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Post by Druid Hills Radio on May 11, 2018 17:51:09 GMT
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Post by Boomer on May 12, 2018 23:05:58 GMT
A ham friend was getting great DX openings on 28 mhz ham frequencies this morning, I don't know if that's related to ducting, but it seems upper bands are hopping.
I've heard it many times on FM, since getting my first stereo. I always liked how AM skipped at night, but read that FM was only line of sight transmission, so I was surprised to get clear reception of stations from other states in between the locals every so often, usually in the summer.
More often I hear changes in propagation on fringe stations close by, where a station is in the static most times, but one night it will come in stronger for hours, almost like they increased the power, then the next day it will be hack in the hiss again. I don't know if it's ducting or what causes it, but I've heard it many times.
Boomer
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Post by thelegacy on May 13, 2018 4:22:12 GMT
Boomer what you are experiencing is known as a temperature inversion or ducting effect. This is when far away stations come in almost as good as the locals do.
When I was doing my FM thing I would tell people especially when they would do slightly over part 15 limits to watch out for temperature inversions because those are the conditions that would make their FM transmitter travel many miles further than what they intended it to travel. Again when I was doing FM I only intended it to go about 2 miles. But the day that I got snagged there was a temperature inversion and I forgot to check. My station got out 21 miles which doesn't happen during normal situations.
Looking at 100.1 megahertz there is a translator around Norfolk but it's very weak and during normal conditions does not come in. But if I go around 21 miles south of where I live that translator can be heard. So when my signal was going further than what I had planned guess who I was interfering with? Well anyway that was one reason I know that I got a visit from the FCC. These conditions are very fun to listen to though getting back to the topic I do like to see how many far away stations I can pick up during the times where the temperature jumps many degrees within a few hours.
Sometimes on FM you can pick up stations for example living in Michigan you might hear some Texas stations. Yes you've heard right it is possible to do that on frequencies that are not overrun by locals but I have actually heard far away stations obliterate the locals during these conditions. Sporadic e layer can cause that and it's a lot of fun you should get into the hobby of dxing is very enjoyable and you will have hours of fun doing it.
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on May 13, 2018 12:28:44 GMT
... what you are experiencing is known as a temperature inversion or ducting effect. This is when far away stations come in almost as good as the locals do. ... My station got out 21 miles which doesn't happen during normal situations. ... Read more: alpb.boards.net/posts/recent#ixzz5FNtmqyfvHowever, such distant signals produced by ducting or temperature inversions are still subject at least to the loss accruing over the physical lengths of their free space propagation paths -- which adds 6 dB loss every time the path length doubles. So a signal that is exactly compliant with FCC §15.239 (250 µV/m @ 3 meters from the transmit antenna) would drop to 0.022 µV/m at the end of a 21 mile, line-of-sight, free space propagation path. An FM signal that weak would be nearly impossible to detect even by the FCC, let alone interfere with the reception of the local FM broadcast signals present at that location. It would take a radiated power of about 1.46 watts to produce a 250 µV/m signal at the end of a free space path of 21 miles. But still that signal would be unlikely to interfere there with local, licensed FM stations of any class of service. |
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Post by thelegacy on May 13, 2018 21:34:11 GMT
What about 7 Watts.
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Rich
Full Member
RF Systems Engr (retired)
Posts: 112
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Post by Rich on May 14, 2018 8:13:44 GMT
Other things equal, the received field will change by the square root (√) of the change in radiated power. In this case, radiated power changes by a factor of 7/1.46 = 4.8 (approx), and √4.8 = 2.19. So the new value of the field intensity existing at a point 21 miles away, and for the previously-stated assumptions, will be 250 µV/m x 2.19 = 547 µV/m, approx. Much more accurate answers can be given for point-point propagation paths near the surface of the earth when/if the GPS coordinates for the ends of the path, and the heights of the transmit and receive antennas above the surface of the earth are used in the analysis.
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