Burst-after-voice Tracker operation


This mode can be very useful for RAYNET operations involving tracking mobile stations, such as sweep vehicles on charity walk events.
It has the benefit that a single (RAYNET) channel is used for voice and APRS, so that each station only needs a single transceiver and antenna (ideal if a temporary station is to be installed in someone else's vehicle).

In practice, stations using this mode will send a short APRS burst at the end of their overs - rather like a pip-tone. In order that these bursts do not disrupt the normal voice communications, they must be kept as short and infrequent as possible, so:
use minimum reporting options (just position and velocity) and compressed mode;
omit text comments, or keep them brief and send them infrequently;
limit the repeat time for APRS bursts (if possible).

It's also possible to use a digipeater on the channel to improve APRS coverage, but remember that this will echo each data burst, so the next person to transmit should allow a short pause before pressing their PTT.

The Kenwood D7-series radios can be set into "PTT mode" - see your user manual for details. For everyone else using Trackers, read on...


Connecting up

This mode requires the "PTT input" on the Tracker (pin 8) to be connected to the microphone, whilst the Tracker's PTT output (pin 3) connects to the radio. When the mic PTT is released, the Tracker maintains the PTT signal to the radio for the duration of the burst.
So the radio's mic input, audio output and PTT need to connect to both the Tracker and microphone (in parallel).

These connections can be achieved either with a custom microphone lead, or by making a "T-adaptor" to insert the Tracker between mic and radio (or alternatively to insert the mic between Tracker and radio).

For handies with the typical "3-wire" speaker-microphones (such as the ICW21E shown below), the interface is merely a parallel connection of these three lines to the Tracker (see diagram). The pin 8 PTT input is also connected to the mic's PTT line, and the "shared PTT" mode must be set in the Tracker.

Icom W21E with Tracker inserted in-line with speaker-mic
Click on the picture to see the schematic diagram.

Some mobile transceivers (usually the older ones!) include pins for audio output (speaker) and DC bias on their front-panel microphone sockets. In these cases, it's relatively easy to wire in the tracker along the lines of the tee-adaptor described above, so long as the DC ouptut pin on the rig's mic socket can source enough voltage and current to run the Tracker (and attached GPS?).
For other mobile sets, it will be necessary to include an external DC supply (7.5 to 13.8V) onto pin 7 of the Tracker.

If the mobile set doesn't route its speaker audio output onto the mic socket, don't worry! For burst-after-voice mode only the Tracker does not need to see this signal (it's only used to prevent the Tracker from making its automatic beacon transmissions until the channel goes quiet).
In this case, the mobile microphone assembly will look like the example below: the original MH31 microphone is retained, by connecting the Tracker into a microphone extension lead made from an ethernet "Cat5" cable and adaptor. This method has the advantage where the set's own microphone includes facility buttons or a keypad.

FT817 and MH31 with Tracker adaptor lead
Click on the picture to see the schematic diagram.

Unfortunately, not all microphones can be connected in this way, as their PTT switch may a high-value resistor (typically 27K) in series, so it will not trigger the Tracker's PTT input (in these sets, the current drawn from the PTT line selects different functions).
The Yaesu MH48 and MH42 (used with the FT8900 series) are known to be in this category.

One solution is to use a PNP transistor as a DC amplifier, to enable the tiny PTT current drawn by the microphone to trigger the Tracker. This circuit (which is as yet untested) shows the arrangement. Note that Tracker draws its power from the FT8900's internal 9V regulator (which is helpfully routed down the microphone lead) so no external battery is needed in this case.
When using the MH42 microphone, it may be necessary to add a resistor into the transistor base circuit, chosen so that the PTT current triggers the Tracker but the other functions on the same line do not.
Please let me know by email how you get on if you try either of these circuits.


Configuring your Tracker

Example configurations for the OpenTracker and TinyTrak units are shown below.

Note that in both cases:
the path is set to "RAYNET", to facilitate digipeating;
the text message is only sent after every fifth (or third) transmission;
the "TX Interval" or "Auto Transmit Delay" is set to zero, to disable periodic beacons;
the (Manual) "TX Delay" is set to a low number, to keep the burst short;
altitude reporting is disabled, to keep the burst short;
"no PTT out on PTT in" is selected, to select "shared PTT" mode.
Additionally, for the OpenTracker, the "use PTT input" and "Compressed" boxes should be ticked.

OpenTracker set for burst-after-voice


TinyTrak set for burst-after-voice

Additionally, for the TinyTrak, the "Manual Transmit Rate" is set to 30 sec, to limit the number of data bursts between brief "rapid fire" overs: i.e. there won't be more than one burst every 30s.

Updated November 2008