The Idea
Ever since I starting foxhunting, I have been very envious of one of my mentor's (Dick, WB4SUV) mobile equipment setup. He has taken a storage oscilloscope, mounted it in his truck, and attached a sine-cosine pot at the base of his antenna to give him polar antenna plots - neat!!! Really hard to beat, too. So, not having the $$$ or the room in my truck, I expended a few brain cells and came up with another solution. I used a PIC microcontroller to do A/D on the pot and the S-Meter, and made serial data to go to a laptop. Since I'm pretty cheap, I made the software run in DOS and it's only about 50k, so it will run on ANY old laptop. Here's how it works:
Equipment
First, you need an antenna. I use a 4 element quad on an 8 foot mast, mounted through the driver side window of my truck (photo 1). At the base of the antenna, I have mounted a continuous turn potentiometer - the value is not important (photo 2). Also, since all continuous turn pots have some dead space, you need some sort of switch, or sensor to indicate when the antenna is in the dead space. I used a small NO switch and a wood screw on the mast (photo 3).
Next, you need a radio that you can tap into the S-Meter, or construct something that will give you a voltage based on the signal strength. I use a Kenwood 7800, and just ran a straight tap into the S-Meter circuit (photo 4). If you need help, please reference the THRDFS bible. Anyways, you need a s-meter voltage between 0-5v.
Now, you can hunt with the items above (I have, frequently!) So, when it all goes to heck, you can revert back to this setup and still win! Anyways, I have never been one to stop there.
Circuit
Now we need a circuit that will convert the S-Meter voltage, the deadspace indicator, and the potentiometer position into a 9600 baud serial stream (and, of course, be based on a PIC processor!).
As you can see, construction is very straight forward - I just used perf-board (photo 5), but I designed a PCB layout anyway.
I chose the 12C671 because of the low cost and the internal oscillator, so that no external clock components are needed. They are available from Digi-Key as part number PIC12C671-04/P-ND for the one-time-programmable, or PIC12C671/JW-ND for the erasable version. The 12C671 will need to be programmed with one of these files:
POLAR_SCH.JPG JPEG of the
schematic
POLAR.HEX Intel format hex file for Potentiometer only
POLAR_PCB.JPG JPEG of the PCB
Layout
POLAR.PCB PC Board layout
in Easytrax format
POLAR.LST
Parts List
Program
Now, you need the POLAR program. This is the DOS based program that displays the data from the PIC circuit on a nice polar display. I have also added some functionality for foxhunting. Here's the basics if how it works:
POLAR {/COMn}
{DSnn} {/BW|/COLOR}
This command line will start the POLAR program. Use the /COMn parameter to
indicate your com port number - the default is COM1. The DS parameter is used to
indicate the percentage of dead space in the potentiometer you are using - the
default is 10. The /BW and /COLOR parameters are for.... you guessed it!
Monochrome or Color display.
Once the program starts, you will see the four sections of the screen. The upper left displays the incoming information and the current limits. Below that, there is a smaller circle with a single line. This indicates the current position of the beam antenna with respect to the front of the vehicle. Along the bottom of the screen there is a long bar display for the adjusted received signal strength, a big S-meter. The large area to the upper right is where the actual polar plotting takes place.
As the A/D converters sense the position of the antenna and the S-meter voltage, and update the individual displays, this information is used to display a real-time polar plot on the main window. A single dot will be placed in the direction of the antenna, spaced out from the center of the display the appropriate distance based on the S-meter. As you sweep your antenna across a signal, a very definite pattern will appear - as well as a number of reflections! Use the C key to clear the display window.
Also, you can place the program into trace mode, where the display will 'connect-the-dots' of the previous display, forming a nice line on the polar display. The line will automatically erase itself over time, and that time is adjustable by the persistence control. The higher the number, the longer the trace line will get. This provides the much better looking display.
There is a README.TXT for the POLAR program that outlines the different parts of the display, the valid keystrokes and what they do, and how to operate. I am always open to suggestions on how to make the program better, more usable, and win more hunts! Please let me know if you have any ideas. One option I'm currently working on is a waterfall-type display used with submarine sonar - very useful for plotting motion and identifying reflection and stray bearings!
Another useful tip is to use the S-meter ranges. By adjusting the slider in the upper left hand window, you can vary the S-meter ranges that are displayed. Here's an example: I took some reading of the 146.670 repeater from my driveway. The standard, non-filtered display looks like this. But, if the upper limit of the S-meter range is lowered, here is the result. If the lower limit is raised, so that only a narrow passband of S-meter readings is displayed, here is the result. Very useful on cluttered hunts!
POLAR.EXE
Program file
README.TXT
Readme file for POLAR with keystroke definitions
Future
There is a lot of ways to improve this concept, and I will be updating this page as I work new things out. Please let me know if you have an idea on how to make this better, or how to win more hunts!!! I'm also working on a touch screen version, for use on the Citadel type computers - I'll post it here when it's finished.
Acknowledgements
I would like to thank Bill Schroeder, WD6ADM and Jim Kasee, KB8UEZ for help
in identifying mistakes in the schematic. Also I'd like to thank Scott Ratchford,
KC5JGV, for facilitating the program running on the Citadel computer.