[Elecraft] P3 generates noise on 144 MHz

[John Lemay]

Roger

 

About a year ago when I was using my K3, K144XV and P3 combination I went
through most of the suggestions you have received . The noise floor was
reduced to almost satisfactory levels, but not quite.

 

Two further things occur to me: The DC cable to the P3 is just over a
quarter wave long on 2m, so making it a different length could help.
Secondly, I'm considering using a shielded cable (some cheap thin coax would
be OK because the current consumed by the P3 is low).

 

Regards

 

John G4ZTR

 

 

  _____  

From: elecraft-bounces at mailman.qth.net
[mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Roger Crofts
Sent: 12 February 2013 00:10
To: elecraft reflector
Subject: Re: [Elecraft] P3 generates noise on 144 MHz

 

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) connectio
 n 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
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