The VK3YNG Foxhunt Sniffer is a specially designed synthesised VHF direction finding receiver covering 120MHz and 144MHz bands.
The receiver is designed for quickly finding the direction of beacons or hidden transmitters. Anything from distant weak signals to very close "sniffing" of transmitters running many watts of output power can be pin pointed accurately without suffering "overload" problems that plague other designs. Full auto-ranging operation allows the operator to quickly and intuitively locate the source of a signal without twiddling knobs or watching meters. The operator is freed to concentrate on more important things such as negotiating terrain or reading maps.
The unit is also available in a fully tested Board level format for those who wish to integrate the unit into other equipment or build their own housing.
number of features have changed since earlier versions of the sniffer were released.
This manual reflects operation of version 3.0 hardware and cannot be used for
earlier versions of the MK4 sniffer. For a copies of earlier sniffer manuals please refer to the VK3YNG
foxhunt web site.
Differences between V2.x and V3.0
Auto Power Down
Low Battery Indication
Reduced Functionality mode
Quick Button Reference
Alternative Key Functions
Detailed Button Operation
Relative Battery Voltage indication
Peak Hold Mode
Peak range memory
Configuring the sniffer
Peak Extend Mode
Scan Ready Mode
Reduced Functionality Mode
Maximising Battery Life
Sample antenna designs
Inside View of Prototype
The VK3YNG sniffer is designed to allow quick, easy and accurate determination of the direction of a transmitted signal in either the 120-123MHz or 143-150MHz bands. The sniffer provides enough sensitivity to determine the direction of a signal from many kilometres or miles away. This is useful for ARDF or Radio Sport and general commercial or Civil Air Patrol use. It also provides enough attenuation to accurately determine the direction of signals right up to the source of the signal without suffering from overload or compression effects.
Attenuation (signal reduction) is provided automatically in steps of approximately 15dB each time a particular signal strength threshold is reached. The number of 15dB steps of attenuation is shown on an LED display. For example, a display value of zero indicates maximum sensitivity, where a value of 9 indicates a very strong signal that requires approximately 135dB of attenuation.
Signal strength indication is provided by an audible tone that increases in frequency (pitch) with increasing signal level. This is done because the human ear is a much more sensitive to changes in pitch than sound level. There is also no inertia or overshoot problems as tend to occur with signal meters. A special software algorithm ensures that the received signal strength tone does not suffer from “compression” effects that occur at higher signal levels with some designs.
Differences between V2.x and V3.0
The main differences between the two versions are:
1) V2.x used a Red display, V3.0 uses an Orange display which is slightly brighter in sunlight.
Headphone balance is much better at low volume levels with V3.0. With
V2.x the one side of the headphones tended to dominate at low volume
levels. This was an issue when efficient headphones were used.
3) V2.x was available in other VHF frequency variants. V3.0 is only available to cover 120-123MHz and 143-150MHz. Though special receiver bandwidths can still be supported in some cases.
4) V3.0 has a special fixed 10 channel option for use at Mt Gambier (AUS) championship fox-or events. This is not supported on V2.x.
5) V3.0 is a single board build. V2.x comprised of 3 separate boards and was more difficult to build.
Other than that operation and specifications are largely identical.
When power has dropped to 2.5 volts or lower, the display decimal point is enabled to warn the user that batteries are in need of replacement.
This mode may be useful for children or first time operators. It allows the sniffer to be used in its most basic format and hides all programmability options from the user. This mode is very useful for scout foxhunting for example. For this reason, many refer to this mode as “Scout Mode”.
||Recall channel 1 (Hold to store frequency/mode)||Recall only channel 1|
||Recall channel 2 (Hold to store frequency/mode)||Recall only channel 2|
||Recall channel 3 (Hold to store frequency/mode)||Recall only channel 3|
||Recall channel 4 (Hold to store frequency/mode)||Recall only channel 4|
||Recall channel 5 (Hold to store frequency/mode)||Recall only channel 5|
||Recall channel 6 (Hold to store frequency/mode)||Recall only channel 6|
||Band Scan or ARDF mode Start of
||Range down Delay Set/Display. 1
to 5 seconds. 0=peak hold mode. (7-9 are special functions)
||Show relative Battery capacity in %|
||Volume Down||No function|
||Volume Up||No function|
||Manual Range set/disable||No function|
||Frequency Entry (4 digits follow)||No function|
||Power/Audio Mode Select (Tone/AM/FM, Hold for power off)||Power On/Off only (instant power off)|
|Button||Display during power-up||Function if pressed during power-up|
||Filter “A”, Minimum Tone filtering. (fastest response)|
||Filter “B”, Medium Tone filtering|
||Filter “C”, Maximum Tone filtering (slowest response)|
||ARDF mode. Synchronised at power-up. Sync key is used to re-sync the transmitter cycle|
||Scan mode. Sync key is used to scan for strongest signal between frequency stored in channels 5 and 6.|
||Filter “D”, Tone Extend Mode. Used for very short duration signals. (New function in V2.1 firmware)|
||(Reset) Normal operation. Resets all options below to default setting:|
||Low-Tone mode (RSSI tones at quarter frequency. Resolution slightly reduced at low tone frequencies)|
||Mt Gambier mode (10-channel special - New in V3.0)
||Enable Reduced key functionality mode (“scout” mode)|
||Enable auto range change announcement tones.|
this button momentarily while the
unit is powered up cycles the unit through its operating modes and
displays the selection on the display. The modes cycle through in the
sequence: “A” – AM reception, “U” – Unmuted FM reception,
– Muted FM reception and “t” – Signal strength Tone. The cycle
repeats. When headphones are used, one channel is always set to give
strength tone, while the other channel follows the selected mode. This
useful for hunting different continuous carrier transmissions where the
transmitters’ identification is given using either AM of FM modulation.
Pressing and holding the Mode button for greater than 1 second will power down the unit. In reduced functionality mode, the unit will power off immediately when the Mode button is pressed. The mode switch operates only as a simple on/off switch in reduced functionality mode.
The recalled frequency is not displayed in reduced functionality mode.
The Store function is disabled in reduced functionality mode.
This facility is used to synchronise the receiver for use in international style (ARDF) foxhunting for a one-minute cycle, five-transmitter system.
In ARDF mode this timer is automatically synchronized when the receiver is switched on. Three short beeps are generated as confirmation of this mode. Pressing the Sync button re-synchronises this timer. Three short beeps are given to confirm this button has been pressed. The sync button should be pressed at the start of transmitter one’s cycle.
At 50 seconds into the cycle, the sniffer will generate three short beeps giving 10 seconds warning that the current transmitter’s cycle is about to end. If the sniffer is currently receiving at range 1 or lower, at four seconds before the completion of the current transmitter cycle the sniffer broadcasts a number of beeps corresponding to the number of the transmitter in the cycle which is about to commence. The pitch of these beeps is set slightly lower than the “50-second” beeps. The display also briefly flashes the number of the next transmitter. If a range down delay (d-Set) of zero is selected and the sniffer is not currently configured for manual ranging, the sniffer will automatically select range zero at the start of the next transmitters’ cycle regardless of the current signal strength.
When ARDF mode is not selected, the “7” key performs a basic band scan operation where the sniffer hunts for the highest signal between the two frequencies stored in channel locations 5 and 6. The highest signal found is stored in channel 4. The scan will ignore any signals within approximately 10KHz of the frequency stored for channel 1 and the signal must be detectable at range 2 or higher to be stored.
This operation takes some time to complete, especially if there is large frequency difference between channels 5 and 6. The scan function works best for continuous signals and may not properly detect intermittent transmissions. Channel 5 must be lower in frequency than channel 6 and both frequencies must be within the same band (i.e. 120MHz or 140MHz)
The SCAN mode is entered by pressing and holding the “5” button during power-up. It is cancelled by entering ARDF Sync mode. Scan mode is the factory default.
In ARDF mode, if a range down delay (d-Set) of zero is selected and the sniffer is not currently configured for manual ranging, the sniffer will automatically select range zero at the start of the next transmitters’ cycle regardless of the current signal strength.
Holding the “7” key during power-up cancels the Low RSSI Tone mode, Range Change Announcement tones* and the Reduced Key Functionality mode. In this mode the signal strength tones function normally and the maximum signal strength pitch is 8KHz. This mode is set as the factory default. (*Note: this facility operates differently in older firmware versions)
This key is disabled in reduced functionality mode.
Ranging up on the sniffer happens automatically with minimal delay. To implement short term peak detection, there is an intentional delay before the sniffer ranges down. This delay is programmable between 1 and 5 seconds using the D-Set button. For beginners, a value between 3 and 5 seconds is recommended. For more advanced users, 1 or 2 seconds gives better results. When hunting intermittent and very short duration transmissions such as those used on wildlife, 5 seconds or “peak hold” mode (see below) is recommended. The factory default is 2 seconds.
To set the range down delay, press the “D-Set” button. The display will respond by displaying the letter “d”. Pressing buttons 0 through 5 will then set and store the new range down delay.
Pressing the D-set (7) button twice will briefly display the current range down delay in seconds. The display then reverts to displaying the current range.
Setting a range down delay of zero disables down ranging. In this case the sniffer operates in a “peak hold” mode and down ranging is disabled. Manual ranging is disabled and pressing the “range” button will reset the current peak hold range to zero. This mode is useful when hunting extremely intermittent signals such as Dog Collar, wildlife or model aircraft beacons.
Holding the “D-Set” key during power-up causes the sniffer to operate in lower tone RSSI mode. This mode may prove useful to those who have difficulty hearing higher audio frequencies. In this mode the signal strength tone frequencies are divided by 4. The top tone pitch is limited to about 2KHz and resolution becomes slightly limited at the lowest tone frequencies. (Note: this mode has no effect on the pitch of supervisory beep and tone frequencies)
This key is disabled in reduced functionality mode.
In version 3.0, if peak hold mode (Dset 0) is selected before the sniffer is set up for reduced functionality (“scout”) mode, the peak hold behavior behaves slightly differently. If a strong signal disappears for more than 5 seconds, the sniffer will range down by one range only. As with normal peak hold behavior it will stay there until the range is reset or the channel or frequency is changed.
This operation was added to give beginners a second go at a signal source if they happen “over shoot” it and the signal falls below the current peak range.
Pressing the “D-Set” button then pressing “7” will display 4 digits indicating the number of hours and minutes since the receiver was powered up. This can be useful for ARDF events where the user may have forgotten to synchronise their watch.
the “D-Set” button then pressing
“9” will display two digits giving an indication of the relative
capacity in percentage terms. 99% indicates a full battery while 0%
the point where sniffer operation is significantly compromised. The
power itself off before reaching 0%.
Pressing the “D-Set” button and then pressing the “F” button will display 4 digits which indicate the installed firmware version.
These buttons are disabled in reduced functionality mode.
Normally the sniffer automatically selects the best range for the currently received signal. In some situations it may be necessary to range the sniffer manually. The range control button can be used for this purpose. This button also controls a number of other features depending on the mode selected.
In auto-ranging mode, pressing this button briefly displays the letter “r” in the display, and then the display reverts to displaying the current range. Pressing any digit then manually selects the range. The display briefly displays “r” followed by the selected range. The sniffer remains in manual ranging mode until defeated by pressing the “R” button a second time. This re-enables auto-ranging.
Holding this key during power-up enables the “range tone announcement” flag. When this flag is set, the sniffer generates a distinct “be-bop” tone sequence whenever the sniffer ranges up or down. The frequency of the tones gives the operator audible indication of the current range and the direction of the change. When this flag is cleared, (see the Sync Button section) range changes happen without announcement. (Note: this mode operates differently in older firmware versions. In older versions this key toggled between the enabled and disabled states. In V3.0 and V2.1.20 or later this key for enable only. The scan ("7") button serves as the disable key)
This key is disabled in reduced functionality mode.
If the range button is pressed twice in succession, the display will briefly show the maximum range the sniffer has achieved since power up or the last successful frequency change.
In ARDF mode, (see Sync button section) the peak range value is automatically reset at 30 seconds into the current transmitter cycle. This is very useful if there is a need to check what range the sniffer got to just before the previous transmitter finished its cycle.
This function is not available when Peak Hold mode (dSet=0) is selected.
The sniffer can operate at any frequency in the range 120.000-122.995 and 143.000 -149.995MHz in 5KHz steps. Pressing the “F” button initiates frequency entry. The display confirms this by displaying “F”. The sniffer then expects four digits to be keyed in to set the desired frequency. If an error occurs while entering frequency, the letter “E” is briefly displayed and the sniffer then reverts to the previous operation frequency. The first digit sets both the 1MHz frequency and the band. 0 through 2 sets the 120MHz band. 3 through 9 set the 140MHz band.
Factory default frequencies
Note: 121.5MHz is used as an
international personal, maritime and aeronautical distress beacon
frequency. With an appropriate antenna, the sniffer can be used to
locate PLB, ELT and EPIRB emergency beacons.
For the scan function to work correctly, the frequencies stored in channels 5 and 6 should be in the same band. This is not the case for the factory default configuration.
frequency entry is not permitted in reduced functionality mode.
*Note: Some special versions of the sniffer operate with different frequency ranges to those indicated above. The label on the rear of your sniffer will indicate the frequency range your sniffer covers.
Some special “narrowband” versions also allow 1kHz resolution. In this case the fourth digit entered can be any number from “0” to “9” instead of only “0” or “5”.
2.5kHz resolution can be available for some special builds, such as in the United Kingdom and areas of Europe. In this case “2” and “7” are also valid and correspond to xxx.xx25 MHz and xxx.xx75 MHz respectively.
The MK4 sniffer has a number of configurable features. Some of these have already been described in the previous sections. The rest are explained here.
There are four levels of filtering available on the MK4 sniffer. These are selected by holding buttons 1, 2, 3 or 6 when powering up the sniffer. The sniffer will retain this setting until the next time it is changed.
This level of filtering is the same as the original MK4. It offers the fastest and most accurate tracking of the received signal level. On transmitters with a high AM component, the tone can become quite “thick” sounding as the tone tries to partially track the modulation. This setting is the factory default.
This level of filtering offers the best compromise between response time and Amplitude modulation (AM) filtering.
This mode provides maximum filtering of the received signal and is similar to that of the VK3TJN/XAJ Ultra-sniffer. While this mode offers the best smoothing of received signals it can tend to “blur” the definition of short duration pulses.
This is a special mode for use with very short duration repetitive signals such as those emitted from wildlife or model aircraft transmitters. The transmitters used are typically very low power and transmit for around 40 milliseconds every second or two. The tone pitch and therefore signal direction can be very hard to determine using the above filtering modes.
This special mode extends the time of the peak level of the received signal so that the user can easily compare the signal level from different directions.
Holding the “4” button while powering up puts the sniffer into ARDF mode. In this mode the sniffer will power up giving three beeps and will synchronise the ARDF timer. For more information see the section on the Sync Button.
Holding the “5” button while powering up puts the sniffer into scan ready mode. In this mode the sniffer gives a single beep during power up and the sync button executes a band scan function. This is the factory default. For more information see the section on the Sync Button.
In this mode the signal strength tone frequencies are divided by 4. See the description of the “Dset” button for more info.
This is a special mode in the V3.0 sniffer for Australian Championships “fox-or” foxhunting. In this case the “7”, “8”, “9” and “0” keys are reallocated as fixed frequency channel recall buttons. The sniffer is set up to recall a set of 10 frequencies that match the transmitters used in the Mount Gambier event. This is done without affecting the frequencies stored in the 6 standard memories and resetting from this mode (by powering up with the “7” key) will recall the previously programmed settings. Note that when this mode is set the functions normally allocated to these buttons (Sync, Dset and Volume) are not available.
|Channel Key||Frequency||Channel Key||Frequency|
In this mode, most of the special features of the sniffer are disabled. This mode is useful when a newcomer or a scout group etc uses the sniffer. (Hence why some users call this “Scout Mode”) Memories can only be recalled and most of the other keys, including volume control are disabled. The power button only acts as a simple on-off and the mode of operation is defined by what is stored for each channel.
To set the sniffer in reduced functionality mode, press and hold the “F” button while powering up.
To cancel reduced functionality mode, press and hold the “7” key while powering up.
This key resets the following modes if set:
1) Low Tone mode,
2) Range Tones Announce,
3) Reduced Functionality (scout) mode,
4) Gambier Mode
There are a number of solutions for extending battery life with the sniffer. The most
critical one is the volume setting. Battery life is reduced considerably when using the
internal speaker on a high volume level with a continuous signal. Use the lowest
volume level possible when using the internal speaker or use external headphones.
The display also operates at a higher intensity level for daytime use that puts more
load on the battery. Typically night time only operation increases alkaline battery life
by about 30%.
Users who would like to get the longest use between battery changes should consider using Lithium AA batteries. These batteries have a very long shelf life and capable of providing well over 14 hours of continuous daytime operation.
Zinc Carbon and General Purpose Manganese, or so called “Heavy Duty” batteries are not recommended. The internal resistance of these batteries is too high to get reliable operation from the sniffer.
This version of the sniffer can run off higher capacity NiMh batteries but this is generally not recommended as their terminal voltage (1.2V) is normally too low. The power supply module in V3.0 of the sniffer is capable of operating off NiMh batteries and their use may be considered if the sniffer is used very regularly. But keep in mind that the sniffer will always indicate the batteries as partially depleted, even when fully charged. When the batteries go flat the sniffer will stop operating abruptly with little or no warning. The sniffer also provides no means of charging these batteries. They must always be removed for charging.
More information on foxhunting and
techniques can be found on the Australian ARDF website Follow this
link for Pricing, Availability and other foxhunt projects.
To contact the author:
The above graph shows the discharge rates of the typical battery options. It shows that the Lithium and 2500mAh NiMh can significantly outperform even the best alkaline batteries.
The lower published per cell terminal voltage of NiMh (1.2V) can become irrelevant as time increases since the voltage is essentially constant. Another downside is that the weight of these high capacity rechargeable batteries is around twice that of Lithium.
NiMh rechargeable batteries are not recommended for infrequent use. The self discharge rate of these batteries will mean that they will go flat after sitting unused in the sniffer for several months where Alkaline or Lithium batteries will maintain their usability for a much longer period.
It is always recommended to remove batteries of any type when the sniffer is not in use for extended periods.
The antenna design is largely a matter of
personal choice. For portable use, designs vary between two and four
elements. More elements give better directivity and more precise
bearings but are more difficult to carry around. For most situations,
three elements offers a good compromise. Here is an example of antenna
designs for 120 and 146MHz.
Standard version: 120-122.995MHz,
143-149.995MHz in 5KHz steps
Better than -120dBm
Maximum signal level
Greater than +30dBm
3VDC (2xAA Alkaline or
Lithium penlight cells)
Typically 8+ hours
(alkaline), 14+ hours (Lithium)
6, programmable (stores
mode and frequency)
AM, FM, Signal strength
Max RSSI frequency
Programmable 2KHz or 8KHz
76mm(W), 80mm(H), 25mm(D)
including BNC connector
Inside view of V1.3 prototype (Left) and the V2.2/V2.4 unit. In both cases not all components are shown as a significant number of them are on the opposite side of the board. The two images to the right are the new V3.0 unit. The later rev units use fine pitch IC's and 0603 SMD components to give maximum functionality in a small package.
Bryan Ackerly, VK3YNG
Phone +613 9752 3641
Fax: +613 8711 8428
23 Jan 2002
Initial Release of manual
Hardware release 1.0, Firmware release 1.0
26 Jul 2002
Manual revision 1.1:
- Added sample antenna designs, block diagram, tips for extending battery life, links and contact information
Hardware release 1.1:
Firmware release 1.1:
- Display shows range instead of Tx number in ARDF mode if range goes up.
- Added ability to see battery voltage in reduced functionality (scout) mode
- Volume control disabled in reduced functionality (scout) mode.
- Relative battery capacity (0-99%) can be displayed.
- Different tones used for ARDF mode transmitter number announcement beeps.
- Transmitter announcment beeps now set to four seconds before minute boundary (was 2 seconds)
- Peak range detector added which holds peak range for 30 seconds in ARDF mode.
- Range hold mode simulates the “Green button” on the “YQN” sniffer by using the range button.
28 Oct 2002
Manual updated to release 1.2 to link with
firmware revision. No code or hardware changes
02 Nov 2002
Hardware Release 1.2:
Firmware release 1.3:
- Recalling a channel multiple times in the presence of a signal would cause the sniffer to range up each time a button was pressed and not range back down if the channel was recalled more than 9 times in a row. Range is now reset to zero if the received frequency is changed or preserved as is if the same frequency is recalled. Frequency will be displayed if no range up is required.
13 Oct 2003
Hardware release 2.0
- Receiver redesigned to remove obsolete Motorola MC13135
- New Rx front end and Discrete MOSFET mixer
Firmware Release 2.0.00
- Added uptime counter
- Added Band Scan function on sync key
- Added ARDF sync at power-up
- Added "FM Mute" as 4th audio mode
- Added tone filtering features
- Added some handling for UHF operation
- Changed battery capacity lookup table
Manual Release 2.0
- released as pdf only
26 Jun 2004
Manual release 2.1a
25 Oct 2004
Hardware release 2.2
- Varactors changed
- Replaced MOSFET mixer with GaAsFET part
Firmware release 2.1.01
- Improved autoranging functionality and range-up speed.
- Added documentation for "peak extend" tone filtering mode
17 Nov 2004
Hardware release 2.2b
Firmware release 2.1.02
28 Nov 2004
Firmware release 2.1.03
2 Dec 2004
Firmware release 2.1.04
5 Dec 2004
Firmware release 2.1.05
24 Dec 2004
Firmware release 2.1.06
- Fixed RSSI filter initialisation (minor bug)
- Added independent range alignments for 120/144MHz bands to improve range 6 overlap on 120MHz.
29 Jan 2005
Firmware release 2.1.07
14 Apr 2005
Firmware release 2.1.08
(Workaround for earlier versions: Do not store "muted FM" mode in memory 1)
- Slight reduction in peak extend time
10 Jun 05
Firmware release 2.1.09
26 Jun 05
Hardware release 2.2d
- Added "dead battery" powerdown below 2.0V (was a hardware function with previous supply)
5 Sep 2005
Firmware release 2.1.11
4 Oct 2005
Firmware release 2.1.12
17 Nov 2005
Firmware release 2.1.13
31 Jan 2006
Hardware release 2.3b
- Slight improvements to front end amp.
- Added initial 243MHz support for trial use.
- Changed default channel 5 and 6 frequencies for 160MHz version.
15 July 2006
Hardware release 2.3c
21 Sept 2006
Firmware release 2.1.16
27 Feb 2007
Firmware release 2.1.17
31 Mar 2007
Hardware release 2.4
- Lower battery voltage sensing. (Mainly affects LCD build, but also useful to get more life from alkaline batteries)
- Change to first IF filter to improve adjacent channel rejection and blocking performance.
- Fixed range alignment sweep timing above range 1. Has no affect for end user.
12 Aug 2007
Firmware release 2.1.19
19 Mar 2008
Firmware release 2.1.20
31 Aug 2008
Firmware release 2.1.21
14 Nov 2008
Firmware release 2.1.22
26 Nov 2008
Firmware release 2.1.23
11 Mar 2009
Firmware release 2.1.24
16 Oct 2009
Firmware release 2.1.25
- Peak range now displayed using Dset-R
- In scout mode only when using dset-0, the range can only go back 1 range from peak hold value unless frequency is changed or channel is recalled. (useful to stop sinffer going "Dead" when an inexperienced user walks right past the signal source.)
- Untested support for manual range keypad LED and button brightness on ADC7 input.
18 Nov 2009
Firmware release 2.1.26
11 Nov 2010
Firmware release 2.1.27
- Added range 10 support for narrowband version 151/173MHz.
17 Feb 2011
Firmware release 2.1.28
15 Dec 2010
Firmware release 3.0.00
30 Aug 2011
Firmware release 3.0.01
1 Sept 2011
Firmware release 3.0.02
9 Sept 2011
Firmware release 3.0.03
2 Mar 2012
Firmware release 3.0.04
- Fixed bug where sometimes channel 1 ended up with "FF00" as the stored frequency. Only occurs when batteries are removed before power is turned off.
25 April 2012
Firmware release 3.0.05
- Changesd range IF offset from 2dB to 1dB per range to improve saturation problems at higher ranges with heavily AM modulated ARDF transmitters.
21 Nov 2012
Firmware release 3.0.06
6 Jan 2013
Hardware release 3.1
22 Jan 2013
Firmware release 3.1.01
5 April 2013
Firmware release 3.1.02
31 Sept 2013
Firmware release 3.1.03
- Note: for some reason this build cannot support 2.5kHz channel step due to code space restrictions from a new compiler version. 2.5kHz channel step versions will stay at version 2.1.02
More information on foxhunting and techniques can be found on the Australian ARDF website
Follow this link for Pricing, Availability and other foxhunt projects.
To contact the author: