[Elecraft] How to Eliminate Power Supply Birdies?

[Jim Brown]

On Mon, 11 Jan 2010 20:05:10 -0700, Bill VanAlstyne W5WVO wrote:

>The RFI is almost certainly common-mode and is being radiated by the AC 
>power cord -- and, by extension, the house AC wiring. 

Yes, this is most common. 

>Your typical 
>store-bought "AC line filter" is a differential-mode device and will do 
>nothing to abate common-mode currents. What you need is a common-mode choke 
>on the AC input cord, installed as close to the power supply case as 
>possible. You make a common-mode choke by obtaining a fairly large ferrite 
>toroid and wrapping it with the AC line cord. 

Yes.

>(Then use an extension cord to 
>reach the wall outlet.) Palomar Engineers and a couple of other vendors sell 
>these, and they have info on their websites that will help you choose the 
>best ferrite mixture for the frequency bands you're most concerned with.

On my website, you will find MEASURED data for common mode chokes of 1-14 
turns wound on 2.4-in toroids of Fair-Rite #31, #43, #61, #77, and #78 
material. The common mode choke shown in Fig 30 is ideal for 160, 80, and 
40M, and still pretty effective on 30M. The capacitor shown in the photo 
forms a differential filter with the leakage inductance of the choke, and is 
"icing on the cake." 

http://audiosystemsgroup.com/RFI-Ham.pdf

The only part of Bill's advice that I disagree with is to purchase from 
vendors like Palomar or Amidon, who have been ripping off the ham community 
for decades with VERY high markups on ferrite parts (typically 4x-5x their 
cost). They get away with this by inventing new part number (for example, FT-
11, FT-240) for parts they buy from Fair-Rite, who is the actual 
manufacturer. Fair-Rite has many industrial distributors who sell for a lot 
less. Further, the so-called technical data I've seen on their websites is 
laughable -- lots of numbers, but none of them relevant.  

The above-referenced tutorial includes buying advice in Appendix One. 

In addition to common mode radiation from external wiring (the AC cord, the 
DC cables), it's also possible that wiring internal to the power supply is 
creating magnetic fields strong enough to be picked up by our antennas. 
Remember, we're listening for very weak signals, so even low levels of 
radiated trash bother us. That field is produced by current flowing in a 
loop, and magnetic fields are NOT well shielded by simply throwing some metal 
around them -- the shield must come very close to totally surrounding the 
field to be effective, and it needs to be thick enough (in terms of skin 
depth) to attenuate the field. This is a MAGNETIC field we're talking about, 
not an ELECTRIC field. Henry Ott's new book has an excellent discussion of 
RFI from switching power supplies, and how to avoid it. 

http://www.hottconsultants.com/

73,

Jim Brown K9YC