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Old 2-Dec-2011, 7:26 PM   #1
GroundUrMast
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Low VHF reception (2 - 6) Experience & Opinions

My personal experience does not include reception of ATSC on the low VHF channels 2 through 6. There are no local low-VHF stations on the air in the Seattle market at this time. I'd like to invite those with real world experience to weigh in on the subject.

In particular, what is a practical minimum NM goal for reliable reception in the low-VHF band?
Though I'm aware that there is more electrical noise in the spectrum between 54 and 88 MHz compared to the higher TV broadcast allocations, I don't consider myself an authority with 'hard data' on the exact difference between each band's average or quiescent noise level. My research leads me to believe that the low-VHF band is particularly prone to impulse noise. Characterization of impulse noise would need to account for at least amplitude and duration, data I have not found as of yet.
What other pitfalls or hurdles are common and how do you overcome them.

What are the differences between generations of DTV tuners, (related to low-VHF reception)?
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Old 20-Dec-2011, 2:54 PM   #2
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Quote:
Originally Posted by GroundUrMast View Post
In particular, what is a practical minimum NM goal for reliable reception in the low-VHF band?
That's not an easy question to answer. The noise on channel 2 is 4-6 db worse than channel 6. The take-off angle of a receive antenna is closer to the horizon on channel 6 than channel 2. During the Sporadic E season channel 2 will get distant co-channel far more often than channel 6. Yet channel 6 can suffer from FM interference.

The FCC planning factor for low band VHF allows 4 db extra noise, but that is not considered on TVfool. Coverage to an indoor antenna is far worse on VHF than UHF.

Next the available low-band VHF receive antennas have less gain than UHF antennas. A slight down-slope in front of a low band VHF antenna can help significantly.

Bottom line, on channel 6 I look for a NM of 10 db or more and then the antenna must have at least 6 db gain and the antenna location must be favorable and the noise level must be at or below normal. If channel 2, I'd be trying for 18 db NM or more.

Rarely does a preamp help low band VHF reception.
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Old 20-Dec-2011, 5:48 PM   #3
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http://transition.fcc.gov/Bureaus/En...7/fc97115a.pdf

pp A3, section III implies that noise in the low VHF band requires at least 13 dB better net NM (per TV Fool prediction corrected for antenna gain and system losses including amplifier NF) than UHF band.

@ TG, This seems quite consistent with your observations and summary.

I generally try to achieve a minimum net NM of +10 dB in the UHF band. So I feel no guilt targeting a minimum of +23 net NM in the low VHF band. When dealing with edge-path signals, I expect these minimums to be inadequate.
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Old 21-Dec-2011, 5:25 AM   #4
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Some where I heard a NM of 15 as minimum for low vhf (but I can't find the reference)

Quote:
Rarely does a preamp help low band VHF reception.
I thought that 15 NM had to do partly from the estimated noise floor of the tuner. IIRC that was 7 or 8 dB for low vhf and 5-6 for uhf for an "average" tuner.
So (even if it's hooked directly to the tuner) how does having a noise floor of 1 or 2 for a low noise amp not better than the stock tuners sensitivity?
And (since I would mast mount not hook directly to the tuner) how is it not an improvement to have the noise sensitive components 15-40+ feet and a house wall/roof away from the noisy interior of the house?
Are these benefits drowned out by some other factor?
I know a lot of consumer antennas have high SWR on low VHF frequency
and even SWR of 2 can raise the "effective" noise floor by 3+ dB.
(for digital signals only) due to vector magnitude error.
And amplified garbage is garbage but there is evidence that a high quality (pre) amp can help stabilize SWR and counter this effect at least some.
On my channel 4 lcd monitor, fans(ac) and microwave oven interfere with reception that I've seen for sure.
My primary plan to get channel 4 includes a "custom cut" short boomed yagi (modeled at 10 dBi ) and mounting an old digital converter box (tuned only to 4 and with the output switch set to channel 4 ) in a grounded metal box on the tower to avoid noise and overcome some cable loss.
Then joinntenna or a/b switch the analog 4
My current antenna is about 6 dBi (by rough modeling)
plan A should improve signal by ~6dB
I was thinking I wouldn't need an amp (other than the one in the tuner)
My NM is 15-16
I hope these measures will prove adequate as this borders on heroic
plan b is truly heroic and would be best avoided.
I used some calculation to arrive at needing 10 dBi.
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Old 23-Dec-2011, 3:54 PM   #5
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So (even if it's hooked directly to the tuner) how does having a noise floor of 1 or 2 for a low noise amp not better than the stock tuners sensitivity?
Unless you live in the country with buried power lines and no part 15 devices in your house, adding a preamp makes the desired signal and received noise go up by the same amount.

In nearly all real-world situations the noise on low band VHF is higher than the internal noise of the tuner itself.

That said, you can decrease the EVM by the use of a preamp, but the net S/N increase is very small. Amplification helps if there are any splitters.

What can screw up the process is lets say that you make a low band signal work fine during the day, but the earth turns and later the Milky Way is in the main beam of your antenna. The higher sky noise from the Milky Way can kill the signal.

The same effect occurs on UHF if the sun rises in the main beam of the antenna. The sun temperature ruins the NM.

Last edited by Tower Guy; 23-Dec-2011 at 4:07 PM.
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Old 24-Dec-2011, 7:45 AM   #6
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Quote:
the earth turns and later the Milky Way is in the main beam of your antenna.
There were patterns that I thought were more heat/cool air turbulence scatter type.Looking back they seem pretty sharp daily time wise.
That would explain a lot.

Quote:
buried power lines and no part 15 devices in your house
Power lines are that noisy? Is it no quieter 40 ft in the air than inside the house?
Since I plan on mounting a tuner on the tower would powering it with 12 v DC help noise interference wise?
I'm a little confused on how the interference gets into the tuner (other than through the antenna ) seems it can leak in every where.

The plot for my locale reports a 1 dB better sweet spot at 23 ft to get a reported NM of 16 again takes a height of over 100 ft.
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Old 24-Dec-2011, 4:50 PM   #7
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Reception

The Milky Way ????????????? This technical hair splitting can go on till the end of everything. The result is the question askers are _____________ out of receiving VHF low band channels 2 thru 6 because of the doubt. The question askers are told not to think about it and do not do it.
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Old 24-Dec-2011, 7:37 PM   #8
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This thread is specifically open to technical hair splitting.

Quote:
Originally Posted by Electron View Post
The Milky Way ????????????? This technical hair splitting can go on till the end of everything. The result is the question askers are _____________ out of receiving VHF low band channels 2 thru 6 because of the doubt. The question askers are told not to think about it and do not do it.
If you believe I or others are overly conservative when making L-VHF reception equipment recommendations, please make your case. Feel free to answer"...what is a practical minimum NM goal for reliable reception in the low-VHF band?"

Please expect to be challenged to provide support for your answer to the above.
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Last edited by GroundUrMast; 10-Jan-2012 at 5:04 AM. Reason: Removed tangential comment
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Old 25-Dec-2011, 1:28 AM   #9
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Power lines are that noisy? Is it no quieter 40 ft in the air than inside the house?
Yes, but the power lines themselves are not the only culprit. The connections between the lines & transformers can emit high levels of noise (static), especially if not properly tightened. It is worse when the humidity is low & barely evident when humid. I've even seen analog UHF affected at my first residence in the winter time. This is not an issue with underground power distribution from my experience.

Once the power is stepped down by the transformer & enters the house, it shouldn't be an issue.
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Old 11-Jan-2012, 1:54 PM   #10
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Lo band tv reception

Here are actual Reception cases of channels 2 thru 6. The question askers need and are getting actual tv reception. Not endless conversations about the merrits of this dB and that dB and and cosmic noise. And the question askers do not need technical hair splitting. The question askers have a right to receive and watch tv. I have Always Advocated the reception of channels 2 thru 6 with out all the hub bub.http://forum.tvfool.com/showthread.php?t=2938 , http://forum.tvfool.com/showthread.php?t=2899 .

Last edited by Electron; 13-Jan-2012 at 2:35 AM. Reason: To stay on topic
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Old 11-Jan-2012, 5:17 PM   #11
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Originally Posted by Electron View Post
The Milky Way ????????????? This technical hair splitting can go on till the end of everything. The result is the question askers are _____________ out of receiving VHF low band channels 2 thru 6 because of the doubt. The question askers are told not to think about it and do not do it.
Please read the following. It specifically states that the Milky Way is the main source of cosmic noise.
http://www.radartutorial.eu/18.expla...s/ex08.en.html

If you want to learn more spend some time Googling. You'll find that the noise temperature of the Milky Way reaches as much as 1400 degrees Kelvin, or an equivalent noise figure of about 8 db. With most good preamps at 3 dB NF or less, an 8 db noise figure due to cosmic noise is not "hair-splitting".
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Old 11-Jan-2012, 5:44 PM   #12
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In the case of http://forum.tvfool.com/showthread.php?t=2938 the predicted NM of WPVI, real CH-6 is 31.6 dB and a 2-edge path. This does not strike me as a particularly challenging situation. I'd expect modest antenna gain and typical distribution losses to result in a slight improvement of the net NM delivered to the tuner(s). If I use a very conservative estimate for the noise floor of channel 6, let's say 15 dB higher than that of a UHF channel, there is more than 16 dB fade margin available in this case.

None the less, this is a valuable example in that it provides a valid data point.
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Last edited by GroundUrMast; 11-Jan-2012 at 7:05 PM. Reason: grammar
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Old 11-Jan-2012, 5:56 PM   #13
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Here are actual Reception cases of channels 2 thru 6. Not technical hair splitting. http://forum.tvfool.com/showthread.php?t=2938 , http://forum.tvfool.com/showthread.php?t=2899 .
In the first thread, we were all in thunderous agreement about trying to get WPVI. In the second thread, you consistantly failed to see that the person already had a low-VHF that worked! We were disagreeing with you only in the sense that you were recommending duplicate capability.

This issue has always been a cost/benefit analysis. If the only station in the lo-VHF band is a low-power station with minor networks, I sometimes make the judgement that it's not worth the hassle. You obviously make a different judgement, and that's OK. What's not OK is to take differing technical judgements personally.
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Old 11-Jan-2012, 6:06 PM   #14
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The case of http://forum.tvfool.com/showthread.php?t=2899 is more interesting. The predicted NM for WMC, real CH-5 is only 3.7 dB and, in addition to the 2-edge path, there is some co-channel interference indicated.

The OP is/was using an antenna described as, "about 20 years old and its huge". That leaves us to estimate/guess at the gain and F/B ratio.

The OP reported less than 100% reliable reception of WMC: http://forum.tvfool.com/showpost.php...2&postcount=13

My 'read' of this situation is, beginning with a NM of less than 4 dB is at best a marginal situation. Such a situation is not hopeless, but calls for realistic expectations to be set, and deserves aggressive or even extreme measures to improve the reliability of reception.

Again, a very useful data point.
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Last edited by GroundUrMast; 11-Jan-2012 at 6:33 PM. Reason: sp. / grammar
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Old 16-Jan-2012, 1:10 AM   #15
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Very interesting
Quote:
WMC always is at about 80%. been at 95% to 100% today. thanks for your recommendations
I wonder if this is the signal level from the tuner or is it viewable 80% of the time. Either answer seems to contradict "common wisdom" of what signal level is receivable.
Yeah and a co-channel to!
But one person's results (one data point) is only really relevant in the see it can happen realm.
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Old 9-Mar-2012, 1:28 PM   #16
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RF 5 using "7-up"?

can anybody relate any experiences they have had attempting to pick up a lo-VHF station with a "7-up" antenna?

I've got just one lo-VHF station in my area (RF 5) my zip is 50036. I'd read on denny's ant page that properly sited a 7-up antenna can "help" with lo-VHF signal, even though it was not designed for it.

up-7 - received 2-6 thru red color?

I've had luck getting *some* hi-VHF signal out of a UHF only yagi (it's the only ant I have right now). The DTV box found RF 5, 8, 11, 13, but only RF 11 & 13 are received consistently. It was more than I expected, and made me wonder what kind of signal I could get on RF 5 using just a hi-VHF ant....

For me RF 5 (WOI) is not really critical (especially now that they have nixed RTV on their subchannel). If we were talking PBS on RF5, the story would be MUCH different (kids need their sesame street).

The terrain around here is best described as "flat as a pancake, with blueberries here and there to make things interesting".
Attached Images
File Type: png 50036-DSM-metro.png (64.3 KB, 551 views)

Last edited by renstyle; 9-Mar-2012 at 1:39 PM. Reason: forgot to add the analysis....
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Old 9-Mar-2012, 2:53 PM   #17
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Two points:
  1. You are asking for help with reception. You should do what everyone else does. Post a request to the Help with Reception area. Include a live link to your TV Fool Radar plot rather than the picture of it.
  2. A TV antenna is an assembly or wires and rods that is designed to receive RF signals over particular bands of the RF spectrum. There is no home for ugly stepchildren who require Lo-VHF antennas. Buy the antenna that you need.
An antenna that is advertised as 2-69 is designed to receive Low-VHF. The engineers who design these antennas are not stupid. It is absurd for anyone to claim that a High-VHF antenna does a better job with Low-VHF signals than a Low-VHF antenna. That said, there are other issues. You may have multi-path reflections or your antenna may be improperly aimed.

Your Low-VHF signal is very strong. Many viewers in your situation have managed to receive these stations with paper clips, coat hangers, or aluminum foil. A UHF-only antenna will do a better job on Low-VHF than a paper clip. A High-VHF antenna will do a better job than a coat hanger.

For a strong signal situation like your RF Channel 5, you have a lot of flexibility. However, your best bet is a full-channel antenna.
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Old 9-Mar-2012, 3:04 PM   #18
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Quote:
Originally Posted by MisterMe View Post
Two points:
  1. You are asking for help with reception. You should do what everyone else does. Post a request to the Help with Reception area. Include a live link to your TV Fool Radar plot rather than the picture of it.
  2. A TV antenna is an assembly or wires and rods that is designed to receive RF signals over particular bands of the RF spectrum. There is no home for ugly stepchildren who require Lo-VHF antennas. Buy the antenna that you need.
An antenna that is advertised as 2-69 is designed to receive Low-VHF. The engineers who design these antennas are not stupid. It is absurd for anyone to claim that a High-VHF antenna does a better job with Low-VHF signals than a Low-VHF antenna. That said, there are other issues. You may have multi-path reflections or your antenna may be improperly aimed.
Thanks for that, my first post to the forum was just that ^^, it's waiting for the moderator to approve for viewing. While I was typing up that post, it made me think about these other possibilities (more theory and conversation than anything else).

Quote:
Originally Posted by MisterMe View Post
Your Low-VHF signal is very strong. Many viewers in your situation have managed to receive these stations with paper clips, coat hangers, or aluminum foil. A UHF-only antenna will do a better job on Low-VHF than a paper clip. A High-VHF antenna will do a better job than a coat hanger.

For a strong signal situation like your RF Channel 5, you have a lot of flexibility. However, your best bet is a full-channel antenna.
Sounds like there was too much "background info" in my post, made it sound like an assistance post, apologies for that.

I was just looking for others that have *tried* something similar, whether on a lark or not. I'd prolly not even thought about it myself if I wasn't using a UHF ant. It's always good to know where your margins are (and that's the thrust of my other as-yet-in-limbo post).

cheers!
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Old 20-Jul-2012, 8:24 PM   #19
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Here's a power point presentation focusing on the general topic of VHF reception:
http://www.google.com/url?sa=t&rct=j...8o0pVbxGO9doXw

And here's OET69: http://transition.fcc.gov/Bureaus/En...et69/oet69.pdf

Though focused on High-VHF, this article has application: http://www.tvtechnology.com/conferen...for-dtv/197955

Last edited by GroundUrMast; 20-Jul-2012 at 9:45 PM. Reason: Added link to OET69 & tvtechnolgy
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Old 10-Oct-2012, 6:47 PM   #20
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A long document, that incorporates quite a bit of information from earlier work. Interesting to see different parties contending for their ideas and interests.

http://www.google.com/url?sa=t&rct=j...yeDhJy1Dw1TGzw
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