More MicroBeacon TX Information

Major Features
Schematics and Pictorials
General Description
Technical Description
Installation and Operation
Programming the Parameters
The Programming Sentence

Major Features :

  • Complete 144 MHz APRS position transmitter
  • 144-148 MHz in 1 KHz steps, 2 Watts output @ 12 VDC
  • Requires user-provided GPS receiver, RS232 NMEA output
  • Small size : 1.2 x 2.2 x 0.13 inches, SMT technology
  • DC power 6 - 14 V, 10 ma "idle"
  • Fractional-N PLL chip, ADF7012
  • Pi output circuit for spectral purity
  • PIC microcomputer ( 18F442 ) to supervise operation
  • MAX5102 8-bit DAC to generate APRS tones
  • Philips PD85004 RF FET PA stage
  • All TX parameters programmable by customer with HyperTerminal
    Specifications :

    Power :
    6 - 14 VDC, 10 mA idle, 0.2-0.6 A TX
    Size :
    1.2 x 2.4 x 0.13 inches
    RF Out :
    Over 2.0 Watts @ 12.0 VDC, 0.54 A
    Frequency :
    144 - 148 MHz, 144.390 MHz standard

    Schematics and Pictorials :

    Download links to schematic diagrams and technical details for the MicroBeacon are provided here. For additional information about the MicroBeacon transmitter, inquire directly by e-mail.

    MicroBeacon schematic ( 15 Kb GIF file )
    MicroBeacon board outline / straps ( 22 Kb GIF file )
    MicroBeacon programming worksheet ( 22 Kb GIF file )


    General Description :

    The MicroBeacon TX is a complete 2 meter, position-reporting APRS “beacon” transmitter which reports the user’s GPS location at regular time intervals. The user must provide an external GPS receiver with RS232 output, ( USB won't work ) an antenna and DC power. All major message parameters can be user-defined with HyperTerminal and are saved in non-volatile EE memory. Over 2 Watts RF ouput from 12 VDC. Reports can ( typically ) be viewed on the web at ( and other websites ) if the signal is detected by an iGate base station, which relays the information to the internet. .

    Several MicroBeacon parameters can be defined by the customer, ( using HyperTerminal ) and are saved in EE memory. These parameters include the following items :

  • Transmit frequency ( 144-148 MHz in 1 KHz steps )
  • TX repeat time interval ( 10-9999 seconds )
  • User call sign and SSID number
  • Zero, one or two VIA call signs and SSID numbers
  • APRS map symbol ( car, van, motorcycle, etc )
  • "comment" text, up to 32 characters

    An RS232 input is provided for GPS data, and for programming the user-defined parameters. The MicroBeacon uses the ( very common ) NMEA $GPRMC sentence, which must be provided by the GPS. The GPS parameters ( automatically ) transmitted by the MicroBeacon include the following items :

  • Present latitude ( DDMM.MM + hemisphere, N or S )
  • Present Longitude ( DDDMM.MM + hemisphere, E or W )
  • Present vehicle course ( true degrees )
  • Present vehicle speed ( knots )


    Technical Description :

    The MicroBeacon TX uses a PIC 18F442 micro running at 20 MHz with about 4K of code, and an Analog Devices ADF7012 LP transmitter chip, followed by a Philips PD85004 RF FET PA stage. APRS audio tones are generated with a MAX5102 8-bit DAC to provide very clean tone waveforms. Output network uses a PI network with a PCB "printed" inductor to maintain a very low profile... overall thickness is only 0.125 inches.

    The measured 2nd harmonic is suppressed more than 50 dBc, ( = 20 uW ) other harmonics are further suppressed. "Close-in" noise is not so trivial, down -40 dBc @ 60 KHz from carrier, due mostly to the ADF7012 "fractional-N" PLL.

    Although intended for use with 12 VDC power, ( up to 13.8 V ) operation at lower voltages is practical, ( with reduced RF output ) all the way down to 3 VDC. The ADF7012 is actually capable of operation up to 1 GHz, ( PD85004 PA stage also ) so this unit has the potential ( with non-trivial hardware and software mods ) to operate on other bands. Source code will NOT be provided, so negotiations will be involved, if this is desired. ( contact me : Bob Simmons )

    It is anticipated that code will eventually be added ( fall of 2009 ) to interpret the $GPGGA NMEA sentence, which reports altitude, which will make this beacon especially suitable for free-flight helium balloons. ( weight = 1/4 oz )

    The actual APRS message generated and transmitted is a standard position report message, with no timestamp or APRS "messaging" features, as described on document page 32 of APRS spec, version 1.0.1, dated August 29 2000. During transmissions, ( which last less than 1 second for APRS messages ) the following values have been measured :

  • Output = 3.2 W @ 13.8 VDC 0.52 A ( = 45% efficiency )
  • Output = 2.4 W @ 12.0 VDC 0.46 A ( = 43% efficiency )
  • Output = 1.6 W @ 10.0 VDC 0.39 A ( = 41% efficiency )
  • Output = 1.3 W @ 9.0 VDC 0.33 A ( = 43% efficiency )
  • Output = 0.6 W @ 6.0 VDC 0.30 A ( = 33% efficiency )
  • Output = 0.3 W @ 4.5 VDC 0.22 A ( = 30% efficiency ) ( NOTE : Vreg running below dropout voltage )
  • Output = 0.1 W @ 3.0 VDC 0.14 A ( = 24% efficiency ) ( NOTE : Vreg running below dropout voltage )

    Installation and Operation :

    Installation doesn't involve very much... for reasons of economy and simplicity, no connectors are provided on the PCB, so external connectors must be hand-wired to the PCB pads provided on the board. The pictorial provided below shows the required connections. As a courtesy to customers, these units are actually shipped with a BNC and DB9F connector provided, and pre-wired to these points with short cables, ( as well as color-coded DC power leads ) but customers may want to replace them with their own connections. No mounting holes are provided on these PC boards, so some "imagination" will be required to secure them. ( velcro / foam tape ? ) Since the transmissions last less than 1 second, these things don't get hot.

    Please note CAREFULLY the polarity of the DC supply... no reverse polarity protection is provided, and accidental reversal WILL cause damage to the PA transistor.

    Operation is equally simple... plug in a GPS, turn them on and walk away. The MicroBeacon will not transmit any messages until the GPS data becomes valid. After that, it will send position reports at regular time intervals defined by the customer in the programming sentence. ( see next section for details of this )

    Programming the Parameters :

    To program the parameters of the MicroBeacon transmitter, the user invokes the PROGRAM mode by installing a wire jumper ( shown in the image above ) and re-starting the unit. ( interrupt and restore the DC power ) Entry into the PROGRAM mode is indicated by a 9 second transmission that can be monitored on a receiver : 3 seconds of dead silence, 3 seconds of 1200 Hz tone, and 3 seconds of 2200 Hz tone. This transmission can also be used to measure the TX frequency and output power, and DC supply load.

    After verifying that the PROGRAM mode has been invoked, the user sends a single line of ordinary text to the unit from an IBM PC host, using the RS232 port. ( disconnect the GPS, connect the IBM ) The text must follow a very precise and exact format, and be terminated by a carriage return character. The carriage return character will trigger the host parsing routines in the MicroBeacon, which will sort out and store the new parameters in the non-volatile EE memory. After this is done, ( which takes only a few milliseconds ) the MicroBeacon will again transmit for 9 seconds, to signal detection and storage of the new parameters. In this new transmission, the unit will transmit on the NEW frequency that was expressed in the parameter message that was just decoded and stored.

    If a receiver is not available to monitor the transmissions, the 9 second duration is long enough to cause finger-detectable heating in the RF amplifier FET. If the DC supply has an ammeter, transmission can also be detected by observing the increase of DC load.

    If an error is made, successive programming sentences can be sent... each one will overwrite the previous one, and trigger another 9 second transmission. To exit from the programming mode, remove the jumper and re-start the unit. ( interrupt and restore DC power )

    The programming jumper can actually be removed anytime after PROGRAM mode begins... it is only examined once, when DC power is turned on. Therefore, a temporary "paper clip" ( or other ) jumper is suitable.

    It is worthwhile to disconnect the GPS ( or turn it off ) BEFORE invoking the PROGRAM mode... the GPS messages are also terminated with a carriage return character, and the MicroBeacon can ( and will ) "get confused" and try to store the GPS messages as though they are command messages. ( the parameters will accidentally be loaded with garbage data )

    The Programming Sentence :

    The programming sentence must follow an exact format and have the proper number of characters in each data field. It must also be terminated with an ASCII carriage return character, to trigger the proper routines in the MicroBeacon transmitter.

    As a convenience for users, the following sentence can be used as a "starting" sentence, with some minor changes :

    1443900180xxxxxx0WIDE1 1WIDE2 B/\COMMENT HERE(carriage return)

    Replace the xxxxxx characters with your call sign, all = capitol letters. If your call sign has less than 6 characters, left-justify it and add a suffix of ASCII space characters to make a total of 6 characters.

    This sentence will cause the MicroBeacon to transmit on 144.390 MHz ( U.S. primary APRS channel ) at intervals of 180 seconds. The map symbol will be a car, ( automobile ) as indicated by the forward slash + backward slash characters.

    The sentence can be sent with the ( Windows ) HyperTerminal utility program, set for 4800 baud, 8N1, no parity, no handshake. It can be sent manually, ( one character at a time ) but it is more convenient to save the sentence as a text file, and then send the file using the TRANSFER/SEND TEXT FILE feature of HyperTerminal. This will allow careful checking of the file before it is sent, and also allows saving multiple files with different parameters, for future use.

    The detailed format of the sentence is described below :


  • FFFFFF ( 6 characters ) : Transmit frequency in KHz ( EX : 144390 = 144.390 MHz )

  • TTTT ( 4 characters ) : Transmit repeat time in seconds ( EX : 0600 = 600 seconds = 10 minutes )

  • SSSSSS ( 6 characters ) : Source call sign, all = capital letters ( = your call sign ) ( see note 1 for details )

  • s ( 1 character ) : Source SSID number ( see SSID table, this is usually "0" = numeral zero )

  • VVVVVV ( 6 characters ) : VIA 1 call sign, all = capital letters ( see notes 1,2 and 3 for details )

  • v ( 1 character ) : VIA 1 SSID number ( see SSID table, this is usually "1" = numeral one )

  • VVVVVV ( 6 characters ) : VIA 2 call sign, all = capital letters ( see notes 1,2 and 3 for details )

  • v ( 1 character ) : VIA 2 SSID number ( see SSID table, this is usually "2" = numeral two )

  • S ( 1 character ) : Symbol table character, either "/" ( primary ) or "\" ( secondary )( see note 4 )

  • s ( 1 character ) : Symbol code character, can be several different characters ( see note 4 )

  • CCC...CC ( 0 to 32 characters ) : Comment text, any printable text is suitable, 32 characters max, 0 characters min

  • (carriage return) ( 1 character ) : Required, this signals the end of the program sentence


    NOTE 1 : SOURCE and VIA call signs MUST total 6 characters each, and MUST be capitalized. If call signs are less than 6 characters long, they must be left-justified and padded with a suffix of ASCII SPACE characters, to bring the total characters ( for each call sign ) up to 6.

    NOTE 2 : VIAs are optional, and can be omitted, if desired. If they are omitted, they MUST be replaced with 6 ASCII SPACE characters in the program sentence, ( and the associated SSID replaced with one SPACE character ) so that the relative locations of the characters that follow ( in the program sentence ) remain unchanged.

    NOTE 3 : The APRS spec allows user-specification of 7 VIAs or less, but the MicroBeacon limits this to 2 VIAs, which should be more than adequate for most people. Since the number of VIAs cannot be predicted by a recieving station, the transmitter is required to identify the LAST VIA with a "special" SSID character. For this reason, the SSID "character table" shown below has 2 columns, one for the LAST VIA, and one for all other preceeding VIAs.

    For example, note in the "example" program sentence ( provided above ) that the SSID character for the SOURCE call sign is "0", ( numeral zero ) and the SSID character for VIA1 is "1", ( numeral one ) but the SSID character for VIA2 ( the LAST VIA ) is "B". This "B" character corresponds to "2", ( numeral two ) but the character "B" is used to further indicate this is the LAST VIA in the station list.

    For the benefit of complete APRS "novices", the following information about VIAS should help. APRS messages are routinely "relayed" ( automatically ) by other nearby APRS stations that have the capabilities to do that... VIAs provide the message originator with some degree of control over how and where their message "propagates" through the nework of nearby APRS stations. VIA call signs can be specific station call signs, to ensure they are ONLY relayed by those particular stations. VIA call signs can also be "generic" call signs, which can be answered ( relayed ) by any station that fits the VIA description. By analogy with other amateur radio methods, "calling CQ" invites ANY station to respond, whereas calling "CQ CONTEST" or "CQ DX" or "CQ OHIO" invites only stations in those specific categories to respond.

    A complete discussion of VIA options and selection ( in particular ) and APRS ( in general ) is beyond the scope of this webpage, and ( frankly ) also beyond the expertise of this author... the reader is invited to enquire elsewhere for mor details, and the actual APRS specification is available on the web, at this address :

    APRS Specification 1.01

    NOTE 4 : The symbol ( icon ) displayed on APRS maps is defined by character 32 ( symbol table ) and character 33 ( symbol code ) in the programming sentence. The APRS specification ( see weblink above ) contains a complete list of the APRS "map symbols" that are available, in appendix 2. The actual map icons ( images ) can be found several places on the web, including here :

    APRS Icons