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Link to top of Page Noise By Tom W8JI The noise that limits our ability to hear a weak signal on the lower bands is almost always an accumulation of many signal sources. Below 18 MHz, the noise we hear on our receivers ( even at the quietest sites) comes from terrestrial sources. Receiver noise is generally a mixture of local groundwave and ionosphere propagated noise sources, although some of us suffer with dominant noise sources located very close to our antennas. Our locations fall into three basic "radio" categories that may or may not be related to our actual communities:
Urban: In urban-type noise situations, noise arrives from multiple random sources through direct and groundwave propagation from local sources. One or more sources can actually be the induction-field zone of our antennas (in most cases the induction field dominates at distances less than 1/2l). Urban locations are the least desirable locations because typical noise floors average 16dB higher than suburban locations. There is often no evidence of winter night noise increase on 160 meters, since ionosphere-propagated noises are swamped out by the combined noise power of multiple local noise sources. Much of the noise sources are utility distribution lines, because of the large amount of hardware required to serve multiple users. Other noise sources are switching power supplies, arcing signs, and other unintentional man-made noise transmitters.
Suburban: Suburban locations average about 16 dB quieter than urban locations, and are about 20 dB noisier than rural locations. Noise generally is directional, arriving mostly from areas of densest population or the most noise-offensive power lines. Utility high-voltage transmission lines are often problematic at distances greater than a mile, and occasionally distribution lines can be problems. The recent influx of computers and switching power supplies has added a new dimension to suburban noise. There is often a small increase in night time winter noise at exceptionally quiet suburban locations. This increase occurs when propagated terrestrial noise equals or exceeds local noise sources.
Rural: Rural locations, especially those miles from any population center, offer the quietest environment for low-band receiving. Daytime 160 meter noise levels are typically around 35-50 dB quieter than urban, more than 20 dB quieter than suburban locations. Nighttime brings a dramatic increase in low-band noise, as noise propagates in via the ionosphere from multiple distant sources. Primary local noise sources are electric fences, switching power supplies, and utility lines. I can measure a 3 to 5dB daytime noise increase in the direction of two population centers, Barnesville (population 7500, distance 6 miles) and Forsyth (population 10,000, distance 6 miles) Georgia. Typical daytime noise levels, measured on a 200 foot omni-directional vertical, are around -130 dBm with a 350 Hz bandwidth (noise power is directly proportional to receiver bandwidth) . Noise power increases about 5 to15 dB at night, when the band "opens". As in the case of suburban systems, directional antennas reduce noise power. Nighttime is the "big equalizer", reducing
the advantage of location as distant noises increase with improved
propagation.
The research, conducted by a team led by Dr. Lene Hau, a Harvard physics professor, is one of 12 research projects featured in a special edition of Scientific American entitled "The Edge of Physics," available through May 31. In their laboratory, Hau and her colleagues have been able to slow a pulse of light, and even stop it, for several-thousandths of a second. They've also created a roadblock for light, where they can shorten a light pulse by factors of a billion. The speed of light is approximately 300,000 kilometers per second (about 186,000 miles per second or 670 million miles per hour). Some substances, like water and diamonds, can slow light to a limited extent. More drastic techniques are needed to dramatically reduce the speed of light. Hau's team accomplished "light magic" by laser-cooling a cigar-shaped cloud of sodium atoms to one-billionth of a degree above absolute zero, the point where scientists believe no further cooling can occur. Using a powerful electromagnet, the researchers suspended the cloud in an ultra-high vacuum chamber, until it formed a frigid, swamp-like goop of atoms. When they shot a light pulse into the cloud, it bogged down, slowed dramatically, eventually stopped, and turned off. The scientists later revived the light pulse and restored its normal speed by shooting an additional laser beam into the cloud. The first slow-light breakthrough for Hau and her colleagues came in March 1998. Later that summer, they successfully slowed a light beam to 38 miles per hour, the speed of suburban traffic. That's 2 million times slower than the speed of light in free space. By tinkering with the system, Hau and her team made light stop completely in the summer of 2000. END of NASA JPL article - now for my thoughts As light is just very high frequency RF, Imagine if this 'goop' could be made into paint, you could build a 160M beam the size of a 70 cm beam! Imagine a 5 element 160M beam you could easily mount on your car roof! Paul VK3TGX Never Take Life Seriously. Gardening Rule: When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant. The easiest way to find something lost
around the house is to buy a replacement. Never take life seriously. Nobody gets
out alive anyway. There are two kinds of pedestrians --
the quick and the dead. An unbreakable toy is useful for breaking
other toys. If quitters never win, and winners never
quit, then who is the fool who said, "Quit while you're ahead?" Health is merely the slowest possible
rate at which one can die. The only difference between a rut and
a grave is the depth. Get the last word in: Apologize. Give a person a fish and you feed them
for a day; teach that person to use the Internet and they won't
bother you for weeks. Some people are like Slinkies . . . not
really good for anything, but you still can't help but smile when
you see one tumble down the stairs. Have you noticed since everyone has
a camcorder these days no one talks about seeing UFOs like they
use to? All of us could take a lesson from the
weather. It pays no attention to criticism. Why does a slight tax increase cost you
two hundred dollars and a substantial tax cut saves you thirty cents? In the 60's, people took acid to make
the world weird. Now the world is weird and people take Prozac to
make it normal. How is it one careless match can start
a forest fire, but it takes a whole box to start a campfire? Amateur Radio to be Exempt from Ban on Driving While Using Mobile Phone The Department of Transport has issued a paper on the results of its consultation on a proposal for the introduction of an offence of using a hand-held mobile phone while driving. The consultation document suggested that "two-way radio microphones" should be included within the proposed ban, which would effectively have banned mobile amateur radio in the UK. In its submission to the consultation document, the Radio Society of Great Britain pointed out that amateur radio had been operated from motor vehicles since 1955 without any accidents being recorded. The Department of Transport report on the results of the consultation states that, "Amateur radio operators, some commercial drivers such as taxi drivers and hauliers, and some of the emergency services use [two-way radio] to communicate with a base station. We accept that such 'press to talk' devices keep conversations short and are likely to have a lower risk. . . While the details of the extent of the exemption remain to be determined, the new offence will exempt the use of such devices." Link to top of Page Switzerland May be First Country to Implement Code-Free HF Licences We reported in GB2RS last week on the major decisions made at the World Radiocommunication Conference in Geneva. These include the immediate removal of the mandatory international Morse code requirement for HF-band access, and the welcome news of an additional 100kHz of spectrum for radio amateurs at 7MHz to be introduced in 2009. Here in the UK, the Radiocommunications Agency has already stated that it will bring in code-free HF licences as soon as it is practical to do so. An announcement will be made on GB2RS and on the RSGB website when Full and Intermediate Class B amateurs can start using the HF bands. However, it looks like Switzerland might be the first country in the world to take advantage of the revised Radio Regulations. According to the website of the USKA, the Swiss national amateur radio society, the licensing authority there will be writing to all Swiss CEPT Class 2 amateurs in the next few days to give them "provisional authorisation" to start using the HF bands with immediate effect. (From the Radio of Great Britain Web
site) Rocket Science? ... ...This actually happened and just as a little extra info to his joke, the gun that fired the chicken was made at Cranfield.....!
Scientists at Roll Royce built a gun specifically to launch dead chickens at the windshields of airliners, and military jets, all traveling at maximum velocity. The idea is to simulate the frequent incidents of collisions with airborne fowl to test the strength of the windshields. American engineers heard about the gun and were eager to test it on the windshields of their new high speed trains. Arrangements were made, and a gun was sent to the American engineers. When the gun was fired, the engineers stood shocked as the chicken hurled out of the barrel, crashed into the shatterproof shield, smashed it to smithereens, blasted through the control console, snapped the engineer's back-rest in two and embedded itself in the back wall of the cabin, like an arrow shot from a bow. The horrified Yanks sent Rolls Royce the disastrous results of the experiment, along with the designs of the windshield and begged the British scientists for suggestions.
You're going to love this......
Rolls Royce responded with a one-line memo: "Defrost the chicken." courtesy Toni (Graeme VK3XTA,s better half) Exercise for Seniors Keith VK3VKS and Shirley will shortly be moving back to Darwin to live. Keith has received an offer of work with in his words "an offer he could not refuse". Keith expects to fly north on the 11th of August. As a way of saying thanks for his input into the clubs life and especially all the work towards the hamfest, it would be good if members and there families could come to a farewell at the guide hall on Friday 1st August at 7.00pm to say there goodbyes. We will arrange nibbles and sweets at the next meeting. Link to top of Page
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