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.

***Note: A number of features have changed since earlier versions of the sniffer were released. This manual reflects operation of version 2.2 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.

Introduction
Auto Power Down
Low Battery Indication
Display Brightness
Reduced Functionality mode
Quick Button Reference
    Alternative Key Functions
    Detailed Button Operation
    Mode/Power switch
       Power On
       Mode Selection
       Power Off
    Memories
       Recall functions
       Store functions
    Sync Button
    D-Set button
       Relative Battery Voltage indication
    Volume Control
    Range Control
       Peak Hold Mode
       Peak range memory
    Frequency Entry
Configuring the sniffer
    Minimum Filtering
    Medium Filtering
    Maximum Filtering
    Peak Extend Mode
    ARDF Mode
    Scan Ready Mode
    Reduced Functionality Mode
Maximising Battery Life
Sample antenna designs
Specifications
Block Diagram
Inside View of Prototype
Links
Revision History

Introduction

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 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.

Auto Power Down

Low Battery Indication

When power has dropped to 2.7 volts or lower, the display decimal point is enabled to warn the user that batteries are in need of replacement.

Display Brightness

Reduced Functionality mode:

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”.

Quick Button Reference:

Alternative Key Functions:

Detailed Button Operation

Mode/Power switch:

Power On:

Mode Selection:

Pressing this button momentarily while the unit is powered up cycles the unit through its operating modes and briefly displays the selection on the display. The modes cycle through in the following sequence: “A” – AM reception, “U” – Unmuted FM reception, “F” – Muted FM reception and “t” – Signal strength Tone. The cycle then repeats. When headphones are used, one channel is always set to give signal strength tone, while the other channel follows the selected mode. This is useful for hunting different continuous carrier transmissions where the transmitters’ identification is given using either AM of FM modulation.

FM Mute:

Power Off:

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. 

Memories

Recall functions.

The recalled frequency is not displayed in reduced functionality mode.

Store functions

The Store function is disabled in reduced functionality mode.

Sync Button

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.

Band Scan:

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)

Other functions:

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 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.

This key is disabled in reduced functionality mode.

D-Set button

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 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.

Uptime indication:

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.

Relative Battery Voltage indication:

Pressing the “D-Set” button then pressing “9” will display two digits giving an indication of the relative battery capacity in percentage terms. 99% indicates a full battery while 0% indicates the point where sniffer operation is significantly compromised. The sniffer may power itself off before reaching 0%.

Firmware version indication:

Pressing the “D-Set” button and then pressing the “F” button will display 4 digits which indicate the installed firmware version.

Volume Control

These buttons are disabled in reduced functionality mode.

Range Control

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 toggles the state of the “range tone announcement” flag. When this flag is set, the sniffer generates a “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, range changes happen without announcement.

This key is disabled in reduced functionality mode.

Peak Hold Mode

Peak range memory

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.

Frequency Entry

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

Memory	Frequency	Mode
1	145.300MHz	Tone
2	145.700MHz	Tone
3	144.250MHz	Tone
4	146.500MHz	Tone
5	121.500MHz	Tone
6	146.575MHz	FM

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 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.

Direct frequency entry is not permitted in reduced functionality mode.

Configuring the Sniffer

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.

Tone (RSSI) Filtering:

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.

Minimum filtering (1):

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.

Medium Filtering (2):

This level of filtering offers the best compromise between response time and Amplitude modulation (AM) filtering.

Maximum Filtering (3):

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.

Peak Extend mode Filtering (6):

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.

ARDF/Scan Mode:

ARDF Mode (4):

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.

Scan Ready Mode (5):

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.

Reduced Functionality Mode:

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.

Maximising Battery Life

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.

Do not use single cell NiCd or NiMh batteries as their terminal voltage (1.2V) is too low. Zinc Carbon and General Purpose Manganese batteries are also not recommended. (For the technically capable however, it is possible to modify the sniffer to run off three AA NiCd or NiMh batteries by the external addition of an extra single cell AA battery holder. Capacity of NiMh batteries nowadays is rapidly approaching that of standard Alkalines)

Sample antenna designs

Specifications:

Links:

 

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:
Bryan Ackerly, VK3YNG
Phone +613 9752 3641
Fax: +613 8711 8428

Revision History

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