has been disconnected. The micro then shuts down the DF circuits and enters a "sleep" mode which consumes very little power... about 100 microamps. Once every 2 seconds, the micro "wakes up" for about 1 millisecond, and re-examines the S - meter signal, to see if it has been restored. If so, the DF is "re-energized" and operation continues.
Display Programs
The Palm PDA display program ( called YagPlot1 ) is written and compiled in NSBasic. This display program should operate properly on any PDA that employs the ( very popular ) PalmOS operating system. You will also need the MathLib program, ( also provided below ) which is a public-domain math "utility" program...
I am using a Palm model M100, ( now obsolete ) but they can still be found, at VERY economical prices. I believe the M100 "successor / replacement" models ( M105 ) now sell for about $120, brand new. "Used" units are abundant, for even less.
A few folks have had trouble downloading and running these files, so they are also available here
in zipped format : ( both files in one zip folder )
Once these programs are downloaded from my website, you can use the Palm Desktop software
( provided with each Palm PDA ) to download them from your IBM to the Palm PDA.
Palm Personal Digital Assistants ( PDAs )
Palm PDAs are really slick devices, and serve the function of a portable DF display VERY well... They have a 160 x 160 pixel LCD graphical display, a touch-sensitive screen, electro-luminescent backlighting, ( for night operation ) and an integral RS232 serial port, ( and serial IR port ) as well as a piezo speaker and a half dozen or so "hard buttons".
At this time, no IBM PC display program has been created for this DF, but
that is the next item on the agenda. It will probably be completed by late
June 2002. It will be quite simple, but will incorporate all the features
of the Palm program. It will be written for MSDOS ( no Windows ) mostly to
allow its use with very old computers. It will be completely free, and
available on this page, when completed.
For those who are willing to write display software, and don’t want to wait for me to finish an IBM program, ( or don’t want an MSDOS program ) I will provide ( on request ) detailed information about the DF data stream format, to allow independant creation of a display program.
For more detailed / technical information about the internal operation of this DF, please proceed to the next page by clicking on the link below...