Found my 20m noise source

About three months ago, I finally put up another 20m antenna—an end-fed, half-wave antenna. Right off the bat, I was flummoxed (I love that word) by the high noise level. It was nearly S9, obliterating all but the strongest signals.

I thought it had to be something to do with the antenna. I was not experiencing any noise on any of the other bands, after all.

As it turns out, though, the source of the noise is the laptop power supply. I had taken the laptop somewhere on Wednesday, and when I returned it to the shack that evening, I switched the rig over to 20m before connecting the power supply back to the laptop. No noise! When I plugged the power supply into the laptop, the noise jumped up to S9 again.

What’s really confusing about all this is that I don’t hear this noise on any other band. They’re all completely quiet. Like I’ve said before, sometimes 40m is so quiet that I wonder if the antenna is even connected.

At any rate, the last couple of days, I’ve been working 20m with the power supply disconnected, running the laptop off the battery, and the results have been quite good. This evening, for example, I worked several DX stations, including 6W/HA0NAR in Senegal.

Comments

  1. Good morning Don, I would say one of the largest hurdles has been overcome when you found out where the RFI was being generated. I did some reading online about the power supplies from laptops and Dell seems to be a big offender. (not sure if yours is a dell) Some had mentioned they used clip on chokes and so on….but as far as I am concerned you can spend a lot of money and still have RFI. I was wondering if you could purchase a new supply say from Radio shack (so it can be returned if it generated RFI) I have 3 laptops in my shack 2 Toshiba’s and a Mac pro and all are good. I have kept many of my supplies when the product they power goes defective and sometimes I find a use for them…but laptop supplies are wonky voltages at times. Good luck with solving the problem.
    Mike

    • The laptop is a Toshiba. There is actually already a choke on the cable that connects the power supply to the computer. I think the easiest thing to do is to disconnect the power supply from the laptop when I’m on 20m. That works just fine for me. I’ll probably also re-orient the antenna to see if that has any effect.

  2. Dan,
    Congrats on finding the noise source.
    I have found that flipping off circuit breakers in my house is a good first step to try and find a noise source. Usually ticks off the family, but what the heck :-)

    Bob

  3. Dave, N8SBE says:

    I attended a 4-hour seminar on EMC design last year at the Freescale Technology Forum in San Antonio. The presenter indicated that it was a very shortened version of a 2-3 day course he normally teaches. Here is the URL to his web site:

    http://www.learnemc.com/index.html

    There are a couple of for-pay seminars listed, but there is also a ‘tutorial’ tab, where a lot of presentation material is free for the viewing.

    The one big takeaway I got from the short course last year, was that all EMC issues stem from poor PCB layout, and blindly following well-worn ‘guidelines’ that often do not address the root cause of the noise. In any event, trying to control the noise after the fact with ferrites and shielding is a sign that the PCB design was defective, and almost never cures the problem completely.

    Of course, by the time most companies find out they have a problem on their hands, they are deep into trying to pass FCC Part 15, and to get their production to market, so no one will listen to the idea that the PCB needs to be re-laid out. The result is things like power supplies with pricey ferrite cores embedded on their cable, a sure sign that the supply is noisy, ill-designed, and rushed to market with bailing-wire type fixes.

    If you have such a product, often the only cure is to take it out and shoot it, to put it out of its misery.

    • Fortunately, I don’t have to take this out and shoot it. All I need to do is to disconnect it when I’m operating 20m. Remember the days when we operated without computers?

  4. David N8SRE says:

    I’m going to posit that because the end-fed antenna is inherently unbalanced, it’s sensitive to noise picked up by the coax shield in the shack, which is no doubt forming part of the counterpoise.

    • I thought about that, especially since that antenna is the only unbalanced antenna that I’m currently using. I was going to fool around with a counterpoise to see if that had any effect on noise pickup. I was also thinking that it might have some effect on the antenna’s feedpoint impedance as well.

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