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post #17431 of 17627 Old 10-17-2018, 08:43 AM
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I made some measurements to compare the gain of my channel 3 folded dipole to the gain of the PET10-8110 FM Dipole on a channel 3 analog signal.

Rabbit73,


There's one issue in your testing (which beyond your control, I'd presume) that will cause rather large measurement errors in a test setup such as you used. When measuring the signals from a source antenna with a receiving antenna in a simplified setup as you used, it's necessary that the receiving antenna be in the far-field region of the transmitting antenna. There are online calculators that can allow you to determine that distance readily, but a good rule of thumb is that a separation distance of 10 times the wavelength is usually a safe assumption. As the wavelength of a 60 MHz signal is 5 meters, that would require a separation of 50 meters to ensure the far-field requirement is met. Obviously, such a dimension isn't going to be available in an apartment.


Just thought I'd drop a note and let you know that your measurements are going to be quite suspect as performed.


Good call on using the RF output of your converter box. I was going to suggest either that or a DVD /VCR player or something else with an RF 3/4 output.

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post #17432 of 17627 Old 10-17-2018, 12:02 PM
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Originally Posted by ADTech View Post
Rabbit73,

There's one issue in your testing (which beyond your control, I'd presume) that will cause rather large measurement errors in a test setup such as you used. When measuring the signals from a source antenna with a receiving antenna in a simplified setup as you used, it's necessary that the receiving antenna be in the far-field region of the transmitting antenna. There are online calculators that can allow you to determine that distance readily, but a good rule of thumb is that a separation distance of 10 times the wavelength is usually a safe assumption. As the wavelength of a 60 MHz signal is 5 meters, that would require a separation of 50 meters to ensure the far-field requirement is met. Obviously, such a dimension isn't going to be available in an apartment.

Just thought I'd drop a note and let you know that your measurements are going to be quite suspect as performed.
Thank you for your note of caution. I am aware of the potential problem with near-field measurements, but far-field measurements are not possible for me. If I were making absolute gain measurements, I would be concerned, but since I am only making a gain comparison, my hope was that it would be fairly valid, and certainly better than nothing. Since the two folded dipoles are full wave loops, I considered them two closely coupled coils; the inherent near-field error would be similar for both antennas under test and would self-cancel.

I was also curious to find out what the measurements would show, and hoped that they would support my advice. In particular, I thought my 88" folded dipole with 14 gauge wire would perform much better on channel 3 than an FM dipole made with 30 gauge wire. I also had a feeling that the FM dipole was much shorter than 6 feet as listed in the Amazon description. That is why I bought one, measured it, and tested it.

When I was making gain tests of my 3 element 14 MHz vertical Yagi mounted on my car to work DX, I set up in a large vacant parking lot. My wife was at the other end of the parking lot with a field strength meter. To make a relative gain measurement of my Yagi as compared to a Hustler mobile antenna, I reduced the power to the Yagi until the reading matched the Hustler. The difference in the power readings gave me the gain difference, so that it wasn't necessary to calibrate the field strength meter. Also, I wasn't concerned about ground reflections because the polarization was vertical.

I never considered my channel 3 antenna comparison anything more than a rough approximation, but hopefully better than nothing.

The true test will be when cyclist44 tests something better than a VHF-High dipole for channels 2 and 4, if he ever does it. He has been struggling with the problem for over a year, and I am the first person on the Canadian forum to give him something useful to try. Others there had told him the obvious, like outdoors would be better, the noise level is high, and the antenna will be big.
Quote:
Good call on using the RF output of your converter box. I was going to suggest either that or a DVD /VCR player or something else with an RF 3/4 output.
Thank you. I do have another modulator that has VHF-low output, but I would have to reset the DIP switches. It seemed easier to use the CM7004, and I have another use in mind for it, so I needed the practice.

I am willing to risk public ridicule for a chance to learn from an experiment. If I have made a mistake with this experiment, maybe I and others will learn from the mistake.


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Last edited by rabbit73; 10-18-2018 at 07:28 AM.
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post #17433 of 17627 Old 10-17-2018, 03:29 PM
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Originally Posted by NRadkov View Post
From experience I know that receiving a signal from two different transmitters is best with two highly focused antennas. I'm currently using an antenna Televes - DAT 790 LR and another one made by me for 482 mhz and amplifier - TGN ULNA 3036. I receiving signals from two distant transmitters of about 75 miles (+local programs), located 60 degrees from my location - Sofia, Bulgaria. Mixing the two antenna I made with this - http: //teroz.cz/produkt/c-211-k-1-x-y-69/
In principle, more antennas can be mixed as long as there is no overlap on the same channels.

best regards - dxing.org
Hello, NRadkov. Thank you for your interesting report. I have made your link active by removing the space:

http://teroz.cz/produkt/c-211-k-1-x-y-69/





I agree that the two antennas should be very directional. The filter that you are using looks suitable for your location. Here in the US, many users are trying to combine two antenna aimed in different directions with a splitter in reverse to avoid using a custom filter. This sometimes works, but often not because the same signals from each antenna interfere with each other at the combiner.
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Last edited by rabbit73; 10-17-2018 at 03:34 PM.
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post #17434 of 17627 Old 10-17-2018, 05:27 PM
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Originally Posted by holl_ands View Post
BTW: It isn't the ARGON that attenuates VHF/UHF Signals....it's the Metallic "LowE" Tinted Coating
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Originally Posted by ProjectSHO89 View Post
It's not the argon gas causing the issue, it's the low-E coating on the glass
Correct. One more footnote to this - argon is about 1% of the air we breathe; if it attenuated RF, radio would be useless for long-distance communication!
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post #17435 of 17627 Old 10-20-2018, 06:04 AM
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Originally Posted by Calaveras View Post
See attachment. I also added data for the Winegard LNA-100 and the Kitztech KT-501.
Have you done any testing on any other Winegard series such as the AP-3800, 4700, 4800, 8275, 2880 etc?
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post #17436 of 17627 Old 10-20-2018, 02:48 PM
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Originally Posted by MSev View Post
Have you done any testing on any other Winegard series such as the AP-3800, 4700, 4800, 8275, 2880 etc?
Only the 8700.

https://www.avsforum.com/forum/25-hd...topic-524.html

The 8275 has about 36 dB gain which is too much for most locations.
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post #17437 of 17627 Old 10-20-2018, 03:56 PM
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Another Way to Combine Antennas

At some locations, combining two UHF antennas aimed in different directions is desirable. If you have just one TV it can be avoided by using an A/B antenna switch or a separate tuner for the second antenna with its output connected to the aux input of the TV.

However, if you have more than one TV, it becomes necessary to combine both antennas in one coax before splitting.

The easiest way to combine two antennas is with a splitter in reverse, but it doesn't always work. It has the best chance of working if the two directions are about 90 degrees apart and you use antennas with a narrow beamwidth, like a high gain Yagi. Forum member budh9534 has proved that it can work if your location is favorable.
https://www.avsforum.com/forum/25-hdtv-technical/3003938-stevensville-mi-receiving-south-bend-chicago-help.html



If combining with a splitter in reverse doesn't work for you as well as it worked for budh9534, you can order a custom filter to insert an odd channel or two with the channels from the main antenna.

Another method of combining uses a converter box for the second antenna. Its channel 3 analog output can be combined with the signals from the main antenna using a HLSJ (VHF-High/VHF-Low Separator/Joiner). The analog signal isn't HD, but it is quite adequate for many viewers. My wife can't tell the difference at the normal viewing distance from our 40" Sony.









To make the information bars at the bottom of the screen image about the same size, the 7004 was set to VIDEO OUTPUT STRETCH and the SONY was set to WIDE MODE > FULL. The sharpness of the image should be judged by the sharpness of the lettering of the information bars. The rest of the screen image isn't as sharp because it was necessary to use a slow shutter speed to capture the screen image.

There is one important consideration you must deal with when using the HLSJ as a combiner with the converter box. A small amount of the channel 3 signal from the output of the converter box will come out of the HI port of the HLSJ and go into the main antenna, making it a transmitting antenna.



The output of the 7004 is about +11 dBmV (-38 dBm). Most of that signal will come out of the LINE port of the HLSJ for the TVs, but some of that signal will come out of the HI port, at about -26 dBmV (-75 dBm). This makes the isolation from the LO port to the HI port 37 dB.

A signal of -75 dBm on channel 3 is quite weak and will most likely be weaker than the ambient noise level which is -67 dBm at my location. A conservative measure, to avoid causing interference, would be to insert an attenuator of 10 to 20 dB between the output of the 7004 and the LO input of the HLSJ. A 20 dB attenuator will reduce the channel 3 signal to the main antenna to -95 dBm. It will also reduce the channel 3 signal sent to the TVs to -9 dBmV (-58 dBm), which is 6 dB stronger than the -15 dBmV (-64 dBm) where the analog signal starts to show snow.

If you are using a preamp for the main antenna, it will attenuate the channel 3 signal coming out of the HI port of the HLSJ. My RCA TVPRAMP1R preamp reduces it to -44 dBmV (-93 dBm) with the preamp OFF and to -47 dBmV (-96 dBm) with the preamp ON. An ultra conservative measure of a 10 dB attenuator after the 7004 output and the RCA preamp for the main antenna reduces it to the Thermal Noise Floor of -106 dBm.

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post #17438 of 17627 Old 10-20-2018, 05:17 PM
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I once used an HLSJ for a similar purpose: combining the RF output of a satellite receiver with my local OTA signals. A remote control extender to control the satellite receiver from any room in the house completed the solution.

BTW, if you want HD output, there are ATSC RF modulators you can hook to the HDMI output of a converter box. They're pricey though: the cheapest ATSC modulator I know of is $299. Also, their output is typically UHF, so you can't use an HLSJ (but most ATSC tuners still go up to RF 69, so you can set the modulator to RF 60-something and use an LTE filter on the OTA feed to make sure your modulator has no interference from LTE signals on those high RF channels).

If your home is wired for Ethernet and you're starting from scratch, you might consider a more radical solution: connect each antenna to an HDHomeRun network tuner, connect the HDHRs to your network, and use DLNA-compatible TVs to stream from the HDHRs. That gives you two tuners per antenna farm; if you need more you can split the RF signals and add more HDHRs as desired.
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post #17439 of 17627 Old 10-20-2018, 07:23 PM
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Originally Posted by Calaveras View Post
Only the 8700.

https://www.avsforum.com/forum/25-hd...topic-524.html

The 8275 has about 36 dB gain which is too much for most locations.
I think those older Winegard preamps are now hard to find. Although I still have a 4700 and 4800 lying around for UHF and an 8200 also for combined.
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post #17440 of 17627 Old 10-21-2018, 05:24 AM
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Originally Posted by Calaveras View Post
Only the 8700.

https://www.avsforum.com/forum/25-hd...topic-524.html

The 8275 has about 36 dB gain which is too much for most locations.
For the 8275 Vhf 29 UHF 28 is what Winegard claimed. I have been told that the 8700's were a low noise preamp and was surprised to see the noise figure your testing revealed.
Also the LN100 is impressively low on noise.

I have a stack of various old stock Winegards and now am curious as to whether or not I am wasting my time with them for DXing purposes based on the noise figure of the 8700's.

According to your figures it is looking like I should have gone with Kitztech in the first place.

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post #17441 of 17627 Old 10-21-2018, 07:15 AM
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For the 8275 Vhf 29 UHF 28 is what Winegard claimed. I have been told that the 8700's were a low noise preamp and was surprised to see the noise figure your testing revealed.
Also the LN100 is impressively low on noise.

I have a stack of various old stock Winegards and now am curious as to whether or not I am wasting my time with them for DXing purposes based on the noise figure of the 8700's.

According to your figures it is looking like I should have gone with Kitztech in the first place.

I think the preamp manufacturers consider 2.5 dB to 3 dB to be low noise. The KT-200 is the lowest noise but it has no filtering. That's how they get the lower noise.

The AP-8275 had much higher gain than Winegard said and it was easily overloaded if you had just one strong station.
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post #17442 of 17627 Old 10-21-2018, 11:57 AM
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I think the preamp manufacturers consider 2.5 dB to 3 dB to be low noise. The KT-200 is the lowest noise but it has no filtering. That's how they get the lower noise.

The AP-8275 had much higher gain than Winegard said and it was easily overloaded if you had just one strong station.
I have an AP-8275 in the air right now with the FM trap on and a Winegard TFM-7 trap inline.
If TVfool is to be believed. All my transmitters are edge 2 even though the locals are on 4 and 7 miles away. No overloading as of yet that I have noticed.
Rabbitears shows my Signal strength on VHF 5 and 11 to be at 100%. While the MER vary from the high 70's to high 90's
My RF ch19 virtual ch27 S.S. is 100 and the MER is high 90's to 100.

I need to go back and see what the meters on the silicon dust software are showing per channel.

I would be happy with the Kitztech 500 as I would have to have FM traps in any case. I have LOS to several FM transmitters that are about 1 mile out.
I have very spotty cell service so I believe I am pretty safe with LTE interference.

Last edited by MSev; 10-21-2018 at 02:11 PM.
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post #17443 of 17627 Old 10-21-2018, 02:25 PM
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I have an AP-8275 in the air right now with the FM trap on and a Winegard TFM-7 trap inline.
If TVfool is to be believed. All my transmitters are edge 2 even though the locals are on 4 and 7 miles away. No overloading as of yet that I have noticed.
Rabbitears shows my Signal strength on VHF 5 and 11 to be at 100%. While the MER vary from the high 70's to high 90's
My RF ch19 virtual ch27 S.S. is 100 and the MER is high 90's to 100.

I need to go back and see what the meters on the silicon dust software are showing per channel.

I would be happy with the Kitztech 500 as I would have to have FM traps in any case. I have LOS to several FM transmitters that are about 1 mile out.
I have very spotty cell service so I believe I am pretty safe with LTE interference.

There's no reason to use a very high gain preamp unless you have a lot of loss between the antenna and the TV. Extra gain just subtracts from the dynamic range of the TV. Unless you have about 20 dB of loss you don't need the gain.

The KT-200 is a better preamp than the KT-500. I don't recommend the KT-500.

Overload is very hard to recognize as it generates 3rd order intermods in the band. It can prevent reception of weaker stations. Most people would simply conclude that the station is not receivable.
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post #17444 of 17627 Old 10-21-2018, 03:39 PM
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There's no reason to use a very high gain preamp unless you have a lot of loss between the antenna and the TV. Extra gain just subtracts from the dynamic range of the TV. Unless you have about 20 dB of loss you don't need the gain.

The KT-200 is a better preamp than the KT-500. I don't recommend the KT-500.

Overload is very hard to recognize as it generates 3rd order intermods in the band. It can prevent reception of weaker stations. Most people would simply conclude that the station is not receivable.
Hmmmm.
The reason I moved to the 8275 is that I was advised that for DXing purposes the extra gain would give me an advantage for distant weak signals.
It is currently hooked to Winegard 8200U.

But in reality I could be defeating the my intended purpose?

I am in pretty geographically challenged location. Luckily my transmitters are aligned approx east to west. When atmospheric conditions are correct I receive consistent decodable signal from transmitters 120 miles out in Knoxville. I have gotten as far out as far as 268 miles in Huntsville AL. Some good skip.

I have about 25 feet of RG6 quad shield between the PSU and the masthead. Then another 25ft behind the PSU and the TV.
My loss should be minimal.

I dont have a Kitztech on hand at the moment. Would be interested in a used one if anybody has one for sale.

What I have on hand are the AP-2870, 3700, 3800, 4700, 4800, 8275 and 8700's
I also have the CM-7775 and 7777 dual inputs.
As well as some Spartan III's. And a Blonder Tongue Galaxy III Plus. Cant remember the exact models off hand.

The AP-2870 probably has the same noise figure as the 8700 as I assume it is dual port version of the 8700.
I had thought to use the 2870 on the 8200U. Remove the circuit board and separate the UHF and VHF. The add either 0089 baluns or Tru-spec baluns and see if I would get a similar gain increase as was done on HDPrimer with the Channel master 3671.
However if the noise figure is that high on the Winegards it would probably defeat any gain that might otherwise be achieved.

But I digress.

What is your take on the Old Stock Channel master 7777 Dual input? Still to much? I have not seen any specs on its overload capacity.
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post #17445 of 17627 Old 10-21-2018, 04:21 PM
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What is your take on the Old Stock Channel master 7777 Dual input? Still to much? I have not seen any specs on its overload capacity.
That was my favorite preamp. It had a little extra gain, was not easily overloaded like the current 7777, and a reasonably good noise figure.


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That was my favorite preamp. It had a little extra gain, was not easily overloaded like the current 7777, and a reasonably good noise figure.

The new dual input Channel Master version that replaced the original 7777 has gotten good reviews. It is called Amplify Plus Pro Grade, and is the new 7778-HD model. And it has adjustable gain. But unfortunately it is rather expensive. And it remains confusing that Channel Master keeps using old model numbers for newer versions. They should have chosen new model numbers to avoid confusion.
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post #17447 of 17627 Old 10-21-2018, 06:08 PM
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Originally Posted by MSev View Post
Hmmmm.
The reason I moved to the 8275 is that I was advised that for DXing purposes the extra gain would give me an advantage for distant weak signals.
It is currently hooked to Winegard 8200U.

But in reality I could be defeating the my intended purpose?

I am in pretty geographically challenged location. Luckily my transmitters are aligned approx east to west. When atmospheric conditions are correct I receive consistent decodable signal from transmitters 120 miles out in Knoxville. I have gotten as far out as far as 268 miles in Huntsville AL. Some good skip.

I have about 25 feet of RG6 quad shield between the PSU and the masthead. Then another 25ft behind the PSU and the TV.
My loss should be minimal.

Whoever told you that more preamp gain would receive weaker signals didn't know what they were talking about. I have an article that covers this subject.

http://www.aa6g.org/DTV/Noise/noise.html

System Noise Figure is all that counts and it's essentially impossible to get that below 1 dB.
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Originally Posted by Calaveras View Post
Whoever told you that more preamp gain would receive weaker signals didn't know what they were talking about. I have an article that covers this subject.

http://www.aa6g.org/DTV/Noise/noise.html

System Noise Figure is all that counts and it's essentially impossible to get that below 1 dB.
Thank you for the article.

It helps clarify a few things.

I'll swap out the 8275 for the 7777 dual input. It might still be to much. But as Rabbit indicated. It has a better margin for preventing overloading.

Looks like a Kitztech may be in my future.

I would imagine that switching to Rg11 on such a short run would not make much of a difference in the amount of noise in the system?
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post #17449 of 17627 Old 10-22-2018, 03:36 PM
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Weblinks to System Noise Figure CHART [shown below], DIY System NF and Distortion Calculators and a bunch more useful info incl. Preamp Specs/Measurements and "typical" Cable Loss for RG-11 vs RG-6 can be found here:
https://www.digitalhome.ca/forum/81-...ml#post1321089

FYI: In fol. (old) Chart, System NF = Vertical Scale + Antenna to Preamp Loss (typ. Balun Loss),
for presumed 6 db Tuner Noise Figure (if better/worse, reduces/increases "Gross Transmission Loss" by the dB improvement).
Preamp NF = Asymptotic Value on Left Vertical Axis.

And "Gross Transmission Loss" = Cable plus RF Splitter Loss between Preamp and Tuner.



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Originally Posted by holl_ands View Post
Weblinks to System Noise Figure CHART [shown below], DIY System NF and Distortion Calculators and a bunch more useful info incl. Preamp Specs/Measurements and "typical" Cable Loss for RG-11 vs RG-6 can be found here:
https://www.digitalhome.ca/forum/81-...ml#post1321089

FYI: In fol. (old) Chart, System NF = Vertical Scale + Antenna to Preamp Loss (typ. Balun Loss),
for presumed 6 db Tuner Noise Figure (if better/worse, reduces/increases "Gross Transmission Loss" by the dB improvement).
Preamp NF = Asymptotic Value on Left Vertical Axis.

And "Gross Transmission Loss" = Cable plus RF Splitter Loss between Preamp and Tuner.


Looking at the 4700 and 4800's. I wonder if the 3700 and 3800's would track the same?
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post #17451 of 17627 Old 10-23-2018, 12:56 PM
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I think those older Winegard preamps are now hard to find. Although I still have a 4700 and 4800 lying around for UHF and an 8200 also for combined.
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There's no reason to use a very high gain preamp unless you have a lot of loss between the antenna and the TV. Extra gain just subtracts from the dynamic range of the TV. Unless you have about 20 dB of loss you don't need the gain.

The KT-200 is a better preamp than the KT-500. I don't recommend the KT-500.

Overload is very hard to recognize as it generates 3rd order intermods in the band. It can prevent reception of weaker stations. Most people would simply conclude that the station is not receivable.
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Hmmmm.
The reason I moved to the 8275 is that I was advised that for DXing purposes the extra gain would give me an advantage for distant weak signals....
What is your take on the Old Stock Channel master 7777 Dual input? Still to much? I have not seen any specs on its overload capacity.

Been a while since I actively posted in this thread, but I'll dust off some old recollections and invite anyone who thinks I am senile to offer alternative numerical values.


In the early 2000s, I used a slew of Winegard 4700/4800s as well as their mongrel 4747(?) which I never found in their catalogs and which came packed with their 4700/4800 flyer. It had a claimed gain of 23dB, putting it at about the midpoint of their low and high gain models, and one person back then claimed he was actually the customer who had commissioned their manufacture.


I had observed that the gain of the Winegard high gain models went through the roof at low UHF frequencies running about 8dB above spec in the 500 -550 MHz range. I also spent a day running the Winegard's and Channel Masters side by side, using two units of each in the tests, and while Winegard's published specs from that era said that they could be driven to about 10dB more than Channel Master's specs, I was running analog signals through them so I could actually see the degradation in terms of sync suppression, and the pictures would start skating across the screen at lower levels with the Winrgards than with the Channel Masters.


I'll grope for some more old numbers here and say that I think the so-called thermal noise figure for the NTSC analog signals was about -59dBmV and they need to maintain about a 45 or 46dB S/N ratio for "TASO Grade 1" pictures, so theoretically, if one had a 3dB noise figure amplifier, the signals, to be recoverable and sustainable as Grade 1 signals, had to hit the preamp at at least about -13dBmV. Does that recollection match the recollections of the other old timers here?


Did we simply use dBm at the same bandwidth for the digital noise floor. Makes sense to me. -59 minus 49.75(?) would be about -110 dBm, but I though sure we were using a figure higher than that but I might be confusing it with my recollection of satellite signal noise floor, which is measured over 27 MHz of bandwidth rather than 6 MHz. For some reason, the numbers -106dBm and -102dBm are stuck i my head. Maybe the first for broadcast TV noise floor and the second for satellite?


So anyway, what is the lowest recoverable level of preamplifier input that a digital signal can be at to still be recoverable? I remember back in the mid 1990s, going to a seminar at Heifner Communications, at which we padded DirecTV satellite signal down to below -76dBm yet of we amplified it back up to -60dBm, the receiver's test feature still gave is a table full of
99%" signal quality, whatever that meant.


I have the economical means to filter broadcast channels using old tunable Tru-Spec UHF-BPFs. so I'm sure that I could recover some very weak broadcast signals using them in conjunction with, say, a 36dB preamplifier. I think the insertion loss might have been in the neighborhood of 4dB for those UHF BPFs. Can someone here do the ballpark calculations and estimate how weak a signal one might be able to sustain using the established thermal noise floor, S/N or C/N ratios?
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Did a quick search on the Tru-Spec UHF-BPFs.
What pops up EscapeVelocity and Rabbit over on DTV mentioning you discussing a similar piece of equipment back in 2009. LOL

I believe I a couple a Channel Masters I picked up along the way.
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Did a quick search on the Tru-Spec UHF-BPFs.
What pops up EscapeVelocity and Rabbit over on DTV mentioning you discussing a similar piece of equipment back in 2009. LOL

I believe I a couple a Channel Masters I picked up along the way.

A decade and half ago, I Googled "DirecTV" and "spotbeams", and, based on results, I am the putative or nominal or self anointed expert on the subject, and if that is true, then the whole world is in trouble.
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A decade or so ago, I tried to get Pico/Tru Spec to make up some more, as I think they ceased production in 1992 or 1993, but they said they had not retained the "tooling" needed to stamp out the chassis and case, so I'd have to order a thousand to make it happen.


I figured that if I placed a guaranteed order for 500 they would at least pay attention, so I penciled myself in for a hundred and contacted a few big dealers but no one else would commit for more than a handful.


In the meantime, I had found a Japanese manufacturer of cylindrical traps who would sell me high pass and low pass filters for a few dollars each if I bought 500, and while they are not tunable, I saw that they had enough models "on the shelf", with no minimum buys, so that if I had been more successful at marketing headend systems with individual channel filtering or narrow band filtering ( like 33-36 from Washington, 38-40 or 38-42 from Baltimore/Annapolis) I could develop a superior filtering system for a song, but the sales were just not there for me to further pursue that.
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Quote:
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Did we simply use dBm at the same bandwidth for the digital noise floor. Makes sense to me. -59 minus 49.75(?) would be about -110 dBm, but I though sure we were using a figure higher than that but I might be confusing it with my recollection of satellite signal noise floor, which is measured over 27 MHz of bandwidth rather than 6 MHz. FOr some reason, the umbers -106dBm and -102dBm are stuck i my head. Maybe one for satellite and one for broadcast?

So anyway, what is the lowest recoverable level of preamplifier input that a digital signal can be at to still be recoverable? I remember back in the mid 1990s, going to a seminar at Heifner Communications, at which we padded DirecTV satellite signal down to below -76dBm yet of we amplified it back up to -60dBm, the receiver's test feature still gave is a table full of 99%" signal quality, whatever that meant.
Back in 2008, the NAB (National Assn of Broadcasters) commissioned MSW to test a group of 18 CECB's (Coupon Eligible Converter Boxes...you know those Low-Rez Boxes that were FREE with $40 Coupon) to see how well they met the Gov't Imposed Specs to qualify for the Coupon program. They ALL exceeded the "TOV" (Threshold of Visibility = 10-6 Bit Error Rate after Coding) performance criteria at an SNR of 15.0 (+/- 0.25) dB....corresponding to Sensitivity measurements tightly packed around -86 dBm [so Tuner NF ~ 5.0 dB].

But that was a NON-FADING BENCH TEST and you wouldn't want to watch a very glitchy picture right at "TOV"....so you would want a 2+ dB increase in SNR for a LOS Link....and for Real-World Fading Conditions (i.e. beyond LOS), you might need another 10-20 dB increase in the SNR.


BTW: NTSC Signals were always measured using a PEAK Reading Meter, whereas ATSC Signals are always measured using an AVERAGE Reading Meter, where the Peak is about 6-7+ dB higher [give or take some Statistical Assumptions].

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With the NTSC, I think our S/N ratio was the number derived by comparing the NTSC carrier peak with the 6 MHz wideband noise floor, and I thought that the 8VSB S/N or C/N ratio was using 5.8 MHz or 5.6-something MHz bandwidth (I think one of those bandwidths was QAM versus the other NTSC) for both the signal and the noise.
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A decade and half ago, I Googled "DirecTV" and "spotbeams", and, based on results, I am the putative or nominal or self anointed expert on the subject, and if that is true, then the whole world is in trouble.
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Originally Posted by AntAltMike View Post
Been a while since I actively posted in this thread, but I'll dust off some old recollections and invite anyone who thinks I am senile to offer alternative numerical values.
I love to hear your old stories, and learn from them. The first meter I used was calibrated in microvolts, and then I graduated to dBmV units for my Sadelco meters. I think in dBmV with 0 dBmV as the magic reference level, but had to learn to think in dBm, because that is what the TVFool reports use. The conversion factor is 48.75, rounded to 49. or 50 for a quick estimate.



I think the Channel Master engineers gave much more accurate specs for their preamps than Winegard did, and the CM shielding was better.

Quote:
Did we simply use dBm at the same bandwidth for the digital noise floor.
Yes, the Thermal Noise Floor for a 6 MHz bandwidth signal is -106 dBm, digital and analog. Of course, if your ambient noise is higher than that (like on VHF-Low), you will need more signal to have the minimum required C/N for analog and SNR for digital.



Quote:
Can someone here do the ballpark calculations and estimate how weak a signal one might be able to sustain using the established thermal noise floor, S/N or C/N ratios?
For some reason, the umbers -106dBm and -102dBm are stuck i my head.
If you put a dipole in a shielded enclosure and connect to it with an F-81 through the wall of the enclosure, the noise level will be -106 dBm at the antenna terminals. If you take the dipole out of the enclosure, the minimum required digital signal will be -85 dBm in a location with low ambient noise. This is the calculation:

-106 dBm + 15 dB for minimum required SNR = -91 dBm
-91 dBm + 6 dB for tuner NF = -85 dBm

This is one way to look at it:



but this is probably more accurate, because the tuner noise is just above the thermal noise floor, and the minimum required SNR is above that:



If you add a reflector and directors, the noise will still be -106 dBm at the antenna terminals, but you have collected more signal to exceed that noise.

However, a TVFool report equates a 0 dB Noise Margin with -91dBm, because it assumes a preamp with a 0 dB NF is used; the difference between the listed NMs and the listed signal powers in dBm is a constant of 91.



When you use a preamp, the tuner NF becomes irrelevant, because it is buried in the amplified noise floor. Then, the preamp NF primarily determines the System Noise Figure as shown by the Friis formula.
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Here is something I encountered last week. Pressure taps rated to 216 MHz, but being used to carry cable TV channels up to 54 (408 MHz).


A pressure tap connects with the center conductor by driving an insulated prick pin into it. When I cut them out, I have to either have old RG-11 connectors with crimp-on center pins, or in desperation, I can wick solder into the center conductor and file it down gently and then fit it into a female F port on an F-81 barrel connector.










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post #17460 of 17627 Old 10-24-2018, 04:41 AM
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How Low can You Go?

Quote:
Originally Posted by AntAltMike View Post
So anyway, what is the lowest recoverable level of preamplifier input that a digital signal can be at to still be recoverable?
Can someone here do the ballpark calculations and estimate how weak a signal one might be able to sustain using the established thermal noise floor, S/N or C/N ratios?
To expand a little further on the answer to how low can you go, the practical limit would be a signal with an actual Noise Margin of -15 dB (-106 dBm, -57 dBmV, 1.4 µV across 75 ohms) as defined by TVFool.

The signal coming out of the terminals of an antenna with a gain of 18 dBd, as shown in the diagram below, would be -88 dBm (-39 dBmV, 11 µV across 75 ohms).

And finally, as you requested, the signal to the preamp input would be -89 dBm (-40 dBmV, 10 µV across 75 ohms).

The Noise Margins listed for weak 2Edge signals are known to be less accurate than for LOS signals, because of software limitations. Also, OTA signals constantly vary in strength and Tropospheric Propagation can temporarily enhance a weak signal. For consistent reliable reception you would want a Fade Margin greater than the one dB indicated in the diagram.



The above diagram assumes that the ambient noise level is at or below -106 dBm. To receive weaker signals, you would need an antenna with more than 18 dBd gain, which is possible, but not easily done.

If you can not measure it, you can not improve it.
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Last edited by rabbit73; 10-24-2018 at 01:41 PM. Reason: corrected math errors
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