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Light,Lazers and HF
What is the main factor that prevents HF radiation from focussing
for extra gain? |
#2
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Light,Lazers and HF
Art Unwin wrote:
What is the main factor that prevents HF radiation from focussing for extra gain? Money. If you can afford to build a 20m parabola about 2,000 feet in diameter and the place to mount it, you'll get lots of gain. -- Jim Pennino Remove .spam.sux to reply. |
#3
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Light,Lazers and HF
On Sep 10, 8:45*pm, wrote:
Art Unwin wrote: What is the main factor that prevents HF radiation from focussing for extra gain? Money. If you can afford to build a 20m parabola about 2,000 feet in diameter and the place to mount it, you'll get lots of gain. -- Jim Pennino Remove .spam.sux to reply. Then are you saying it is the antenna size that is the main factor?. So my antenna which is physically small can be focussed on a dish which would provide straight line radiation or a radiation beam? Working on a single element on the ground with a optimizer instead of a half sphere I got a straight vertical line at the sides which suggested a gun barrel radiation with a perfect earth as the reflector. Gain was around 8db vertical which is why the question regarding focussing! If it was properly focussed the gain should be more. 2000 foot dish seems somewhat odd, probably based on a "straight" wavelength and not a small volume in equilibriumas the directer right? Art |
#4
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Light,Lazers and HF
On Sep 10, 9:23*pm, Art Unwin wrote:
On Sep 10, 8:45*pm, wrote: Art Unwin wrote: What is the main factor that prevents HF radiation from focussing for extra gain? Money. If you can afford to build a 20m parabola about 2,000 feet in diameter and the place to mount it, you'll get lots of gain. -- Jim Pennino Remove .spam.sux to reply. Then are you saying it is the antenna size that is the main factor?. So my antenna which is physically small can be focussed on a dish which would provide straight line radiation or a radiation beam? Working on a single element on the ground with a optimizer instead of a half sphere I got a straight vertical line at the sides which suggested a gun barrel radiation with a perfect earth as the reflector. Gain was around 8db vertical which is why the question regarding focussing! If it was properly focussed the gain should be more. 2000 foot dish seems somewhat odd, probably based on a "straight" wavelength and not a small volume in equilibriumas the directer right? Art Let me ask the question another way. Whether it is believed or not, if a 80 Metre antenna was compressed to the size of a couple of shoe boxes would the dish be reduced in size accordingly? Regagards Art |
#5
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Light,Lazers and HF
"Art Unwin" wrote in message ... What is the main factor that prevents HF radiation from focussing for extra gain? HF can be focused for gain. Main factor is that it takes a big antenna to focus a signal on 3.5 MHz or so. Even a 2 or 3 element beam is too large for most people , but it has been done. To get any gain from a dish at that frequency would take a dish larger than 400 to 500 feet in diameter, maybe much larger. Hard to put that up a tower. |
#6
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Light,Lazers and HF
Art Unwin wrote:
On Sep 10, 8:45?pm, wrote: Art Unwin wrote: What is the main factor that prevents HF radiation from focussing for extra gain? Money. If you can afford to build a 20m parabola about 2,000 feet in diameter and the place to mount it, you'll get lots of gain. -- Jim Pennino Remove .spam.sux to reply. Then are you saying it is the antenna size that is the main factor?. Of course. Everyone knows the gain of a parabola is directly proportional to the size in wavelengths, or: G=10*log k(pi*D/L)^2 Where G= gain in DB over an isotropic, k ~ .55 for most real parabolas, D is the diameter, and L is the wavelength (wavelength and diameter in the same units. So a 2,000 foot parabola on 20m would have just about 58db gain. -- Jim Pennino Remove .spam.sux to reply. |
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Light,Lazers and HF
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#8
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Light,Lazers and HF
On Sep 10, 10:29*pm, Art Unwin wrote:
On Sep 10, 9:23*pm, Art Unwin wrote: On Sep 10, 8:45*pm, wrote: Art Unwin wrote: What is the main factor that prevents HF radiation from focussing for extra gain? Money. If you can afford to build a 20m parabola about 2,000 feet in diameter and the place to mount it, you'll get lots of gain. -- Jim Pennino Remove .spam.sux to reply. Then are you saying it is the antenna size that is the main factor?. So my antenna which is physically small can be focussed on a dish which would provide straight line radiation or a radiation beam? Working on a single element on the ground with a optimizer instead of a half sphere I got a straight vertical line at the sides which suggested a gun barrel radiation with a perfect earth as the reflector. Gain was around 8db vertical which is why the question regarding focussing! If it was properly focussed the gain should be more. 2000 foot dish seems somewhat odd, probably based on a "straight" wavelength and not a small volume in equilibriumas the directer right? Art Let me ask the question another way. Whether it is believed or not, if a 80 Metre antenna was compressed to the size of a couple of shoe boxes would the dish be reduced in size accordingly? Regagards Art- Hide quoted text - - Show quoted text - No. The shoebox size antenna would approximate an isotropic if it did radiate. It would still have to be placed at the focal point of a very large parabola due to the size of the wave length. Such an antenna, I believe, on the island of Puerto Rico (the SETI antenna) although it is currently used primarily as a receiving antenna. That parabola is positioned to have a very high radiation angle and might not be be that good for terrestrial DX. |
#9
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Light,Lazers and HF
Roy Lewallen wrote:
wrote: Of course. Everyone knows the gain of a parabola is directly proportional to the size in wavelengths, or: G=10*log k(pi*D/L)^2 Where G= gain in DB over an isotropic, k ~ .55 for most real parabolas, D is the diameter, and L is the wavelength (wavelength and diameter in the same units. So a 2,000 foot parabola on 20m would have just about 58db gain. Hm. I get 47. Roy Lewallen, W7EL Hmm, when I use 14 Mhz and 6 decimal places I get 37; must have fat fingered it the first time. Working backward from 47 I get a wavelength of 21 feet. -- Jim Pennino Remove .spam.sux to reply. |
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Light,Lazers and HF
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