This article explains a method of setting the tx audio level of a Foxtrak using an SSB/FM receiver exploiting Bessel zeros to calibrate the modulator. Whilst the Foxtrak is used in this example, the method can be used with any tracker or TNC that allows single modem test tones to be transmitted. Continue reading Adjusting Foxtrak tx audio using an SSB/FM receiver exploiting Bessel zeros to calibrate the modulator
Some AFSK sources such as TNCs, APRS trackers etc do not have facilities for sending single modem tones for the purpose of level adjustment which must be done using data packets which are a complex arrangement of high and low tones. Continue reading Adjusting AFSK FM tx level using a PC oscilloscope
Hams have used 1200Bd AFSK FM as the physical layer of AX.25 for decades, though with variable implementation quality.
Whilst the AX.25 packet networks of yore have all but died, the protocol is currently used with APRS, and so the related issues remain relevant… well as relevant as APRS remains. Continue reading AX.25 1200Bd AFSK FM – to pre-emphasise or not
I spent some hours today trying to receive a balloon travelling nearby on 2m 1200Bd AFSK.
At the end of the day, I note that the balloon was decoded by only two receivers located in or near Sydney and that resulted in a record to APRS-IS. One was my own station which decoded three packets, and VK2RHR-1 which relayed 16 packets.
Given the balloon’s flight, it is notable that it was not relayed by VK2AWM-1 (which was 40km closer), VK2RAG-1 and VK2RTZ-1 at Gosford and Newcastle, and at least four igates in Wollongong, Sydney and Newcastle. Continue reading Diagnosis of APRS reception problem
Appropriate setup of audio drive of an AFSK APRS FM transmitter is important to ensuring that the signal can be copied by all receivers in range.
Excessive audio drive results in a signal that cannot be decoded by some receivers event though it may be very strong.
The TNC-X does not have any facilities for producing steady tones for adjusting the tx audio output for the desired deviation level, so alternate firmware was developed solely for this lineup function. The regular MCU chip is replaced by the adjustment chip for the lineup then the regular MCU chip is reinstalled. Note that this is not an augmented version of the regular firmware, it is for adjustment purposes only… do not overwrite the original chip. Continue reading Audio level setup on TNC-X (MFJ-1270X)
This article shows how to analyse 1200Bd AFSK FM signals off-air to identify defects in setup that will degrade system performance, and the shared channel generally. Continue reading Are you getting the best performance from your FM AFSK APRS transmitter
This article documents the build of a TNC-X, a basic KISS only TNC using modern components, including option of an on board USB interface.
Above, the PCB after assembly and cleaning. Continue reading TNC-X build
Foxtrack is an inexpensive kit form basic APRS tracker, taking a GPS NMEA feed and producing audio frequency shift keyed output to key an FM transmitter. It incorporates a limited TNC function and some intelligent position reporting algorithms
Above is the top view of the Foxtrack PCB. Continue reading Foxtrak build
I recently built a FoxDelta FOXTRAK APRS tracker.
FoxDelta supply the FOXTRAK with firmware by DK7IN, but also recommend APRSTRACKER by PE1RXQ and PE1ICQ.
APRSTRACKER v0.11 was trialled as it offered promise of a better SmartBeaconing implementation and support for altitude, course and speed reporting. Continue reading Review of APRSTRACKER v0.11firmware
In a QST column in 2008, a correspondent asked the question
… I have the ladder line terminated to double coaxes that run about 12′ (4m) inside the house to an antenna tuner. Should this pair of coaxes be grounded at one end or both ends?
The Doctor gave a detailed diagram (above) and his advice was… Continue reading Spoiling balun action with ‘shielded twin’