The 136 kHz (2200 m) band is now officially available to all Australian amateurs that hold an advanced license, but there are not many commercially made receivers or transmitters that will cover that band. It is true that some equipment will allow tuning on that frequency, but their performance is really horrible and are not particularly good for serious work.
The transmitter problem can be easily addressed by the many designs that are available on-line, which will make use of a FET and a few other components to make a simple CW capable transmitter which can be assembled in a day with minimal cost. But the receiver remains a problem.
The kit from ICAS Enterprises in Japan addresses exactly this problem by providing a simple design which is specifically designed to work well on the LF part of the spectrum. The kit is designed to be used as an SDR (Software Defined Radio) system which needs a computer to translate the I and Q signals into meaningful information but can also be used as a DC receiver with the BPF/preamplifier kit which is also available from the same company. With the addition of the BPF kit, this receiver can be used as the basis of a grabber together with an old PC and without using the SDR software.
This receiver is used with great success at the Tokyo 2200 m grabber. See the references section at the end of this page for a link.
The kits arrived very quickly and come complete with all the components, high quality PCBs, plugs and sockets you will need. The assembly manuals are available online in both English and Japanese, however there is only a Japanese version of the BPF kit manual currently. Fortunately, all you will need is the schematic and the component list, which are available in the Japanese document!
This is not a simple kit for the beginner, but it is not one of the most complicated ones either. Firstly, the only SMD component (FST3253) is already soldered on a little PCB converter board! Every other IC is of the pin variety. You will have to wind a transformer, but the instructions provided are very clear. Some sockets are provided to enable quick replacement of ICs if needed in the future, but that's up to the individual to decide if they want to use them or not.
There is one piece of advice I have to offer to the assembler. Read and understand the schematics provided before you attempt the assembly! This sounds like an obvious statement, but in my excitement on the arrival of the kits, I jumped on the soldering table straight away which resulted in a mistake or two. The designer of this receiver system has allowed for many optional configurations, for example the ability to use an external LO instead of the oscillator provided or the ability to control the receiver which is required in the case that it will be mated to a transmitter in order to produce a complete transceiver. With a careful study of the schematics, all details will be obvious and the assembly will be successful.
It took me three or four hours from opening the bags to the successful test of the receiver and BPF. I would not consider myself a beginner but I am not the most experienced soldering iron expert either. You will need a good quality iron with a fine tip, a multi-meter (I always measure the resistors rather than try to read the color code!) and either a spectrum analyser with a tracking generator or a scope/sig gen to tune the band pass filter. In fact, it is probably possible to simply use a signal source on the desired frequency and then peak the signal from the BPF to a receiver, but I went for the easiest option of the spectrum analyser, as this was available to me.
I first started with the BPF/preamp kit assembly. This was very straight forward as the schematics are quite simple to follow. The result is this:
Then I continued with the assembly of the receiver. There are two PCBs, a smaller one for the IF part and a bigger one for everything else. The small board connects to the bigger board through two header connectors, as can be seen in the photo below:
The first thing I tested was the BPF/preamp. This consists of a sharp band pass filter with two variable "slug" type inductors. I connected the input to the tracking generator, the output to the spectrum analyser input and provided 12 V. I immediately saw the familiar picture of a BPF frequency response, only it was on a higher frequency than the one desired. Within seconds, I managed to get the following frequency response:
In this test, the tracking generator was providing a -30 dBm signal and the spectrum analyser was configured to display the spectrum from 100 kHz to 200 kHz with a 5 dB/division setting on the Y axis. This shows a gain of about 20 dB and a very sharp BPF frequency response! I was quite happy with that, as this can be used with any receiver that has low sensitivity and gets severe interference from strong NDBs and MW broadcasting stations. With that in mind, I changed the stop frequency of the spectrum analyser to 1 MHz and the Y axis step to 10 dB/division, which resulted in the following:
This looks like it will take care of the station at 666 kHz which is a few kilometres from my QTH, as there seems to be over 50 dB of attenuation at that frequency!
After the BPF, I tested the receiver. At this point, all I wanted was the confirmation that the receiver works. So I put a 130 kHz signal to the input and looked at the I/Q output with the scope. Surely enough, I saw a nice sine wave with a 5 kHz frequency as expected. So, the whole system seems to work as expected.
With the difficult bits done and dusted, I am now working on putting everything in a box. My plan is to place the BPF and the receiver in the same box, but connect them together via an external BNC jumper cable that allows me to use the BPF/Preamp with a different receiver. I am also planning to put the pot which is provided for the BPF kit to control the gain of the op amp. Signals are generally quite strong and so is the noise, so in some cases the high gain of the preamplifier may not be necessary.
Here is the final result of this weekend's hard work:
There are three BNC connectors at the back because there is one for the SDR antenna input, one for the pre-amp/BPF input and one for the pre-amp/BPF output. There will be a jumper BNC cable between the receiver input and the preamp output and the antenna will connect to the preamp input.
Remaining now is the printing of some nice labels to identify each and every BNC, socket and switch!
Overall, I am very happy with this kit, so far everything has worked as promised. I am looking forward to setting it up as a grabber for the 137.777 kHz QRSS frequency and capturing some signals from far far away! Hopefully, this will also help with the local amateurs setting up their transmitters. It's always nice to have a local grabber in order to test a new transmitter or antenna system.
Please make sure you visit the grabber archive section (see References) to see some of the more interesting captures of the VK1SV grabber.
ICAS Enterprises web site in Japanese
ISDR-136-KIT web site in English
BPF-136-KIT web site in Japanese
Tokyo 2200m grabber
VK1SV grabber and VK1SV grabber archive