Next Fox Hunt
How does 29 May 2021 sound?
At Kent Park at 10:00 hours
This will be a driving hunt.
Saturday 29 May 2021 10:00
Kent Park
2048 Hwy 6 NW
Oxford, Iowa 52322
On the
Friday evening prior to a hunt,
we will conduct an informal
net
at 19:30 on 146.850
to get an idea of participant count.
We will confirm location and time.
On the equipment front
I have picked out
five of the higher power transmitters (FOX5, FOX6, FOX10, FOX11, FOX12).
They all run from around 30mW to just shy of 50mW and they're all a bit chatty.
I hope they are
loud enough to to allow
them to be located throughout the park.
There will be two FOX
groups used for this hunt,
one group on 144.250 and the other on 144.285.
Transmit cycle is 5 minutes
Each transmitter runs for about a minute
The
Fox Hunt will start at 10:00 AM.
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.
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.
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
1⁄
2
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
Operation
Connect the switch to the receiver antenna input.
With power off, the receiver will act normally.
If you have mounted the antenna elements 1⁄2
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.
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
Back to the Top
102-73161-7 ZiLog zNEO SOC
1 unit produced, this is the
proof-of-concept, the first one.
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.
Back to the Top Back to Transmitters
102-73161-12 ZiLog zNEO SOC
3 units
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.
Back to the Top Back to Transmitters
102-73161-25 ZiLog zNEO SOC
4 units
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.
Back to the Top Back to Transmitters
102-73181-0 ZNEO ICS307
Boards in 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.
Next Model.
The ICS525-02 is now listed as
end-of-life which will make the
chip hard to find in the future (DigiKey lists stock as of 2020).
This artwork update swaps out the ICS525 for an ICS307,
the same clock device used on the Raspberry-PI model that follows.
The audio channel
haywire on the 102-73161-25 is incorporated into
the board with all the other features of the 102-73161-25 board retained.
The 12-pin external radio connector from the 102-73176-0 design
is used here with the extra pins connected to provide 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.
The FRAM in the parts list is specified as a Fujitsu MB85RS4MT.
Two large Cypress parts may be substituted, the CY15B104Q or the
CY15B108Q.
The Fujitsu part is the less expensive selection. Other large
FRAM devices require additions to the device table in the operating code.
Another change is in the USB connector.
It is now vertical mount.
This is intended to allow the battery cover to be used to access the USB
conenctor without having to remove screws. The 6-cell AAA packs will
not fit through the battery door so no access has been lost with the change.
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 common
software. It is ready to transmit in just a few seconds, so a unit
alive message can be broadcast immediately following power on.
This design also adds a new daughterboard connector to route the
time network (i.e. serial channel) to the allow control of the
SA818/DRA818 walkie-talkie module.
Back to the Top Back to Transmitters
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
Raspberry PI Zero-W.
2 major changes to the system:
- Change ZiLog zNEO SOC to Raspberry PI Zero.
- Change from ICS525 to ICS307 (ICS525R-02 is an end-of-life part).
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.
Back to the Top Back to Transmitters
ICARC/KC0JFQ Power Amplifiers
102-73161-24 60mW Class D Amplifier
Class-D Amplifier
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.
Back to the Top Back to Transmitters
102-73161-27 90mW Class D Amplifier
Class-D Amplifier
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.
Back to the Top Back to Transmitters
102-73181-22 1W walkie talike module
Module Schematic.
This amplifier daughterboard 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 102-73181-0 board.
This RF module provides a fixed output power of 500mW or
1000mW as selected by resistor R2.
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..
