137 kHz antenna preamplifier

One of the principal problems an amateur operator finds when tries to receive in the 137 kHz band is the really poor performance on typical amateur transceivers. Almost any modern transceiver tunes down to 500 kHz, many also tune down to 150 kHz, and some of them tune under 100 kHz.

To tune the frequency is not guarantee to work at that frequency. Most transceivers have sensibilities in the range of volts, and or are really prune to several overloading, so intermodulations wipes out any real signal on the band. In my case I tried to receive 137 kHz signals with the FT-817. It was not a easy task. First, the FT-817 have a antenna preamplifier (IPO) that attenuates at 137 kHz ( ! ) so the first thing to do is enable the IPO function (this is, disable the antenna preamplifier). Now you will hear some signal between the incredible high intermodulation level. Medium Wave broadcast stations does not like very much to the little transceiver, so I started to figure out how to improve the usability in the 137 kHz band.

The design goal were high gain and narrow bandwidth. High sensibility because the FT-817 does not have too much, and narrow bandwidth to keep out the strong broadcast signals, and therefore, the intermodulations. The first prototype worked as well that I made the definitive version in just a few days.

The preamplifier

The antenna preamplifier consists in a narrow 137 kHz filter followed by an operational amplifier. This is the schematics:

For the operational amplifier, I choose the TL-081. It worked just fine!

The filter

The filter is made with two standard IF yellow coils. They must be modified prior to be used here.

As you can see, all you must to do is to remove the original coil's capacitor. The easiest way to do it to broke it with the help of a small screwdriver, as seen at the most right image. Once removed, the coil will tune nicely at 137 kHz with the aid of a 2nF capacitor, as seen in the schematics. Use the primary winding, the one with three terminals.

Mounting

The amplifier works with a high impedance input, so you must shield it or you will have problems. I use a metallic box from an old TV tuner with great success

I used a small 9 volts battery to supply the preamplifier. You can try to supply it with a common 12 volts power supply, but check for noises.

Adjustment

Once mounted, you must to tune it. It's an easy task if you have a signal generator. Just apply a weak 137 kHz signal at the input and tune both coils alternatively to get the maximum output signal. If you don't have a signal generator, and you live in Europe, you can use the powerful DCF-39 to tune it. The DCF-39 is an utility station, located in Germany and very easily to receive in all Europe, specially at night time. It uses a FSK signal with a mark frequency of 138.830 Hz. You can use it to tune your preamplifier. The bandwidth of the filter is narrow enough to be adjusted at 138.830 but not at 137 kHz band. It will not be perfect, but pretty close. You can make the first attempt with the DCF-39 signal, and later, make the final adjustment with a real in band signal.

On air

The first signal received with the preamp, the FT-817 and a 35m long wire antenna was the DCF-39 with almost 50 dB over the noise signal. Pretty nice.

The DCF-39 signal

 
Some days later, I received my first amateur signals: CT1DRP and RN6BN:

My first amateur signals: CT1DRP and RN6BN

 
The signal in the upper part is CT1DRP, very strong here and sometimes audible in the speaker both day and night. The lower signal is RN6BN. Note its fading corresponding with the sunrise at RN6BN location.

Conclusion

Receiving the 137 kHz amateur band with an amateur transceiver is a very frustrating experience. Manufacturers do not make reliable receivers in that part of the spectrum. Fortunately, many transceivers are usable with the aid of a tuned antenna preamplifier, so you can try your firsts steps in this band. I have used this preamplifier with other amateur transceivers and it has prove it is a very useful accessory for the LF amateur.