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’
An Antenna Tuning Unit (ATU) performs a simple but important function in many transmitting systems.
Almost all things called an ATU are simply impedance transformers, and almost always, narrow band impedance transformers (meaning that when adjusted, they achieved the desired transformation over only a narrow frequency range).
ATUs come in a range of configurations, each designed for a specific set of characteristics. Above is the heart of a Palstar AT2K T Tuner, just three real passive components that are fully explained by conventional linear circuit theory. Continue reading What does an Antenna Tuning Unit (ATU) do?
An article in a series documenting measurements on an IC-7410.
This focuses on receiver Tx power and SWR meter.
Continue reading IC-7410 tests – 04 – Tx power & SWR meter
W5WSS describes his antenna at (eHam 2014). It is essentially a shortened dipole with capacity hats for 20m.
The configuration appears from several postings to be this shortened dipole with a Balun Designs 1115du balun at the center and an adjacent LDG Pro 200 automatic ATU.
Continue reading Feed point voltage – W5WSS 7′ dipole
Balun designs has a warning to users of baluns on a full wave dipole.
It must be pointed out that a 1:1 balun should never be used on the second harmonic of a half-wave center-fed dipole fed with coax (like an 80 meter dipole being used on 40 meters). The impedance can be as great as 10,000 ohms creating very high voltages which can bring about voltage breakdown and/or excessive heating. This exception ONLY applies to Coax Fed HALF WAVE CENTER FED DIPOLES WHEN USING A 1:1 BALUN AT THE FEEDPOINT.
Whilst differential voltage can be an issue in antenna systems (Duffy 2011), the warning above is a bit dramatic for this case.
Firstly, it is very difficult to measure the impedance of a full wave centre fed dipole in the worst case, but modelling suggests it is unlikely to have an impedance at resonance greater than about 4200+j0Ω.
Lets suppose there is a balun located at the feed point of an 80m half wave dipole, and the antenna is fed with 25m (~80′) of RG58C/U feed line. Using TLLC, the transmission efficiency of that section of line at 7MHz with load of 4200+j0Ω is just 9.4%.
If we have a 100W transmitter, we might get 90W out of the ATU in this scenario, and 9.4% or 8.5W of that reaches the feed point.
It is a simple matter to calculate the RMS voltage as V=(P*R)^0.5=190V, or 270Vpk. This is not going to strain any balun!
They outcome here is due to the extreme loss on the coax under very high standing waves results in very little power reaching the balun anyway.
This is one of those cases that if the antenna was half as long, the system would be ten times as good!
- Duffy, O. 2001. RF Transmission Line Loss Calculator (TLLC). VK1OD.net (offline).
- ———. Jul 2011. Avoiding flashover in baluns and ATUs. VK1OD.net (offline).