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Author Topic: Some random things I've observed about QRM/etc. by sitting outside  (Read 2003 times)

Offline ChrisSmolinski

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1. The touchpad on my MacBookPro generates considerable QRM, right in the DGPS band. Only when using it.

2. You still need a ground when set up outside, even if everything is run off batteries.

3. The AC adapter on the MacBookPro also generates considerable QRM. I tried a 3rd party adapter, it of course was even worse. I need to see if there's some other options.

4. I also (sometimes) get QRM when the power supply for the resonant amplifier is plugged in. If I unplug it, the QRM goes away, well for a few seconds while the capacitor inside the amp holds a charge, then of course it stops working. Something I need to investigate. Maybe some inductors on the input will help?

5. Aiming a big loop at your house also generates a lot of QRM.
Chris Smolinski
Westminster, MD
eQSLs appreciated! csmolinski@blackcatsystems.com
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Offline Pigmeat

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Re: Some random things I've observed about QRM/etc. by sitting outside
« Reply #1 on: August 14, 2017, 1150 UTC »
Aim it Al's head and see if anything turns up? My guess is nothing will, but you never know.

Offline jFarley

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Re: Some random things I've observed about QRM/etc. by sitting outside
« Reply #2 on: August 14, 2017, 1304 UTC »
Keeping noise out of LF antennas here is a never ending endeavor.  Cupla things I have learned.

The biggest thing I watch is keeping ground loops out of the system, which can generate mysterious noise from time to time.  An active antenna which may have multiple grounds - power for the antenna amp and an RF ground represented possibly by the coax shield - is something I avoid.  I try to keep the power ground as the "reference", and break up other grounds if possible.  The addition of an RF isolating transformer in the signal return path has worked wonders for me.  I also have a USB digital path out to the site, and I use a USB isolator  from time to time as required.

Many NDB DXers use a common mode choke(s), with at least one out at the antenna.  Here, a ground rod rod near the loop my be required.

Joe Farley, Near Chicago
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Offline R4002

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Re: Some random things I've observed about QRM/etc. by sitting outside
« Reply #3 on: September 03, 2017, 1742 UTC »
Chris,

I've tested several different laptops (and their AC power supplies, all of which are switching power supplies) and they all seem to generate the same kind of broadband noise that is the worse between roughly 5-8 MHz but affects higher frequencies, going up into low VHF band, messing with 11 meters pretty badly.  Local receive on 27 MHz is totally obliterated when my laptop is connected to the same AC circuit as the radio's power supply (Astron RS-20A).  Putting RF chokes in the DC line for the CB/11 meter radios and HF equipment helps a little bit, but doesn't eliminate the problem.  I find that connecting the switching supply to a different AC mains power plug reduces but doesn't eliminate the noise, especially when comparing noise levels to a "clean" DC power source (12 volt car battery - direct connection).  These tests were only done using 27 MHz CB equipment, as I don't have a proper HF mobile setup, but I imagine they apply for most of the HF band. 

I've noticed that its not just switching power supplies used with laptops, but switching supplies in general.  One particularly terrible offender is the cheap switching supply that ships with the Radio-Tone RT-SRC1+ simplex repeater controller.  I swapped one out for an older-generation RadioShack AC-DC converter and the difference is crazy.  The Radio-Tone switching supply adds 7-9 S-units of noise to the noise floor on 11 meters / 27 MHz.  
« Last Edit: September 03, 2017, 1743 UTC by R4002 »
U.S. East Coast, various HF/VHF/UHF radios/transceivers/scanners/receivers - land mobile system operator - focus on VHF/UHF and 11m