San Francisco, CA - OTA - Page 219

I was thinking about my tests yesterday with adding attenuation after the preamp and how that affects the setup, especially the system noise figure, which takes into account losses between the preamp and the TV.

It was a cold, wet, and ugly day up here today so I worked on an Excel spreadsheet where I could enter all the parameters to calculate system noise figure and also a graph to see what the affects of adding attenuation after the preamp were.

It's just a small Excel file and you can download it here:

http://images.aa6g.org/System_NF.xls

Enter your parameters in the blue boxes. The green box is the system noise figure. The graph shows the affects of adding attenuation beyond your coax loss, same as increasing the length of coax. The default parameters are the actual numbers for my setup at 700 MHz.

Total Coax Loss is determined form the length of your down lead plus any jumpers. Misc Loss in my case is the loss of the diplexer I use to combine separate VHF & UHF antennas. Splitter loss is entered there. TV Tuner Noise Figure is a guess based on what I've read ATSC tuners should be. If you're using a distribution amp enter the noise figure for it instead of the TV.

I included a couple of tables showing the typical loss of high quality RG-6 and RG-11 coax for VHF and UHF. I also included typical losses for 2 and 4 way splitters.

You can gauge if you have about the right preamp gain by looking at the beginning of the line. If it's quite flat you have more gain than you need. If it's steep then you don't have enough gain. The default example is set about right.

In the default example, adding 30dB of attenuation after the preamp is about the same as putting a 20 dB attenuator ahead of the preamp.

Those other numbers at the top are notes to myself so I can remember what I did sometime in the future.

Chuck

Edit: I improved the spreadsheet by expanding the table showing attenuator settings and the corresponding Signal Margin. Signal Margin (dB) is the amount the signal is above the minimum level at which the TV will lock that station.

I hate the idea of virtual channels, but it's too late to change now.

Larry
SF[/quote]

If a tv station does not program a virtual number on a "Ongoing" basis.
My equipment will default to the RF number only.
Sometime last year, KTVU did not send the virtual number 2
So that "Bumped" KBCW off my channel list.
The converter box saw the conflict and selected KTVU for 44 (Because it had no mapping info)

Last year KCSM & KTLN did not send virtual channel information.
They defaulted to 43 & 47 no matter what you did.

If a tv station does not send virtual channel mapping. They can wipe
other stations off your scanned list.
They are very careful to avoid doing that ....
They are really ..... really careful. ( At least this year ... they are)
Ben

If KVIE "stopped" sending the virtual channel number.
That would bump KQED off of 9
The RF without a virtual map would take priority.

I always scan KVIE first, it maps to 6, THEN 9 is open when detected.

It all works properly if the tv stations are ... "doing it right"

Please check the URL... the file won't open. I get a failure notice.

Larry
SF

I clicked on my link and the file downloaded and opened fine. It's an older version of Excel and from a Mac but I've never had any trouble before with PCs opening it. Excel files are cross platform.

Is it a failure to download or a failure to open?

Chuck

Just tested it out on my Windows 7 machine and was able to download and open it just fine.

I opened it on XP just fine.

Hi,

I right clicked and selected "Save target as". The file downloaded just fine.

I then opened it with Lotus 123 and got the usual problem messages I have always gotten with xls files.

I do have Microsoft office installed but I do not wish to use it, only 25 times allowed before I have to pay for it!

Sheet1 looked not too bad but without seeing the original I do not know what to expect. It might be just fine.

Sheet2 and Sheet3 appear to be empty.

MS had a viewer program for free at one time and may still.

It might be worth a try.

SHF

I copied and pasted the address into my browser and was then able to download the file okay. Clicking on the link here in the forum gives me an error..."could not be opened, because an unknown error occurred." I'm using Firefox on an I-Mac.

Now to study it and figure out what it all means.

Larry
SF

A little more explanation..... There's really two parts.

Part 1 is calculating the actual noise figure of the system by including 2nd stage noise figure which is coax loss + tuner NF + misc losses like splitters. You enter the numbers for your system in blue boxes and read the system noise figure in the green box. The difference is how much 2nd stage noise figure has degraded the actual preamp noise figure. My personal rule of thumb is I don't want to see it degraded by more than 1 dB.

Part 2 is really about using the table to find the signal margin above the digital cliff disregarding multipath. If your system does not have a preamp, then all you have to do is add attenuation after the antenna until the signal drops out and that attenuation is your signal margin. If you enter 0 for the preamp noise figure and preamp gain you'll see the Signal Margin increase dB for dB and the line on the graph will be straight with a slope of 1.

If you have a preamp, there is not a 1:1 correlation between adding attenuation after the preamp and Signal Margin. The purpose of the preamp is make sure this is not the case. in order to determine Signal Margin by adding attenuation after the preamp, 2nd stage noise figure has to be calculated for the amount of attenuation that causes the signal to fall off the digital cliff. That's what's in the table under Signal Margin. Look in the table under Attenuator for the value at which the signal dropped out and then read the corresponding value under Signal Margin. The calculated values under Signal Margin are equivalent to adding that amount of attenuation ahead of the preamp.

The graph is just there because some people such as me like to look at graphs. Once you become familiar with how preamp gain affects the system noise figure, the graph gives you an at-a-glance idea of whether the preamp gain is too high, too low, or about right.

The table at the top for coax loss is for convenience so no one has to go look up up cable loss to calculate their number to enter. The Splitter table shows typical losses for the two most common splitters.

The point of this exercise is to help you select a preamp of appropriate gain for your system, and if you're so inclined, to determine signal margin with tens of dollars worth of attenuators instead of $1000+ for a spectrum analyzer.

The attenuator method will work in the presence of mutipath as long as you can lock a signal. The trouble with Signal Quality or SNR on your TV is that they are a combination of signal strength and multipath effects.

To cite a couple of personal examples at the extremes, I tested KMMD and KOVR a few days ago in the rain. KMMD with on SNR of 17 dB on the TV started breaking up with 7 dB attenuation, 6 db being the greatest amount it would lock. Reading from the table showed the KMMD signal margin was just 1.1 dB indicating a very weak signal and not multipath. KOVR showed an SNR of 25 dB on the TV. 40 dB attenuation was the greatest amount that it would still lock without errors. Reading the table showed the signal margin was 29.7 dB. The SNR was greatly limited by multipath, not signal strength.

Chuck

Try OpenOffice.org
That works with this spreadsheet, and it is free. (GNU license)

Hi all,

I'm finally ready to go strictly OTA, but need a bit of guidance.

I've tried an RCA omnidirectional antenna (one of those flat white rectangles) that doesn't ever seem to get more than 75% of the local major networks. A usb tuner I just bought came with a tiny set of rabbit ears that work slightly worse. I also found some old rabbit ears in the garage that actually yielded the best results, but don't get everything.

Note that I'm really stuck with indoor options. My wife will lose it if she sees me drilling a hole in the side of the house for the coax.

Here's my antennaweb map:


Q1: Although more channels is always better, I'd be happy if I could get everything in paths D and E. Does that suggest I'd be better off with a uni-directional antenna? Most signals seem to be between 1-15 miles from me.

Q2: My current setup has the antenna right behind the tv (about a foot away) and right next to a window. I understand that proximity to the tv can cause interference. Moving it elsewhere in the room would be tough to do without exposing an ugly mess of cords, but I could try it if it's likely to make a big difference. Is it?

Q3: Once I have the right antenna, I hope to split the signal to get to both my tv and htpc (via usb). Will that kill the signal?

Any other info I could provide that would make it easier to help me?
Anyone want to suggest an antenna given my situation?

Yes, absolutely move the antenna away from the TV. TV's are a significant source of noise and are especially problematic for VHF.

Is there any chance you can get an antenna in the attic? If you must use a small indoor antenna, the one I like to use is:

http://www.solidsignal.com/pview.asp...u=034405001690

I am wary to use any kind of indoor amplified antenna if you have noise problems or if you are close to the tower. The one above does not have an amp.

Finally got my antenna properly mounted and pointed. We're getting all the channels we expected at 95% strength or better (TivoHD).

We're located near the San Jose Rose Garden and have the following setup.

Terk HDTVO antenna (small but amplified), 5" Mast on a 3" tripod roof mounted on a single story home. I would guess that the antenna is between 20'-25' above ground level.

Suffice to say, we're very happy with the results.

Did that ever work. I moved it about ten feet across the room and got every channel I had a reasonable chance at. Still using the RCA onmidirectional antenna. I thought failing to plug it in would eliminate the amplification feature, but I couldn't pick up anything until I plugged it in.

Since its a good strong signal, I ought to be able to use a splitter and send the signal to both the tv and htpc, right?

If you split the signal, you'll be sending half of what you are getting to each tuner. That doesn't mean it won't work though. Another good non-amplified antenna, that can be hidden fairly easy, is the single classic bowtie. While it says "uhf" on the packaging, I've had good success with vhf channels on rf 9 and 10. For KGO, I've received 33% on a Zenith converter. To compare apples with apples, check out the TVFool website. The noise margin predicted for KGO here is about 24. I'm sure your noise margin will be much higher. My channels 9 & 10 have noise margins of 54 and 53. This is just another option to consider.

Here's a story about Sutro Tower and other underrated San Francisco landmarks by Peter Hartlaub from the Chronicle's web site, sfgate.com:

http://www.sfgate.com/cgi-bin/blogs/...id=62656&tsp=1

Larry
SF

First, thank you to everyone for hosting/posting to this forum. I decided to try HD OTA and this is by far the most informative site I've found.

I'm looking for an antenna recommendation. I've tried a coupld Radio Shack and Best Buy specials that have worked but each getting different stations and none getting them all.

I live in an apartment in Nob Hill (Clay at Leavenworth facing North). According to antennaweb.org, Sutro Tower stations are at 202 degrees and 3.2 miles away. Mt. San Bruno stations are at 173 degrees 7.5 miles away.

I only need the basics, ABC, NBC, CBS. I'm on the third floor of a five story complex and I have ran coax to the roof when experimenting with an RS indoor/outdoor, which performed drastically different moving it a centimeter one direction vs. another. I tried two indoors (RS and BB) and neither could get KNTV. The RS indoor/outdoor mentioned above would get KNTV but not other stations, and then when moved would not get KNTV and KTSF. There are pipes shooting up on the roof. Ideally, I could strap an antenna on to the pipes on the roof, unless someone knows of an indoor that will do it all. The distance from roof to apartment is not 100 ft., but I use a 100 ft. chord.

Is there an easy solution (that doesn't require a permit)? If there isn't an easy solution, is there a guy in town that does this for the cost of the antenna and $50 for labor kind of thing?

Thanks in advance. Any info is appreciated.

GregW

Before I recommend anything or make suggestions, what is your line of sight from the top of your building? Can you see Sutro Tower (202 degrees) and/or Mt. San Bruno (173 degrees)? Are there other buildings, hills or other structures blocking your view in those directions?

Larry
SF

I don't know what RS antenna you have but moving an antenna a short distance and losing the station is a sure sign of multipath. You'll almost certainly need a directional antenna.

There are no magic antennas. If the ones you have don't work indoors then it's likely that none will work. A directional outdoor antenna is the next step.

Chuck

Thanks for the prompt replies!


Larry: I'm going to say "I have LOS to both" I think. I'm on the West/left bank/sunny side of Nob Hill, just down from the top, with no buildings obstructing my view in the SouthWest direction. I'm no explorer but I am pretty sure I can see with my own eyes Sutro Tower. I don't know if I can physically see Mt. San Bruno, or see 7.5 miles period, but I am fairly high up and have a largely unobstructed view in that direction (South/SouthWest).

That said, I do have St. Francis hospital 3.5 blocks away in the South/SouthEast direction, which might block my LOS to Mt. San Bruno. Here is a Google Map with a line showing the angle, and it seems I just miss it. http://maps.google.com/maps/ms?ie=UT...0e2b214cd671f2. I guessed as to where the towers are on Mt. San Bruno though. Maybe if they are further East I am blocked.

Note, to the East is kind of a disaster - not only the top of the hill but a wall of high rises. West and North are perfectly clear.


Chuck: I was using a Radio Shack Model: DA-5200 | Catalog #: 15-2186
http://www.radioshack.com/product/in...ductId=2253765 on the roof and moving it slightly would bring in some channels while losing others. It may be an alright antenna but seemed unreliable to me.

Thanks again, all, for your help.

Gwood...

I think you're in good shape as far as location goes. I don't see anything obstructing your line of site from your rooftop. I also plugged your general address into tvfool.com - which is probably the best sight for surveying reception - and you got green for all of the Sutro and Mt. San Bruno stations, plus the one on Mt. Diablo, as well as the stations up in Marin and Sonoma counties. I think you're going to have good results.

I recommend getting the Channel Master 4228. Even though it's a UHF only antenna, I think both KGO and KNTV will be strong enough that you don't need an antenna for VHF too. I'm located between the Castro and Noe Valley and get KNTV fine with my 4228. You're not that much farther away.

The 4228 is somewhat directional, but has a wide enough reception area that you should be able to get the Sutro stations and Mt. San Bruno stations with one position. If you want to get the other stations, then you'll need to invest in a rotor so that you can turn the antenna in other directions.

Once you get it up on the roof, you'll need someone to watch your TV down below as you adjust it. If you peak it so that you have a good signal from KNTV 11, then the others should come in fine. You might need to tweak it some toward the west, but you shouldn't have much trouble finding a position where you get the stations from both locations - Sutro and Mt. San Bruno.

You might get some other stations, too, like the ones from the south day - 14, 36, 48 and 54.

Let's see if Chuck agrees with me on this.

Larry
SF

[quote=gwood312;18609202]First, thank you to everyone for hosting/posting to this forum. I decided to try HD OTA and this is by far the most informative site I've found.

I'm looking for an antenna recommendation. I've tried a coupld Radio Shack and Best Buy specials that have worked but each getting different stations and none getting them all.

I like the EZ-HD model from www.dennysantennaservice.com
also sold as the RCA- 751 model.
Sold locally at Walmart. (Pleasanton)

This antenna (and many others) work well in the San Francisco green zone.
Ben

I downloaded the manual and found it has a 20dB gain amplifier in it. I wouldn't recommend any kind of an amplified antenna where you are. The signals are extremely strong and the amplifier is likely to get overloaded.

The antenna Larry is suggesting is fairly large but it will help a lot with multipath and will be good enough for 7 and 11 at your location.

Chuck

[quote=888CALLFCC;18612514]
I'm not familiar enough with that one to recommend it. I don't know of anyone who's tried it around here. Have you experienced how well it works yourself, Ben? What kind of gain does it provide? I found several pages showing the antenna, but couldn't find any specs.

Larry
SF

Larry,
on page 205 of this forum #6150 deejaysham talks about it.
also page 206 # 6155 and # 6162

I do not have the exact gain figures.

The CM 4228 sounds good. I assume I will get channel 2/KTVU 4/KRON and 5/KPIX? I don't know enough, unfortunately, to know if this is a stupid question, but I thought I'd make sure since the description for the 4228 (and many other antennas I've seen) reads about getting channels 7 and up. "The Channel Master CM 4228HD is a 8-Bay HD and UHF outdoor TV antenna which also picks up High VHF. This antenna has a High VHF signal range of 45 miles picking up available local channels 7 thru 13 and a UHF reception range of 60 miles picking up channels 14 thru 69. This antenna is optimal for receiving the highest quality UHF and HD signals available. With an HD television you will pick up any crystal clear HD signals being broadcasted in your area."

Also, is one online or local reseller better than another?

THANKS ALL!

Greg

Greg...

I need to fill you in about the virtual channel vs the actual channel of a station. To keep viewers from being confused when stations changed over to digital transmission, the FCC ruled that stations will continue to ID by their old analog channel numbers. We all know about FOX 2, KRON 4, CBS 5, ABC 7, etc. and that's what the stations use, but most of the stations don't actually transmit on those channels anymore.

KTVU 2 transmits on channel 44, KRON 4 is on channel 38, KPIX 5 is on channel 29, KQED 9 is on 30, KNTV 11 is on 12. You'll note that I skipped KGO 7. It's actually on channel 7. When they turned off their analog transmitter they moved their digital operation to 7. KICU 36 is another one that's actually on their old analog channel, so they're on 36. For a list of the transmitter channels, check the DTV station list linked below.

The antenna doesn't care about the virtual ID channel. It's looking at the transmitter channel. So to answer your question, yes, the 4228 will do a great job for all of the stations on Sutro and on Mt. San Bruno. There actually isn't anything on channels 2 through 6 anymore.

As for on line sellers, I haven't had any problem with any of them. I've used Solid Signal and Warren Electronics for antennas and have received good service from both.

Larry
SF

Larry:

Thanks very much. I did not understand the above/below 7 thing until now. Time to get my credit card out of my wallet.

Greg

Ah, spring is here. Sunny skies, warm temps, and poor signal in the north bay.