Elecraft KX3 Station Accessories

Earlier this summer I purchased an Elecraft KX3 to use as a portable radio as well as my main station radio when paired with my Hardrock 50 amplifier. To facilitate both of these scenarios I had to make a couple of accessories to adapt my existing gear to the KX3.

The KX3 has several 3.5mm and 2.5mm jacks on its left side for the microphone, headphones, accessory connections, RX I/Q data, etc. Since the mic jack is the only way to get audio into the radio and I operate both voice and digital interchangeably quite often, I didn’t want to have to unplug the mic connection every time I changed modes. To solve this problem I built a microphone connection switch box. The box allows for the connection of a common 8-pin round microphone plug (in this case wired to the Kenwood/Elecraft wiring convention) as well as a 3.5mm jack for the output of the sound card I use for digital modes. The two are switched via a DPDT toggle switch and connect to the KX3 with a TRRS 3.5mm right angle plug. Both connections also feature 0.1uF DC blocking capacitors and the sound card side includes a 10:1 voltage divider which results in a 20dB reduction of the sound card output. This allows for much easier control of the audio going into the radio and helps prevent over-driving of the mic input. The blocking capacitors were intended to prevent any mic bias voltage from affecting my dynamic desk microphone, however, I found that some still got through. To solve this issue I made some macros for the KX3 to switch between desktop voice, digital, and portable voice configurations. This allows me to turn the microphone bias on or off and adjust the mic gain and compression depending on whether I am using the desk mic and foot switch in my station or a handheld microphone while portable.

The other accessory I made is for portable digital operation. I wanted to keep my setup as compact and light as possible, so I wanted to avoid having to take my SignaLink USB with me in the field. I found a good compact USB sound card, but I wanted to add some isolation from the radio as well as some signal reduction from the sound card output to the radio similar to what I did in my microphone switch box. For isolation I used ground loop isolators to reduce the possibility of interference between the radio and the sound card. To reduce the sound card’s output I used another 10:1 voltage divider to reduce the output by 20dB. In this case I only did it to the left channel since this is the default used by most digital programs and it is the same position on the 3.5mm mic jack as the microphone input on the KX3. Fortunately I was able to install the voltage divider inside of the isolator case itself by carefully prying apart the two halves. The voltage divider resistors were then soldered directly to the isolator circuit board and fit inside the case with plenty of room. The isolator case snapped back together when I was done modifying it without any glue necessary.

So far my KX3 accessories have been performing well and they make an already great little radio that much easier to use.

HobbyPCB Hardrock 50 Amplifier & Auto Tuner Kits

Now that I have a couple of QRP radios, especially the very capable mcHF, I became more interested in amplifier options to boost my power output when the need arises. I also wanted to build the amp myself to add to my collection of kit built equipment. After looking around, I decided to go with the HobbyPCB HARDROCK-50 Amplifier and ATU.

At $300 for the amplifier and another $180 for the tuner this is not the most economical kit, however, this is a very full featured amplifier and the antenna tuner adds a lot of additional functionality. The amplifier can be driven by a PTT signal or a carrier and also features rig interfacing with some QRP radios for automatic band switching. Both the amplifier and tuner can be inline or bypassed. This is a great feature since the tuner can be used with your radio at QRP levels when you don’t need to use the amp. It is also a very wide range tuner, covering roughly a 10:1 SWR range. The tuner board also adds 60 meter band and standalone Watt/SWR meter functionality.

The kits themselves are very well put together with well made boards (pre-populated with surface mount components) and good quality components. The case is super rugged and all of the machining and fit and finish were very well done. I really like that the amp is completely air cooled and should only require a fan under extreme circumstances. The instructions (HR-50, ATU) are also very well put together with lots of pictures and a ton of detail to step you through the assembly, calibration, and operation of the amplifier.

The amplifier section took about 7 hours to build and calibrate. Most of this involves soldering all the components (including all 15 relays) as well as winding and mounting the 14 inductors. The auto-tuner board took another 3 hours to assemble. This board is almost entirely relays (17) and inductors (9). Integrating the two boards together is very straightforward. It’s a testament to the design of these kits that you can build the amplifier as a standalone kit and then add the auto-tuner later and only have to modify one coax connection.

These kits have been around for a few years and various corrections and bugs have been ironed out, so my kits didn’t require any hardware mods and came pre-loaded with the current firmware revisions. The amp’s screen displays the hardware, amp firmware, and ATU firmware versions on boot-up.

Once everything is running the screen displays the current keying mode, the band selected, heatsink temperature, and power supply voltage. When the amp is keyed the LED turns red and the display shows a bargraph of power output as well as the SWR and output power in PEP. To initiate the auto-tuner you press the Key Mode button after keying the amplifier.

In order to integrate the amp with my mcHF I adjusted the full power setting of the radio from 10 watts output to 2-4 watts output depending on the band. This was done to provide the appropriate drive level for 50 watts out of the amplifier. Using this setup I tuned around the bands and found a Belgian station calling CQ on 80 meters and received a good signal report when I responded to him. Not a bad first contact. I look forward to using this amp and tuner more in the future especially with my mcHF since I am no longer limited to QRP operating.

Boafeng UV5R USB Soundcard Interface

I recently purchased a Baofeng UV5R5 to throw in my Go Kit as a backup handheld and I decided to build an interface to be able to send and receive digital signals. The interface was intended to be as simple and inexpensive as possible, much like the radio itself.

VHF/UHF digital EMCOMM transmissions in my area typically use the MT63 mode which is very robust and can work quite well using only acoustical coupling. While this technique works surprisingly well, it has limitations. If the area you are operating in is too noisy, your audio is too weak, etc. the data transmission can have issues getting through correctly. It also doesn’t work very well for modes other than MT63.

USB soundcard interfaces are very common, I have multiple SignaLink USBs myself, but they are definitely overkill for this application. After some experimentation, I built this simple interface for under $20.

Parts

Construction

The main idea for this project was to replace the external speaker microphone functionality with that of the USB soundcard. In order to do this I used the speaker mic cable and wired it to two 3.5mm stereo audio cables such that the speaker output from the radio connects to the microphone input of the soundcard and vice versa. Each splice was soldered and insulated with heat-shrink tubing. The entire joint between the three cables was then secured with more heat-shrink tubing. Each 3.5mm plug was marked with colored electrical tape to make it easy identify which cable plugs into which port of the soundcard (red for microphone, green for speaker).

Operation

To operate, I plug simply plug in the cables and connect the USB soundcard to my computer (a big advantage of this model of soundcard is that it does not require special drivers for Windows 10 or Linux, it is truly a plug-and-play device). When I am ready to send data I simply key the radio using the PTT switch on the side and click the transmit button in the digital software. When the transmission is finished I unkey the radio. I had originally played around with an external VOX circuit as well as the UV5R’s internal VOX feature, however, neither of them would reliably key the radio and stay keyed throughout an entire data transmission and I decided they were unnecessary. Using manual keying is actually somewhat of an advantage since it simplifies the interface, reduces complexity, and doesn’t require changing the radio’s configuration.

Calibration

I used FLDIGI to test the interface over simplex to another radio. After some experimentation I found that with the radio’s speaker volume set at a comfortable level (about 1/4 turn) a setting of 50% for the soundcard’s microphone gain was a good audio level for receiving data. For transmitting, I found that a setting of 1% from the soundcard’s speaker produced the cleanest output.

If I was going to build more of these I think I would add a 10K ohm resistor at the connection between the soundcard’s speaker output and the radio’s microphone input. This would attenuate the signal somewhat and allow for finer control over the transmit audio level. Even so, as it stands now the audio is clean and data transmission worked flawlessly. I have used this interface on my local digital net and it performs very well. This has definitely found a place in my Go Kit.

Update (March 2017)

I recently discovered that FLDIGI has a built in TX Audio Attenuator feature. Using this I can achieve much finer control over my transmit audio level, even with the soundcard’s speaker output set to 1% volume. This makes adding a resistor in the transmit audio wiring unnecessary.

PSK31 Sound Card Interface

psk_interfaceIf your sound card has Line In and Line Out jacks, and your radio has an accessory port, this is the easiest way to interface between the two. Using your radio’s accessory port eliminates the need for more circuitry to control the audio level going to and from your radio. This interface has worked perfectly during regular use and several Field Days. All the parts are available from RadioShack, and it can be constructed in under an hour.