Using the MK4 foxhunt receiver to hunt
short duration pulsed transmissions.
The VK3YNG MK4 foxhunt sniffer (V2.2
or later) has a special mode for hunting very short duration "pulsed"
These beacons have transmission times of typically around 40
milliseconds and are often used for tracking animals, rockets and
These short transmissions can be very hard to deal with when
listened to in the standard tone modes on the VK3YNG sniffer. The
is usually so short that the user cannot distinguish the audio tone
determine a direction. The MK4 sniffer has a new “peak extend” mode
out the received pulse so that its signal level and resultant tone can
Setting up Peak Extend mode
To successfully look for these very
duration transmissions, the sniffer needs to be set up in “peak
with a range down delay of 5 seconds. To set this mode:
The peak extend mode only needs to be
set once. The sniffer
will remain in this mode for each subsequent use until another filter
- Make sure the sniffer is turned off
- Press and hold the “6” button on the
- While holding this button, power up
the sniffer with the “M” button
- The sniffer will beep and briefly
- Press the “7” or “dset” key. The
sniffer will display “d”.
- Press the “5” key
- The sniffer is now configured for
“peak extend” mode
For pulsed transmissions that only
occur every few seconds
or longer apart, it may also be useful to activate “Peak hold” mode.
information on this mode, consult the sniffer
Note that if the
sniffer is to be
used for finding longer
pulsed intermittent or continuous transmissions, the peak extend mode
confusing. For hunting these signals, filter modes 1 through 3 are
Filter mode “1” is the factory default. Also a range down delay of 5
may be too long. A setting of around 2 seconds may be more appropriate.
more information please refer to the sniffer
Using the Sniffer
Using the sniffer to locate a signal
straightforward and intuitive. Just remember the motto “highest range
then highest tone”. Once the
sniffer is powered up and the frequency or
channel selected, operation is effectively hands free.
Power control and receive mode.
The sniffer is capable of receiving
in 4 standard modes.
These modes are Tone, AM,
Unmuted FM and Muted FM. Pressing the
“M” button powers up the receiver and cycles through these receiving
most cases the “Tone” mode will be used to determine signal
The sniffer indicates this mode by briefly showing “t” on the display
“M” button is pressed. Pressing and holding the “M” button for more
than a few
seconds will power down the receiver.
The receiver always powers up on the
mode and frequency
stored in channel 1. There are a total of 6 programmable channels on
sniffer. Recalling any channel is just a matter of pressing the
channel button. For information on setting channel frequencies please
The range display
The range display is effectively a
measure of how much
signal the sniffer is receiving. In effect it tells you how close you
the signal source.
The maximum number displayed when you
point directly at a
very close signal source will vary depending on how strong the
A bit of practice with a typical transmitter will help you determine
range display indication varies with distance.
- 0 = Extremely weak or no signal
- 5 = Close to a medium power transmitter
- 9 = Very close to a very strong
Note: When you
first set the
frequency in peak extend
mode, it may take a few pulses before the tone is properly heard,
the pulse duration is very short. It can take a few transmissions for
sniffer to determine the proper range to use. Once it has done so, the
will be heard clearly.
Direction Finding Techniques
Using the sniffer for direction
finding takes a little
practice, but it doesn’t take long at all to become proficient enough
at it to
find any transmitter.
A few tips to remember
- Always hold the receiver so the
display can be easily seen and the speaker easily heard.
- Always keep sweeping the receiving
antenna from left to right over a 90 to 180 degree arc while
approaching a transmitter or signal source.
- Every few minutes, do a slow and
complete 360 degree spin to ensure that you have not “overshot” the
signal source or been fooled by a strong or local reflection.
- Always remember the motto “Highest
range then highest tone” while using the sniffer.
The Multipath problem
A major problem to contend with at
VHF and higher
frequencies is a phenomenon called “multipath”. This is where signals
off other objects such as metallic structures, hills and even
Signals in this case appear either skew or appear to come from multiple
directions (hence the
“multipath”). A good antenna can help sort out some reflections
wanted signal, but in many cases reflections become something that you
have to deal with.
The useful thing to know about
reflections is that they come
and go. In most cases if you always make the effort to determine the strongest
signal, you will eventually end up at the signal source. With a bit of
you can get fairly good at working out which signal is a reflection is
a direct signal. The major thing to remember is to keep moving. Even if
start following a reflection, you probably won’t be doing so for long.
Multipath is a bit more difficult to
deal with for
intermittent signals but the techniques are the same.
With a bit of practice, hunting any
signal source should be
simple and straightforward with the MK4 sniffer. The peak extend mode
hunting for short pulse signals almost as easy as continuous ones.
Once the receiver is properly set up,
there should be no
need to do any adjustments from the time the signal is first received
to when the transmitter is found. Using the MK4 sniffer to quickly and
successfully find any signal source should be easy for anybody
age or technical background