144.8 Mhz TX for APRS

The "Prototype"

Been busy at work lately, but still had some time for myself. Latest idea: a low-cost APRS beacon, done with a classical 144.8 Mhz TX and a μController. AX.25 on air. Automatically. So these days I’ve been busy making this:

144.8 Mhz for my APRS Beacon project

The prototype. Schematic below. Two transistor 400 mW power output, 7 dBV. Enough to fry a tiny egg on the output amplifier transistor that heats like hell. Anyway, it works, and it works very stable. See below.

Winding coils, soldering, measuring. Fun. The frequency is remarkable stable, given the rudimentary nature of this. For example, with mask limit on on my Agilent DSOX 2002A, tolerance set at ± 500 mV amplitude deviation and ± 2.5 ns jitter:

And even better, with scope mask limit tolerance set at ± 500 mV amplitude deviation and ± 50 ps frequency jitter (first below) and ± 300 mV amplitude deviation and ± 50 ps frequency jitter (second below):

Regions coloured in red are „error” regions, out-of-tolerance. Pretty good, huh ? The schematic below:

144.8 Mhz TX proto schematic

The 144.8 Mhz TX Prototype schematic; a two-transistor TX, based on cheap components (2N2222) and some spare parts found in the junk box. At 7 dBV output (400 mW) Q2 is getting quite hot. Quite expected as these are not RF transistors. Capacitors labelled {C} are adjustable; had to use this symbol because the schematic is drawn in LTSpice that does not have a symbol for adjustable capacitors.

This is a promising start. Both receivers I own — the UV-5R Baofeng and my Kenwood TM-D710 E dual-bander — have no problem in locking on 144 Mhz, the carrier transmitted by this prototype. Some problems, though:

  1. The orange trimmer (see the picture), connected in series with the 48 Mhz quartz crystal does not work as it should. Probably the value is too large because the adjusting its value does not change the TX frequency, just the harmonics’ amplitude and overlapping. Which is not ok. I have to make further tests with some fixed-value capacitors, but for now this is an issue.
  2. The „power” transistor (the second one, the one in the middle of the picture) is getting hot as hell; not a surprise given the 7 dBV output of this and the fact that 2N2222 is not a power transistor, not even a RF one; I should — probably — replace them with something more appropriate, like a BFR or BFX etc, but for now, it is ok. Anyway I have to change the schematic to accommodate the mixer, which is not in the scheme at the moment.
  3. Some spurious 2.25 and 2.5 Ghz oscillations:

I don’t know where these come from. Maybe from the layout, like parasitics in the PCB placement of parts etc. I have to dig further and get rid of this.

For now I am pretty impressed of the results. Next step is to bring in the dual-gate mosfet, and add the µC part, together with AX.25 routines and the support for automation. Maybe I will increase the power, but, for now, I will test in QRP mode.

Other similar projects:

More to add here, as documentation is found. Have a great weekend. AP

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