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Killing RF Noise for Field Day and CQP
Jim Brown K9YChttp://k9yc.com/publish.htm
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The Fundamental Problem
• RF noise is generated inside equipment
• The wires inside equipment, and cables that interconnect equipment, are antennas, and can transmit that RF noise
• The same problems that let RF into the box also let it out of the box– Pin One Problems
– Poor shielding and poor circuit layout
• Our antennas receive it like any other signal
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General Strategy• Don't bring problems with you – check
out every piece of gear for RFI before you leave home
• Prepare for known common problems–Most generators are noisy–Switching power supplies for gear,
battery chargers, wall warts–Noisy equipment
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The Generator Filter• Most of the noise is common mode, and
power line filters don't work on RF common mode
• Power industry's definition of common mode is voltage between neutral and green
• The true definition of common mode is current flowing in the same direction on all conductors
• Common mode current radiates trash to our antennas
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The Generator Filter
• Use the same cookbook guidelines for power line common mode chokes as for coax of the same diameter
• Make cable between choke and generator very short
• You can add a commercial line filter, but it is much less important than the choke
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A Generator RF Noise Filter
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Ferrite Choke More Important Than Line Filter Inside the Box
Very Short Cable to Generator
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Sources of RF Noise• Generators, including Hondas
• Switching Power Supplies, including Battery Chargers
• Equipment with digital circuitry– Computers, audio and video gear, ham gear
• Degraded Insulators in Power Systems
• Variable Speed Motors
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What is Digital Noise?• Most digital noise results from oscillators
or clocks that produce square waves
• Square waves have lots of harmonics
• Faster rise times = stronger harmonics
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Typical noise signature of a switching power supply
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Why a Hump Instead of a Steady Carrier?
• Oscillators are dithered (modulated by random noise) to skirt FCC RFI rules
• That noise causes them to wobble around in frequency or drift, and the modulation makes them broad
• FCC rules limit the strength of carriers, so the noise modulation moves some of power from carrier to sidebands
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The same switching PSU drifting after being switched on
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A closer look at one of the peaks
10dB
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Same picture, a minute later
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And another minute later
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And another minute later
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And another minute later
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That’s the PSU for my SteppIR• I’d already suppressed the noise by more
than 20dB before I took these pictures! • I’ve worked a lot of guys who don’t move
my S-meter• 10 dB of noise makes a 1kW signal seem
like a 100W signal• 20dB of noise makes 1kW seem like 1W• You can’t work ‘em of you can’t hear ‘em! • It’s really worth it to chase and kill RX
noise
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Finding Noise Sources• Run your station on a battery and kill
power to your home–Be sure to turn off any UPS units
• Listen on all bands
• Any noise that goes away is your noise
• Restore power, and turn off one breaker at a time until noise stops (or gets weaker)
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Killing RF Noise• Noise must be killed at the source
• So we must find the source
• Exception – use antenna location and directivity to reduce noise–Move antennas away from noise sources–Use serious chokes on your feedlines at
the feedpoint (that is, up in the air)
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When You Can't Attack the Source• Use serious chokes on your feedlines at the
feedpoint (that is, up in the air)
• Chokes prevent RF picked up on the feedline from filling in the nulls in your beam's pattern–Use antenna directivity to reject noise
• Follow guidelines in my Choke Cookbook–http://k9yc.com/RFI-Ham.pdf
• Benefit typically 3-6 dB
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Probing For Noise SourcesHam talkie that
receives HF
Low cost AM-FM- Short-wave receiver (this one has DSP IF, $45 at amazon)
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Probing For Noise Sources• Tune the portable receiver to the
range where you hear RF noise
• Move antenna around suspected noisy equipment–For lower bands, antenna is a
loopstick (in base of talkie)–For higher bands, it's the duck or
rabbit ear of the Tecsun
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CQP Sites Are Often Much Quieter• S2-S3 is common at remote locations unless
we screw it up with our own trash
• An S5 noise level at home may prevent your RX from hearing noisy equipment
• If your probe receivers hear trash, kill the trash or leave the noisy gear at home
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Switching Power Supply Wall Warts• Identifying a switcher
–Much smaller and lighter for same power rating
–Probe with the receiver –Most have hash below 3 MHz –Worst ones have noise extending to
high HF bands
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Is This Switcher a Problem?• Set it up with the equipment it powers,
turn that equipment on, with all cables attached
• Probe all cables (including both AC and low voltage power) with the RX
• If the cables are noisy, they are carrying RF current that can radiate to our antennas (it's normal for noise to vary along length of cable)
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Noisy Switching Power Supplies• Try to replace with a linear supply• Most switchers are regulated
• Most linears are not regulated, just a transformer, rectifier, and filter cap–No load voltage will be 30-40%
greater than rated voltage, will drop under load
–Some gear may not turn on at higher voltage (internal protection circuit)
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Finding Linear Power Supplies• Your junk box (you don't save stuff?)
• Cheap ($ .25- .50) at second hand stores–Goodwill, Salvation Army, etc.
• Electronic Flea Markets–I see hundreds of them laying on the
ground at DeAnza Swap
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Replace a noisy switching power supply with a vintage linear supply
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Replacing Switching Power Supplies• Buy a bunch of Power Pole Connectors• Cut cables of both supplies• Put Power Poles on linear supplies,
and on the plugs that fit the gear• Make a Power Pole Y-cable so you can
measure voltage under load
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Buying Power Pole Stuff
• Powerwerx is cheapest by far if you buy individual housings and contacts
• They're easy to install
• I've never used a crimper for this stuff
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Some Useful Power Accessories
$60
$18
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RFI From Switching Power Supplies• If you cannot replace one with a linear
supply:– Wind turns of the DC cable through
a ferrite core to form a chokeand
–Plug supplies into choked multi-outlet boxes or wind AC power cable through a toroid to form a choke
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Plug Noisy Power Supplies and Gear Into Filtered Power Outlets
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RFI From Digital Equipment• Noise must be radiated for us to hear it
• What are the antennas? –Every interconnecting cable
–The power cable
• With the portable RX, probe the gear, and along each cable
• If you hear lots of trash on a cable, it needs a choke
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RFI From Digital Equipment
• Wind multiple turns of AC cable through a ferrite core to form a choke
• Wind every interconnect cable through a ferrite core to form a choke
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This 4-turn choke is about right for 15-30 MHz
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This 5-turn choke is about right for 10-30 MHz
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An Effective Choke for 2-10 MHz
14 turns around a #31 core
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ZN = N2 * Z1
#43 mix, 1-5 turns
HF Bands
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#43 mix, 1-14 turns
HF Bands
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#31 Mix is best for HF bands
HF Bands
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If you can’t easily remove the connector
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Biggest Clamp-On, #31
Sometimes you can’t remove the connector
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Other Noisy Gear• Video Monitors
–Don't bring it if it's noisy – find one that isn't noisy
–If you must use it, choke both video and power cables
–Chokes can't kill trash radiated by internal wiring – if that's the problem, leave it at home!
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Solar Systems Can Be Noisy• Most charge regulators use pulse-
width-modulated square waves
• Most DC to 120VAC converters are square wave generators that are filtered and shaped to approximate a sine wave
• Nearly all are very noisy unless the designer has worked on making them quiet – and, quiet ones are expensive
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A Quiet Solar Charge Controller for Small Systems • $ 17 at amazon
• Hysteresis regulator is a simple switch – no pulses
• Charges until battery hits 14.2V, starts charging when voltage drops below 13V
• Max panel open ckt 24V • Thanks, AB6VU
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RFI From Battery Chargers• What are the antennas?
–The AC power line–The DC cable, if there is one
• Treat it like any other switching power supply – replace it with a linear supply, or choke the antennas!
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Ethernet Birdies• Identifying Ethernet birdies
– Crystal controlled, wide tolerance, modulated– Around 14,030 kHz, 21,052 kHz, low end of
10M CW, low end of 6M– Often multiple signals – we hear our neighbors
too, each on a slightly different frequency– Kill power to your router to see if birdies go
away, work on those carriers– Many other frequencies, but these will tell you
if you have a problem and if you're fixing it
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Killing Ethernet Birdies• Wind each cable around a toroid
–6-8 turns usually about right
–Don’t forget power supply cable–Choke both ends of cables > 0.2λ
• Use shortest cables practical–Longer cable is better antenna
• There is no fix for trash radiated from a badly shielded box–Leave those boxes at home
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Try Wireless Networking• The short cable to internet modem, and a
poorly shielded box are only causes of RFI
–No QRM from our rigs to wireless
–Modern WiFi good for 200 ft or more
• The downside of wireless networking–Configuration conflicts between wireless
routers and an ad hoc collection of computers are all too common, and can be very difficult to solve
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The Biggest Myths
Myth: “I need a better ground”
Fact: A connection to earth almost never reduces noise or RFI, and it will often make it worse, because the “ground wire” can act as an antenna.
Fact: A connection to earth is very important for lightning protection.