First QSLs received through buro

I have just received my first two way QSL cards through buro. It took only half a year :)

Couple of months ago I also asked for one QSL to be delivered direct as it was my first contact with an Asia station. So here it is.

Remote symmetric matchbox

Here is my second attempt to create a symmetric matchbox for my QRP purposes. I quickly realized that it is quite annoying to run to the antenna to retune after a frequency change. So I decided to create a remote matchbox based on my previous manual one.

There are some commercial balanced matchboxes. The single remote one (HamWare AT-615B) is VERY expensive and uses a lot of control wires.

When I saw that I decided on three main design goals for my prototype:

• no micro controller on the antenna side
• immunity to interference
• simple wiring

So I started thinking about the control interface and realized that if UTP 5e is good for 100 meter long lines up to gigabit speeds it must surely be good for low frequency as well. UTP gives me four differential pairs to use. SPI then needs four wires and can control a shift register directly. This combined will allow me to reuse my stash of UTP 5e cable and connectors and when I do not have any more UTP is quite cheap anyway.

Here are the first steps I took.. schematics and PCBs. I could have used ground plane construction, but I was a bit worried about the capacitance it would create.

I have already mentioned that I am using SPI as the control protocol and here is how I envision it to work:

• A micro uses SPI to send 24 bits over to the tuner. Each bit controls one bipolar relay, together they control three sets of eight relays each.
• Each SPI signal is converted to a differential pair (5V/0V levels) to prevent interference and transfered in that form to the tuner side controller. Each signal is using one twisted pair in the UTP cable.
• At the tuner side the pair is decoded to SPI signals again and those are connected to three 74HC595 latching shift registers (24 bits).
• The shift registers hold the control value and pass the bit values to 74ACT04 invertors that drive the bipolar relays (FTR-B4).
• The ACT family can source or sink more than 20mA per leg. That is more than enough to switch the state of my relays.

Transceiver side controller - SPI to UTP

Tuner side controller - UTP to relay controller lines

The controller boards use AM26C32CD receivers and ST26C31B drivers. I know I am wasting one perfectly good receiver/driver, but I have quite lot of those in my stash :)

The characteristic impedance of UTP twisted pair is 100 ohms, the same as the output impedance of the ST26C31 driver (see Table 6 in the datasheet). For that reason there are 100 ohm terminating resistors just before the receivers.

Coil switching board

Capacitor bank board

Coil and capacitor switching circuits are not special at all:

• Coils are connected in parallel and single coils can be bypassed by the respective relay.
• Capacitors are organized into banks where each bank can have 16 capacitors - four parallel capacitors four times in series to make the bank withstand higher voltages (I have assortment of 500V NP0 capacitors).
• All relays are bipolar (latching) Fujitsu FTR-B4 4.5V with 17mA latching current. I am only using QRP power so the current carrying capability (1A per section and I wire both sections in parallel). I am using the same trick to drive them as in my manual matchbox from the previous article.

I am planning to use balanced-Pi configuration and haven't decided on the capacitor values yet so I planned for the worst case. I won't probably be populating most of the capacitors. All of them will be the NP0 kind though (zero temperature coefficient).

Panel with controller boards (tuner and trx sides) + one unrelated micro-controller board

Two capacitor bank boards with relays

The boards were made by Seeedstudio and it took about a month to get them. I was busy during that time so it was not really holding me back.. but I am glad I have them at home. You might have also noticed that there is no board for the coil part. That is because I am planning on reusing the board I already have in the manual matchbox.

That I all I can describe today as I haven't started soldering yet. Once I have a prototype ready, I will post the results.

Multiband dipole, ladder line and homebrew manual symmetric matchbox

I recently found out that my dipole was a bit shorter than it should be. Since I use 75 ohm HDTV coax (Nordix CM407 Cu PE) as my feedline, I was losing power in the impedance mismatch. I also started to be a bit annoyed by it being single band only. So I started looking for a possible solution.

The only reasonable change I could do regarding antenna was to use a bit longer wire (about 44 feet), ladder line and a matchbox to match it to the 75 ohm feedline. This is the basic concept of non-resonant multiband dipole as described in ARRL Handbook and Antenna book.

I built the feedline out of 0.5mm^2 insulated stranded wire (because I have plenty..) and cut 6mm diameter hard irrigation tubing (black, 15m for 99Kč - 4€ in a local hobby store) to pieces to make the separators. I then cut slots to the separators, inserted wire and secured using black plastic cable ties. I tried many different ways of making the ladder line, but this way was the fastest and cheapest.

Then I started looking at the available ATUs and quickly figured out that the prices are very high. I won't describe the details as DJ0IP already dedicated many pages to this topic at his website.

Since I am only interested in low power (10W max) it was quite easy to come up with home-brew solution. It is a switchable L network with 16 coils (2 times 8 to make it symmetric) and a variable capacitor I got at ebay wired as split stator (the two sections in series). I also added one additional fixed 200pF/1000V (2x4x100pF/500V NP0) capacitor that can be switched in and out of the circuit.

There is also an 1:1 current balun (7 bifilar turns on FT50-43) at the input side of the tuner.

Components before wiring. There is the 200pF/1000V capacitor at the top (the PCB)
and the relay controller board in the middle.

Front panel (I added one more switch later).
You can also see the current balun at the back side right from the BNC connector.

Coil switching is accomplished by FTR-B4 4.5V locking relays (or bipolar as we call them in CZ) that are directly controlled by front panel switches. I decided to do it this way to minimize wiring length and parasitic effects. The circuit is powered by single cell LiPol battery (3.7V nominal) that is used only when relays change state and so will stay charged for a long time.

Coils, relays and the control cable on a copper clad board.

Simulation circuit of the relay controller. R1 is the relay coil and V1 is the switch.

All the coils were wound on toroids from the Ferroxcube 4c65 material. I used three different sizes of cores (9mm, 14mm and 23mm OD) and I wound the inductances so they are about 0.125uH, 0.25uH, 0.5uH, 1uH, 2uH, 4uH, 8uH and 16uH. Since there are two coils of the same inductance in symmetric series it gives me an inductance range of about 0.25uH to about 64uH.

The finished matchbox

The result was an "ugly" box that is able to match my antenna at least on 20m and higher. Those are the bands I wanted to use. I used my miniVNA with vna/J in Smith chart mode to find the settings for different frequencies. The settings for 20m and my dipole were 3.75uH and minimal possible capacitance connected to the antenna side. The fact that I tried to learn how a Smith chart works helped me a lot when figuring out the values :).

Baofeng UV-3R in packet world using ArduinoTNC

Since it is so cheap and works reasonably well for 2m/70cm band in FM mode, I wanted to test also it's capabilities as FM modulator for AFSK digi modes and APRS.

First I needed a cable to connect to the radio. Fortunately that is relative easy and requires only four ring 3.5" jack and some standard components.

The cable pinout is easy:

Baofeng UV-3R TNC cable

To get to the packet world a TNC is needed. I was experimenting with different software solutions (soundmodem, agwpe), but this test was done with Arduino based TNC (because I had one laying around and it has USB to serial chip on board).

ArduinoTNC shield and cable

The design is based on KI4MCW's, but the software was updated to work well with latest BertOS (2.7.0) and to support KISS escapes and command structure.

Arduino TNC schematics (the filter cap on PTT line is missing here)

You can also download the complete source codes (complete with configured BertOS).

The TNC was connected to Xastir running on Mac OS X with the proper settings - Serial KISS TNC, 19200baud, /dev/cu.usbmodem24311 and "Transmit now" was triggered.

The radio switched to TX mode and... stayed transmitting. Oops. RF noise induced enough energy on the unshielded Arduino to trigger the PTT optocoupler and keep it triggered.

All that was needed to remedy this was to connect one SMD ceramic 100nF cap between PTT line and ground.

With that done, I triggered Transmit now again and measured what happened:

• Blue line (CH1) measures the PTT line on the PCB (pin 11/PORTB3)
• Red line is the audio output of the simple DAC, measured before the pot

With STE turned on in the menu, the radio showed about 230ms PTT to TX delay and 300ms TX tail as you can see on the attached pictures (watch the RF induced noise on the blue PTT line).

TxDelay with STE on

TxTail with STE on

The TX tail was pretty long so I turned the STE (Tail tone elimination) function off and tried again:

TxTail with STE off

With full power, STE turned off and resistance between the pot wiper and ground measured as 572 ohm the transmission was finally properly received by our local IGATE (9.2 km). So the UV-3R is indeed usable as an APRS transmitter.

My first foreign QSL card

I have just received a QSL card from Japan. It is my first QSL card from abroad and also my first satellite reception ever. :)

Not bad considering I have used the first version of FUNCube dongle and a corner reflector antenna mounted in the middle of the wall of an apartment building.

HF direct conversion receiver

Since I recently got my HAREC HAM licence, I decided I need a radio for HF. So here are a description and some pictures of the current contraption I use to listen on the 40m band.

The RX part was inspired by the popcorn DC receiver mainframe published by VE7BO on his site http://www.qrp.pops.net/popDC.asp. I used slightly different diplexer (similar to B variant) and created both filters with bistable relay switching circuit. ADE-1 from Mini Circuits serves as the main mixer.

DC receiver, groundplane construction,
the switch was later replaced by relay

The receiver used to be controled by Arduino (with Diecimila bootloader to support eeprom flashing), but has been since replaced by much smaller cpu board holding ATmega644PA microcontroller running at 8Mhz on internal oscillator. This board connects to all controls, LCD and a DDS box, which is the source of RF signal for the ADE-1 mixer in the receiver. The CPU board also provides 5V regulated power to all digital parts.

ATmega644PA with pin headers, voltage regulator (upper right)
and SFH5110-38 IR remote demodulator (bottom left)

A sample of AD9835 was used as a basis for the DDS oscillator, together with a low pass filter based on a design by maxit91 (http://hem.passagen.se/communication/dds.html). The whole module is built in RF tight box to prevent RFI.

DDS is the silver box next to the knobs.
You can also see the placement of LCD ad CPU boards,
power switch, fuse and back ground plane with connectors.

Filter switching relay is controlled using couple of transistors by two of the ATmega's pins.

Relay control board

There are four buttons, volume knob and rotary encoder for user input. Currently only volume, rotary and two buttons are used as the firmware still contains only very basic functions. It is possible to switch between CW and SSB filters and change the frequency (using 7 different steps from cca 1Hz to 1Mhz per revolution).

The insides.. just before the new CPU board was installed.

I will publish some schematics as I draw them as most of the work was experimental and the circuit changed often.

The current firmware lives at https://github.com/MarSik/HF-SSB-CW-direct-conversion-receiver including the bootloader that I had to adapt from Sanguino (m644p port) and Arduino sources (Diecimila's bootloader supports proper upload of eeprom data and reports correct signature).

In the future I'd like to use the free space I got by replacing Arduino to add CW transmit capability and about 5W PA.