Is there a breakeven point in cable run length to justify antenna amplifier vs. noise added to signal? How do you do the dB math?

I have a 100' run of RG-6 with a 4-bay UHF bow tie antenna, and no splitters. Would adding a Winegard HDP269 at the antenna with 12dB gain, 3 dB noise figure be beneficial or detrimental in a remote signal area?

More info please . a TVfool.com plot would be most useful in answering your question.

However, in general - if you don't need any amps - don't use it. Just one more piece of equipment to troubleshoot.

Reason (besides long runs) for amps - running splits to multiple locations.

Plot attached.
Let assume 5-6dB loss in the coax from antenna to TV. What does a 12dB gain, 3 dB noise add from an amp do for this?

You're in Canada? What channels can you watch reliably or intermittently now? That preamp will definitely help, and a higher gain preamp will help even more, but you need to develop the best antenna that you can. You should put your money there first.

HDTVPrimer recommends a pre-amp on any cable run over 20 feet.

Personally I think it's a bit arbitrary. If you're in a remote area (no local strong signals to cause overload), a low noise pre-amp would usually be of benefit regardless, because the (low) noise figure of the amp is better than the noise figure of your TV's tuner. TV reception is mostly about signal-to-noise ratio, and a low noise figure translates into better signal-to-noise ratio.

You're in Canada?

This location is in Michigan on the shore of Lake Superior where I will be camping this summer. I cannot tell you the present digital reception condition.

Let me talk my way through this and please correct as necessary:

Let's figure my antenna has a gain of 12dB over HDTV frequencies.

So, I guess we consider which stations can be pulled out of the noise given the TVfool data. What threshold, before line losses, is considered a usable signal for an ASTC tuner?

Then, if we use a preamp we need to consider its noise contribution (3db), right?

So, after that it becomes amp gain (12db) if used vs. transmission line loss (6db)??

What is gained here with a preamp?

If you're in a remote area (no local strong signals to cause overload), a low noise pre-amp would usually be of benefit regardless, because the (low) noise figure of the amp is better than the noise figure of your TV's tuner. T

What would be a relevant noise figure for a DTA converter box tuner?

What does a 12dB gain, 3 dB noise add from an amp do for this?

Assuming your TV has a noise figure of 6 dB (being generous), with the 5-6 dB of cable loss, you'd have an overall noise "factor" equivalent to 11-12 dB.

Adding the amp lowers the overall "factor", not all the way to 3 dB (because the TV noise figure still adds in to some extent, and because there are still some losses before the amp), but probably to somewhere in the range of 5-8 dB.

A higher gain amp might improve that somewhat (because it decreases the impact of the TV noise figure), but it's a game of diminishing returns. Don't buy more amp than you need. With a single cable run and no splitters, I'd stick with a 20 dB amp.

What would be a relevant noise figure for a DTA converter box tuner?

About the same as a run-of-the-mill TV - probably somewhere between 6 and 10 dB. Just like TVs, some converter boxes are better than others.

If you search out some old posts from forum member dr1394, he has posted the formula calculating the improvement in S/N ratio when using a low noise preamp. Based on your situation, I'd be inclined to go with a 23 dB gain Channel Master, or the ultra low noise Research-somethingorother preamp that several members here have had good luck with, but you could gain almost another 10dB of noise-free antenna gain with a long UHF Yagi antenna.

If you search out some old posts from forum member dr1394, he has posted the formula calculating the improvement in S/N ratio when using a low noise preamp...

Found one:

The minimum signal level for terrestrial (antenna pointed at the horizon) receivers is:

thermal noise floor + noise figure + required S/N ratio

The thermal noise floor is:

p=ktb

p = noise power
k = Boltzmann's constant (1.3807x10E-23)
t = antenna temperature (290K = 17C)
b = bandwidth (6 MHz)

p = 1.3807x10E-23 * 290 * 6000000

noise power in dBm (10 log p + 30) = -106.2

The CM-7000 requires 15 dB of SNR, so the minimum signal is -106.2 + 15 or -91.2 dBm. Since the specified sensitivity is -83 dBm, then the noise figure is -91.2 - -83 or 8.2 dB.

For the TVFool numbers, you have to add your antenna gain and estimate any additional loss. For example, the WLNS-DT signal level is -85.2 dBm. You'll need at least 2.2 dB of antenna gain with no additional losses...

Ron

Would adding a Winegard HDP269 at the antenna with 12dB gain, 3 dB noise figure be beneficial or detrimental in a remote signal area?

The HDP-269 would be extremely beneficial in that location. In fact, a higher gain preamp would help even more and would not be overloaded unless there is a nearby FM station.

The verbose math is here: http://www.microwaves101.com/encyclopedia/noisefigure.cfm

The thermal noise floor in dr1394 formula is actually the noise picked up by the antenna and is often called antenna noise. This formula would be correct for an antenna aimed at the ground. For one aimed at the horizon, in a quiet location it could be half that. It could be even less for an antenna tilted up a few degrees.
John

The thermal noise floor in dr1394 formula is actually the noise picked up by the antenna and is often called antenna noise. This formula would be correct for an antenna aimed at the ground. For one aimed at the horizon, in a quiet location it could be half that. It could be even less for an antenna tilted up a few degrees.
John


Not true. That calculation of noise floor does not rely on the antenna picking up anything. It's the basic noise floor calculation that would be true in any device, even a resistor. It's -174 dBm/Hz everywhere the temperature is 290 K. -174 dBm/Hz = -106.2 in a 6 MHz bandwidth.

The thermal noise floor in dr1394 formula is actually the noise picked up by the antenna and is often called antenna noise. This formula would be correct for an antenna aimed at the ground. For one aimed at the horizon, in a quiet location it could be half that. It could be even less for an antenna tilted up a few degrees.
John

I wonder if anyone has tried putting dry ice on the ground in front of an antenna?

I realize that you are mostly making a joke but that won't help. Unless the antenna is very badly designed it produces almost no noise at any reasonable temperature. All of the antenna noise is picked up from the warm ground, other warm materials and maybe the atmosphere or man made sources. In RF quiet locations it would be mostly the ground. In the winter it would be a little lower than the summer. If you point the antenna up at a high angle its noise pick up will get a lot lower. Satellite antennas are often specified at a noise temperature above a provided elevation angle. This value is way below the ambient ground noise temperature.
John

if the antenna and receiver were placed in a RF anechoic chamber it would still result in a 290K noise temperature at the receiver input.

the warm earth, sky noise, atmospheric losses and man-made noise result in a complicated frequency dependent noise temperature which is not accounted for in the dr1394 formula.

The noise temperature Tn of two amplifiers in cascade is the noise temperature of the first plus the noise temperature of the second divided by the gain of the first:

Tn = T1 + T2/G1.

To convert Noise Figure(f) to noise temperature (Tn):

Tn = 290((10^(f/10)) - 1)

The 2.0 dB CM7777 preamp has a noise temperature of 170K at UHF.

Let's say that the Noise Figure of the receiver is 10 dB and the 200' RG6 cable loss is 10 dB for total Noise Figure of 20 dB. The receiver noise temperature is 28710K

Then the system noise temperature for the 2.0 dB NF 26 dB gain preamp is:
170 + 28710/398 (26 dB) = 242K

To convert from noise temperature back to Noise Figure:

NF = 10 log(1 + Tn/290)

So the system Noise Figure with the CM7777 preamp and 200 feet of coax is 2.64 dB. The -0.64 dB difference over 0 feet of coax is almost imperceptible.

Ron

Rather than do a lot of spot calculations, it's probably more useful to develop some "rule of thumb" from the equations.

"To only suffer 1 dB additional system noise figure over the noise figure of the preamp itself requires that the preamp gain is 4 dB more than the total of the coax loss and receiver noise figure."

In other words, if your coax loss is 3 db and your receiver noise figure is 7 dB (for a total of 10 dB), the preamp gain required is 14 dB.

Let's test the premise with a 2.0 dB NF preamp at 26 dB, 20 dB and 14 dB gain.

170 + 2610 / 398 (26 dB) = 176K or 2.06 dB

170 + 2610 / 100 (20 dB) = 196K or 2.24 dB

170 + 2610 / 25 (14 dB) = 274K or 2.89 dB

The idea is to match the preamp gain to the coax loss and receiver noise figure. More gain buys you very little improvement in signal to noise ratio but a lot of degradation in overload performance.

Ron

Some links:

http://www.qsl.net/yu1aw/vhfnoisetemp.pdf

http://www.punawai.net/Articles/Antenna/Basic%20Concepts%20of%20Antenna%20Noise%20Temperature.pdf

Ron

TV reception is mostly about signal-to-noise ratio, and a low noise figure translates into better signal-to-noise ratio.

Bingo. A lower noise figure on your first amplifier and everything before it is equivalent to (for purposes of SNR) the same amount of additional antenna gain. I don't know the actual NF of a TV tuner can, but the numbers usually kicked around are in the 5-10dB range; then you add the coax loss and any splitters.

Rather than do a lot of spot calculations, it's probably more useful to develop some "rule of thumb" from the equations.

"To only suffer 1 dB additional system noise figure over the noise figure of the preamp itself requires that the preamp gain is 4 dB more than the total of the coax loss and receiver noise figure."

In other words, if your coax loss is 3 db and your receiver noise figure is 7 dB (for a total of 10 dB), the preamp gain required is 14 dB.

Let's test the premise with a 2.0 dB NF preamp at 26 dB, 20 dB and 14 dB gain.

170 + 2610 / 398 (26 dB) = 176K or 2.06 dB

170 + 2610 / 100 (20 dB) = 196K or 2.24 dB

170 + 2610 / 25 (14 dB) = 274K or 2.89 dB

The idea is to match the preamp gain to the coax loss and receiver noise figure. More gain buys you very little improvement in signal to noise ratio but a lot of degradation in overload performance.

Ron

This is the "dB math" I'm looking.

Wouldn't the final step after this calculation be to compare the noise increase of the amplified system vs. the noise (loss) of the transmission line itself to see if you're better off with the amplifier or not?

I am not sure what you are asking but you are always better off with the preamp if the following conditions are met. The preamp noise figure is lower than the receiver's and the preamp does not overload. The first is generally true for most of the commonly recommended preamps. The second will be met if all signals coming from the antenna are not too strong.
John

Wouldn't the final step after this calculation be to compare the noise increase of the amplified system vs. the noise (loss) of the transmission line itself to see if you're better off with the amplifier or not?

That's essentially whats you are doing when you calculate system noise figure. The loss of the cable contributes to the system noise figure.

Once the cascade noise figure is calculated, then you can subtract it from the NM column of tvfool to figure out how much noise margin you will have with a particular antenna. (Or how big of an antenna you need for a given noise margin.)

I am not sure what you are asking but you are always better off with the preamp if the following conditions are met. The preamp noise figure is lower than the receiver's and the preamp does not overload. The first is generally true for most of the commonly recommended preamps. The second will be met if all signals coming from the antenna are not too strong.
John

Preamp overload when signals are at different signal strengths is a topic that has never been adequately addressed in this forum and probably never will be.

Published preamp and amplifier overload figures are referenced to engineering benchmarks of input levels that develop a certain amount of sync compression, cross modulation, 2nd or 3rd order IMD, etc. With preamps, that reference level may serve as a useful practical input level for analog signals or when digital signals are of equal strength, but with digital signals that vary greatly in strength, as is common when one is pre-amping to aid the reception of a weak signal, the input at which the combination of intermodulations of the strongest signals beats the weaker signals to paste is always well below any of the published overload threshold levels for any preamps.

A few years ago, I posted my tale of woe incurred when I was tryimg to mix weak off air signals from one market with strong local signals. I build multichannel, master antenna system headends and have a lot more filtering resources at my disposal than do most of the members here. I was trying to boost up a filtered UHF channel 59 signal that was probably down around -30dBmV coming into my preamp, but two other local signals were as strong as +10. If I fed that antenna signal unfiltered into a Winegard or ChannelMaster 23dB gain preamp, the channel 59 signal became unusable even though the preamp outputs was maybe 15 to 20 dB below the published overload specs, but if I prefiltered down those two stronger signals, using either a filter that most consumers wouldn't want to pay for, or even a more economical discontinued Tru-Spec BPF-UHF, I could enjoy the benefit of a net gain about 20dB on channel 59 after subtracting the filter insertion loss with no discernible qualitative degradation of the channel 59 signal.

BTW, it may not have been published here recently, but newbies should be made aware that the gain of a Winegard AP-4800 is actually around 38dB in the 500 MHz range, versus about 28 dB at 600 MHz and above, which probably explains why there have been more reports of Winegard high gain preamps making situations worse than ballpark overload calculations would otherwise predict.

I have some experience with overload here. This is my location http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d7f145f823dd95c
Anywhere I look overload is spelled ION.

This post from andy.s.lee, with graphs, helped me to understand how pre-amps help the signal-to-noise ratio (or more accurately, help to preserve SNR). It's a visual depiction of noise figures, cable losses and such.

Visual explanation of Noise Margin (http://www.avsforum.com/avs-vb/showthread.php?p=15700679#post15700679)

The graphs are a simplification - they largely ignore the affect of the tuner's noise figure - but I think they're a good approximation, and simple is good.

I have some experience with overload here. This is my location http://www.tvfool.com/?option=com_wrapper&Itemid=29&q=id%3d7f145f823dd95c
Anywhere I look overload is spelled ION.

I couldn't get your link to fully open, but when I entered the Mystic, CT zip of 06355 into TV Fool, I see that your ION channels 17 and 26 are brutally strong. They are prime candidates for notch filter attenuation, since there is no adjacent channel to get damaged in the process. Did you wind up notch filtering them?

Try this attachment.
I have two dual channel TINLEE notch filters that will reduce each channel by about 30 dB. Now that that station on 26 has left 34 I am trying to get two antenna systems so I can get both Providence and Hartford.
John

Thanks all for the great information and advice.

I have ordered a Winegard AP-8700 preamp: Middle ground with respect to gain vs. overload susceptibility, and OK noise for the buck at 2.8dB.

Try this attachment.
I have two dual channel TINLEE notch filters that will reduce each channel by about 30 dB. Now that that station on 26 has left 34 I am trying to get two antenna systems so I can get both Providence and Hartford.
John

Wow, if tvfool is correct, you are overloading off of a -31dBm station. I wonder if the tvfool calculations are a bit off in this case. What pre-amp are you using?

Wow, if tvfool is correct, you are overloading off of a -31dBm station. I wonder if the tvfool calculations are a bit off in this case. What pre-amp are you using?

It is plausible and in fact quite likely that ctdish's system was overloading before he inserted the notch filters. -31 dBm is about +17dBmV calculated to be the strength coming off an isotropic antenna. If he has 10 dB of antenna gain, then that would be +27 dBmV.

With a 26dB gain Channel Master, it would be amplified to 53dBmV. With a Winegard 28 dB gain amp, it would be 55dBmV.

I don't have any Channel Master overload specs handy, but as I recall, they used to claim that their preamp would attain either 5% sync compression, or, in later printings, -46 dBc cross modulation" with something like two channels at 54 dBmV or 4 channels at 51 dBmV. If those are the numbers, then you would attain those same compression and cross-mod levels at 57 dBmV

dBc means decibels below carrier. Cross modulation that is 46 dB below ctdish's 55dBmV ion signal would have a strength of 9 dBmV. such interference would have no discrenible effect on any 55dBmV signal, but would totally decimate weaker signals. I suggest re-reading my post #25 in this thread, in which I cite an instance where I tried two 0064DSBss and two Winegard AP4727s, 23 dB gain preamps, had output levels well below the published overload specs for those devices and yet the intermodulation distortion ruined the weaker signal I was trying to improve.

Here is a post I wrote a couple of years ago when my recollections were clearer than they are now:

Eventually, we're going to start paying a lot more attention to dBc than we are now. Cable companies, when assessing the suitability of their amplifiers, have the luxury of dealing with a spectrum of signals that are "flat", but our residential antenna arrays develop signals that may vary by about 40dB from one another. Just yesterday, I had to process adjacent digital channels that varied in signal strength by as much as 25dB. While my amplifiers weren't considered overloaded by any of the benchmark overload levels that are referenced in manufacturer's literature, a digital signal that is 20 to 25 dB stronger than an adjacent one develops strong enough intermodulation distortion to render the weaker signal unusable even when the amplifier load is safely under the rather arbitrary maximum output level.

I have an off-air antenna targeted at Baltimore and located between Baltimore and Washington, such that the stronger, Washington signals hit the back of it at about 150 and 160 degrees (150 degrees from the towers themselves, and 160 degrees is the angle of the signals that bounce off Byrd Stadium, which are about equal in strength to the "direct" signals). The strongest signal developed by that antenna are my undesired analog channels 22 and 50 (25dBmV) 26 (24dBmV) and 20 (20dBmV). My undesired digital channels 48 and 50 are around 10dBmV, and everything else is weaker.

I have to recover and process Baltimore channels 38 (-18dBmV) and 40 (-23dBmV) even though there is a strong, Washington channel 39 (+2dBmV off this same antenna) in between them. Now, I can tune unamplified channels 38 and 40 with my Radio Shack Accurian tuner and develop signal "strength" percentages of 80% on one and 75% on the other, but if I preamplify them using any Winegard product - even my AP4727, which is supposed to have the same output capability as the AP4700 and AP4800, but with just 23dB of gain - and then bandpass filter and attenuate the single channel outputs, those channel 38 and 40 signals have been decimated by the amplifier beyond recovery.

Believe it or not, I get better performance from a Channel Master 0064DSB than from the Winegard that has identical, 23dB of UHF gain. Winegard's published overload ratings are a bad joke. Yet even with the 0064DSB, my analog channel 24 (3-4dBmV, coming off the antenna) suffers from some other analog picture sliding across it from left to right when it is amplified along with other analog signals that are a little over 20 dB stronger than it is.

Until we get a better handle on the development of intermodulation byproducts, we are going to be frustrated in a lot of situations in which the rated maximum output of an amplifier is not exceeded yet it makes weak signals worse, because a byproduct that is substantially weaker than the carriers it is derived from can still overwhelm a significantly weaker channel, and often does.

According to the applications notes I got relating to my Sencore Signal Analyzer, the then-current crop (ca. 2005) of DTV tuners risked tuner overloads with input signals above +20 dBmV.

While I don't know if that was an overly cautious recommendation on their part or not, I routinely hear about folks with big antennas, a pre-amp, and a location in the backyard of a 1 MW blowtorch UHF station complaining that their antenna can't pick any channels. Go figure...

It is quite plausible and in fact quite likely that ctdish's system was overloading before he inserted the notch filters. -31 dBm is about +17dBmV calculated to be the strength of an isotropic antenna. If he has 10 dB of antenna gain, then that would be +27 dBmV.

With a 26dB gain Channel Master, it would be amplified to 53dBmV. With a Winegard 28 dB gain amp, it would be 55dBmV.

I don't have any Channel Master overload specs handy, but as I recall, they used to claim that their preamp would attain either 5% sync compression, or, in later printings, -46 dBc cross modulation" with something like two channels at 54 dBmV or 4 channels at 51 dBmV. If those are the numbers, then you would attain those same compression and crioss-mod levels at 57 dBmV

dBc means decibels below carrier. Cross modulation that is 46 dB below ctdish's 55dBmV ion signal would have a strength of 9 dBmV. such interference would have no discrenible effect on a 55dBmV signal, but would totally decimate weaker signals. I suggest re-reading my post #25 in this thread, in which I cite an instance where I tried two 0064DSBss and two Winegard AP4727s, 23 dB gain preamps, had output levels well below the published overload specs for those devices and yet the intermodulation distortion ruined the weaker signal I was trying to improve.

Here is a post I wrote a couple of years ago when my recollections were clearer than they are now:

Mike, I actually read all of your post concerning antennas/amp/signal strengths a while back, GREAT info! This stuff has helped me out a LOT!

As to this person's problem, I would if he like a CM7777 or such with super high gain across the board, that I would be t think would overload... I was curious if he has a lower gain pre-amp though, an HDP-269 may be a good solution, and maybe some signal channel TinLee filters....