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Old 14-Jun-2011, 3:17 PM   #20
Dave Loudin
Senior Member
Join Date: Jan 2010
Location: King George, VA
Posts: 659
You've received a torrent of advice on what to do, so let me help you recap:

The bulls-eye chart depicts a combination of strength and direction of signals. The longer the bar, the stronger the signal. If you look carefully, you'll see the bar ends at the color where you'll find that station's details in the table below. Stations in green are projected to have lots af signal in the air to the point an indoor antenna may be used.

In the table are the details you need to pick an antenna. First is the "real channel" column that lists the channel actually used to transmit the signal. (The virtual channel provides a way to preserve a station's identity. A station that broadcast on Channel 9, say, in the analog days can still be tuned as "9" even though the digital signal may have ended up on another channel.) Second, is the Noise Margin column, which tells you how much signal is predicted to exist referenced to the signal necessary for decoding by a receiver.

Good design practice that accounts for interference and signal fading modes not accounted for in this site's prediction model leads to design for +15 dB NM. Since you are mounting outside, there are no losses for going through walls and the like (often 20 dB or more), so, even accounting for 5 dB loss in the cable, you don't need any antenna gain to capture enough signal for your TV. Hence my recommendation for a small UHF antenna. Going a little overboard with antenna gain does not hurt anything, to a point. Once you have enough signal to properly drive your TV and overcome the unusual propagation events and local noise, the picture does not get any better.

ADTech knows your area and knows more than any of us about the antennas you ordered. He is telling you 1) the very last bit of terrain may knock the signals down more than what is predicted. Having the extra gain of the CS 4 may be needed, and 2) antennas do not have zero sensitivity outside their design range. You can see from the PDF document that he linked to that the C5 will respond to UHF frequencies and deliver most of what it captures into the cable downlead. Most importantly, the C5's gain over the UHF band in the forward direction (+- 20 degrees from center) never dips below -6 dBi.

What does this mean for you? Let's consider the case where TVFool's predictions are reasonably accurate for your side of the lake. The strongest VHF signals are KQTV (-6.3 NM), KTWU (-18.4 NM), and WIBW (-21.7 NM). The C5 has 5 dBi gain at channel 7, and 7 dBi at the others. Doing the math (including 1 dB cable loss) leaves you with NMs of -2 for KQTV, -12 for KTWU, and -15 for WIBW. This means you will see KQTV on occasion (maybe 30 to 40% of the time) and the other two stations rarely if at all. On the UHF side, the weakest station, WDAF, is still predicted to have an NM of +34. Assuming the worst case gain of the C5 (-6) and 3 dB cable loss, you will still have an NM of +25, which should be rock-solid reception. Bottom line, with the C5 only you would get all the UHF stations, some bonus reception of KQTV, and FM radio.

If your situation is what ADTech fears, then lets try to model that by shaving 25 dB off the available signals. The VHF stations will be totally closed to you, and the CS4 would be the antenna that would fit the bill. You would have UHF only reception.

Last edited by Dave Loudin; 14-Jun-2011 at 3:19 PM.
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