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Old 18-Sep-2015, 8:02 PM   #6
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,738
Quote:
I found that virtual channel 35.1 is 596 to 602 MHz
Quote:
36.1 is 602 to 608 MHz. close enough?
Well, yes, but when you design an antenna for a certain channel you should use the real channel number and not the virtual channel number. The real channel frequency is what the transmitter uses. The virtual channel number is a holdover from analog days to maintain the identity of the station for the viewer. In this case, the real channel numbers and virtual channel numbers are the same, but it isn't always that way.

I did a profile for a location further north at Roxborough Ave and Cope St. The signal just clears the escarpment:



Quote:
I wonder about what might be involved with respect to azimuth in order to capture RF within the Fresnel zone. (as you suggested) If you think that might work, what degree of tilt would you suggest?
I estimate the tilt at +6 to 7 degrees. Hams that do satellite contacts use an AZ/EL mount for their antenna. The AZ is for azimuth aim and the EL for the tilt.

There aren't many antennas that have a tilt adjustment, but the Antennas Direct 91XG does.

Keep in mind that the wavefront presented to the antenna might not be uniform. Sometimes making a small adjustment in the location of the antenna can make a difference, like raising the or lowering the antenna a 6" to 12". When I was doing a temporary setup across the street I setup a 2-bay UHF antenna, my Sadelco DisplayMax 800 signal level 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. Another example of layering in a non-uniform field. This is most likely because of the tree line in front of the antenna about 200 ft away.

When I was doing another temporary setup with a 4-bay UHF antenna, I noticed that it was necessary to tilt the top of the antenna back a little for max signal.

When I adjust the aim of an antenna, I use my signal level meter (SLM) for max signal. I then adjust the antenna aim for max signal quality as defined by SNR and errors; they don't always happen at the same azimuth. When I was first using my Apex DT502 converter box just after the transition to digital TV, I noticed that difference. The Apex has two signal bars, one for signal quality and one for signal strength. I now use my Sony TV that has a Diagnostics Screen that gives signal strength, SNR and errors.
Attached Images
File Type: jpg 4.6 ExplorerTVFprofile4.JPG (123.4 KB, 2073 views)
File Type: jpg CH42setup.jpg (132.5 KB, 4224 views)
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Last edited by rabbit73; 19-Sep-2015 at 10:54 PM.
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