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Old 4-Jul-2015, 5:07 PM   #1
Stereocraig
Senior Member
 
Join Date: Sep 2012
Posts: 472
FM Overload?

I'm not having any problems w/ the majority of my stations, but am wondering if those two stations could be hampering any of my DXing.

Are there any hard and fast rules, or is it going to vary among frequency and other variables?

My traps are set to IN, but I've seen recommendations for two in some cases.

Thanks, people!

FM fool won't c/p here, so they are 3.1 mi away at 50kw and 37 kw at -8.7 and - 10.4.
And if somebody could set me straight w/ the link, please?

http://www.fmfool.com/index.php?opti...pper&Itemid=29

http://www.tvfool.com/?option=com_wr...8e03d95f4f8005

Last edited by Stereocraig; 4-Jul-2015 at 5:18 PM. Reason: FM link broken
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Old 4-Jul-2015, 9:03 PM   #2
rabbit73
Retired A/V Tech
 
Join Date: Aug 2012
Location: S.E. VA
Posts: 2,657
Hi, Craig:

FM Fool doesn't link like TV Fool does. You have to post an image.

I did an FM report for your area based on 53158 zip, which is centered at 42.521693,-87.884431, near the intersection of route 165 (104th st) and route 31; or more exactly 105th st and 65th ave. See attachment 1.

It shows some very strong FM signals, but is probably different from your location. I'll try to do a closer estimate, unless you want to send me a PM with your address or coordinates.

I did another estimate based on your TVFOOL report, which is a little better. It puts you south of Anderson Park near 93rd st and 29th ave near St Joseph's Home. See attachment 2.

Quote:
I'm not having any problems w/ the majority of my stations, but am wondering if those two stations could be hampering any of my DXing.
It's possible, but I would need to know a little more about your setup and which weak stations you are hunting. There are a lot of factors involved.
Quote:
Are there any hard and fast rules, or is it going to vary among frequency and other variables?
No hard and fast rules without precise lab measurements, but a lot of general guidelines.
Quote:
My traps are set to IN, but I've seen recommendations for two in some cases
The cases where two are needed is when the FM signals are much stronger than your strongest TV signals, using the Pwr dBm scales on the reports like yours. But, there are other factors, like the frequencies of the strongest FM signals and the frequencies of the weak TV channels; the second harmonic (2x FM freq) of FM signals can interfere with VHF-High channels.
Attached Images
File Type: jpg StereocraigTVF FM est.JPG (108.5 KB, 186 views)
File Type: jpg StereocraigTVF FM est2.JPG (118.1 KB, 171 views)
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Last edited by rabbit73; 4-Jul-2015 at 9:55 PM.
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Old 5-Jul-2015, 11:11 AM   #3
Stereocraig
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Join Date: Sep 2012
Posts: 472
Yes Rabbit, I am 2 blocks east of the 2nd attachment.

I have a DIY bowtie on my rear tower fixed at due North w/ something like an old CM Spartan, about 100' away.

My front tower has a 91XG, rotor and a Winegard AP8275 and a bout a65' run.

I sometimes like to just aim across Lake Michigan, to see what I can receive, from Michigan and Indiana.

I probably should be happy w/ all of the channels I'm getting, but you know how it is.
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Old 5-Jul-2015, 4:01 PM   #4
rabbit73
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Join Date: Aug 2012
Location: S.E. VA
Posts: 2,657
Quote:
Yes Rabbit, I am 2 blocks east of the 2nd attachment.
OK, I think I got it now. Thanks for the further INFO. See attachment 1.
Quote:
I have a DIY bowtie on my rear tower fixed at due North w/ something like an old CM Spartan, about 100' away.

My front tower has a 91XG, rotor and a Winegard AP8275 and a bout a65' run.
Nice setup, Craig. You must have a lot of fun with it.

I see trees to the north and power lines to the south, that might be a problem. Your strongest TV signal is WLS with a signal power of -40.7 dBm which allows you to use a preamp for DXing, but your FM signals are stronger so you do need FM attenuation.

Fortunately, the two strongest FM signals are from the SW and NW, so your TV antenna patterns help you when hunting N, E, and S.

All external FM filters are not the same. You have to check the frequencies of your strongest FM signals and pick the best filter for that part of the FM band. A HLSJ often makes a good FM filter because it blocks everything below TV channel 7, including the FM band. See attachments 2, 3, and 4.

When you are hunting for weak ones, you are limited by your strong signals to a certain dynamic range between the two. Strong signals can cause fundamental overload which creates distortion products. But two or more strong signals can create IMD products called spurious signals, or spurs for short, that can wipe out your weak signals. See attachment 5.

Good hunting!

Best regards,
rabbit
Attached Images
File Type: jpg StereocraigTVF FM est3.JPG (107.2 KB, 232 views)
File Type: jpg ADvsRSFMfilter.JPG (135.6 KB, 397 views)
File Type: gif MCM%20FM%20Trap%20Attenuation.gif (15.5 KB, 265 views)
File Type: gif ADTechHLSJs3.GIF (22.5 KB, 254 views)
File Type: jpg SFDRdiagramJ3.jpg (103.6 KB, 229 views)
__________________
If you can not measure it, you can not improve it.
Lord Kelvin, 1883
http://www.megalithia.com/elect/aeri...ttpoorman.html

Last edited by rabbit73; 5-Jul-2015 at 5:59 PM.
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Old 5-Jul-2015, 5:41 PM   #5
Stereocraig
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Join Date: Sep 2012
Posts: 472
Thank you for your efforts w/ this great info, Rabbit!

I see they haven't updated my street view in 3 years.
That was the summer that I didn't mow the lawn for months, b/c of the drought.
I also hadn't finished up that tower, yet, either.

The thing that prompted this post, is that while checking if HBU 55s were on closeout, I picked up 3 of the FM trap cubes from Rat Shack, on impulse.
Like many of my other projects, they'll probably just sit in a drawer for a while, but hey, I have them
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