Noise - Origins, Effects and Mitigation Strategies

Transcription

1 Noise - Origins, Effects and Mitigation Strategies Prepared for the Oro Valley Amateur Radio Club March 16, 2018 By KK6MC James Duffey KK6MC@amsat.org

2 Noise - I will be discussing: Origins Cosmic Natural - lightning Manmade - switching power supplies, light dimmers, arcing power line insulators Effects - makes copy difficult or impossible Mitigation Noise blankers Narrow bandwidth modes - SSB vs FM, CW vs SSB, Digital vs CW Identify 2

3 What is noise? Noise is a random unwanted disturbance or error of a signal - static Noise is a natural phenomena: Thermal noise is generated in any resistive (dissipating) device due to Brownian (random) motion of carriers in an electric field 3

4 Noise is conventionally stated relative to the thermal noise in a 50 Ohm Resistor Noise Figure is defined as: Fa = 10 log fa db Where: f a = k p T n 0 b pn : available noise power from an equivalent lossless antenna 4

5 Receiving System Noise = external noise from theoretical lossless antenna + noise due to antenna losses + noise due to transmission line losses + noise due to receiver Any loss in the receiving system will translate to noise External noise consists of both natural, including cosmic sources, terrestrial sources, and man-made sources 5

6 Typical Receiver Noise Figure Values Receiver losses are usually low, and in a well designed receiver, should be dominated by the first active device after the antenna. Noise figures can vary from 15 to 20 db in a typical HF transceiver to less than 1 db in a VHF/UHF/microwave front end. External noise dominates front end noise below 30MHz At higher frequencies external noise decreases and front end noise is more important 6

7 A F a (db) C T a (K) B E D Frequency (Hz) A : atmospheric noise, value exceeded 0.5% of time B : atmospheric noise, value exceeded 99.5% of time C : man-made noise, quiet receiving site D : galactic noise E : median city area man-made noise minimum noise level expected Fa versus frequency (10 4 to 10 8 Hz) P Noise Sources 10kHz to 100MHz Fa is db above noise in 50 Ohm reference 7

8 A D F a (db) C B E (0 ) F E (90 ) T a (K) (1 GHz) Fa versus frequency (10 8 to Hz) Frequency (Hz) A: estimated mediancity area man-made noise B: galactic noise C: galactic noise (toward galactic centre with infinitely narrow beamwidth) D: quiet Sun (½ beamwidth directed at Sun) E: sky noise due to oxygen and water vapour (very narrow beam antenna); : upper curve, 0 elevation angle; lower curve, 90 elevation angle F: black body (cosmic background), 2.7 K minimum noise level expected P Noise - 100MHz to 100GHz Note noise decreasing with frequency and very low noise at very high frequencies 8

9 Median values of man-made noise power for a short vertical lossless grounded monopole antenna A B C 60 F am (db) D 40 E f (MHz) Environmental category: Curves A: Curves B: Curves C: Curves D: Curves E: city residential rural quiet rural galactic (see 6) P Man-made noise from 200kHz to 300MHz 9

10 Antenna Noise Most losses from properly designed, and even poorly implemented HF antennas are small and don t contribute to the noise because the external noise is so high For example, consider a 160M mobile antenna that has 99% losses. That will add 20dB to the noise seen at the antenna terminal, but the external noise is already 70dB. With a receiver that has a noise figure of 20dB, this additional loss from the antenna is neglible At VHF and above, the antenna pattern can drive noise pickup if side lobes are not suppressed Recent VHF/UHF antenna work has emphasized antennas with low sidelines 10

11 Low noise VHF antenna Figure of merit for antennas that reflects noise is G/T, or gain over Temperature High G/T antennas are best This figure of merit includes the gain and temperature integrated over the entire 4pi steradians of the sphere Typical modern high G/T antennas include those designed by YU7EF, G3KSC, and DK7ZB Low noise antennas most important for weak signal use, EME, terrestrial troposcatter, and meteor scatter 11

12 Modern low noise 2M antenna Note all side and rear lobes are low, below 20dB G/T= -1.2 VE7BQH has G/T table of popular 6M, 2M, and 70cm antennas 12

13 Pattern of NBS Yagi (ca. 1970) Numerous side lobes, many above -20dB 13

14 Coping with man-made noise Identify whether noise source is radiated, that is coming in through the antenna or conducted; that is coming through connections to AC power line, power supply, computer or network connections Identify noise sources Develop mitigation plan NM Section Technical Coordinator can help with identifying an mitigating noise sources ARRL HQ can also help contact Ed Hare W1RFI 14

15 Some things to do before you go looking for noise Make sure that outlets are wired correctly - use inexpensive outlet checker Make sure that all of your equipment is powered from the same neutral, preferably from the same outlet Make sure that the grounds (cases) of all of your equipment are tied together See Grounding and Bonding for the Radio Amateur by N0AX, ISBN for discussion on good grounding practices Also, K9YC s web page can be useful, particularly his discussions on the pin one problem and his discussion on ferrites 15

16 Before trying to mitigate noise, try to quantify it Record noise levels on different bands, with different antennas Try to identify noise qualitatively; that is, is it just high static, is the static modulated (pulsed), is it synchronous with line (60Hz or harmonics), is it fixed in frequency or does it drift, are there noise peaks that occur at definite frequency intervals, look on bands you don t usually operate SDR or band scope can help Take measurements of noise amplitude S meter is OK, some modern rigs have input signal level calibrated Be sure to use the same bandwidth, at CW bandwidth there will be less noise evident Take notes and put them in your log book to establish a baseline 16

17 Identifying whether noise is conducted or radiated Power radio with Battery Disconnect antenna and any peripherals to radio If noise goes away when antenna is disconnected, then the noise is radiated, if not it is likely conducted 17

18 Make sure your QTH is quiet Power rig from battery Throw main breaker and see if noise decreases This can have some consequences, so be sure you can restart everything in the house that needs restarting If noise decreases or goes away, then at least part of the problem is in your house Turn main breaker on and then turn off breakers for each circuit to see if noise goes away If it does, then look 18

19 Things to look for that typically generate copious amounts of noise Switching power supplies - replace with linear if possible Switching wall warts are particularly bad, replace with analog supplies, thrift stores have boxes of them cheap Battery chargers - unplug when operating Light dimmers and motor speed controllers - replace with regular lights switch or non digital speed controller Plasma TVs Electric fences - often arc to weeds 19

20 Outside of coax can act as antenna, picking up noise A good choke balun will help in these cases - a balun should not only be at antenna, but another in the shack as close to the rig as possible is also a good idea Verticals are often thought of as noisy, but much of that is due to noise pickup on the outside of the coaxial cable - the braid of the coax is acting as a radial, which is part of the antenna There should not only be a choke balun at the antenna, but also one where the coax exits the radial field and one at the rig 20

21 Locating radiated noise sources Use a portable AM receiver or transceiver and turn off AGC Use a directional antenna if possible Often driving down the street with an AM radio on an empty frequency can identify the general area VHF is useful as a small handheld Yagi can be used to pinpoint noise sources 21

22 Power line noise PNM has a reasonably competent expert to deal with noise He is overworked and doesn t have a work crew assigned to him, so even if he identifies the noise, it can take a long time to fix it If you can pinpoint the source and give them a pole number, that will speed things up They have a legal obligation to resolve noise problems 22

23 Solar installations can be noisy Usually the problem is with the inverter Manufacturers should have solutions for this A heavy dose of toroids can help in many cases April 2016 QST article on this Avoid microinverters There are ham savvy solar installers that can help with keeping new installations 23

24 Incandescent Light bulb replacements LEDs are generally pretty quiet - buy one to make sure Cost is going down Compact Fluorescents (CFLs) can be noisy, but not all are Replacing noisy fluorescents with LEDs is a good strategy and is now reasonable in cost 24

25 Dealing with conducted noise AC noise filters are available commercially Use one at station AC Use on at offending device, often a motor or controller DC filters are also available, but because of the current requirements are often not cheap 25

26 When the noise source cannot be eliminated Some rigs have effective noise blankers or noise reduction software Different rigs have different noise blanking/reduction capabilities, some are good with some types of noise, others are good with other kinds of noise Noise elimination at rig often comes at a cost, noise blankers often reduce dynamic range, digital noise reduction often colors or distorts signal. Often these are less annoying than the noise though Phasing noise reduction devices, such as the Timewave ANC-4 can be effective if the noise source is localized Directional antennas, while not practical on all bands, are very effective on localized noise sources 26

27 If you find a particularly noise device, let the ARRL know, also, if you have a success story let them know as well 27

28 Unfortunately, living in a urban environment means more noise, as does the ease with which devices that do not meet FCC requirements can be imported 28

29 Resources used in preparing this talk ITU-R P Radio Noise NBS Circular 688 YU7EF Web page < > W8IO web page < > VE7BQH Gain tables < w7gj/6mtable.htm > 29