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[aprssig] Mic-E and non ham aprs use

Stephen H. Smith wa8lmf2 at aol.com
Thu Jun 7 17:51:09 UTC 2012

On 6/7/2012 1:08 PM, Andre wrote:
> besides being ham I'm also communication volunteer for the dutch red cross 
> and my region is looking into assets tracking, currently google earth is used 
> but this is manual so I figured APRS could play a good roll in that especialy 
> as it is bidirectional.
> The need is not a constant update of every station as posts only rarely move 
> so only a position burst after a transmition is more then enough like it was 
> done with the original Mic-E trough repeaters, also the frequenties and 
> radios are limited so it is likely that data bursts will have to go trough 
> the voice channel.
> So the question is what trackers there are that still have the original Mic-E 
> function of appending the data burst after the transmition and wich ones are 
> capable of 9k6?

The TinyTrack III and Tigertronics TigerTrack TM-1 can both do this burst on 
unkey, as can the Kenwood D700, TH-D7, D710 and TH-D72  APRS radios with 
built-in TNCs.  You enter the Kenwood APRS menu and by change their beacon mode 
from "AUTO" to "PTT".

The standalone trackers can only do 1200 baud.  The Kenwood radios with their 
built-in TNCs can also effortlessly do 9600 as well.   The 9600-baud 
bursts-on-unkey are almost un-noticeable to the users -- the 9600-baud data 
stream sounds almost like white noise and blends into the receiver squelch 
crash that follows it.

The 1200-baud bursts are quite noticeable to the user.  The traditional 
approach, if doing voice operation through a repeater is to attach a TNC, 
configured as a digipeater, to the repeater receive audio. When it hears packet 
CARRIER DETECT, it mutes the repeater retransmit audio briefly. The transmit 
side of the TNC keys up a separate transmitter at the repeater site that 
transmits on the usual APRS frequency (144.800 in your case??).

The external trackers have to be able to monitor the state of the radio's 
microphone PTT line to trigger the burst-on-unkey.  In the case of the 
TinyTrack, the device must be in SERIES with the mic PTT line; i.e. the TT has 
separate MIC PTT-in (that monitors the state of the MIC button) and TX PTT-out 
(that actually keys the radio) pins.

(For this type of Mic-E operation, I have a TinyTrack packaged in a small metal 
box with a female mic jack on one end,  a male mic plug on a 15 cm cable (to go 
to the radio) coming out the other end, and a DB-9 male connector (to mate with 
a serial GPS) coming out the side.  The unit is powered by the 8 VDC present on 
one of the conductors from the radio mic jack. I was able to power a 
very-low-power GPS plugged into the box from the same 8 VDC source. You unplug 
the existing mic from the radio, plug it into the jack on the box, and then 
plug the box into the radio's mic jack.)

The TigerTrack has a single tri-state I/O pin that bridges the mic PTT line in 
parallel.  Normally it presents a high-Z CMOS-type input as it monitors the 
state of the PTT line.  When it sees the PTT line go low (mic PTT button 
pushed) and then return to the high state (mic button released), the single pin 
changes function and becomes an active-LO output that shorts the PTT line to 
ground to key after the voice transmission.  I.e it's edge-triggered when it 
sees a LO-to-HI transition on the mic PTT line.


1200 is easy to interface - basically just parallel the microphone and tracker 
TX audio at the radio mic jack.     For 9600, you need direct DC-coupled access 
to the transmitter's FM modulator.

This means cutting into the radio somehow, unless the radio has the 6-pin 
mini-DIN "data" or "packet" jack --AND-- configuration or menu support for 9600 
baud mode.    The DIN connector has two receive outputs:  One is normal 
de-emphasized  RX audio for 1200-baud operation.  The other is a direct 
DC-coupled connection to the receiver discriminator that supports 9600-baud 

However on TRANSMIT,  the DIN jack has only ONE connection.  The radio must 
provide an explicit jumper or configuration menu option for switching this 
single pin's function from audio input for the 1200 & 2200 Hz  audio tones used 
for 1200 baud to the direct TTL logic-level data stream used for 9600-baud 



Stephen H. Smith    wa8lmf (at) aol.com
Skype:        WA8LMF
Home Page:          http://wa8lmf.net

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