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ICARC Fox Hunting

Last updated 18 Sep May 2022 by KC0JFQ

        Quick Links:
  1.   KC0JFQ Fox Hunt Projects Page       (KC0JFQ website)
  2.   ICARC Fox Hunting       (this page)
  3.   Next Fox Hunt
  4.   Fox Hunt Instructions       (log sheets w/frequency lists)
  5.   Fox Hunting Hardware
  6.   Fox Hunting Transmitter Software
  7.   ICARC Fox Hunting Tools
  8.   DTOA Switch
  9.   DTOA Receiver
  10.   NVARC RF Detector
  11.   ICARC Fox Hunting Infrastructure: Transmitters       (Fox Hunt Transmitter)
  12.   WB6EYV MicroHunt Foxhunting Transmitter
  13.   ICARC/KC0JFQ Transmitters       (Fox Transmitter)
  14.   New Transmitter       boards have arrived (Talking Fox Hunt Transmitter)
  15.   ICARC/KC0JFQ Power Amplifiers       (Fox Hunt Transmitter RF Amplifier)
  16.   October 2023 Presentation (KC0JFQ) Fox Hunting Tips

Next Fox Hunt

How does 06 May 2023 sound? Hickory Hill Park at 10:00 hours

This will be a close-in walking hunt.

On the Friday evening prior to a hunt,
we will conduct an informal net at 19:30 on 147.150
to get an idea of participant count.
We will confirm location and time.

On the equipment front

We will again have our huge stable of transmitters, 11 of them!

Some of the transmitters are CW only, some are voice only, and the remainder are a mixture of CW and voice.
Keep in mind that KC0JFQ, the hardware source, is a devious bastard!
These transmitters will try to trick you in various ways, so beware!

Transmit cycle is 5 minutes
Each transmitter runs for about a minute
As you should realize, we are operating multiple frequencies, so grab a copy of the Known Operating Frequencies to use as a checklist.

I will setup several transmitters to be more-or-less continuous to try to make life easier for the beginners...

The Fox Hunt will start at 10:00.

You may also contact KC0JFQ at any time using this crude email obfuscator: Sorry, but you need Javascript on to email me (or at least expose the email address).
  and let me know your schedule and venue preferences.

Fox Hunt Instructions

These are the handout sheets for the fox hunt:
                Print a copy for the upcoming hunt

        102_73161_25-13   Multipurpose Checklist for multi channel hunt (log sheet w/frequency list)
        Known Operating Frequency List (Alternate log sheet)
        6-up log sheet (Avery 5164)

        Hickory Hill Operating Scenario Assignment and Orders
        City Park Operating Scenario Assignment and Orders
        Terry Trueblood Operating Scenario Assignment and Orders
        Kent Park Operating Scenario Assignment and Orders

Fox Hunting Hardware

Some projects that you may be interested appear below.
Circuit boards are on hand and for all of these and you may send an email message to the email link above if interrested.

Don't miss the blue button on the right at the top in the navigation bar. This links to more details for the ICARC fox projects.

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ICARC Fox Hunting Tools

These are local ICARC projects.

DTOA Switch

There are circuit boards available for this project!

The first project is a direction finding assist.
The DTOA acronym is "Differential Time of Arrival".
The DTOA switch is connected between a pair of antennas and a handheld receiver. When the antennas are normal to the transmitter (i.e. electrically equidistant) the receiver doesn't notice the DTOA switch. As the antennas turn away from the transmitter, a squeal is introduced into the audio.

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DTOA switch Schematic
Antenna End Schematic
DTOA switch Parts List (web page)
DTOA switch Master Build Record
DTOA switch DigiKey order spreadsheet This file can be dropped directly into the DigiKey ordering system
KC0JFQ Pages for this project

The last images in the group show the base of the prototype antenna.
The antenna is semi-rigid (hobby store) brass tubing. The main part of the vertical element slips into the fixed tube mounted to the base. The long antenna elements are stowed for transport in this image.
Click the image to get to view a high resolution image.
The two antennas are mounted on a yardstick about 12 wavelength apart, although this spacing is not particularly critical.
The fixed portion of the antenna has a brass spacer that is tapped 4-40 solderd at the base to provide secure attachment, this being visible in the image.
The coax connector in this image is a BNC, but an SMA connector may be substituted (SMA part numbers follow).
The connection from antenna base to the coax, shown in the far right image, is cut from the main circuit board (that is two circuit boards are nominally required).
You see on the trimmed coax boards that a 0.125" hole is provided to attach the antenna base. Either a BNC or an SMA may then be attached (SMA part numbers follow).
The two non-plated holes provide for mechanical attachemnt. In our prototype #4 pan head sheet metal screws along with some short nylon spacers are used.
A short piece of 1" stock is glued to the inter-antenna spacer (i.e. the two yardsticks glued together) to provide a bit of support for the antenna base.
The circuit board is used as a drill guide to set the holes before being assembled to the antenna and the spacers.
The two nylon spacers are present only to provide clearance for the solder joints on the circuit board.

The antenna in the image has not been trimmed to resonance. There has been no effort expended in trying to achieve an impedance match.
A simple telescoping whip antenna should work equally well and probably be less expensive than the brass tubing used for the prototype.
Antenna spacing is also not particularly critical. The prototype antennas are mounted on a garden variety yardstick from the local hardware store with a spacing of 34 inches (1 inch from each end).

Misc. Substitute Parts SMA

J1, J2, and J3 on the 102-73170-A board may be populated with SMA connectors, if desired:
Right angle SMA Connector
Another Right angle SMA Connector
18 inch SMA Cable
24 inch SMA Cable

Misc. Substitute Parts BNC

Here are various BNC parts you may find useful:
Right angle BNC Connector (from parts list)
BNC to SMA Cable 18 inch
BNC to SMA Cable 24 inch
BNC Cable 18 inch
BNC Cable 24 inch
SMA to BNC Adapt
SMA to BNC Adapt

Misc. Parts

Here are some parts for the antrenna element termination boards:
0.125(ID)x0.188(OD) by 0.100 (Hgt)
0.125(ID)x0.188(OD) by 0.200 (Hgt)
0.125(ID)x0.188(OD) by 0.250 (Hgt)
Round Standoff, 4-40, BRASS, 0.250 dia. 0.625 long

Misc. Parts, Antenna

K&S brass tubing (0.014 wall) for antenna elements:
8132 9/32 brass tubing K&S, 0.281 dia. 12 inch
1149 1/4 brass tubing K&S, 0.250 dia. 36 inch
0.250 OD tubing requires working the threaded spacer diameter to fit!
8131 1/4 brass tubing K&S, 0.250 dia. 12 inch
1148 7/32 brass tubing K&S, 0.218 dia. 36 inch


    Connect the switch to the receiver antenna input.
With power off, the receiver will act normally.
If you have mounted the antenna elements 12 wavelength apart, the receiver will null with the elements lined up with the signal.
That is to say, when the DTOA switch is not powered, and the element spacing is close to 1/2 wavelegth, the combined signal from the two antenna elements will destructively combine thereby reducing the signal seen by the receiver.

    Once switched on, the receiver will receive normally when the antenna elements are normal to the signal.
Your line-of-position is perpendicular to the line between the two antenna elements (i.e. normal).
As you sweep your line-of-position away from the source, a squeal will be introduced and be heard in the received audio.
The pitch is fixed by R2/R8 but the volume increases as the phase change, introduced by the DTOA switch, increases as the antenna elements differential distance to the source increases.

    The system is sensitive to reflections, much more so than a single antenna.
As you move further away from the transmitter you may see reflections that have signal strength similar to the transmitter.
In an area with metal structures, power lines, equipment cabinets, power lines, anything that will reflect the carrier, a false line-of-position is easy to encounter.

    Swing the antenna array back and forth to find the point where the squeal introduced by the switch is minimized.
As you get closer to the transmitter this will be more pronounced.
Keep in mind that the line-of-position has front-to-back ambiguity, so you need several lines-of-position to establish the direction of the transmitter.
Consider: after establishing your first line-of-position, move some distance perpendicular to that line and take another line-of-position to start to resolve the front-to-back ambiguity.
When used with low power transmitters you may see the effect your body has on reception, the front-to-back sensitivity may change.
You will still see the null if you turn around, but it will sound slightly different.
You can also simply power off the DTOA switch and use the null off the end of the antenna array to establish your line-of-position.

    Once you establish the general direction of the fox transmitter, you can sweep and walk to find the transmitter.

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DTOA with receiver

Yet another project is in the works, this using an inexpensive DRA818/SA818 walkie-talkie module and the zNEO used in the Fox Transmitters to form a complete DTOA subsystem.
The two antenna inputs connect to a matched antenna pair spaced about 1/2 wavelength apart.

The DRA818/SA818 is capable of transmitting, and the DRA818/SA818 control signals are driven by the zNEO. This design, however, does not incorporate a low-pass filter on the output so the software does not provide a transmit capability.

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DTOA receiver Schematic
DTOA receiver Parts List (web page)
DTOA receiver Master Build Record
DTOA receiver DigiKey order spreadsheet This file can be dropped directly into the DigiKey ordering system
DTOA switch Manual

October Meeting Presentation Slideshow PDF

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NVARC RF Detector

" The second project is a simple RF detector.
This is rather boadband, but works well in proximity to the transmitter as a signal strength sniffer.
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RF Detector Schematic
RF Detector Parts List (web page)
RF Detector Master Build Record
DTOA switch DigiKey order spreadsheet This file can be dropped directly into the DigiKey ordering system

These both fit into the same Hammond project box. The power switch is in the same location on both projects.


    Connect headphones to the 18" headphone jack and connect your directional antenna.
The tone in the headphones is proportional to signal strength. The tone increases in pitch with increasing signal strength.

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ICARC Fox Hunting Infrastructure: Transmitters

We have a growing collection of transmitters that are used for our fox hunting events.
Follwing the initial purchase of 3 low power transmitters, KC0JFQ started a project to build a more capable transmitter.
The trasnmitters in use are detailed below.

WB6EYV MicroHunt Foxhunting Transmitter

ICARC owns 3 of these units
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WB6EYV 50mW transmitter.
This uses the Integtated Device Technology (now Renesas Electronics America) ICS525R-02 to generate the RF carrier.
The PLL frequency multiplication is fixed with traces in the artwork.

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ICARC/KC0JFQ transmitters

    There are two basic systems described below,
one being a low power unit and a second that uses a Raspberry-PI as a control element.
    The first design, the zNEO SOC based units, was concieved to provide an easily programmable FOX that can be deployed in the field by simply turning the unit on as it is placed in its hiding place.
Low power was also a consideration in the design.

    The second design was an outgrowth of someone casually remarking that is should be able to talk.
The ubiquitous line of Raspberry-PI computers provides a convenient and cost-effective solution for this.
The downside of the Raspberry-PI being that is is not designed to be power efficient.
In spite of the power hungry nature of the Raspberry-PI, it is capable of operating on battery power for about 8 hours.

    After some development effort, a means of producintg audio was developed for the zNEO based unit. Although the zNEO can now produce audio, it does require external software support to generate and convert normal audio files into the low bandwidth, limited resolution, file required to meet the limits imposed by the 20MHz speed of the zNEO processor.
    ZiLog zNEO SOC -7
    ZiLog zNEO SOC -12
    ZiLog zNEO SOC -25
    ZiLog zNEO SOC ICS307
    Raspberry-PI Zero
    60mW Class D Amplifier
    90mW Class D Amplifier
    500mW walkie talkie module

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102-73161-7 ZiLog zNEO SOC

1 unit produced, this is the proof-of-concept, the first one.

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The prototype board.

A few haywires required to deal with missing parts.

The ICARC fox transmitter project started in early 2019.
Using the same ICS525R-02 to generate a carrier, it produces about 1mW through the low power RF section.
Ther zNEO drives the control pins on the ICS525R-02 to allow dynamic frequency selection. The frequency may change at any time, includeing in the middle of a message.

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102-73161-12 ZiLog zNEO SOC

3 units
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First Revision.

This revision adds the missing missing parts from the -7 board, above.
The RF section was changed in an attempt to improve RF amplifier performance.
Mechanical changes on the board moves the network jack to make room for a charging jack.
A 10 pin connector (not populated on this board) is added to allow the board to control an external tranceiver.
A battery voltage monitor allows the unit to transmit its battery condition.

Bare boards and build documents are available for this revision of the project.

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102-73161-25 ZiLog zNEO SOC

4 units
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Second Revision.

This revision slightly improves the fit in the case while remaining mechanically compatible with the -12 revision.
This major update for this revision moves the RF amplifier to a daughterboard.
The modulation control circuit is changed to allow the use of inexpensive crystals rather than a VCMO.
The zNEO SOC also switches to using a crystal to reduce cost.
The first stage regulator changes to a switch-mode device to improve battery life (run time is ow in excess of 24 hours).
The ICS525R-02 can now be powered from the 5V rail to increase the barefoot output power to around 30mW. You can see the RF daughterboard shown has no active parts.

After some careful deliberation, I have managed to find a means of processing audio clips.
Upgrading the FRAM to a 4Mb device allows space for about 100 seconds of audio.
The existing units have room for 4 to 6 seconds of audio. The audio clips are digitized at 4KHz so thet sound a bit muddy.
Although they need an FRAM upgrade, the callsign and unit name are announced.

Sample voice output from this FOX:
Bare boards are available for this revision of the project.

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102-73181-0 ZNEO ICS307

Boards on hand and slowly working through assembly.
These will make use of the DRA818/SA818 tranceiver module running at 500mW.
I hope to be able to move back to Kent Park and possibly to the area around Greencastle and Swan Lake Road.
If this works as expected, we can car-hunt, so our less mobile members can participate.

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Next Model.

SA818/DRA818 Amplifier Daughterboard

The ICS525-02 is now listed as end-of-life, stopping production of 102-73161 units.
The ICS307 suffers the same fate, making further production of the 102-73181-0 units no longer viable.

102-73181-5 ZNEO SI5351

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Revision 5 Model.

SA818/DRA818 Amplifier Daughterboard (w/software power control)

The -0 artwork used the ICS307, while the -4 artwork uses the SI5351.
The PWM audio channel haywired on the 102-73161-25 is incorporated into both of these boards with all the other features of the 102-73161-25 board retained.
We also incorporate the current sense from the 102-73176 board.
The 12-pin external radio connector from the 102-73176-0 design is broughtt forward into these units, the extra pins providing the 102-73176 functionality (i.e. 2 additional A/D channels).
Mechanical compatibility with the 102-73161-25 external radio port may be achieved by installing a 10-pin header (pin-1 justified) on the pads.

This unit is provisioned with 2 memory devices, one being a smaller FRAM which is easy to alter manually (as it doesn't require sector/device erase) and a large Flash device that is used to store audio waveform data. The FRAM in the parts list is specified as a Cypress FM24V01. The Flash device is a Winbond W25Q64.
The FRAM is reasonably large relative to cost and the Flash is simply inexpensive (and large!).
The Flash device requires sector/device erase to reprogram, but this is not an issue as programming requires the use of a host computer (due to the size of the waveform files).

The -5 board also adds a backup battery maintenance circuit to keep the backup battery for the clock fully charged.

The host interface, that is the USB connector, when used, is vertical mount to give access through the battery door.
The -5 adds a vertical stereo jack to allow the FT232RL and the USB connector to be eliminated (to reduce unit cost). An FTDI-chip serial USB adapter is used to connect to the unit for programming.

This design uses much less power than the 102-73176 (Raspberry-PI) design, while keeping the ability to speak.
Startup time is the same as the 102-73161 units as it uses similar software. It is ready to transmit in just a few seconds, so a unit alive message can be broadcast almost immediately following power on.
This design also adds a new daughterboard connector to route the second serial channel (i.e. time network) to the allow control of the SA818/DRA818 walkie-talkie module.

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102-73176-0 Raspberry-PI Zero

Some wag asked if it could talk. Well now it can!
This design makes use of a Raspberry-PI running Linux.

3 units
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Raspberry PI Zero-W.

2 major changes to the system:
Keeps the same mechanical interface to the enclosure as the 102-73161-25.
Modulation control voltage comes from a PWM channel in the Raspberry PI that is configured as an audio DAC (i.e. a sound card).
A class-D amplifier daughterboard is mounterd in the above picture.
It makes use of a pair of 74LVC04 buffers which produce about 60mW.
Operating barefoot will produce about 30mW.
A battery current monitor was also added to allow battery power monitoring and analysis.
An audio amplifier drives an on-board speaker for debugging and to allow the unit to talk.

The Raspberry PI Zero W is a power pig! Run time on six "AAA" batteries is about 8 hours, so fresh batteries are generally required for each hunt.
The Raspberry PI Zero, lacking WiFi capability, is mechanically and electrically compatible.
The WiFi on the Raspberry PI Zero W provides a convenient way to access the unit to download software and .wav files.

Sample voice output from this FOX: Sample CW output from this FOX:
Bare boards and build documents are available for this project.

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ICARC/KC0JFQ Power Amplifiers

102-73161-24 60mW Class D Amplifier

Class-D Amplifier
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Class D Amplifier Schematic

This is a trivial Class-D amplifier that uses a high speed low voltage CMOS inverter.
All three active devices are 74LVC1G04W5-7. This specific device is chosen for its fast propogation.
U1 is an input buffer while U2 and U3 are the output drivers.
The 74LVC1G04 device is supplied by Diodes Incorporated boasting a 1.6nS propagation delay along with an output drive of about 30mA.
At VHF frequencies the 74LVC1G04 is operating near its maximum speed resulting in relatively slow rise and fall times. Much of the high frequency content is attenuated by the device itself. The output from the drivers is passed through a low-pass filter on the main board and then on to the antenna.

R2,R3, and R4 provide a pi network to attenuate the signal should that ne required. Nominally R3 is populated with a 0 Ohm resistor.

D1 is provided as a debug aid. It is powered from the 5V rail that powers the 74LVC1G04. D1/R1/JP1 would typically not be populated.

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102-73161-27 90mW Class D Amplifier

Class-D Amplifier
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Class D Amplifier Schematic

This is a 3 gate implementation of the Class-D amplifier design.
RF traces are all rounded and trace length into and out of the gates are matched.
This is on a 4-layer board. Three layers are ground and one is power.

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102-73181-22 500mW or 1000mW walkie talkie module

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Module Schematic.

102-73181-24 500mW/1000mW walkie talkie module

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annotated Module Schematic.

Thse amplifier daughterboards uses either an SA818V or DRA818V walkie-talkie module (sourced from China).
Connectors on the left edge of the board provide access to the serial port on the walkie-talkie module when used with the 102-73161 and 102-73176 boards (or for experimenting).
A write only connection can also be configured using the spare pin in the lower right hand corner of the 102-73161-25 board (spare pin on the RF output connector).
The 102-73181 board adds three pads to route the bi-directional serial port through an additional connector on the bottom edge of the board.
This connection only appears on the 102-73181 boards.
This RF module provides a fixed output power of 500mW or 1000mW. On the 102-73181-0 this is selected by resistor R2 while on the 102-73181-5 board it is software selected.
Nominally it wouold be jupered low power position to conserve battery power.

For most Fox Hunt applications, the audio amplifier would not be populated. As the SA818/DRA818 module supports split operation, it is possible to populate the audio section to provide a voice channel for the event organizers to verbally taunt the participants when they are close to the transmitter..

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Fox Hunting Transmitter Software

Here is the operating manual for the fox transmitters. These are the configuration files that are loaded into the KC0JFQ fox transmitters.
These are the commands that configure and control the operation of the transmitter.
They are loaded into the trnamsitter through the USB interface using a simple terminal emulator (i.e. no special control software or USB drtivers are required).
Audio files for the zNEO system are gathered together into an Intel HEX file for download into the transmitters FRAM.
The audio records are located after the configuration commands in the FRAM.
The hex file processing in the zNEO tolerates the extra whitespace that makes the Intel HEX Record a bit easier to read.
Extended addressing is managed using a type-4 extended address record. The type-2 and type-3 records are ignored and must not be used in the audio load image.

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Links on the KC0JFQ FOX wep page for search engines to stumble into

Fox Hunting       Fox Transmitter       Amateur ARDF       Fox Hunting
ICARC FOX HUNT       Next Fox Hunt       DTOA Switch       Raspberry PI FOX Transmitter
FOX_PI Features       FOX Transmitter       Features       Description
Board Status       Software Status      

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Last verified 19 Jan 2021, email obfuscator incompatible!