Why does signal strength fluctuate? - AVS Forum
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post #1 of 10 Old 05-27-2007, 05:56 AM - Thread Starter
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I recently got a Panasonic plasma and put up a CM4221. I get all the local channels with signal strength in the mid to upper 90's (antenna farm 20 mi away/ all within a few degrees). The exception is the NBC affiliate which is on channel 10. The signal varies from the 80's down to the 30's, even on dead calm days. I lose the signal for maybe a second or two every few minutes. I know the 4221 is a UHF antenna, but why does the signal vary so much? I have read where people say the Panny tuner is no good (not sure if that's the cable part or OTA also) but I get stations I didn't know were there (including one 60mi away). Anyone have any ideas about this?

Thanks

Dan
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post #2 of 10 Old 05-27-2007, 08:10 AM
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You usually need a larger antenna for reliable VHF reception. A CM 4228 UHF usually works well for VHF chs 7 thru 13, plus UHF. Or buy a highband VHF antenna and combine it with the 4221, using a V/U antenna combiner.

A fluctuating signal is usually caused by multipath interference - bounced multiple signals arriving at the receiver at slightly different times. A more directional antenna may help reduce multipath, or a better tuner that ignores multipath.
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post #3 of 10 Old 05-27-2007, 08:50 AM
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The CM4221 is no good on Channel 10. In Philadelphia, with an analog channel 10, I get a poor and snowy picture from my 4221. A simple curved folded dipole (the VHF section of a gutted Terk HDTVo) works fine, though.
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post #4 of 10 Old 05-27-2007, 08:56 AM
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Or one of these, combined with the 4221.

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post #5 of 10 Old 05-27-2007, 01:11 PM
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Quote:
Originally Posted by nybbler View Post

The CM4221 is no good on Channel 10. In Philadelphia, with an analog channel 10, I get a poor and snowy picture from my 4221. A simple curved folded dipole (the VHF section of a gutted Terk HDTVo) works fine, though.

Depends on the signal strength and his distance to the digital channel 10 broadcast tower. I get a WWPX-DT Ion/Pax station on VHF 12 at 46 miles with a CM 4221, but it is one of the odd stations that comes in stronger than expected. I also get decent picture quality for the Washington DC analog WJLA 7 and WUSA 9 stations with the CM 4221, but those are high powered stations at only 16 miles. But I get lousy picture quality for the Baltimore analog WBAL 11 and WJZ 13 stations at 43 miles. If you look at the charts for VHF reception for the CM 4221 at hdtvprimer.com, it has some gain at VHF 13, but falls off rapidly down to VHF 7. The CM 4221 can work for VHF 11 to 13 if the stations are close by, but, yes, it is a limited antenna for upper VHF use.

xrdan, just how far are you from the NBC digital channel 10 station and what is the call sign for the station? If you give us your zip code, we can look the station up and what power it is at. If you are getting the station with drop outs with the CM 4221, you may be able to simply add a rabbit ear or simple dipole antenna which will have more gain for the upper VHF station along with a VHF/UHF combiner. arxaw enclosed a photo of one of the Winegard YA-1713 or YA-6713 upper VHF antennas which you can get if the rabbit ears don't work. I have been thinking of experimenting with adding a rabbit ear / dipole VHF antenna to my setup as I will have the four upper VHF stations in DC and Baltimore that I listed above switching to upper VHF in 2009. Easy and inexpensive experiment for me to do.
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post #6 of 10 Old 05-27-2007, 03:01 PM - Thread Starter
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I'm 19mi from the tower and it looks like the power is 16.5 kw. The station call sign is WXIA.
I guess I could live with it- I would hate to drag another antenna up there for one channel.
Thanks for the help

Dan
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post #7 of 10 Old 05-27-2007, 03:16 PM
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Quote:
Originally Posted by afiggatt View Post

...you may be able to simply add a rabbit ear or simple dipole antenna which will have more gain for the upper VHF station along with a VHF/UHF combiner...

Start simple, because a dipole may be all you need to add.

Extend the dipoles horizontally, about 29" total tip to tip for channel 10. Aim dipoles perpendicularly to direction of the tv transmitter.


If it works indoors, you can probably add the simple dipole and V/U combiner in the attic.
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post #8 of 10 Old 05-27-2007, 07:49 PM
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Quote:
Originally Posted by xrdan View Post

I'm 19mi from the tower and it looks like the power is 16.5 kw. The station call sign is WXIA.
I guess I could live with it- I would hate to drag another antenna up there for one channel.

The power for WXIA-DT on VHF 10 shown at the FCC database is 80 kW which is cranking for digital upper VHF. Have you tried to tweak the aim of the CM 4221 to see if that helps? Is your CM 4221 located indoors in the attic or outside? If it is indoors, adding a VHF rabbit ear antenna and a VHF/UHF combiner should be pretty straightforward. Since you want to pick up a VHF 10 station, you only need to extend the rabbit ears a total of ~2.5' long.

I should try this myself to see what happens, but you could add some crumpled aluminum foil to the end of two of the bowties. That could degrade performance for the weaker UHF stations, but would extend the antenna length for one bowtie pair for better performance for the longer wavelengths of upper VHF. But I don't recall reading any posts from people who have tried this for a Cm 4221, only for the small Radio Shack 2 bowtie antenna.
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post #9 of 10 Old 05-28-2007, 04:51 AM - Thread Starter
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I don't know which site I got the 16.5kw from(did sound a little low). I do have the antenna on the roof but I pointed it with a compass and haven't tweaked it. I'll try that first and see what happens.

Dan
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post #10 of 10 Old 05-28-2007, 06:17 AM
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I've heard of people cable-tying #8 or #10 copper wire to the upper bowtie of a CM4221 to increase the gain on high VHF. I haven't tried it, but it would be easy to do. Coathanger wire would also work but it would rust.


As afiggatt said, modifying may degrade UHF reception slightly, but it shouldn't be enough to matter if the location is not too far from the transmitters.
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