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Curious to know if low-band VHF will still be used when ATSC 3 is implemented. Being ATSC 3 is supposed to offer better reception, wondering if low-band reception will be as much of an issue.

Thoughts?
 

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AFAIK, yes; there are stations planning to use ATSC 3.0 on VHF-Lo.

I alluded to an intriguing proposal when I bumped the thread:
I was browsing the FCC's application database, and came across an interesting experimental STA application from a church over in Canton. (Canton is in Van Zandt county, and Van Zandt county is part of the DFW DMA/media market, hence why it's in this topic)

Link: https://enterpriseefiling.fcc.gov/dataentry/public/tv/publicFacilityDetails.html?facilityId=745554

Looks like they want to stand up an experimental low-power TV station on RF6, to test a hybrid ATSC 3.1 / analog NTSC signal. They have supplied antenna type & location & mounting information already as well. Application is still pending, so no callsign yet.

Given Canton's distance from the metroplex, and the presence of Franken-FMs KBFW and KZFW on RF6, I don't think any of us in this topic would be able to pick it up normally. But would be interesting to check out once we start seeing some tuners come out.
I think ATSC 3.1 is a typo or misunderstanding, and they actually mean ATSC 3.0. Google couldn't find anything about "ATSC 3.1." But at any rate, their idea is to take advantage of the fact that ATSC 3.0 isn't wedded to a 6 MHz bandwidth the way ATSC 1.0 is. So AIUI, they want to broadcast ATSC 3.0 in the first 5.5 MHz of their channel and traditional analog FM audio in the top 0.5 MHz. Thus, the audio could continue being received on FM radios tuned to 87.7 MHz. (Currently they have to remain an analog TV station to do this.)
 

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Vertical Radiation Angle on Low VHF

Despite all the discussion of reception of low VHF stations, there is one issue that has been completely overlooked: Vertical Radiation Angle.

Antennas over flat ground do not have their main lobe aimed at 0 degrees unless they are high enough to act as though they are in free space. Their main lobes are always aimed up some amount. In practice, antennas act as though they're in free space if they are at least 10 wavelengths high. All models of TV antennas I have ever seen are done in free space.

Let's look at how antennas actually perform over average flat ground. See the images below for reference. The antenna modeled is a 10 element yagi. According to a couple articles I read, it doesn't matter how many elements the antenna has. The angle of radiation over ground is the same whether the antenna is a dipole or a multi element yagi.

First I did some calculations for what angle your typical TV station comes in at. One useful formula is the reduction is apparent elevation with distance from the transmitter tower due to the curvature of the Earth. When you know the reduction in height, you can calculate the elevation angle. At 20 miles from a 2000' tower above sea level and with the receiver at sea level, the angle to the top of the tower is 0.94 degrees. At 40 miles it's about 0.25 degrees.

http://tchester.org/sgm/analysis/peaks/how_to_get_view_params.html

For most people their stations are coming in at less than 1 degree. People closer than 20 miles generally don't have reception problems because the signals are so strong.

The bottom of channel 2 is 54 MHz. A wavelength at 54 MHz is 18 feet. From the image below, the main lobe at 1 wave is elevated 13 degrees. At 1 degree the antenna gain is down around 20 dB! It's not until the antenna is up at 5 wavelengths that there's some gain at 1 degree elevation. Unfortunately this is 90 feet. Free space performance on channel 2 requires the antenna to be at 180 feet, totally impractical for everybody. Even at channel 6 free space is still 120 feet.

One can only imagine how horrible a low VHF dipole is indoors. Most of the pattern will point straight up. Only the strongest signals have any chance of being received. Add the poor building penetrating characteristics of low VHF along with higher noise levels and prospects for reception are dismal.

This reinforces what I've been saying, low VHF requires an outdoor antenna and it requires an antenna designed for low VHF reception. It also needs to be as high as possible. A good height to shoot for is 55 feet or 3 wavelength. I think most people would be amazed at how good their low VHF reception would be with such an antenna.

Let's briefly look at high VHF and UHF. The bottom of channel 7 is 174 MHz. 5 wavelengths is just 28 feet, a very reasonable height for an outdoor antenna. Free space for channel 7 is 56 feet. The bottom of UHF channel 14 is 470 MHz. Free space is only 21 feet, a very easy height to obtain.

I have one anecdote to illustrate that poor response to low angle signals is real. I've been fighting a noisy power pole about 1/2 mile from here. It's 35' high while my ham and TV antennas are 55'-66' high. This places the noise at about -0.5 degrees. I cannot hear a trace of the noise below 30 MHz where the antenna heights are 1-2 waves. At high VHF the noise is awful where the antenna is in free space with the main lobe at essentially 0 degrees. At UHF I can still see the noise but it's much less. Noises are much weaker on UHF. With my 50 MHz antenna at 3 waves I expected the noise would be bad, but it's quite weak. Apparently the antenna gain is way down at 0 degrees even though the main lobe is around 5 degrees.


Edit1: I discovered a simple formula that you can use to calculate the radiation angle of a horizontally polarized antenna over flat ground.

Radiation Angle = .25 / Height of Antenna in Wavelengths

Wavelength in Feet = 984 / Frequency in MHz

A lot more information on Radiation Angle can be found in this ARRL document:

https://www.arrl.org/files/file/antplnr.pdf

It specifically talks about HF antennas but it applies the same to VHF antennas, just a lot lower.


Edit2: Correct typos. Correct info on different sized antennas over ground.

Edit3: Replace images with improved versions.
 

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My Winegard 8200U mounted at about 40 feet above the ground on a 315 foot hill, works quite well for Low VHF. KQRO is transmitting on channel 2 with about 3 kw from Mt. Chual southwest of San Jose 55 miles away and I get a solid 17-18 dB SNR signal from the station.

Larry
 

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Okay. I didn't think I had to say this specifically but I guess I do. I didn't say in my post above that you cannot receive low VHF stations over a fair distance. Obviously that's not true. People have been receiving low VHF over long distances since the invention of TV. What I said was that no matter how good you think your low VHF reception is, it's not nearly as good as it could be if the antenna elevation was such that the main lobe was down around 1 degree. Like higher noise levels, the higher than optimum radiation angle is another low VHF issue that makes reception more difficult.

The angle of radiation of antennas over flat ground has been known for a long time but for some reason is generally ignored for TV reception. The issue didn't just disappear for TV. It's a real issue and impacts reception whether it's acknowledged or not. The models show that in most cases signals are down in the range of 20 dB for low height antennas.

My low VHF antenna was at 3 waves in Mountain Ranch and I was able to receive 2 stations at over 110 miles (KFTY & KTVJ) and one station at 14 miles (KCSO which later moved to 54miles away). Two of the stations had SNRs in the mid to upper 20's. See attached spectrum analyzer image of channels 2-4 from 2015 comparing nominal signals to above average conditions.

At my current location I have two identical high VHF antennas, one at 4 1/4 waves and one at 10 waves (free space). My one high VHF station on RF 9 is about 6 dB stronger on the high antenna even though both have LOS to the first diffraction edge. The lower antenna should have a radiation angle of around 4 degrees while the higher antenna should be near 1 degree. A 6 dB increase on the high antenna is not an unreasonable difference based on the models.
 

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Despite all the discussion of reception of low VHF stations, there is one issue that has been completely overlooked: Vertical Radiation Angle.

Antennas over flat ground do not have their main lobe aimed at 0 degrees unless they are high enough to act as though they are in free space. Their main lobes are always aimed up some amount. In practice, antennas act as though they're in free space if they are at least 10 wavelengths high. All models of TV antennas I have ever seen are done for free space.

Good point, I should re-think the order of antenna placement on my single mast. I was going to place the VHF full-band on the bottom, at about 4' above the roofline. I should probably shift it to the top position placing it 12' above the roofline. Still far less than ideal, but could make a difference. Hopefully the two UHF X-beam antennae don't suffer from the lower positions.
 

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Calaveras,
Some thoughts on vertical radiation angle. The loss at low received angles is caused by the out of phase ground reflected wave canceling the direct path wave. This means that the field strength is actually reduced by the amount shown in your elevation pattern thumbnails. I think the deep nulls would be less deep in real life because the reflected signal comes from lossy and probably irregular ground which produces imperfect cancellation. This might be the reason that my Rabbitears prediction for WSBE shows little change from 20 to 50 foot height.
https://www.rabbitears.info/searchmap.php?request=result&study_id=110771
https://www.rabbitears.info/searchmap.php?request=result&study_id=110775
John
 

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Calaveras,
Some thoughts on vertical radiation angle. The loss at low received angles is caused by the out of phase ground reflected wave canceling the direct path wave. This means that the field strength is actually reduced by the amount shown in your elevation pattern thumbnails. I think the deep nulls would be less deep in real life because the reflected signal comes from lossy and probably irregular ground which produces imperfect cancellation. This might be the reason that my Rabbitears prediction for WSBE shows little change from 20 to 50 foot height.
https://www.rabbitears.info/searchmap.php?request=result&study_id=110771
https://www.rabbitears.info/searchmap.php?request=result&study_id=110775
John

There's no doubt that the antenna interacting with the ground causes the nulls and the main lobe shifted above 0 degrees. The free space pattern of the models don't show any such nulls. I have read comments on the effects of an imperfect ground but it's not so dramatic that the nulls don't exist. An imperfect ground may reduce the nulls but it also reduces the ground gain. It's well known in amateur radio circles that the height of the antenna makes a huge difference in the performance. The ground definitely causes the main lobe to be tilted up and the higher you go the lower the main lobe gets. I see no reason why this wouldn't be true at low VHF. An imperfect ground doesn't make an antenna act like it's in free space independent of the elevation. How I wish it was so! Rabbitears doesn't show the effects of the elevated pattern because it doesn't take the receive antenna pattern into account. Same goes for the transmitter vertical antenna pattern. It's not taken into account. That's the reason it will show noise margins of 70 or 80 dB near the tower. This is in error because the ERP far below the main lobe is greatly reduced.

If your antenna is not over flat ground then things are different. An antenna looking over a downslope has a lower angle of radiation. An antenna over irregular ground can be modeled but you need a detailed ground profile to enter into the model.
 

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You are correct that the Rabbitears model does not include transmitting antenna vertical pattern info.

The model does attempt to calculate actual field strength at the received location's height. The elevation pattern over a reflecting ground is not really an antenna pattern but the effect of the received field strength at the antenna's height. This should be calculated by the propagation model. One description of the Longly Rice model describes how it is used here: https://www.softwright.com/faq/engineering/prop_longley_rice.html in the Path Parameter Calculations section.
John
 

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You are correct that the Rabbitears model does not include transmitting antenna vertical pattern info.

The model does attempt to calculate actual field strength at the received location's height. The elevation pattern over a reflecting ground is not really an antenna pattern but the effect of the received field strength at the antenna's height. This should be calculated by the propagation model. One description of the Longly Rice model describes how it is used here: https://www.softwright.com/faq/engineering/prop_longley_rice.html in the Path Parameter Calculations section.

John

I can accept that the model is calculating the correct received signal based on the antenna height. That doesn't affect my point that an antenna with a main lobe tilted up will generate a weaker signal than if the main lobe was pointed at the horizon where most signals come from. This is effectively the same thing as the antenna being off pointed horizontally. The angle of arrival of the signals and the peak response angle of the antenna are different things and need to be pointed at each other for maximum signal.

I'm not sure what you're saying about the elevation pattern not really being an antenna pattern. :confused: Every discussion I've ever seen refers to it as the antenna pattern. Maybe you're saying that the elevation pattern is the result of the antenna interacting with the ground and not just the pattern of the antenna itself?
 

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Despite all the discussion of reception of low VHF stations, there is one issue that has been completely overlooked: Vertical Radiation Angle.

This reinforces what I've been saying, low VHF ... needs to be as high as possible. A good height to shoot for is 55 feet or 3 wavelength. I think most people would be amazed at how good their low VHF reception would be with such an antenna.
Thank you for this. It explains a lot.

Unfortunately, for many 55 feet is simply impractical. The FCC's OTARD rule only protects antennas up to 12 feet above the roofline. My roofline is 16 feet; that means that I can legally go to 28 feet, but beyond that I'm at the mercy of my community's ordinances, any HOAs (not a factor for me but a big issue for many), etc. My community is pretty forgiving but a 55-foot tower in a residential area, even behind the home, would probably raise a lot of eyebrows.
The loss at low received angles is caused by the out of phase ground reflected wave canceling the direct path wave.
That makes me wonder: are there any antenna designs that could circumvent this issue by blocking the ground reflection somehow? Some sort of reflective screen mounted below and in front of the main antenna, perhaps? It'd be huge but it wouldn't necessarily have to be 55 feet high.
 

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Calaveras,
I like your last sentence. Also, regardless of the verbiage, the calculated response of a horizontal antenna over a flat perfectly conducting ground is as you have indicated. However the 6 low VHF stations in my Rabbitears propagation prediction show very little change between 25 and 50 feet of antenna height.

John
 

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Thank you for this. It explains a lot.
That makes me wonder: are there any antenna designs that could circumvent this issue by blocking the ground reflection somehow? Some sort of reflective screen mounted below and in front of the main antenna, perhaps? It'd be huge but it wouldn't necessarily have to be 55 feet high.

The screen would just become a better ground. The only way to reject the reflected path is to design an antenna with a vertical beamwidth less than a degree which is more impractical than a couple of hundred foot tower.
John
 

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However the 6 low VHF stations in my Rabbitears propagation prediction show very little change between 25 and 50 feet of antenna height.
The Rabbitears report is interesting and very helpful but it's only a starting point. The real world test is the final word which is a good thing because Rabbitears says my reception should be difficult at best. My report:

https://www.rabbitears.info/searchmap.php?request=result&study_id=11616

Rabbitears predicts that my strongest UHF signals are with the antenna 5-10 feet off the ground. An antenna that low is useless here because it's looking through miles of vegetation. The antenna starts to work when it gets up to about 25' where the diffraction edge becomes LOS.

One of the nice things about having a crank-up tower is that it's easy to compare an antenna at various elevations. Attached is a spectrum analyzer image showing three stations with the antenna at 47, 57 and 67 feet. Higher is definitely better even though Rabbitears says the opposite. Looks like I'm getting to the point of diminishing returns at 67'.

Even in the real world you might not see that much difference on low VHF between 25 and 50 feet. The elevation plots are hard to read at 1 degree but it looks like it could be as little as a few dB.
 

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Unfortunately, for many 55 feet is simply impractical. The FCC's OTARD rule only protects antennas up to 12 feet above the roofline. My roofline is 16 feet; that means that I can legally go to 28 feet, but beyond that I'm at the mercy of my community's ordinances, any HOAs (not a factor for me but a big issue for many), etc.
The rule does not say that HOAs can prohibit antennas higher than 12' above the roofline. It says:

Masts higher than 12 feet above the roofline may be subject to local permitting requirements for safety purposes.

If the antenna needs to be higher to say clear a tree for reliable reception, then they must permit that as long as you meet any legitimate safety concerns.


My community is pretty forgiving but a 55-foot tower in a residential area, even behind the home, would probably raise a lot of eyebrows.That makes me wonder: are there any antenna designs that could circumvent this issue by blocking the ground reflection somehow? Some sort of reflective screen mounted below and in front of the main antenna, perhaps? It'd be huge but it wouldn't necessarily have to be 55 feet high.
55' is only my suggestion. You do what you can do. A 3 section push-up mast on top of a house with a rotor and a 5' mast on top will get your antenna up around 50'. No tower required. This kind of setup used to be common before cable, especially in fringe areas.
 

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55' is only my suggestion. You do what you can do. A 3 section push-up mast on top of a house with a rotor and a 5' mast on top will get your antenna up around 50'. No tower required. This kind of setup used to be common before cable, especially in fringe areas.
I grew up on the East Coast, and worked for a paper dryer company in the late 70's. I'll never forget the first time I visited California on a business trip to Hollister (which is deep fringe to SF). All the houses were single story with a 50' push up mast on top. The edge of town was a little elevated. From that vantage point, it looked like an aluminum forest.
 

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55' is only my suggestion. You do what you can do. A 3 section push-up mast on top of a house with a rotor and a 5' mast on top will get your antenna up around 50'. No tower required. This kind of setup used to be common before cable, especially in fringe areas.
I'm thinking I could do 35' (which is a bit over 2 wavelengths at RF 3) without raising too many eyebrows. At 21' RF 3 is right on the cliff, so 35' may be enough even if it isn't ideal.

There are plenty of tall oak trees in my neighborhood: a problem for higher frequencies, but they mean a 35' mast and antenna in the back might not even be visible from the street!
 

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The screen would just become a better ground.
To be clear, I wasn't thinking of a level screen! I was thinking of an angled one with the antenna at the crest. The reflections would be directed away from the antenna and the polarization angle would be changed.

But if the interfering signal comes in at a 1-degree angle (!) it would have to be hundreds of feet long to work. So, yet another impractical solution. Rats.

There's just no good answer to this. No wonder stations take VHF-Lo (especially RF 2-4) only as a last resort.
 
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