Help with Reception / Antenna Setup

[cmihai]

Originally Posted by Billiam

cmihai. See if a friend or neighbor or two will bring over a HDTV for you to set up to compare. If you still don't get this channel on at least two other televisions (preferably a different brand from your existing unit) then we could probably rule out the problem is with your tv tuner. Start here first. This is a cheap way to troubleshoot the problem.

This is a great idea to rule out the TV tuner Billiam, although it might be difficult to implement - I'll see what I can do.

I really want to try GroundUrMast's suggestion this evening, to remove the splitter, as it is incredibly easy to implement.

Would get back to you when I'll have new information, do not want to start another fire. Thanks again!

[GroundUrMast]

Back to your opening post:

How can this be explained? Do I need a more powerful preamp?

... and a subsequent post:

Is is possible to know in advance what the ideal preamp gain should be? On one side we have cable loss (which is relatively straightforward to calculate). A few db on top of that should be fine, however too much would lead to overloading. How can someone figure out the sweet spot?

The explanation has already been offered, there is more to reliable reception than signal strength. Noise, interference and multipath are all potential challenges. An amplifier has no ability to remove or correct any of these transmission impairments because they occur before the signal arrives at the amplifier. You are correct that an amplifier is able to overcome cable and splitter losses. An amplifier can also aid in the case of poor receiver noise figure if the signal going into the amplifier has sufficient noise margin and the amplifier does not distort the signal or add excessive noise.

Look at your TV Fool report... you'll see that many signals are predicted to arrive at your location at power levels as high as -21 dBm. The tuner should be able to automatically adjust it's input gain to deal with the wide range of signal levels that may be present. An amplifier offering 29 dB of gain will not be at risk of over-amplifying a signal arriving at -80 dBm... It will however cause trouble when fed a signal at -25 dBm. (The 'sweet spot' is quite broad.)

If there is a solution to reliable reception of weak, distant signals... it lies in the selection and placement of the antenna. Quit looking to an amplifier as a fix for a too small, or poorly located antenna. If reception of WUTV is of upmost priority to you, select an antenna that has maximum performance at the frequency of real channel 14. The Winegard HD7698P and the Antennas Direct XG-91 come to mind... as does the enormous Wade PB-82-BB (8 feet by 16 feet and 155 lbs... and a bunch of money). Also look at placement options that would improve the reception conditions... possibly tower mounting.

[cmihai]

Allow me to ask a dumb question: should I rescan after a / any change in antenna's direction? I've only performed this step after significant changes in the overall setup (e.g. added the preamp or splitter). Does a change in direction warrant a full channel rescan? Thanks!

[GroundUrMast]

Originally Posted by cmihai

Allow me to ask a dumb question: should I rescan after a / any change in antenna's direction? I've only performed this step after significant changes in the overall setup (e.g. added the preamp or splitter). Does a change in direction warrant a full channel rescan? Thanks!

If available, scan for additional channels.

Most TV's seem to prevent the user from directly tuning the real channel number... a feature the cable companies would like outlawed I suspect. Hopefully your TV allows scanning for additional channels. Some sets force you to rescan, loosing all previous channels (the cable company's preferred feature set because it make OTA reception more difficult).

[cmihai]

Sorry for not being explicit enough. I have Fox in the list from previous scan, however current level is 0 while peak level is 98. Status reads Unlock, SNR (dB) is 0 and AGC (%) is 62. Since the channel is in the list, do I need a rescan? Thanks again!

[GroundUrMast]

Originally Posted by cmihai

Sorry for not being explicit enough. I have Fox in the list from previous scan, however current level is 0 while peak level is 98. Status reads Unlock, SNR (dB) is 0 and AGC (%) is 62. Since the channel is in the list, do I need a rescan? Thanks again!

It sounds like the answer is 'no, you don't need to rescan'. With an indicated SNR (Signal to Noise Ratio) of 0 dB, you have significant evidence of low signal level, noise, interference, multi-path or some combination of those.

But sometimes, after trying everything else...

[cmihai]

Originally Posted by gcd0865

...I've seen Tower Guy's interesting calculations of total signal strength (but not sure how to calculate it myself from my TVFool report)...

If I got it right, here's how GroundUrMast calculated signal strength from TVFR. Keep in mind that Noise Margin (NM) is another important metric to look at.

Start from the absolute Signal Power level (dBm) on your TVFR, e.g. WDIV-TV on 4.1 virtual channel at -23.3 dBm - this is the signal level at the antenna input (in the air at your location).

Add your antenna gain for that channel / frequency (or a conservative average gain if not known), let's assume +10 dBi (it looks like you do not need to translate from dBi to dBm): you should have -13.3 dBm signal level at your antenna output.

Assuming you factor in a preamp, -13.3 dBm is what your preamp receives in input.

Add the preamp gain if you have one (e.g. 12 dB) to that figure to get your signal power at the preamp output: -1.3 dBm in this case (again, it seems that you do not need to convert from dB to dBm). From GroundUrMast calculation, it seems that the preamp noise only affects NM and not signal level.

Subtract total signal losses (cable, splitters, misc), e.g. 10 dB and you get signal level at the tuner input, -11.3 dBm.

Let me know if you notice differences versus Tower Guy's calculations.

[GroundUrMast]

TV Fool uses a dipole reference. Antenna gain is assumed to be specified in dBd.

dBi - 2.15 = dBd

If you start with a TVFR NM value, you can add the antenna gain (dBd) . The sum will be 'net NM' at the antenna terminals.

Amplifier gain will not affect net NM. Amplifier noise figure will subtract from the net NM.

Passive losses such as cable and splitters will not affect net NM.

A stable net NM of +10 or better should offer reliable viewing with most tuners. (Multipath, noise, adjacent and co-channel interference all work to reduce or destabilize the net NM.)