VK3YNG Foxhunt Sniffer Instruction Manual

Introduction

The VK3YNG sniffer allows quick, easy, and accurate determination of signal direction within the 120-123MHz or 143-150MHz bands. It offers sufficient sensitivity for long-range detection and precise attenuation to prevent overload effects. Signal strength is indicated by an audible tone that increases in frequency (pitch) with signal level, leveraging the human ear's sensitivity to pitch changes. This design avoids inertia or overshoot issues common with signal meters.

Attenuation is applied automatically in 15dB steps as signal strength thresholds are met, indicated on an LED display (0 for maximum sensitivity, 9 for ~135dB attenuation).

Using the MK4 Foxhunt Receiver

The VK3YNG MK4 foxhunt sniffer features a special mode for hunting very short duration "pulsed" or "pip" transmitters, often used for tracking animals, rockets, or model aircraft. These transmissions, typically around 40 milliseconds, are difficult to discern in standard tone modes. The "peak extend" mode stretches these pulses, allowing for easier determination of signal level and direction.

Setting up Peak Extend Mode

To use "peak extend" mode for short duration transmissions:

1. Ensure the sniffer is turned off.
2. Press and hold the "6" button.
3. While holding "6", power up the sniffer using the "M" button.
4. The sniffer will beep and briefly display "J".
5. Press the "7" or "dset" key; the display will show "d".
6. Press the "5" key.

The sniffer is now configured for "peak extend" mode. This setting persists until another filter mode is selected.

For transmissions occurring every few seconds or longer, "Peak hold" mode may be useful (refer to the operation manual). If hunting longer pulsed or continuous transmissions, "peak extend" mode might be confusing; filter modes 1-3 are recommended, with mode 1 being the factory default. A range down delay of 2 seconds may be more appropriate than the default 5 seconds for these signals.

Using the Sniffer

Locating a signal source is intuitive. The motto is "highest range then highest tone". Once powered on and frequency/channel selected, operation is hands-free.

Power Control and Receive Mode

The sniffer supports four modes: Tone, AM, Unmuted FM, and Muted FM. The "M" button cycles through these modes, indicated by "t" for Tone mode. Pressing and holding "M" powers down the unit.

Channel Selection

The receiver defaults to channel 1. Six programmable channels are available. Recalling a channel is done by pressing the corresponding button.

The Range Display

The range display indicates signal strength, showing proximity to the source:

• 0: Extremely weak signal or no signal
• 5: Close to a medium power signal
• 9: Very close to a very strong signal

The maximum displayed number for a close signal varies with transmitter strength. Practice is recommended to understand range display variations.

Note: In "peak extend" mode, it may take a few pulses to hear the tone clearly, especially with very short pulses. A few transmissions might be needed for the sniffer to establish the correct range.

Direction Finding Techniques

Proficiency in direction finding is achieved with practice. The core technique involves swinging the antenna over a 90 to 180-degree arc to find the "peak" of the tone pitch.

For continuous signals: Swing the antenna back and forth while moving towards the transmitter.

For intermittent pulsed signals: Try different directions, wait for each transmission, and move towards the direction yielding the highest tone pitch.

Perform a 360-degree rotation every 10 steps to avoid overshooting or following reflections.

Visual Description of Direction Finding:

Imagine two scenarios:

• "Off bearing" Receiver (Lower tone pitch): The antenna is not pointed directly at the transmitter. The tone pitch is lower.
• "On Bearing" Receiver (Highest tone pitch): The antenna is pointed directly at the transmitter. The tone pitch is highest.

As you approach the transmitter, the tone pitch increases until the receiver saturates. The processor then applies "Attenuation" (signal reduction), causing the range display to increase by one and the tone pitch to drop. This effect becomes more frequent as you get closer, providing an indication of proximity.

Tips for Direction Finding:

• Ensure the display is visible and the speaker is audible.
• Sweep the antenna from left to right over a 90-180 degree arc.
• Periodically perform a 360-degree spin to confirm position and avoid reflections.
• Remember the motto: "Highest range then highest tone".

The Multipath Problem

Multipath is a common issue at VHF and higher frequencies, caused by signals reflecting off objects like structures, hills, and vegetation. This can make signals appear to come from multiple directions. While a good antenna can help, managing reflections is often necessary.

Visual Description of Multipath:

Two diagrams illustrate multipath:

• Diagram 1: Shows a transmitter, a reflecting object, and a receiver. A "Direct Path" and a "Reflected path" are shown. The "peak" direction can vary based on the phase of the reflected signal, potentially pointing towards the direct signal, reflected signal, or an angle between them. A good beam antenna might distinguish signals, but often isn't sharp enough. Reflected paths can have signal loss; a signal appearing stronger than normal might be the direct signal, especially over varied terrain.
• Diagram 2: Shows a transmitter and a reflecting object. After moving some distance, the reflected signal's angle changes, and only the direct signal is received on the true bearing. In real environments, multiple reflections occur, but the direct signal is usually present.

Reflections are transient. Consistently seeking the strongest signal will eventually lead to the source. With practice, distinguishing direct signals from reflections becomes easier. Continuous movement is key; even if following a reflection, it's usually temporary.

Multipath is more challenging with intermittent signals, but the techniques remain the same.

Final Comments

With practice, signal hunting using the MK4 sniffer becomes straightforward. The "peak extend" mode simplifies hunting short pulse signals. Once set up correctly, no further adjustments are typically needed. The VK3YNG MK4 sniffer facilitates quick and successful signal finding for users of all ages and technical backgrounds.

VK3YNG Sniffer 4 Configuration and Features

Key Button Functions (Quick Reference)

Alternative Key Functions (Power-Up Configuration)

Default operation modes can be reconfigured by holding specific keys during power-up:

Detailed Button Operation

Mode/Power Switch (M)

• Power On: Pressing "M" once powers the unit to the frequency and mode stored in memory channel 1.
• Mode Selection: Momentarily pressing "M" cycles through modes: "A" (AM), "U" (Unmuted FM), "F" (Muted FM), "t" (Signal strength Tone).
• FM Mute: Muted FM mode mutes receiver audio when the signal is below a set threshold.
• Power Off: Pressing and holding "M" for over 1 second powers down the unit. In reduced functionality mode, a single press powers off the unit.

Memories

Buttons 1 through 6 store and recall memory modes and frequencies. In normal operation, pressing a button recalls the stored data, displaying the last 4 digits of the frequency in kHz. The first two MHz digits are inferred by the first entered digit (0-2 for 120MHz band, 3-9 for 140MHz band).

To store, press and hold a button (1-6) until two short beeps confirm storage. The store function is disabled in reduced functionality mode.

Sync Button (7)

ARDF Sync: Used to synchronize the receiver for international ARDF foxhunting (one-minute cycle, five-transmitter system). Pressing "Sync" re-synchronizes the timer. Three short beeps confirm the action. It should be pressed at the start of transmitter one's cycle.

At 50 seconds, three short beeps warn of the cycle's end. If receiving at range 1 or lower, the sniffer broadcasts beeps indicating the next transmitter number. If "d-Set" is zero and manual ranging is off, range zero is automatically selected for the next transmitter.

ARDF mode is entered by holding "4" during power-up; it's cancelled by returning to Scan mode.

Band Scan (7)

When not in ARDF mode, the "7" key performs a band scan, seeking the highest signal between frequencies in channels 5 and 6. The highest signal is stored in channel 4. Signals within 10KHz of channel 1 are ignored, and the signal must be at range 2 or higher to be stored.

Scan works best for continuous signals. Channel 5 must be lower in frequency than channel 6, and both must be in the same band (120MHz or 140MHz).

SCAN mode is entered by holding "5" during power-up; it's cancelled by entering ARDF Sync mode. Scan mode is the factory default.

Other Functions (via Power-Up Key Combinations)

• Holding "7" during power-up: Cancels Low RSSI Tone, disables Morse code tones, sets LED brightness to "auto", and disables Reduced Key Functionality ("scout" mode).

D-Set Button (8)

Range Down Delay: Programmable from 1 to 5 seconds (default 2 seconds). Beginners may prefer 3-5 seconds; advanced users 1-2 seconds. For intermittent/short duration signals, 5 seconds or "peak hold" mode is recommended.

To set: Press "D-Set" (display shows "d"), then press 0-5 to set the delay. Pressing "D-Set" twice shows the current delay.

Setting delay to zero enables "peak hold" mode, disabling down ranging and manual ranging. Pressing "Range" resets the peak hold to zero. Useful for intermittent signals.

Holding "D-Set" during power-up enables lower tone RSSI mode (tones divided by 4, max pitch ~2KHz), useful for those with hearing difficulty.

Special Note: "Dset 0" in "Scout" Mode: In Scout mode (v3.0+), if "Dset 0" (peak hold) is selected, a signal disappearing for over 5 seconds causes range to drop by only one level, allowing beginners a second chance.

Uptime, Battery Voltage, Firmware Version

• Uptime: Press "D-Set" then "7" to show hours/minutes since power-up.
• Battery Voltage: Press "D-Set" then "9" for relative battery capacity (0-99%). 99% is full, 0% is critically low. In Scout mode, a single "D-Set" press shows capacity.
• Firmware Version: Press "D-Set" then "F" to display the firmware version.

Volume Control (9, 0)

The "9" and "0" buttons adjust volume. These are disabled in reduced functionality mode.

Range Control (R)

Automatically selects the best range. Manual ranging is possible by pressing "R" then a digit. Pressing "R" again re-enables auto-ranging.

Holding "R" during power-up enables Morse code announcements for range, frequency, and other functions, assisting blind operators. Powering up with "7" disables Morse announcements.

Peak Hold Mode

Resets peak range hold if "D-set" is zero. Manual ranging is disabled.

Peak Range Memory

Pressing "Range" twice shows the maximum range achieved since power-up or last frequency change. In ARDF mode, peak range resets near the end of the next transmitter cycle.

This function is unavailable when Peak Hold mode (dSet=0) is selected.

Frequency Entry (F)

The sniffer operates from 120.000-122.995MHz and 143.000-149.995MHz in 5KHz steps. Pressing "F" initiates entry, displaying "F". Four digits set the frequency. An "E" indicates an error, reverting to the previous frequency. The first digit sets the 1MHz frequency and band (0-2 for 120MHz, 3-9 for 140MHz).

Factory Default Frequencies

Note: 121.5MHz is the international distress beacon frequency. The scan function requires channels 5 and 6 to be in the same band.

Direct frequency entry is not permitted in reduced functionality mode.

(Special versions may have different frequency ranges or 1kHz resolution.)

Configuring the Sniffer

Tone (RSSI) Filtering

Four filtering levels are available, selected by holding buttons 1, 2, 3, or 6 during power-up:

• Minimum filtering (1): Fastest, most accurate tracking. Can sound "thick" on high AM transmitters. Factory default.
• Medium Filtering (2): Best compromise between response time and AM filtering. Recommended.
• Maximum Filtering (3): Provides maximum filtering, smoothing received signals but potentially blurring short pulses.
• Peak Extend mode Filtering (6): For very short duration repetitive signals (e.g., 40ms pulses). Extends peak level time for easier signal comparison.

ARDF/Scan Mode

• ARDF Mode (4): Holding "4" during power-up enters ARDF mode, synchronizing the ARDF timer.
• Scan Ready (Non-ARDF) Mode (5): Holding "5" during power-up enters scan ready mode, disabling ARDF. The sync button executes a band scan. Factory default.

Other Modes

• Low Tone Mode (8): Signal strength tone frequencies are divided by 4.
• Gambier Mode (9): Special mode for Australian Championships foxhunting, reallocating numeric keys to fixed frequency recalls. Functions like Sync, Dset, and Volume are unavailable.
• Display (and keypad) LED brightness (0): Cycles brightness: "Dull" (d), "Bright" (b), "Off" (o), "Auto" (A). "Off" and "Dull" save battery. "Auto" is factory default.

Reduced Functionality Mode (F)

Disables most special features, useful for beginners or scout groups ("Scout Mode"). Memories are recallable only; other keys, including volume, are disabled. Power button acts as on/off. Mode is set by channel configuration.

To enter: Hold "F" during power-up. To cancel: Hold "7" during power-up.

Function Reset (7)

Resets or cancels modes: Low Tone, Range Tones (Morse) Announce, Reduced Functionality, Gambier Mode, sets keypad LEDs to "dull" brightness, and ARDF mode (firmware v4.1.00+).

Battery Selection and Maximizing Battery Life

Battery Life: Typically 3+ hours (alkaline), 14+ hours (Lithium). Affected by volume and display intensity. Night use increases alkaline battery life by ~30%.

Recommended Batteries: Use 2xAA Alkaline or Lithium penlight cells (3VDC).

Not Recommended: Zinc Carbon and General Purpose Manganese ("Heavy Duty") batteries are not recommended due to high internal resistance, potential leakage, and voiding the warranty.

NiMh Batteries: Higher capacity NiMh can be used but may offer lower performance than alkaline. Their lower terminal voltage (1.2V) causes the sniffer to report them as partially depleted (~50%) even when fully charged, as the reading is based on the alkaline discharge curve. They stop operating abruptly when depleted and require removal for charging.

Lithium Batteries: Single-use Lithium batteries are supported in v4.2. Rechargeable Lithium-Ion batteries MUST NOT be used as their terminal voltage is too high and will damage the sniffer.

Battery Discharge Comparison

Visual Description: A graph shows AA battery discharge rates at 160mA for Lithium Disulphide, Alkaline, and NiMH types. Lithium and 2500mAh NiMH generally outperform alkaline. Low self-discharge NiMH (e.g., Panasonic Eneloop) are good for regular use. Standard NiMH are not recommended for infrequent use due to self-discharge.

Lithium batteries report full charge for longer and give less warning before depletion.

Recommendation: Always remove batteries when the sniffer is not in use for extended periods, especially for aircraft transport.

Sample Antenna Designs

Antenna choice is personal. Portable designs typically use two to four elements. Three elements offer a good compromise between directivity and portability.

Visual Description: A diagram shows a "Typical 3-element Beam Antenna (based on VK3VT design)". It includes a BNC connector, director, driven element, reflector, balun, and sniffer mounting plate. A table provides dimensions in millimeters for 121MHz and 146MHz frequencies, including element lengths, tap length, element spacing, and balun length. Units can be converted to inches by dividing by 25.4.

Specifications

*Note: Battery life depends on sound level and display intensity.

Visual Description: A block diagram labeled "VK3YNG Sniffer MK4 - Block Diagram" illustrates the internal components and signal flow, including RF, Atten, VCO, PLL, Detector, CPU, Power Supply, and connections for Antenna, Keypad, Tune, Gain, and Detector modes (FM, AM, Tone, RSSI).

Links

For more information on foxhunting and techniques:

• Australian ARDF website: http://www.ardf.org.au
• Joe Moell's Foxhunting website: http://www.homingin.com

Author: Bryan Ackerly, VK3YNG

• Web page: http://www.foxhunt.com.au
• Email: info@foxhunt.com.au
• Phone: +61 438 573 686