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SWL for Newbies (was: Balcony Antenna for Shortwave Listening)
On 13 dic, 22:01, Sébastien MEDARD wrote:
On Sun, 12 Dec 2010 12:25:59 -0800, Richard Clark wrote: On 12 Dec 2010 13:59:06 GMT, Sébastien MEDARD wrote: What you are talking about is common to DXing. *Get yourself a globe of the world. *Light it up with an artificial "sun." *The edge of the dark/light should be half way between you and the transmitter you want to hear. *So, during the late day, your best signals are probably from Eastern Europe, Turkey, and Asia. *During the early-to-late night, your best signals are probably from the Americas. *During the late afternoon to early evening you best signals are from the south or the north. I understand better what is the purpose of this kind of map (on SDR- Radio.com software for example). Where you can see an OM, you are pretty sure to see this kind of map. I need to know a little bit more about digital transmission modes... To be able to recognize digital patterns (on the waterfall display) to be able apply the good settings for decoding... Easy for CW... Easy for RTTY used by HAM... But what is the pattern for Meteo Faxes for example...? When I used to teach RTTY there were only two tones. The reason why they call you OM ? ) By the way... I searched for good free software to decode meteo faxes.... But it seems not easy to find something free and easy to use... Do you know shere I could find something interesting? Nope. OK. I will try that later... For now I get some problems to use Linrad with the SDR-IQ... Painful to boot on windows every time I want to listen to shortwaves... And using a virtual system is not very efficient on my old computer... My next tries will be on a multiple turns loops. With or without matching loop inside. I will try with simple wire but connection ribbon cables could be a good alternative... (like old hard drive ribbon cable) This will reveal the "Law of Diminishing Returns" (the more you add, the less you get back). Interesting. What would be the best ratio? I was wondering... It seems loop antennas are a never ending story. So, there you have a century of technical perspective on the topic. ) Perhaps the "Joe Carr's Loop Antenna Handbook" may be interesting, but it is quite expensive and an Amazon reviewer says it is not so interesting... What about the "Joe Carr's Receiving Antenna Handbook" ? He is a respected author. *I always look at the bad Amazon reviews to see if something is terribly wrong with the product or terribly wrong with the customer. *Quite often the customer is disappointed because that customer is stupid. *If I encounter a smart customer that is disappointed, I can well imagine all the good comments are not very reliable. I will try to find one when they are cheaper. The thing about all these -um- books is that many of them serve up old wives' tales about low noise and other magical characteristics. Remember the lessons of a century worth of experience with loop antennas within an inch of the guts of any radio. *That worked fine for a billion radios and radios were not bought if they were "noisy." OK. But today we probably ask more to radios than yesterday, no? The real science of low noise is found in the distance to the source. Lapalisse would have said exactly the same thing. He would have said that the real science of good reception is found in the distance to the source too ) Is the loop quieter because of its magic design - or simply because it is 10 times further away (20dB) from the noise? OK for that. We get writers here who pound the table because their magic loop in the shack is noisy and their long wire 10M away in the back yard is quiet. They have to be told to turn off the flourescent light in the shack if they want their magic loop to work. * *(Their magic loop low score on Amazon) = (stupid.) Lesson one = learned. Thanks. Sebastien. Hello Sébastien, Regarding tuning, loops and (long) wires. You probably locate your antenna far away from noise sources. Mostly this is not where your receiver is. In case of tuned loops, you have to go outside for each significant frequency change. This is one of the reasons that I want my preselection / tuning adjacent to the receiver. Loops aren't magic things. A wire from your balcony to a tree or other tie point may give better S/N ratio then the best (expensive) loop on your balcony. Best matching isn't required for reception at HF, just sufficient signal. In case of a PI tuning / preselector device, you need to adjust two capacitors in case of major frequency change. In my opinion a single capacitor device is more pleasant to use. (deep) Nulls in the radiation pattern at low frequency (AM BC and below) are the main benefit of the loop if you are willing to change the orientation frequently when tuning across the bands. At increasing frequency, the nulls are less pronounced because of propagation issues. Off course a small horizontally oriented dipole has also figure-of-eight radiation pattern, but polarization isn't good for AM BC and lower. For experimentation / comparison, I used a 1m diameter non-tuned loop in the attic with preselection / tuning adjacent to the receiver. In most cases a 5 m outdoor wire with same preselector and receiver gives similar or better S/N ratio. This may be because of the attic has mains wiring and I didn't want to go to the attic for reorientation of the loop. If you want to experiment with some long wire (unbalanced) antennas, make sure to have a ground (counterpoise) provision outside; otherwise your coaxial cable will be part of the antenna. This may result in more interference from indoor sources. Whether or not you have sufficient signal from your antenna is easy to check. Listen to a known station. Tune your preselector to maximum signal. Now tune to a free frequency close to your station's frequency so that you hear noise. Remove the antenna, the audio noise should be less now. If you use a tapped coil preselector (like the one in my link), and have sufficient noise from the antenna, you may even change the taps. The preselector will give more insertion loss, but it will be more selective also. This can be of use when you have strong signals in your neighborhood. Because of portability, I frequently use an AOR AR8200. Without preselection, such a wide band receiver is useless with an external antenna. Best regards, Wim PA3DJS www.tetech.nl without abc, PM will reach me very likely |
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