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Old 19-Apr-2015, 11:57 PM   #7
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,747
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I never thought to search for FM channels that might interfere.
Glad that I was able to call your attention to something that might be a problem.
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It seems the antenna installers are all over 70, as am I
Hello old-timer, I'm 81.
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About needing an FM trap I was told that both the LNA 200 from Winegard and the RCA TVPRAMP1R had FM traps so I assume I will not have to had another one.
The built-in FM trap is usually enough to prevent overload from FM signals, but sometimes two in series are needed with extremely strong FM signals.
FM Filter
http://www.avsforum.com/forum/25-hdt...fm-filter.html

Overload and FM Trap
http://www.hdtvprimer.com/ANTENNAS/g....html#overload

A HLSJ (VHF-High VHF-Low Separator-Joiner) instead of, or in addition to, an FM Band Elimination Filter (AKA FM Trap) is very effective in eliminating FM interference. It attenuates all signals below CH 7, including the FM Band with low insertion loss (0.5 to 0.7 dB) because it has a high-pass filter between the high and common ports.
Holland Filters and Diplexers

ATTACHMENTS:

There are some attachments you and your installer might find interesting.

The first attachment was done by Antennas Direct, but I was having trouble understanding the screen shot of the spectrum analyzer display. I added some notes to improve my understanding of the results of the FM filter comparison test, and added a credit line for Antennas Direct. Credit, where credit is due.

The second attachment compares the analog signal with the digital signal. The comparatively rapid loss of the signal at the "Digital Cliff" is caused by the FEC (Forward Error Correction system) that has a limited ability to correct errors in the digital stream. When that limit is exceeded, the signal drops out. If it weren't for the FEC, we would see the errors increasing as the signal got weaker, and a "glitch" (picture freeze or pixilation} even once a minute would be more irritating than a dropout.

The third attachment illustrates the Noise Margin concept. I drew it to help me learn the concept, and thought that it might also help others.

A signal must have a NM of at least 0 dB to be received. If it is listed on your tvfool report as having a negative NM, you can add the gain of your antenna. You can also add the gain of your preamp, but must subtract the Noise Figure of the preamp, because its internal noise reduces the SNR of your signals.

The fourth attachment is a multicolored NM (Noise Margin) chart that refers to the NM column on your tvfool report. Credit to Calveras of AVS.

The fifth attachment is a calibration chart, showing equivalent dBmV and dBm values, of the Signal Strength scale on the Diagnostics Screen of my latest Sony TV. What is really neat about it is that each step is equal to one dB, so it is very useful for antenna experiments. I got a signal level meter (SLM) and a TV for the price of a TV!

The last two are charts from an old copy of the CATJ. If your installer has been doing antenna work for a long time, he might enjoy them. When I started doing antenna work, I had to make signal field strength measurements in microvolts (μV). Later, I had to make measurements in dBmV, which is signal power, not voltage, in spite of the fact that the reference level of 0 dBmV is defined as 1 mV across 75 ohms. And now I have to think in terms of dBm, which you see in the signal power column of your tvfool report.

Quote:
Thanks again Rabbit 73 for your unusually sophisticated response. It was considerably more help than I anticipated from this forum and I thank you for taking the time to do it.
My pleasure. Please let us know how it works out; we need the feedback to let us know if our advice was accurate.
Attached Images
File Type: jpg ADvsRSFMfilter.JPG (135.6 KB, 646 views)
File Type: jpg AnalogVSDigital_1.jpg (59.7 KB, 1562 views)
File Type: jpg NoiseMargin.jpg (60.1 KB, 649 views)
File Type: jpg NMChartC.jpg (69.0 KB, 627 views)
File Type: jpg SSCHART KDL32R400A_1.jpg (170.7 KB, 843 views)
File Type: jpg CATJ SIGNAL LEVELS (2).jpg (182.3 KB, 1977 views)
File Type: jpg dBmV To uV Conversion Table (2).jpg (187.1 KB, 661 views)
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Last edited by rabbit73; 20-Apr-2015 at 3:41 PM.
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