I watched a cable TV system installation on a 300 ft tower many years back, and it was interesting to see where the receive antennas were placed. We had channels 4,5,8,11, and 13.
The tech took a portable antenna and receiver up the tower and watched the signal strength for each channel as he climbed, and placed a marker on the tower for the FIRST peak he found in signal strength. He started on ch 4 and that antenna position was at about 100 ft. We were about 70 miles from the TV stations. As he worked his way up the channels the ch 13 antenna wound up at about 200 ft. All these points were at the first peak he found going up the tower. As he climbed above that point, the signal strength would go down for a given channel, as the ground reflection was starting to cancel out the direct signal, or was no longer adding to the direct signal, whichever way you want to look at it. I thought this might be interesting to the group - 73 - Jim W5ZIT --- On Sat, 8/9/08, Jeff DePolo <[EMAIL PROTECTED]> wrote: From: Jeff DePolo <[EMAIL PROTECTED]> Subject: RE: [Repeater-Builder] Height Gain figure To: [email protected] Date: Saturday, August 9, 2008, 4:25 PM > Where can you experience a situation where you have > line-of-sight, but poor > fresnel clearance, and have no communications path (assume > VHF and UHF, not > microwave)? Is there ever a case? A couple of simple examples: 1. If you have an exactly-wrong height as far as Fresnel clearance goes, you have complete cancellation. Hard to realize in the real world though, at least not a consistent basis, but you can get close... 2. Ducting and inversions that "bend" the signal, either horizontally or vertically. I have 950 MHz STL paths over water and marshland with full LOS and Fresnel clearance that, when conditions are just right, will go from > 1000 uV to below squelch threshold (about 5 uV), about a 50 dB drop. 3. Multipath. Signals arrive out-of-phase at equal amplitude. 4. Antenna pattern issues, nulls, etc. Probably self-explanatory, and obvious. > We've got a link path that an analysis indicates "absolutely > no way" yet it > works just fine (UHF). The obstructions are numerous and > large, but they are > some distance from either site. Then I've seen other paths > (UHF) where a > somewhat mild obstruction causes grief, but the obstruction > is close-in to > one of the sites. As far as path obstructions go, diffraction is one of the biggest factors. The further from either endpoint the obstruction is, the more of a chance diffraction will work in your favor, "bending" the signal over the obstruction. Think of it as a "shadowing" or "shielding" effect - if you're up close to the obstruction, the shadowing effect is much more pronounced as compared to if you are further away from it. In a bit of a hurry, sorry for the short responses. --- Jeff WN3A __
