Amateur radio, RF design, electronics, uC, software, Arduino, AVR, Antennas
I purchased a couple of Anytone AT-5189 4m FM radios at a flea market. As the radio is of Chinese origin I was interested in seeing how the chinese engineers is coming along regarding design, waterproofing / IP degree, PCB layout, internal shielding, component selection and general workmanship.
Summary: the radio is surprisingly well designed. A cast alu chassis is made with milled grooves for O rings, professional layed out PCB, proper ground vias and compartmentization, no stray cabling inside. The integrated chips used in the design are LMX1511 synthesizer, MC3311 compander, M62364 DA, KIA278 LDO, RDO7MVS18 driver. Well known integrated circuits.
I have been working on a trackball based controller for my HDSDR SDR project lately. This is a small R&D project that is run on my spare time where the goal is to determine if it is possible to use a trackball as a VFO for software defined radio (SDR) in contests. The project started out based on a demand for a more ergonomic way to operate a mult receiver in a contest environment that is less fatiguing during 48hours duration of a major contest like CQWW or CQWPX. The goal is that it should be possible to operate all radio functions you need from one hand only: VFO, speed of vfo, band, mode, filter width, volume, gain. I have modified a Marconi trackball and the controller is a Trinket Pro controller (Arduino)
I blasted my LDG antennatuner some time ago. Or …. I thought I blasted it….. It appeared that it was only the resistor in the SWR detector circuit that got burned out. I replaced that resistor and now its ok again.It was easy to repair. However these small LDG tuners dont take more than 100W max. The designers have used ferrite cores, whereas it would have been a much better idea to use carbonyl cores or air core inductors. The latter doesnt get so easily saturated.
However I must say that the design of the LDG equipment I have seen so far is not very impressive. Why use that BIG chasis when you dont need it? Why use DB9 style connectors on a chassis that is supposed to be watertight? Look at that coax termination there. Both on the board and on the PL259 chassis connector. Why use RG174 teflon coax when you have such crappy terminaions? Perhaps it would be better with no coax at all However when the tuner works it works fairly OK. Just dont trust this kind of equipment in a contest or on a dx expedition.
I have long had the need to measure antenna current. This can be done by a rf-current transformer and a rectifier made by a shottky diode or fast silicon diode and an integrator driving a mechanical instrument. However the dynamic range is very limited with such a setup.
Therefore I made a new rf-amperemeter design with a log detector chip from Analog Devices driven from a current transformer made out of a split core ferrite material that has response in the HF and lower VHF frequency range.The current trafo is terminated in a few ohms and the RF current conductor (actually the primary) sees only a fraction of an ohm so very little influence is done on the circuit you are measuring. (Apart from capacitive coupling and a slight leakage inductance from the current transformer windings/ferrite combination). The ferrite core is a split type and is epoxied to a clip.
I calibrated the instrument by terminating the generator in a fancooled 100W dummy load and measured the voltage over the load by an oscilloscope. Since it has a log output the dynamic range from milliamp range up to 1,4A RF current. That would peg a mechanical instrument if you would at the same time want to be able to detect a significantly lower RF-current without having to change scales with switches / pot meters etc.
Next I plan on making a OLED display on this design with an arduino controller. Its a sparetime project so lets see how long tiem it takes before I implement that.
I recently got hold of a motorized roller inductor that is made PROPERLY. This is a MIL-SPEC unit that has been on stock for many years. It features a silver foil that is rolled on to a ceramic former, pretension, a shortcut cylinder that prevents eddy currents and arcing on the unused coil section under QRO operation. Due to that the unused coil is shorted and that the shorting is not a single turn, that the foil is wide, this coil has a very high Q.
F5OEO has recently written some code to transmit a SSB signal using just the hardware in the Broadcom SOC chip on the Raspberry Pi. You can find more info on this link http://www.rtl-sdr.com/transmitting-fm-am-ssb-sstv-and-fsq-with-just-a-raspberry-pi/
I tested the code on my RPI on 6m via a cable connection. It worked OK. If you look at the S-meter you can see that the envelope is constant. This is due to that the RPI has no way of modulating the envelope so the software actually modulates frequency. It is kind of constant envelope SSB.
Below you can see what the signal looks like. (This is received on a SDR via an attenuator). The signal is a bit too wide. However cool test.
Warning: do not connect the RPI GPIO output running this code to an external antenna without a bandpass or lowpass filter and a valid amateur radio license. Never transmit any signal outside the amateur bands.
I just got myself a new roller inductor. This one is designed on the principle of a silver foil that is rolled away from and onto a ceramic form with guides for the silver form. The ceramic coil forms the coil. There is a large shorting cylinder that the unused silver foil is rolled onto. The effect of this is to significantly increase the Q of the inductor. For high power QRO applications there may either be arcing from the end of the unused part of the coil or heat loss in this part of the coil. How the unused coil is completely shorted with an inner conducting cylinder and the unused part of the coil has no flux thru it.
Here is a review of SDR radios currently on the market in the low cost segment: http://www.rtl-sdr.com/roundup-software-defined-radios/