[Elecraft] P3 generates noise on 144 MHz

[Roger Crofts]

Thanks to everyone who responded. With your help, I have already reduced the noise level from S9+5db down to S6. The receiver noise floor is S3, so I still have a little way to go. There is now no doubt that the P3 power cord is responsible for most of the noise radiation. Putting the ferrite clamp-ons on the power cord as suggested by David, G4DMP, had the biggest effect. I also obtained a small improvement by removing some accessories that were sharing the 12V switched output from the K3. I am using the power cord supplied with the P3, but I had a splitter at the K3 end to feed power to a keyer and pre-amp. The extra cables for these devices were acting as additional antenna wires to radiate the 144MHz noise. However it seems to me that the noise should not be getting onto the P3 power cable in the first place. I looked at the P3 circuit diagram and was surprised to see that there is no series inductor in the power lead as it enters the box. Every other DC (or AF) connection I could find in the K3/P3 was protected by a series inductor and usually multiple bypass capacitors. There is a single bypass capacitor in the P3 power line. It is C500 with a value of 100nF. It seems to me that the effectiveness of this bypass capacitor very much depends upon a very good and direct connection between the I/O board printed circuit and the case. This connection relies on the 4 screws associated with the two 9pin D connectors. On re-reading the construction manual, I notice there is an instruction at the top of page 41, to check the surface of the metal work which mates with these connectors. At the time of construction I only gave this a cursory glance. I may have been remiss here. I will polish this surface with emery cloth and see if it makes any difference. I also tried powering the P3 from another 12Volt power supply, but the noise remained at S6. If I stretched the power cord in a certain direction the noise could be made to drop to the receiver noise floor (S3). I guess that this is because the power cord "antenna" has a null towards my 2 meter antenna in this particular arrangement. This is a solution, but not a very good one!  I will continue my experiments and let you know how they go.

Roger Crofts, VK4YB