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Magnetic Loop Antenna

This article presents my magnetic loop antenna (MLA). The antenna covers 7 MHz, 10 MHz and 14 MHz HAM radio bands. Magnetic loop antennas are based on a resonation circuit made of a loop and a tunable capacitor. In my case, the diameter of the loop (coil of two turns) was 52 cm and a butterfly air variable capacitor (12 - 125 pF, 3 kV) was used. The antenna can be tuned by hand but there is high voltage at the capacitor when transmitting, so the tuning knob must be properly isolated. The resonation frequency can be shifted when the hand is placed near the antenna, also due to the high Q of the antenna the resonation is very sharp and it can be difficult to fine-tune the MLA. Therefore, I decided to build my MLA with a motor. For precise positioning, I chose a stepper motor with a gear unit (100:1). As I wanted to avoid an external cable from the antenna to the control unit, a Bluetooth connection was used. The control unit features an LCD touch screen. The firmware for it was written in AVR C and an interesting fact is that I wrote the firmware in a way that the same control board based on ATmega 2560 can be used for both the antenna and the control unit. It allowed me to debug the entire system with just one board. I have already made many contacts around Europe from my flat with this antenna. Usually, 5 - 10 W is sufficient for CW contacts. I have a lot of ideas for further improvements, so I will update this article once I have new modifications.

A short video can be found here (Vimeo).

OK-OM DX Contest 2020 (YouTube)

USBasp with Atmel Studio 7

This short article presents my setup of Atmel Studio 7 for USBasp programmer. USBasp is a simple low-cost programmer that does not offer any advanced features such as on-chip debugging. On the other hand, the cost of this programmer is significantly lower in comparison with Atmel-supplied debuggers such as Atmel-ICE. USBasp can be added into Atmel Studio as an external tool. The following steps present the installation process.

1. Plug in your USBasp.


2. Install libusbK driver using Zadig software as presented in the following picture. (If you cannot see USBasp device you might need to click on Options -> List All Devices.)

2. Download AVRdude http://download.savannah.gnu.org/releases/avrdude/?C=M&O=D (in my case avrdude-6.3-mingw32.zip and avrdude-doc-6.3.pdf).

3. Unzip the downloaded file to a chosen folder.

4. Open Atmel Studio and click on Tools -> External Tools as shown in the screenshot.

5. Choose a title you like.

6. Command is the path to your downloaded (unzipped) avrdude.exe file.

7. Field Arguments is specific for your type of microcontroller. In my case I used ATmega2560 (m2560) . More information can be found in avrdude-doc-6.3.pdf file.

avrdude -c usbasp -p m2560 -U flash:w:$(ProjectDir)Debug\$(TargetName).hex:i

8. $(ProjectDir) sets the Initial directory as a project directory.

9. Check "Use Output Window" for status messages of avrdude.exe.

10. Write a simple code and click on Build -> Build Solution (F7). The following code blinks an LED on the MEGA 2560 board.

11. Press Tools -> USBasp to program your microcontroller. Good luck!

RF Solid State Switch

In this project, I wanted to design an RF switch that I could use for low power switching, especially at intermediate frequencies. Solid-state switches have several benefits in comparison with electromechanical switches such as better reliability, longer lifetime, resistance to mechanical vibrations and shocks, and they allow fast switching. On the other hand, solid-state switches have greater parasitic resistance, so they have higher insertion loss. Good quality mechanical relays can be very expensive therefore this could be a good alternative for some projects. My first proptotype can be seen in the following pictures. I chose a non-reflective DC to 4 GHz GaAs MESFET SPDT switch (HMC435AMS8GE). Soldering of the package (HMSOP-8) was a bit challenging and I had to use a microscope for it. The switch is connected to the SMA connectors through blocking capacitors (100pF, 0402 package). Based on the measurement I found that there is better impedance matching needed between the SMA connector and the coplanar waveguide, so it will be improved in my future design.

1kW 144 MHz amplifier with GS-35B vacuum tube

This is a short article presenting my first serious RF project. I started with electronics and HAM radio when I was 10 and since then it has been my main hobby. When I was finishing my high school, I had to do a final project and after some consultations with my friends I decided to build an RF amplifier and I was lucky enough that I had very clever and experienced people around me who were willing to help me with this, at that time, very challenging project. It was also very hard to find a supervisor at my high school as the amplifier works with a high voltage. At the time I was a member of OK1KZE radioclub and Mila OK1VUM (the president of the club) had already built several of those PAs and agreed to become my official external supervisor. Many thanks for the supervision and help! A long story short, I learnt a lot. One day I tried to switch on the main power source (those 3 big toroids that you can see in the photos) and something was seriosly wrong as I could hear some sound effects that really should not be there, so I switched it off immediately. The same day in the evening I told this problem to my friends (also HAMs) in the pub where we had regular meetings :) and we found that I had created a shorted turn in the transformer as both my chassis and the mounting bolt passing through the center of the transformer were conductive. It was easy to fix  but I will never make the same mistake again. I think it is nice to see work that I did 10 years ago. I can remember that I spent all my savings from my summer jobs on this project and even my parents supported this project (a big thank you). It was a lot of work but I think it was worth the effort.

This is just a presentation of my project. This device can kill you and I don't accept any responsibility for your construction! 

A short video from testing: https://vimeo.com/341928536

30 cm dish (RX) for Es’hail-2 / QO-100

AMSAT Phase 4-A is the first geostationary amateur radio transponder which is placed on Es'hail-2 communication satellite (launched aboard a SpaceX Falcon 9 rocket on November 15, 2018).
The uplink is on 2.4 GHz (S band) and downlink on 10.5 GHz (X band). I received the signals using a downconverter from an LNB and a small parabolic antenna (30cm) placed on a chair pointing to the satellite through an open window. As the downconverter had a free-running oscillator without any temperature stabilization, you can hear that the receiving tone is not stable. Using a disciplined oscillator will be the next step. The record of the Beacon can be found here.

In the next step, I modified my LNB. I removed the internal crystal (25 MHz) and replaced it by an SMA connector. Then I used an arbitrary waveform generator that was disciplined to GPS (10 MHz clock) and injected the signal into the LNB.

This solution worked significantly better in terms of frequency stability but I was not happy with the mechanical construction and with the RF path on X-band where an adaptor from circular waveguide to a rectangular waveguide was connected to an adaptor form rectangular waveguide to SMA connector. Then another SMA to SMA adaptor was used for connection of the LNB as can be seen in the picture above. This was the reason why I decided to modify Quad Octagon LNB based on this description. My modification (before and after) can be seen in the following photos.

Once the modification was finished I designed and 3D printed an adaptor for my dish. I cut the original horn antenna with a pipe cutter. The goal was to keep the front face of the circular waveguide as flat as possible. Then I glued the 3D printed adaptor to the LNB. The shape of the LNB's waveguide is slightly conical, so there is a small gap between the waveguide and the adaptor. Fortunately, it is on the side where the adaptor is not connected to the antenna, so it does not have any significant effect. The connection is rigid and the LNB can be connected to the dish antenna with four screws. So far I am happy with this setup.