Understanding dB & Amplifiers

[rsw1941]

Thanks for the opportunity to request a bit of schooling.

The attached screenshot is from the best screen I ever owned, a 60" plasma by Panasonic. (it's gorgeous) The feed comes from a TiVo Roamio OTA Series 5. (Perhaps later we can discuss their use of % as a unit of measure.)

I'm located in Rockport, Maine, 40+ miles from RF Channels 2, 7, 9, & 13 in Bangor. The antenna is the VHF section, only, of a ChannelMaster 3020 with their new version 3 of the CM7777 preamp. I'd like to add something like the variable gain SKY38323 distribution amp ahead of the TiVo. Specs say it can add 25dB to VHF signals.

Signal levels, expressed as a %, vary from day-to-day, hour-to-hour, from a low of 37% to a high of 60% and better. Of course, SNR tracks that, predictably. Most days, most channels are virtually free of dropouts. There are 4 ATSC tuners in the TiVo. Even for quite low signals, the TiVo reports Signal Lock, Program Lock, and Search Complete to be "Yes."

The caution I often hear is, "No amplifier can create signal. The pre-amp can take what is at the antenna and "Boost" it in order to help with the signal loss over the down lead coax cable coming in to the house."

My question:

IF the distribution amp can increase signal level between its input and output sufficient to overcome the signal loss about to be added by X-teen feet of cable before a splitter and cable headed off to a couple of mythical destination receivers with ATSC tuners,

THEN isn't that increased RF level available and in play, regardless of whether the destination is 100 feet away or 1 foot away, and we're hitting a 4-way splitter before ATSC tuners in the TiVo box?

In other words, wouldn't the Channel 2 RF signal level be increased by some net amount, measurable with the proper gear, and register as both a Signal Level and SNR improvement on the TiVo? My assumption has been that both the preamp and the amp add signal and noise, but each adds far less noise, for a net gain. Is that my mistake?

Or is it the fact that we're no longer living in an analog world that's driving the difference? Is digital reality somehow that much different from my analog recollections?

Be assured, I like learning stuff even when it contradicts what I think I know. : )

Thank you

[rabbit73]

Originally Posted by rsw1941

I'm located in Rockport, Maine, 40+ miles from RF Channels 2, 7, 9, & 13 in Bangor. The antenna is the VHF section, only, of a ChannelMaster 3020 with their new version 3 of the CM7777 preamp. I'd like to add something like the variable gain SKY38323 distribution amp ahead of the TiVo.

In other words, wouldn't the Channel 2 RF signal level be increased by some net amount, measurable with the proper gear, and register as both a Signal Level and SNR improvement on the TiVo? My assumption has been that both the preamp and the amp add signal and noise, but each adds far less noise, for a net gain. Is that my mistake?

Hello, rsw1941; welcome to the forum.

An amp will make the the channel 2 signal stronger, but it will also amplify the ambient noise at your location, resulting in no improvement in the SNR.

Here is a sample signal report from rabbitears.info showing very poor signals at the intersection of Ship St and Pascal Ave in Rockport:
https://www.rabbitears.info/searchma...tudy_id=135056

You can do your own report for your location here:
https://www.rabbitears.info/searchmap.php

It is true that a preamp usually has a lower internal noise figure than a tuner, but on VHF the noise levels are higher than on UHF. This means that the noise figures become irrelevant because they are buried in the much higher local noise. The NFs of the amp and tuner are stated as noise above the reference level of the Thermal Noise Floor at about -106 dBm for a digital TV signal with a bandwidth of 6 MHz.





The TiVo signal strength is just a relative scale; 40% is very low. An SNR of 16 dB is just barely enough. Uncorrected errors should be zero; 111728 is way too many. Corrected errors are never shown, for some unknown reason.



VHF-Low is even worse at my location:





Channel 3 must be extremely strong to have enough SNR above the very high noise level.



The SNR is measured from the noise level to the top of the signal.



So, to answer your question, more amplification is not likely to solve your reception problem for channel 2, but you are welcome to try it. It will not improve the signal quality (as defined by SNR and errors) coming from the antenna. It will be necessary to improve the signal quality coming from the antenna terminals before any amplification.

The CM3020 is quite large in order to receive VHF-Low channels. There aren't many antennas with more gain for channel 2 than the 3020. Review of the 3020 by Tyler the Antenna Man:

https://www.youtube.com/watch?v=jpjcO8PUC88

You will have to find a possible location for your antenna that has more signal strength for channel 2 and less noise for an improved SNR.



The WLBZ terrain profile from the sample report shows a very difficult signal path with your location behind a hill:

[jrgagne99]

The voltage signal that the amplifier sees consists of "usable signal" (the part that the TV tuner needs to decode) and "noise". I think in broad terms, amplifiers increase both components equally, so that the ratio of the usable signal to the noise (SNR) stays the same (except for a bit of added noise which is characterized by the amplifier's noise figure). On the way down an your transmission line, the voltage peaks drop, but the thermal noise floor stays the same. So the SNR is decreasing with distance. The ratio between the peaks and the noise floor must be greater than 15 dB (about 32x) at the tuner to decode properly.

So, amplifiers can't improve SNR between their input jack and their output jack since they amplify the whole signal (usable signal + noise) indiscriminantly.

[rabbit73]

Originally Posted by jrgagne99

I think in broad terms, amplifiers increase both components equally, so that the ratio of the usable signal to the noise (SNR) stays the same

That is correct

(except for a bit of added noise which is characterized by the amplifier's noise figure).

The preamp does add its NF which reduces the SNR of the signal coming from the antenna, but on VHF the preamp noise and tuner noise are buried in the much higher amplified local noise. On UHF, a low-noise preamp might help because the preamp NF is lower than the tuner NF, but not usually on VHF, especially VHF-Low.

So, amplifiers can't improve SNR between their input jack and their output jack since they amplify the whole signal (usable signal + noise) indiscriminantly.

Correct

[rsw1941]

Gentlemen,

Thank you both for your interest and replies.

rabbit73, I see you're from S.E. Virginia. I grew up in Mechanicsville, just north of Richmond, back when we actually watched with rabbit ears. With a great deal of effort I managed to graduate from Virginia Tech in '63 after about 5.5 years and a switch from EE to Marketing.

When you say, "The SNR is measured from the noise level to the top of the signal." do you mean from 0, or from the top of the noise level?

I'd hate to replace the antenna, but would consider doing so if there were one with a better VHF section. Can you suggest one? Keep in mind that I removed the UHF section and attached the balun directly to the end of the connectors that reached from the VHF section thru the UHF section. Might a gain of even 2 or 3dB on Channel 2 be significant? SNR of 17dB and better generally delivers a rock solid signal.

Because I was having issues with an earlier preamp and thought I might replace it, I located the connection at the bottom of the 10-foot mast, not up directly under the antenna. This added at least 10 feet to the cable from the balun to the preamp. BAD idea?

I should have posted a version of the map and table of data from TV Fool. The file is attached.

Thanks,
Steve

[jrgagne99]

Originally Posted by rsw1941

Because I was having issues with an earlier preamp and thought I might replace it, I located the connection at the bottom of the 10-foot mast, not up directly under the antenna. This added at least 10 feet to the cable from the balun to the preamp. BAD idea?

I think that was a good idea. Distance of 10-feet isn't very much, and if it makes it easier and safer to troubleshoot and service, it is worth the small penalty (fraction of a dB).

My pre-amp is located at the bottom of the pine tree that my antennas are in. Makes it way easier to service and worth the hit.

[rsw1941]

I was sure the difference was trivial. But, surprise, I've been wrong about stuff before.

Thanks, again.

[rabbit73]

Originally Posted by rsw1941

Because I was having issues with an earlier preamp and thought I might replace it, I located the connection at the bottom of the 10-foot mast, not up directly under the antenna. This added at least 10 feet to the cable from the balun to the preamp. BAD idea?

I agree with jrgagne99; go for safety and convenience. The coax loss (attenuation factor) for RG6 coax is much less for VHF than UHF. It's only 1.4 dB per 100 ft for channel 2, 2.8 dB for VHF-High, and 5 dB for UHF.

[rsw1941]

rabbit73,

Thanks for the reply. When you get one of those round toits, I'd also be interested in your thoughts on the related questions:

When you say, "The SNR is measured from the noise level to the top of the signal." do you mean from 0, or from the top of the noise level?

I'd hate to replace the antenna, but would consider doing so if there were one with a better VHF section. Can you suggest one?

Keep in mind that I removed the UHF section and attached the balun directly to the end of the connectors that reached from the VHF section thru the UHF section. Might a gain of even 2 or 3dB on Channel 2 be significant?

Regards

[rabbit73]

Originally Posted by rsw1941

rabbit73, I see you're from S.E. Virginia. I grew up in Mechanicsville, just north of Richmond, back when we actually watched with rabbit ears. With a great deal of effort I managed to graduate from Virginia Tech in '63 after about 5.5 years and a switch from EE to Marketing.

Previously, I'm from NJ. I started at Rutgers to be an EE, my father died, had to leave school and joined the Army during the Korean war (radio operator in Germany). Was an AV Tech for the Government for 30 years. I've been a ham since the early '50s. I started doing antenna experiments when I was 8; I'm now 87 and still learning.

When I was a kid our family had a vacation in ME on Orr's Island. That's when I learned to love blueberry pie. After I was married, my wife and I went to EXPO '67 in Montreal. On the way back we stopped to see Cadillac Mountain in Acadia National Park; she loved it.

When you say, "The SNR is measured from the noise level to the top of the signal." do you mean from 0, or from the top of the noise level?

From the top of the adjacent noise level. This scan was made with an RST-SDR.COM V3 dongle and free open-source spectrum analyzer software from GitHub. The software is not available ready-to-use. It is necessary to assemble it in a folder. I found that difficult to do, but finally got it to work. The dB scale numbers are only relative, not absolute.



This is the screen shot that goes with that test. The SNR given by the tuner is actually MER based on the constellation diagram. It would be difficult to measure the noise level in a channel that's in use:

I'd hate to replace the antenna, but would consider doing so if there were one with a better VHF section. Can you suggest one? Might a gain of even 2 or 3dB on Channel 2 be significant? SNR of 17dB and better generally delivers a rock solid signal.

I find it difficult to suggest another antenna because of the lack of reliable references. I can't find data for the 3020 that I trust. Yes, I know that CM gives gain figures for the 3020. Even if you had a better antenna, your reception of channel 2 wouldn't be 100% reliable because it is a Tropo signal subject to the whims of propagation. Since channel 2 is your weakest desired channel, you might improve reliability from say 50% to 70%.

I found the net gain for the 2020; the channel 2 gain is poor. Since the 3020 has more elements in it's LPDA VHF section, I assume that it would do better on channel 2.
http://www.hdtvprimer.com/ANTENNAS/comparing.html



I see that CM has brought back the 3671 (Ultra-Hi Crossfire 100):
https://www.channelmaster.com/Digita..._p/cm-3671.htm

This is a gain chart for the Winegard HD8200U, note gain is in dBd, not dBi

Keep in mind that I removed the UHF section and attached the balun directly to the end of the connectors that reached from the VHF section thru the UHF section.

It's difficult to predict how much that mod might have changed the VHF gain. In the review of the 3020 by Tyler that I linked above, he said (at 1:03) that the location of the balun affected the performance of the antenna.
https://www.youtube.com/watch?v=jpjcO8PUC88

I should have posted a version of the map and table of data from TV Fool. The file is attached.

Thank you for the image of your report. My guess was pretty close. I prefer a link to the report, so that I can click on the callsign to see the terrain profile. I had hoped that you would have done a rabbitears.info report, which would have been more accurate because the reports generated by TVFool use an outdated database.

Sometimes I ask a poster for his address and antenna coordinates in a PM for privacy. That allows me to look at the sat view of the location to see if there are any objects in the signal path like trees.

[OTAFAN]

It's difficult to predict how much that mod might have changed the VHF gain. In the review of the 3020 by Tyler that I linked above, he said (at 1:03) that the location of the balun affected the performance of the antenna.
https://www.youtube.com/watch?v=jpjcO8PUC88

If you're looking to squeeze out every dB you can especially for that tough Channel 2, here's a link to a previous thread rabbit73 authored that offers an excellent tip:

http://forum.tvfool.com/showthread.p...ed=1#post58266

Please let the forum know how it turns out for you, rsw1941. I'm sure it would be helpful. Thanks and all the best.....

[AKADAP]

SNR is the ratio of signal power to noise power. Spectrum analyzers are deceptive in that the apparent noise floor changes with the resolution bandwidth setting. The narrower the resolution bandwidth, the lower the apparent noise floor. One can't just look at the spectrum plot and measure the signal power to noise floor. One must integrate the signal power over the bandwidth, and noise power over bandwidth, and take the ratio of the two results.

Often when people discuss preamps, they focus on a single channel of reception. This is dangerous since preamps are wide band amplifiers and there could be high power signals within the bandwidth of the preamp. One must also pay attention to what happens in the time domain. All amplifiers have a limited output voltage range. If the instantaneous input voltage times the gain of the amplifier causes the result to be outside that range, the output of the amplifier will clip. During the time that the amplifier is clipping, nothing gets through. This means that if there are any high powered signals, it can be a bad idea to try and use a preamp. In my case, I found that local FM stations were hugely more powerful than the TV stations I was trying to receive. I found that I needed an FM trap. There are two types of FM traps, and marketing seldom telly you the kind they are selling. One type kills all FM, but also kills channel 6. The other allows channel 6, but does not kill the low end of the FM band.

For antennas, I think that the shape of the reception pattern would be more important than the absolute gain. If you know you are having trouble with interference from an unwanted signal, you could pick an antenna that has a null an that frequency at an angle from your station of interest that would let you aim the null at the problem interference source. Unfortunately, antenna manufacturers no longer publish the plots of gain vs angle.

For digital television I wish I could find a low cost tool that would display the constellation plot of the signal. This can give good clues as to why you are having trouble with a signal. The closest I have seen is the HDHomeRun Tech, but that is not a full constellation plot it is a histogram of the I signal rather than a plot of I vs Q.

[rabbit73]

Originally Posted by AKADAP

SNR is the ratio of signal power to noise power. Spectrum analyzers are deceptive in that the apparent noise floor changes with the resolution bandwidth setting. The narrower the resolution bandwidth, the lower the apparent noise floor. One can't just look at the spectrum plot and measure the signal power to noise floor. One must integrate the signal power over the bandwidth, and noise power over bandwidth, and take the ratio of the two results.

The resolution bandwidth used for the scan will definitely affect the power reading. The thermal noise floor of -106 dBm for a DTV signal is determined by its bandwidth of 6 MHz.

However, since the same resolution bandwidth is used for the DTV signal and the adjacent noise floor, the power ratio between the two is preserved.

A $25 SDR dongle used as a spectrum analyzer is not lab-grade equipment, but when the SNR given by the scan agrees with the SNR given by my SONY TV as shown in post #10, that's good enough for me. However, it is necessary to adjust the SDR gain to the proper level. With the RTL-SDR.COM V3 dongle and the free open-source spectrum analyzer software, it should be increased to the point where the noise floor just starts to rise. With the RTL-SDR.COM V3 dongle and SDR# (SDRsharp) software, SDRplay RSP1A and Airspy R2, the gain should be adjusted for max SNR, which is usually not max gain.

Often when people discuss preamps, they focus on a single channel of reception. This is dangerous since preamps are wide band amplifiers and there could be high power signals within the bandwidth of the preamp.

Very true. The front end is wide open; there is no preselection. One very strong signal will cause clipping and gain compression; two or more very strong signals will cause IMD that creates spurs which will raise the noise floor and wipe out weak signals.

All amplifiers have a limited output voltage range. If the instantaneous input voltage times the gain of the amplifier causes the result to be outside that range, the output of the amplifier will clip. During the time that the amplifier is clipping, nothing gets through.

Yes; I learned that lesson the first time I connected two preamps in series; nothing got through.

This means that if there are any high powered signals, it can be a bad idea to try and use a preamp. In my case, I found that local FM stations were hugely more powerful than the TV stations I was trying to receive. I found that I needed an FM trap. There are two types of FM traps, and marketing seldom telly you the kind they are selling. One type kills all FM, but also kills channel 6. The other allows channel 6, but does not kill the low end of the FM band.

That would be the Radio Shack FM filter and the Antennas Direct FM filter:



FM filters are getting hard to find. A HLSJ makes a good FM filter if you don't need VHF-Low. FM filters are made for SDRs, but they are designed for 50 ohms and have SMA connectors.

For digital television I wish I could find a low cost tool that would display the constellation plot of the signal. This can give good clues as to why you are having trouble with a signal. The closest I have seen is the HDHomeRun Tech, but that is not a full constellation plot it is a histogram of the I signal rather than a plot of I vs Q.

Majortom on the Canadian forum has worked out a low-cost method. It requires Linux software manipulation which is a little beyond my ability. I'm happy with just signal strength, SNR and errors. The shape of the signal when finding the best location for an antenna is also useful. Fewer notches seems to be better.
https://www.digitalhome.ca/threads/o...5/post-2919953

I modified the linux driver for this usb atsc tuner to allow plotting the received constellation, using updateDVB.
Working pretty good here...

a weak distant cblt

https://www.dropbox.com/s/p59zq8f1dh...qplot-cblt.png

strong local wned

https://www.dropbox.com/s/w9ue0jg7fg...qplot-wned.png

[rsw1941]

OTAfan, AKAdap, rabbit73,

Many thanks for all of the detailed information. (Been meaning to acknowledge Lord Kelvin's assertion. Like it!)

rabbit73, perhaps there's a way we can communicate offline, and not bore others. Open to exploring that.

All,

I'm back to looking at how to improve the GoesInta signal to the mast-mounted preamp. The next time I can persuade my son to pull the mast down, I'll focus on that bit.

I see that CM reports their CM-3203 balun is superior to all others, but doesn't give actual specs. A lower insertion rate would certainly be desirable if it's measurable. There are at least two changes that can be made to the manner of connecting the balun to the VHF section of the CM-3020. Remove the feed-thru connectors entirely and drill and attach at the element joint, and/or replace the balun with the best I can buy.

rabbit73, I'd seen the video you referenced but ignored it because he discussed UHF signals only. However, the observation about the balun touching an element is basically what I did, and that will be changed. I've attached an image that shows detail on the current arrangement

I covet feedback.

This has all been incredibly informative. Though it'll be awhile before we get back up on the roof, I'll be sure to report what I learn.

Regards

[rsw1941]

Is there ANY chance analog modulation of RF channels back in the late 60s behaved any differently, with regard to ambient noise?

When installing a mini-CATV system on a 10-acre school campus way back, I vaguely recall using a distribution amp at each building.

[rabbit73]

Originally Posted by rsw1941

Is there ANY chance analog modulation of RF channels back in the late 60s behaved any differently, with regard to ambient noise?

Hello, again rsw1941.

Since I don't know your specific concern about ambient noise, I will have to give a general answer.

The ambient noise at any location will reduce the SNR of both analog and digital TV signals.

With analog signals, as the noise level increases, you will see more and more white specks in the picture of VHF signals. As the noise level increases even more, there will be even more white specks until you reach the point where the picture is nothing but snow. The picture never drops out like it does with a digital TV signal.

With a digital TV signal, as the noise increases more errors are created, but the picture quality doesn't change until you reach the "Digital Cliff."



The FEC (Forward Error Correction) corrects the errors, but it has a limited ability. Once that limit is exceeded, the errors increase and you have pixelation, picture freeze, and finally dropout. If there were no FEC, the picture quality would gradually deteriorate, which would be even more annoying than the sudden loss at the cliff.

I have been helping a poster in NJ with low VHF on the satelliteguys forum. I taught him how to use a $25 RTL-SDR.com dongle and SDR# (SDRsharp) software to check the SNR of his signals. I suggest you take a look at that thread:
Low VHF interference
https://www.satelliteguys.us/xen/thr...erence.367781/
The 2020 posts start at post #51





Newer version of software. The display doesn't have to be dark, you can set it to a lighter theme, which I actually prefer. I set it dark to match the scans done by freeisforme on the satelliteguys forum:



One of these videos should work:

https://www.dropbox.com/s/w62g62arag...02-32.mp4?dl=0

https://www.dropbox.com/s/w62g62arag...02-32.mp4?dl=1

One nice advantage of having an inexpensive spectrum analyzer is when you are doing a channel scan. If a channel doesn't show up, you don't know if it isn't there or just too weak. The spectrum analyzer will give you the answer.

When installing a mini-CATV system on a 10-acre school campus way back, I vaguely recall using a distribution amp at each building.

That is the correct procedure. You want to make the signals stronger before they are split. If you don't, the SNR will be reduced as the signals get weaker. Once the SNR is reduced, you can't get it back.