Note
This description applies to the exixting sniffer kit operation. A new version is due soon which may vary a little from this as we improve the software. See Current Status
The Buttons
The only user interface is a Red button (LHS), a Green (RHS) button and a 3.5mm stereo headphone jack. There is also a speaker and a BNC connection for the RF input. This minimal control approach allows the sniffer to be properly waterproofed.
To turn it ON press the LHS button. You are now in Whoopee mode as required for sniffing on the FOX frequency. Whoopee mode is a tone that varies in pitch with signal strength. A variation in pitch is much easier for the ears to detect than a variation in tone amplitude.
The sniffer ranges UP automatically in Whoopee mode. That means as you get closer to the fox transmitter, the sniffer will put in more attenuation as appropriate. There are 9 attenuation ranges altogether, about 15dB apart.
To Range the sniffer Down (more sensitive) in Whoopee mode Press the RHS button. You will need to do this if the fox gets weaker for some reason, or when you start sniffing. Another tone can be heard whilst the button is depressed, the pitch of which lets you determine the range currently selected.
The sniffer will always select the optimum range for DFability. Within each range the whoopee tone will vary from a low buzz to a high squeal over about a 40dB RF input variation (this means the ranges overlap).
The LED will flash the Range number, with the lowest number (1) Range being the most sensitive range. The Range can also be heard as Clicks in the audio on top of the whoopee tone.
To listen to the fox on AM press the LHS button again. The automatic ranging and the range down button works exactly the same as in whoopee mode.
To listen to the FM LIASON frequency press the LHS button again. If the TX option is installed (only possible on 2m sniffers) the RHS Button is now the PTT button. Speak into the speaker to talk. When no FM signal is present the output is muted. This squelch operation is based on signal strenth.
To get back to whoopee mode press the LHS button again...and so on.
To turn the sniffer OFF hold the LHS button in for about 1.5 seconds. The sniffer will turn OFF. Release the LHS button.
The sniffer will also auto Power off after two minutes of neither button pressed or any auto range changes.
Multi-Purpose Plug
The 3.5mm stereo socket is used for many tasks. Normal mono headphones (a mono 3.5mm jack is assumed) can be used, a small tunable radiating coil for FM wireless operation instead, and battery charging is performed using the same socket when the sniffer is OFF. The upcoming version will also have a serial port provided through the same socket.
The FM wireless link allows a set of commonly available FM radio headphones to be used with the sniffer. The coil plugged in is both the FM radiator and the tuning element (so the FM frequency can be tweaked without taking the sniffer apart). No more broken headphones cables !
The Nicad charger is a simple diode and series resistor which is adequate for 12V charging. It's a good idea to use an external constant current source set to 10mA for more accurate Nicad charging. In this case the internal resister has no effect.
Programming
There are two frequencies that need to be programmed into the sniffer, one for the FOX frequency, and one for the LIASON channel for FM. This is done with a special power-up sequence.
The sniffer remembers these frequencies when it is turned off, so this task need only ever be done once, or if your frequencies change.
Of course the two frequencies can be programmed the same when the FOX is known to be transmitting FM if you wish to be able to monitor the FOX audio as well as DF it.
By holding in the RHS button whilst turning ON the sniffer by pressing the LHS button you can put the sniffer into programming mode.
Now that you have both buttons depressed which one you let go of first determines whether you program the "hunting" channel or the "liason/FM" channel.
Graham, VK3KOA, likes to keep commonly used frequncies shown in binary on the lid of his sniffer:
