Deep fringe with two edge diffraction-low tech way of finding the sweet spot? - AVS Forum
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post #1 of 14 Old 01-23-2012, 09:28 AM - Thread Starter
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NYC TV for me is a challenge due to a nearby mountain ridge, but after reading a great article posted by a member here: http://www.hdtvexpert.com/?p=134 I'm up for the challenge. What is the best way of looking for the "sweet spot?" I suppose in some cases you could just walk around with an antenna hooked up to a small TV with a long length of coax to see where the signal is strongest, but my setup is going to require twin 91XGs with a pre-amp if I'm going to pull NYC UHF stations in at all. VHF high and low should be a lot easier because those signals are much stronger so those will just go in the attic. Should I maybe just try different spots in the yard/on the roof with one 91XG reasoning that wherever I get decent reception with one antenna should be even better with two?
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post #2 of 14 Old 01-23-2012, 10:26 AM
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Is this important enough to you to spend a couple hundred bucks? The signal characteristics you most need to optimize, strength and "flatness across the channel", are impossible to dynamically compare without equipment that evaluates them in "real time". You really need to use a spectrum analyzer to do that. Here is a modestly priced used spectrum analyzer listed on eBay.

AVCOM PSA-65A Portable Microwave Spectrum Analyzer

Time left: 1 day 23 hours (Jan 25, 201208:15:30 PST)

Current bid: US $144.00

For portable use, it needs a battery, which you can get for about $20. Unfortunately, someone will likely place a "snipe" bid at the last second and drive the price up into the mid two hundred dollar range.

Basically, you just put your antenna on a light weight test mast and walk around aiming it while looking at the squiggles that appear on the CRT screen of this device and find the spot where their plateaus are highest and flattest. It doesn't require any engineering knowledge for you to do that.
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post #3 of 14 Old 01-23-2012, 02:34 PM
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Here's something to consider: portable signal finder setup .

It would cost you about $100, and some time to build the rig.
But the assumption is that a small antenna that you can lug around will be powerful enough to at least give you a feel for where a sweet spot is. Some of these little TVs have decent strength meters, so you can get an idea without actually locking on to the station.
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post #4 of 14 Old 01-24-2012, 08:38 AM
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Post your tvfool results so we can have a better idea of reception in your area.

If you can pull in stations with 1 antenna, then thats all you need, but now you need a VHF antenna for the VHF stations. So maybe a high VHF/UHF antenna would work.

But for now, try the 91XG on the roof and see what you can get. Remember to point it correctly towards the Empire State Bldg.
In this pic, the left side of the picture would be pointing to the Empire State Bldg.
http://img3.prosperent.com/images/25...1XG_medlrg.jpg
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post #5 of 14 Old 01-24-2012, 08:51 AM - Thread Starter
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Quote:
Originally Posted by mikepier View Post

Post your tvfool results so we can have a better idea of reception in your area.

Here is the report:

http://www.tvfool.com/?option=com_wr...c&t=ALLTV&n=34

As you can see Philly TV is easy-NY is hard. I'm honestly not sure if one of those little TVs would be able to pull a signal at my location, but I suppose I could try it.

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Originally Posted by mikepier View Post

If you can pull in stations with 1 antenna, then thats all you need, but now you need a VHF antenna for the VHF stations. So maybe a high VHF/UHF antenna would work.

Right now I'm thinking of putting the 91XGs with a VHF High Yagi (for WABC-7) on the roof fixed pointing toward NY. Later on if I decide I want Philly I could put a VHF Low in my attic for WPVI-6 out of Philly and handful of others. I'm just more concerned with placement than anything. In terms of gain I think the XGs will do it, but the question is will the best location be on my roof or somewhere else?
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post #6 of 14 Old 01-24-2012, 10:09 AM
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Quote:
Originally Posted by Badfish740 View Post

Here is the report:

http://www.tvfool.com/?option=com_wr...c&t=ALLTV&n=34

As you can see Philly TV is easy-NY is hard. I'm honestly not sure if one of those little TVs would be able to pull a signal at my location, but I suppose I could try it.



Right now I'm thinking of putting the 91XGs with a VHF High Yagi (for WABC-7) on the roof fixed pointing toward NY. Later on if I decide I want Philly I could put a VHF Low in my attic for WPVI-6 out of Philly and handful of others. I'm just more concerned with placement than anything. In terms of gain I think the XGs will do it, but the question is will the best location be on my roof or somewhere else?

Thats going to be a tough one. But try out the XG91 on the roof and see what you get. The higher you put the antenna the better. And aim the antenna East towards NYC. If you have an HDTV with built-in tuner nearby, run a small length of RG6 coax to it, and do a scan. If you get nothing, try connecting an amp, if you have one, as close as possible to the antenna.

If you have a laptop, you can get a Hauppauge tuner stick, and bring the laptop up to the roof. Thats what I do when trying to find a sweet spot.

Do you get Philly stations now?
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post #7 of 14 Old 01-24-2012, 10:45 AM - Thread Starter
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Originally Posted by mikepier View Post

Thats going to be a tough one. But try out the XG91 on the roof and see what you get. The higher you put the antenna the better. And aim the antenna East towards NYC. If you have an HDTV with built-in tuner nearby, run a small length of RG6 coax to it, and do a scan. If you get nothing, try connecting an amp, if you have one, as close as possible to the antenna.

If you have a laptop, you can get a Hauppauge tuner stick, and bring the laptop up to the roof. Thats what I do when trying to find a sweet spot.

Do you get Philly stations now?

Right now I have Dish Network-I'm looking to get rid of them and replace with OTA/internet streaming. I plan on mounting the XGs at 30' and seeing what I get there, but I'm wondering about trying other locations because of the article I linked to in my OP. The author of the article ended up mounting a couple of antennas at near ground level and gets great reception. This is due to the diffraction caused by some nearby mountains, which is the same thing I'm dealing with-for all I know the best spot could be in my yard somewhere. The tuner stick sounds like a great idea-thanks for that.

I actually just took a walk around my neighborhood and took note of the folks that do have antennas. They may or may not be in use, but all were pointed SW toward Philadelphia. Most were old looking all band types. I'm hoping that the twin 91XGs will have enough gain (along with a quality pre-amp) to pull in NYC TV, but if not, I'll just point them toward Philly and maybe even remove the pre-amp depending on the signal strength I end up with. I figure at the very least if I pull in Philly TV with the twin antennas it should minimize the potential for signal strength problems. I'm still going to get a VHF High Yagi for WABC-7 though-WABC is one of the only NY channels in the red zone-with that mounted on the roof up high I should have a reasonable shot at getting it. Convenient since channel 7 news is the wife's favorite
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post #8 of 14 Old 01-24-2012, 12:34 PM
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Quote:
Originally Posted by Badfish740 View Post

Right now I have Dish Network-I'm looking to get rid of them and replace with OTA/internet streaming. I plan on mounting the XGs at 30' and seeing what I get there, but I'm wondering about trying other locations because of the article I linked to in my OP. The author of the article ended up mounting a couple of antennas at near ground level and gets great reception. This is due to the diffraction caused by some nearby mountains, which is the same thing I'm dealing with-for all I know the best spot could be in my yard somewhere.


Everyone's situation is different. What worked for the author of that article might not be necessary for you. The only way to find out is to start experimenting.
If your neighnbors antennas are pointing towards Philly, that might tell you that NYC is hard to receive.
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post #9 of 14 Old 01-24-2012, 01:12 PM
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I would not mess with two antennas except as a last resort. Go with one XG91 and a good preamp. Your ranges and TVFool values are in the same general range as I get for Providence and Hartford. I have done experimenting with several different positions and antenna heights. I get absolutely no signal anywhere near the ground. I barely get the strongest UHF signals about 10 feet above the roof of a one story house. At about 25 feet everything is about 10 dB stronger and I get intermittent reception on the weaker UHF stations and pretty reliable reception on the stronger ones. It does not matter much where the antenna is located it's all about how high I can get it. I have tried stacking two Winegard HD-9022 antennas and have never gotten an improvement over one alone. A XG91 is about two dB better at the same location on channels 21, 22, 49, and 51 than the Weingard. This is not a very significant improvement when the day to day signal strength varies an easy 10 dB from one day to the next. The VHF channels are much more reliable with an old large VHF antenna about 10 feet above the roof.
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post #10 of 14 Old 01-24-2012, 03:08 PM - Thread Starter
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Quote:
Originally Posted by mikepier View Post

Everyone's situation is different. What worked for the author of that article might not be necessary for you. The only way to find out is to start experimenting.

Sure, but it did open my eyes to the possibilities-I think the laptop idea is probably the most simple way of checking.

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If your neighnbors antennas are pointing towards Philly, that might tell you that NYC is hard to receive.

Maybe, but this is an old neighborhood (built in 1960s) and most of the antennas looked pretty old as well. I'm betting a lot of them either aren't in use anymore, or are merely adequate for receiving Philly signals-most of them looked like small (5-6' long) all band units. Certainly nothing that looks anything like a 91XG Hopefully once we get some nicer weather I can do some experimenting.
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post #11 of 14 Old 01-24-2012, 05:20 PM
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Sure, but it did open my eyes to the possibilities-I think the laptop idea is probably the most simple way of checking.

The only problem with the laptop idea is catch-22. You can't find a sweet spot unless you can see it first. It's like scanning your TV for available channels, if it can't be seen, you will not be able to tune to it later.

Only a spectrum analyzer will work, it see all signal, even the weak ones.
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post #12 of 14 Old 01-24-2012, 07:58 PM
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Originally Posted by ctdish View Post

...I get absolutely no signal anywhere near the ground...

... it's all about how high I can get it.

This is certainly not always the case, especially with nearby hills in the way.
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post #13 of 14 Old 01-25-2012, 06:02 AM - Thread Starter
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This is certainly not always the case, especially with nearby hills in the way.

Just for kicks I plugged my address back into TV Fool along with a hypothetical 100' tall antenna mast The results were nearly the same:

http://www.tvfool.com/?option=com_wr...67d9738bf8dfe7

The Philadelphia stations are mostly in the green, a few in red, and the networks in NYC are all at the bottom of the red heading into the grey. In the abovementioned article that arxaw so kindly posted, I learned that the ridge 3 miles to my east may be bending the signal in an unpredictable way. I of course can't find the link now, but there was tongue in cheek illustration of this with an old Beetle Bailey cartoon. Beetle is on the roof trying to aim the antenna and Sarge is yelling at him to go here, go there, etc... Finally Beetle falls off of the roof tangled in the cable and the antenna ends up hanging (with Beetle) upside down beside the open window where Sarge is watching TV. Sarge yells "that's perfect!" The caption explains that when diffraction is involved the ideal antenna location is not always where you think it might be, hence the problem. The point of the spectrum analyzer is well taken, but this is a budget project-I think I might just have to hunt around with an antenna and a portable TV/laptop for now. I just don't want to go to the trouble of building a sturdy mast on my roof only to find out that the roof is not the ideal location.
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post #14 of 14 Old 01-25-2012, 06:38 AM
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Quote:
Originally Posted by Badfish740 View Post

... I of course can't find the link now, but there was tongue in cheek illustration of this with an old Beetle Bailey cartoon. Beetle is on the roof trying to aim the antenna and Sarge is yelling at him to go here, go there, etc... Finally Beetle falls off of the roof tangled in the cable and the antenna ends up hanging (with Beetle) upside down beside the open window where Sarge is watching TV. Sarge yells "that's perfect!" ..

http://www.hdtvprimer.com/ANTENNAS/siting.html

It's the fifth image from the top.


According to "the internet", it can also be found reproduced in the book, "Antennas" (not too original title) by John Kraus - W8JK.
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