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Old 26-Feb-2018, 11:37 PM   #1
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Join Date: Nov 2017
Posts: 1
VHF high troubleshooting

I'm trying to receive NBC on 8.1 from a tower about 4 miles away. It's a 2 edge path, and there's another tower about 1/2 mile from me basically in the same direction... it doesn't have anything VHF on it but it could have tons of other stuff.

I have an RCA ANT751R on the roof, running through a TVPRAMP1Z with the FM filter turned on. I have an additional FM filter after the amp, and that terminates in an HD Homerun 5.

Some days, I get 8.1 completely fine, and other days I can't even decode a signal on that band. It's not completely weather dependent, but seems worse when there is moisture in the air. Wind matters somewhat... on windy days I never get NBC, but it can be dead still like today and I don't have anything.

I have been thinking of adding an antenna that is specific for VHF high to see if that helps, but I'm worried that the problem is related to multipath interference and not signal strength. Does anyone know of good resources to continue to try and figure out what's going on on this one band in Portland?
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Old 27-Feb-2018, 11:13 AM   #2
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Move the external FM filter so it's before the amplifier. It does little to no good after the amp. The internal filter in your amp is ineffective at the low end of the FM band and it's the low end of the FM band (90-93 MHz) that has the greatest potential to cause 2nd harmonic interference into channel 8's bandwidth.

I have no idea if this will solve your particular issue, but its a glaring error in the connection scheme. I always recommend fixing the obvious first to get those issues off the table.
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Old 27-Feb-2018, 6:25 PM   #3
Tower Guy
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Location: Delmar, NY
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EGADS. There are 11 FM stations on that tower .5 miles away. Some are 97KW ERP. In addition to ADTechs suggestion of moving the FM filter, you may want to try only the filter and no preamp.
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Old 28-Feb-2018, 2:17 AM   #4
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Good advice from ADTech and Tower Guy.

Your FM signal report; the second strongest FM signal report I have ever seen:

Very likely to cause interference to TV reception.

You must ground the coax with a grounding block connected to the house electrical system ground to make the FM filter (or two FM filters in series) effective.

If the antenna is outside, the coax shield should be grounded with a grounding block that is connected to the house electrical system ground with 10 gauge copper wire for electrical safety and to reject interference. For further compliance with the electrical code (NEC), the mast should also be grounded in a similar manner to drain any buildup of static charge which will tend to discourage a strike, but the system will not survive a direct strike.

Unfortunately, FM filters are no longer in stock at Antennas Direct. I hope they will reconsider their decision not to carry them anymore; they were the last remaining regular supplier of FM filters.

It is now necessary to scrape the bottom of the barrel, or use a HLSJ, high and line ports, that is also hard to find.
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Last edited by rabbit73; 28-Feb-2018 at 3:30 AM.
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