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Old 20-Sep-2016, 11:36 PM   #9
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,450
Quote:
I mean if I move the antenna 5-feet to the right or left it can make the difference between 40% signal for WCAX and nothing. Oddly enough, in some locations I can receive WCAX at 5ft-AGL, but when I raise the antenna tripod up to 8ft-AGL, reception dies, until I move to the right or left about 6 feet. I have confirmed that this is a spatial anomaly somewhat repeatable from day to day, not a temporal one. Very strange behavior, but I figured it would go away when I permanently mounted the antenna on the roof at 30ft.
I have noticed the same thing at my location. The wave front presented to the antenna has a non-uniform field.

I was having trouble receiving CH 42 because my antenna was facing the wrong direction. I went across the street and setup a 2-bay UHF antenna, my meter, and a preamp. I was able to get a nice scan and a stronger signal with the antenna aimed at the transmitter for CH42.

Interestingly, when I moved the antenna a few feet left or right, without changing the height or azimuth, there was a big difference in the signal strength and scan quality. This is most likely because of the tree line in front of the antenna about 200 ft away which created the non-uniform field.



I understand the need to mount the antenna in a location that "looks nice," but the antenna must be mounted where the signal is strongest, which is often not where you want it. Antenna installers call it walking-the-roof, signal level meter and test antenna in hand, to find a hot spot.

There are several possible things going on with the height. When the antenna is close to the ground, it can receive a reflection of the signal from the ground which is added to the direct signal in phase; called "ground bounce." When you raised the antenna, you lost the reinforcement from the reflection and your antenna was then behind the electrostatic field that surrounds the power line from the pole to the house.

Quote:
If this is a multi-path "null", would an antenna with a wider capture area (e.g. DB8e) improve reception of WCAX?
Maybe not.
http://www.hdtvprimer.com/antennas/siting.html

scroll down to Non-uniform fields
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Last edited by rabbit73; 21-Sep-2016 at 12:00 AM.
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