Signal analysis: I'm missing something here

[davodavo]

http://www.tvfool.com/?option=com_wr...dfaf1406db412d

I live in a deep fringe area (literally, no transmitters of any kind above 1 kW within 30 miles) and have a nice setup (stacked yagis with a CM7777 amp). When the wind is blowing the right direction, I can get 70 channels or more from as far as 150 miles away. Rock solid for everything from San Francisco, 75 miles away. Cool.

But looking at the TV signal analysis page, I'm confused by some things:

• Several repeaters that should be showing up just aren't. They're marked as 1Edge, but with big positive NMs.
• I put in my street address, but something is messed with the altitude calculation because the results assume I can see through big hills that put me in the signal-shadows.
• Some channels that I'm getting fairly consistently don't show up at all (most notably KTEH 54 and KSBW 8) on the list. These channels have been around for a long time, should be "right" in the database.
• Meanwhile, I essentially get nothing that's listed below an NM of -10...even though I have stacked Yagis (should be good for at least 10 dBd) and a preamp that's good for at least 28 dB.

I know YMMV (google it), but what I'm actually experiencing matches the analysis less than 50% of the time.

Is this a common experience for rural, somewhat hilly areas? Or am I just a dolt who doesn't understand (as opposed to a dolt that does...)?

[Tower Guy]

In order to determine the advantage of stacked antennas you should add 2.5 db to the gain of a single antenna. But even then the spacing must be optimum, which can't be true on all channels at once. Next you SUBTRACT the 3 db noise figure of the 7777 preamp. The gain of the preamp can overcome the loss of the coax and splitters, but never add to the NM.

As for the low power stations, most of them are on low band VHF. Perhaps your stacked yagis are not intended to receive channels 2-6.

Next, verify your location using the online maps scheme.

[davodavo]

Thanks so much for your reply.

Originally Posted by Tower Guy

In order to determine the advantage of stacked antennas you should add 2.5 db to the gain of a single antenna. But even then the spacing must be optimum, which can't be true on all channels at once. Next you SUBTRACT the 3 db noise figure of the 7777 preamp. The gain of the preamp can overcome the loss of the coax and splitters, but never add to the NM.

Well, I do have a Kitz preamp that advertises a lower noise figure and nominally higher gain...but the CM actually pulls in more stations when I do an A/B comparison.

Is NM the only determinant of "viewability", or is the dBm equally important? Is NM stated in terms of a dipole reference? In that case, stacked Yagis should give you something better than10 dB extra martin, right? (Sorry for the newbie questions, but NM is a new term for me in this context.)

Originally Posted by Tower Guy

As for the low power stations, most of them are on low band VHF. Perhaps your stacked yagis are not intended to receive channels 2-6.

Nominally, they are full-VHF band...but none of the elements is longer than 40 cm so it's not going to have a bunch of gain down at 80 or even 60 MHz...but it won't have zero gain. With the NM of the repeaters (and a couple of them are LOS) I'm not understanding why I don't see their signal. Of course, I don't have a spectrum analyzer so I only have the TV to go by...

[/QUOTE]Next, verify your location using the online maps scheme.[/QUOTE]
Did that, the system places me correctly in the terrain. But the signal analysis indicates I should be "seeing through hills" that have me in the shadows for sure.

In particular, channel 3 (second item on the report) has a huge NM and PWR rating...so it should be screaming thru, right? Yes, there's an overlap of a weaker signal (sixth one down in the list), it's 30 degrees off in azimuth so nicely attenuated in the antenna's side-lobes. Similar story with channel 6, third one on the list.

[rabbit73]

Originally Posted by davodavo

I put in my street address, but something is messed with the altitude calculation because the results assume I can see through big hills that put me in the signal-shadows.

A terrain profile will show the direct path from the transmitting antenna, but some TV signals can make it to your antenna by atmospheric refraction which bends the signal down to your antenna and diffraction as it encounters the peak of a hill. VHF signals are more common in rough terrain because they can make it over terrain obstructions more easily than UHF signals. The tvfool simulation takes some of that into consideration, but its accuracy with 1Edge and 2Edge signals isn't as good as for LOS signals. The report assumes your antenna is outside and in the clear with no trees or buildings in the path.

Your report says your antenna is only 10 feet AGL; is that true?

If you click on a callsign in your report, you will see a terrain profile:
KNTV
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d10

KTVU
http://www.tvfool.com/?option=com_wr...ALLTV%26n%3d15

Here is a terrain profile for KNTV using different software. Note the curvature of the earth makes reception more difficult.

[rabbit73]

You are on the extreme fringe for KNTV. Small changes in location can make a big difference in signal strength. Don't be misled by the implied accuracy of the Longley-Rice coverage map; the computer simulation isn't any more accurate than about the length of a football field.

[rabbit73]

Several repeaters that should be showing up just aren't. They're marked as 1Edge, but with big positive NMs.

TV repeaters are called translators because they translate the original transmitted signal on a different frequency.

Some channels that I'm getting fairly consistently don't show up at all (most notably KTEH 54 and KSBW 8) on the list. These channels have been around for a long time, should be "right" in the database.

As near as I can tell, KTEH is the former callsign for KQEH, real channel 50, virtual channel 54.1.
http://www.rabbitears.info/market.ph...&callsign=kqeh
click on historical ownership

https://www.google.com/#q=kteh

KSBW
http://www.rabbitears.info/market.ph...&callsign=ksbw

KSBW is 161 miles away in Salinas. The only way you can receive it is during a tropospheric propagation event.

[davodavo]

Thanks, Rabbit, for running the other simulation software, and for the clarifying explanation. Yes, my antenna is no more than 15 feet above the ground. The XYL won't allow a tower...so can't experiment that much.

So...just for fun...Channels 2 (KTVU) and 11 (KNTV) that you simulated come in rock solid on my gear. In general, the TVFool model seems pessimistic, as I get 94 (virtual) channels from as far away as 150 miles when the wind is blowing the right direction.

Where I'm confused are the situations that the model indicates a huge positive NM for a station, but there's either "nothing there" or there's a 200 foot hill right next to me that doesn't show on the terrain map. For the second issue, I'm guessing there's just not enough detail in the terrain database for my particular location.

But for the signal strength seeming to be misleadingly strong, let's take a super-clear example: channel 2 in Sacramento. The model indicates I should have a -2.8 dB NM...but this is for a 3 kW transmitter 76 miles away with the terrain blockages as shown?? Seriously?? Now...that signal has a 5-meter wavelength, but I don't think low-VHF bends that much.

[ADTech]

Several repeaters that should be showing up just aren't. They're marked as 1Edge, but with big positive NMs.

Just because it's on the TVFool list doesn't mean that the facility is on the air or has ever been built. The database picks up any issued construction permits (CP) and automatically includes them as if they were really on the air.

K06QM is licensed as a 100 watt station with a CP to 3 KW with a completely different coverage pattern and direction. Same for K03IC. Both are owned and operated by the same entity.

In general, the TVFool model seems pessimistic

For 2-edge calculations. the simulator is about as accurate as this past election's political pundits and prognosticators.

[rabbit73]

I have a little more to answer before I do CH2.
For the images I posted, I had to know your exact antenna location coordinates, which I don't have, but I think my estimate of your location is pretty close.

Meanwhile, I essentially get nothing that's listed below an NM of -10...even though I have stacked Yagis (should be good for at least 10 dBd) and a preamp that's good for at least 28 dB.

It is difficult to receive any signals with a NM of less than -10 dB. If you do receive a signal that has a NM of less than -10 dB, you have an excellent antenna system, your tvfool report is wrong, or they have been enhanced by Tropospheric Propagation.

[rabbit73]

But for the signal strength seeming to be misleadingly strong, let's take a super-clear example: channel 2 in Sacramento. The model indicates I should have a -2.8 dB NM...but this is for a 3 kW transmitter 76 miles away with the terrain blockages as shown?? Seriously?? Now...that signal has a 5-meter wavelength, but I don't think low-VHF bends that much.

KFTY is 45.6 miles away; do you receive it?

For the same coverage area, VHF-Low requires less power than VHF-High, ignoring the higher noise level on CH 2-6, which the FCC did when it calculated power allowed by a transmitter. The FCC also assumed everyone would have a receiving antenna outside, in the clear, at 30 feet AGL.

Is NM the only determinant of "viewability", or is the dBm equally important? Is NM stated in terms of a dipole reference? In that case, stacked Yagis should give you something better than10 dB extra martin, right? (Sorry for the newbie questions, but NM is a new term for me in this context.)

Viewability is determined by the signal strength and signal quality at the input of the tuner. Signal quality is defined by SNR and uncorrected errors.

Most tuners will drop out a signal weaker than -85 dBm at its input.

I recently did a tuner sensitivity comparison between a 32" Sony and a Channel Master 7003 converter box.
http://www.avsforum.com/forum/186-co...erter-box.html

Yes, NM is stated in terms of a dipole reference. NM 0 is at -91 dBm, but if you don't have a preamp, you must have a NM +6 dB, to allow for the 6 dB average NF of a tuner. The report assumes the dipole is outside and in the clear. You should read the FAQs here:
http://www.tvfool.com/index.php?opti...ask=view&id=57

Is this a common experience for rural, somewhat hilly areas?

YES

[rabbit73]

The XYL won't allow a tower...so can't experiment that much.

You must be a ham.

My wife tolerates my antenna experiments because she wanted a battery operated TV for use during a power failure. She even bought a signal level meter for me.

An indoor antenna worked for an analog TV near a window, but it doesn't work for digital TV because the insulation in the outer walls has an aluminum foil vapor barrier. We are living in a Faraday Cage. I had to set up an outdoor antenna on the porch.

73,
W4...
ex W2...
ex DL4..

[WIRELESS ENGINEER]

probably what you should be using is a ch2- 51 antenna system

UHF is strictly line of sight and severly attenuated by foliage
the only way you can receive UHF is by a reflection if no line of sight Path is available

high vhf (7-13) frequencies will bend slightly over hills and covers better than UHF due to lower path loss and is much less affected by foliage

low vhf (2-6) cover best in hilly terrain are virtually impervious to foliage
path loss is very low so coverage is very good

a large combo antenna like the Winegard hd8200u would likely do a much better job for you

and noise figure of amplifiers is a hair splitting debate that needs to stop

the fact is that your system. noise figure is set by the TV tuner which are ALOT higher than the routine 3 or so db that most preamps have

it like most of these made up specs I see these days is just another of many sales gimmicks

[rabbit73]

Originally Posted by WIRELESS ENGINEER

probably what you should be using is a ch2- 51 antenna system

Correct; his stacked yagis certainly don't sound like they would be any good for VHF-Low.

Originally Posted by davodavo

I live in a deep fringe area (literally, no transmitters of any kind above 1 kW within 30 miles) and have a nice setup (stacked yagis with a CM7777 amp)......Nominally, they are full-VHF band...but none of the elements is longer than 40 cm so it's not going to have a bunch of gain down at 80 or even 60 MHz...but it won't have zero gain. With the NM of the repeaters (and a couple of them are LOS) I'm not understanding why I don't see their signal.

Originally Posted by WIRELESS ENGINEER

the fact is that your system. noise figure is set by the TV tuner which are ALOT higher than the routine 3 or so db that most preamps have

Not true. The system noise figure is set primarily by the preamp at the head end of the system.

The calculation of the System Noise Figure is done with the Friis Cascaded Noise Figure Formula.
https://en.wikipedia.org/wiki/Noise_figure

I use this calculator:
http://imageevent.com/holl_ands/files/ota

COMPARE System Noise Figures
With and Without Preamps

It's now listed as File Number 8, but might change if more files are added.
http://imageevent.com/holl_ands/file...=0&w=1&s=0&z=4

Here is the calculation I did for the Noise Margin diagram in post #6:



The NF of the tuner doesn't matter as much when using a preamp because its noise is added AFTER the Noise Floor has been amplified, so it is buried in the amplified noise.

it like most of these made up specs I see these days is just another of many sales gimmicks

Yup, and it's getting worse.

[Stereocraig]

If I had the option of purchasing a better OTA stand alone tuner based on specs like I always have w/ my AM-FM Stereo tuners, I certainly would.