Radio Sweden on 1179khz mixes with a station in Norway on 1314 and sometimes also with a station in Germany on 1044. Both giving a difference frequency of 135,0khz. The sidebands can be heard in the lower part of the lf-band and can be very strong in the evening with me, sometimes with the swedish, norwegian and german languages to be heard at the same time ! A wonderful experience !

How the mw signals come in to the preamp is a mystery as the amplifier is connected close to the loop and the loop Q is about 110. I have tried to suppress 1179 with a resonant circuit in various configurations however no change was noted.

Anyway, I have changed the preamplifier circuit somewhat and the cross modulation now seems to have decreased a lot (not bothersome anymore) while the gain seems to be about the same (I have no means of measuring the gain). At least DCF has the same strength. Besides the mains noise seems to have decreased very much, possibly due to the much higher standing current in the FET:s.

One example is the noise generated by the diodes in the rectifiers for the TX power supply (yes, I have capacitors, 0.1uF, across each diode). Earlier I always had to switch off mains to the tx while listening. This is no longer necessary. This noise has completely disappeared. The band seems overall much quieter.

The following items have been changed:

The 2N3819:s are replaced by high current FET:s BF246C (the same pinout).
R3 and R4 are decreased from 2.2k to 270ohms
The source lead in each FET is broken up and 150 ohm (see below though) in parallel with 0.1uF are inserted.
The leads between the drains of the FET:s to the bases of the 2N2222:s are broken up and 47k in parallell with 0.1 uF are inserted.
R5 is decreased from 1k to 180 ohms.

In my version I had to adjust one of the 150ohm resistors in the FET sources to obtain approximately the same currents through each FET resulting in that on one of the FET:s 150ohms had to be replaced by 120 ohms. To check this, measure the voltage on each drain. On the one with 150 ohm the drain voltage was exactly 6V while on the other 6.6V. Decreasing to 120 ohms resulted in 6V on both drains. No other adjustments seem to be necessary.

The current consumption has now increased to about 72mA at 13.8V, divided into about 26mA through each FET and the rest through the 2N2222:s. The resistance of the 1mH choke now has some impact on the voltage to the transistors so I had to replace mine with a one with lower resistance than the one originally used..

Hope this might help someone with the same problems as me.

Have a nice weekend !

73 de SM5EUF / Urban