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Old 21-Feb-2010, 10:29 PM   #1
kernow
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Join Date: Feb 2010
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Orlando area reception of WUSF Tampa

New here, so please be tolerant if I make a newby mistake in my post.

My situation is I have a DB4 mounted on a J-mount on the peak of my single story home in south west Orlando. I pointed the antenna toward the main cluster of Orlando stations approx. 70 degrees. I noticed that at certain times I was getting a PBS station from Tampa, WUSF 16 which is 34. I like the station very much so thought perhaps I could aim the antenna towards that station 230 degrees and probably pick up WUSF better as well as get my local Orlando stations from the back of the antenna.

So I proceeded to reposition the antenna, no improvement on WUSF, although I did get WMOR and WTOG. So i bought a CM7777 preamp and installed it, not a real improvement, perhaps a little.

So I dug out a rotor that I have had for years, hooked that up and proceeded to test. OK now I get WUSF with almost no dropouts, just when a low plane goes over I think. No here is the odd thing to me, I get best reception with the DB4 pointing 15 degress.

So my question is; why am I getting the best reception of WUSF with the antenna pointing 15 degrees instead of the expect 229 degrees? I assume it may be something related to the 2Edge type of reception.

My signal analysis is below.

http://www.tvfool.com/?option=com_wr...27ee07aff68654
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Old 22-Feb-2010, 11:02 PM   #2
mtownsend
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Quote:
Originally Posted by kernow View Post
why am I getting the best reception of WUSF with the antenna pointing 15 degrees instead of the expect 229 degrees? I assume it may be something related to the 2Edge type of reception.
Hello and welcome!

There are lots of possible reasons why a particular channel will/won't come in, so without detailed analysis of the signal coming out of your coax, we can only speculate what the most likely causes might be...

My first guess would be that you're getting overload on your CM 7777 amp. Some of the nearby stations are strong enough to put the 7777 into overload, and you're probably better off without it. When you turn your antenna to a heading of 15 degrees, you're probably getting the strong local stations into the first deep null of your DB4 antenna. This cuts down on the amount of signal power going into the amp and might be enough to bring it out of overload.

When you point the DB4 directly at WUSF, there is probably enough of a back lobe of the antenna pattern to still leave your amp in an overloaded condition due to the strength of the local stations.

Another possibility is that you're getting a combination of channel harmonics (by-products of frequency mixing of some of your strong stations) that are affecting WUSF. If the right combination of frequencies is causing noise on channel 34, it might clobber that channel. Again, adjusting your antenna will alter the relative strength of signals, which also changes the relative strengths of their harmonic by-products.

There are many other possibilities (or contributing factors), and there's no way to know for sure what's affecting your situation without further testing.



The first thing I would try to do is remove the amp. If WUSF peaks out at heading 235 degrees (as expected) instead of 15 degrees, then you'll know that the amp was being overloaded by the local stations.
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Old 23-Feb-2010, 12:00 AM   #3
kernow
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thanks.

I should also mention that WUSF came in strongest about 180 degrees out from 229 degrees even without the CM7777. The problem was it was very variable, I might have a watchable signal for an hour then it would break up every few minutes.

If I am understanding correctly though you are saying that even when not tuned to a particular station the preamp is overloading and causing odd behavior?

None of my locals has the signal strength meter pegged in its current position.

I shall disconnect the preamp tomorrow and report back.
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Old 23-Feb-2010, 4:51 AM   #4
mtownsend
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Yes, an overloaded amp can affect channels whether you are watching them or not. Amps amplify the whole TV spectrum all at once. That is, all the signal going to its input gets boosted to a higher level at its output.

Antennas and amps have no idea who's watching which channels "downstream" from them, so they operate on the entire spectrum as opposed to being tuned according to the channels people are watching. For example, an apartment complex might share one rooftop antenna plus pre-amp and split the signal to many tenants. Any tenant could be watching any channel at any time because the antenna and pre-amp are delivering the entire TV spectrum down to every end-point.

If the total input signal energy exceeds the design capacity of the amp, then you end up with overload. When you over-drive the electronics this way, the amplification is no longer "linear" and you get signal distortion in the output. If the distortion gets bad enough, the channels can no longer be decoded.

Your amp might go in and out of overload depending on how your antenna is aimed. If the antenna is pointed at your strongest local stations (you have very strong signals compared to some people), then too much signal energy will be picked by the antenna and delivered to the amp. This will surely be too much for the amp to handle. If you point away from the strong signals, you might be able to get the signal levels (at the input to the amp) down far enough to avoid overload. It all depends on where the peaks and valleys are in your antenna's gain pattern.

Without the amp, you won't have overload problems, but then your weaker signals will not have the benefit of a signal boost (to compensate for things like cables, splitters, and other components that cause loss of signal). In some cases, if you cannot use an amp due to strong locals, but still have weaker stations you want to get, it may be necessary to go with a bigger antenna to make up for the lack of a pre-amp.
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Old 26-Feb-2010, 3:02 PM   #5
Dave Loudin
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Another factor to consider - possible co-channel interference from WTVX. Tropospheric enhancement is fairly common down your way, so it's entirely possible that as you aim the main lobe of the antenna at WUSF, you're picking up enough interference from WTVX to knock it out. Aiming the antenna at 15 degrees is probably putting a null towards WTVX while having enough sensitivity towards WUSF to pull it in.

Stranger things have happened.
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