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-   -   Reflections on preamps and signal loss (http://forum.tvfool.com/showthread.php?t=16689)

BrooklinOTA 16-Oct-2019 3:57 PM

Reflections on preamps and signal loss
 
Calling all OTA Gurus for confirmation.

I have a setup where I have a DB8E pointed at Buffalo from Ajax, Ontario. Initially I had a long cable run that was probably around 75 feet. I used an RCA TVPRAMP1R to take the signal from my antenna to the distribution block in the basement.

So I have been thinking about my preamp for a while and here are my thoughts:

- The TVPRAMP1Z brings around a 2.6 db signal loss immediately from the antenna.

- It then boosts that signal to take it down the line.

- But the signal that is boosted is the signal that already had 2.6 db of loss!

Therefore the preamp is boosting an inferior signal....

I ended up re-running my RG6 cable and was able to cut about 35 feet off the run. The result: much more stable US channels!

JoeAZ 16-Oct-2019 9:17 PM

You probably won't have good U.S. reception from Buffalo, NY
for long..... When the repack is completed in Buffalo, NY, toronto
and hamilton, there will be Canada adjacents for several Buffalo
stations......

Tim 17-Oct-2019 2:05 AM

Quote:

Originally Posted by JoeAZ (Post 62314)
You probably won't have good U.S. reception from Buffalo, NY
for long..... When the repack is completed in Buffalo, NY, toronto
and hamilton, there will be Canada adjacents for several Buffalo
stations......

I was wondering how the repack would affect adjacent channels in my market. We have stations on channels 18 & 19 as well as on 31 & 32...all located on the same tower. No problems at all so far.

ADTech 17-Oct-2019 2:26 AM

Adjacent channels that are of similar strength (at the receiving location) are not an issue with ATSC. However, when one is considerably stronger than the other, reception may fail. IIRC, the number was around 35 dB differential back when the standard was set up.


FWIW, it's far more likely that the described mechanism and conclusion in the first post is incorrect. It's far more likely that the amplified signals from the CN tower were strong enough to "desensitize" the tuner to the far weaker Buffalo signals.

JoeAZ 17-Oct-2019 2:01 PM

The Buffalo, NY tv stations are many, many miles apart.
They range from a distance of about 55 miles to more
than 90 miles from ajax, on. The Ion station, WPXJ, is far East
and WNYB is far West. Interesting to note that the DB8E
does a good job of receiving all of them.....

ADTech 17-Oct-2019 2:24 PM

The angular spread of the Buffalo UHF stations is about 20 degrees. The HPBW of the DB8e varies from about 25 degrees at the low end of the UHF band to about 16 degrees at the high end of the (current) UHF band, assuming the panels are in parallel alignment. The numbers from the mathematical model would appear to accommodate this condition just fine and his reception results would appear to validate the models.

Note that WNYB is actual a VHF-5 station, so we're ain't going to see that one with a DB8e unless you were pretty close to the station's tower.

BrooklinOTA 17-Oct-2019 3:00 PM

Here is my TV Fool report if it helps: http://www.tvfool.com/?option=com_wr...9038b49f8b2436

Tower Guy 17-Oct-2019 4:51 PM

Quote:

Originally Posted by JoeAZ (Post 62318)
The Buffalo, NY tv stations are many, many miles apart.
They range from a distance of about 55 miles to more
than 90 miles from ajax, on. The Ion station, WPXJ, is far East
and WNYB is far West. Interesting to note that the DB8E
does a good job of receiving all of them.....

Hey Joe;

The repack did allow WPXJ to move from Batavia to Buffalo. The adjacent channels in Toronto will be tough for those close to the CN tower.

A trick to reduce the adjacent channel signals is to aim the DB-8e so that the CN tower is in a null while the Buffalo stations are slightly off the main beam.

I love the Bills every week. How often to you get to see them? Here near Albany CBS usually carries the Jets.

BrooklinOTA 17-Oct-2019 5:52 PM

Quote:

Originally Posted by Tower Guy (Post 62321)
Hey Joe;

The repack did allow WPXJ to move from Batavia to Buffalo. The adjacent channels in Toronto will be tough for those close to the CN tower.

A trick to reduce the adjacent channel signals is to aim the DB-8e so that the CN tower is in a null while the Buffalo stations are slightly off the main beam.

I love the Bills every week. How often to you get to see them? Here near Albany CBS usually carries the Jets.

Yes, this is essentially what I am doing.

JoeAZ 17-Oct-2019 9:07 PM

Quote:

Originally Posted by Tower Guy (Post 62321)
Hey Joe;

The repack did allow WPXJ to move from Batavia to Buffalo. The adjacent channels in Toronto will be tough for those close to the CN tower.

A trick to reduce the adjacent channel signals is to aim the DB-8e so that the CN tower is in a null while the Buffalo stations are slightly off the main beam.

I love the Bills every week. How often to you get to see them? Here near Albany CBS usually carries the Jets.

I thought WPXJ was near Pavillion, NY and served both the Rochester
and Buffalo, NY markets.. If the transmitter moved closer to Buffalo,
how is Rochester, NY being served??? Please check your private mesages.

rabbit73 17-Oct-2019 9:39 PM

3 Attachment(s)
https://forum.tvfool.com/attachment....1&d=1571358639

Quote:

Originally Posted by BrooklinOTA (Post 62320)
Here is my TV Fool report if it helps: http://www.tvfool.com/?option=com_wr...9038b49f8b2436

https://forum.tvfool.com/attachment....2&d=1571358893

Thank you for the TVFool report. Here is a rabbitears.info report based on my estimate of your location. It should have a more accurate list of channels.
https://www.rabbitears.info/searchma...study_id=20077

You can do your own report here. You might need to use coordinates.
https://www.rabbitears.info/searchmap.php

Extract from rabbitears.info report:

https://forum.tvfool.com/attachment....3&d=1571360017

Quote:

Originally Posted by BrooklinOTA (Post 62313)
I used an RCA TVPRAMP1R to take the signal from my antenna to the distribution block in the basement.

- The TVPRAMP1Z brings around a 2.6 db signal loss immediately from the antenna.

- It then boosts that signal to take it down the line.

- But the signal that is boosted is the signal that already had 2.6 db of loss!

Therefore the preamp is boosting an inferior signal....

It is not clear to me where you think that 2.6 dB loss is coming from.

Is it because there is a long length of coax between the antenna and the input of the RCA preamp, or some other reason?

BrooklinOTA 18-Oct-2019 1:11 PM

Quote:

Originally Posted by rabbit73 (Post 62324)
https://forum.tvfool.com/attachment....1&d=1571358639



https://forum.tvfool.com/attachment....2&d=1571358893

Thank you for the TVFool report. Here is a rabbitears.info report based on my estimate of your location. It should have a more accurate list of channels.
https://www.rabbitears.info/searchma...study_id=20077

You can do your own report here. You might need to use coordinates.
https://www.rabbitears.info/searchmap.php

Extract from rabbitears.info report:

https://forum.tvfool.com/attachment....3&d=1571360017


It is not clear to me where you think that 2.6 dB loss is coming from.

Is it because there is a long length of coax between the antenna and the input of the RCA preamp, or some other reason?

My quess is that the RCA preamp causes an initial 2.6db of signal loss as the signal from the antenna goes into the unit. Is that accurate?

rabbit73 18-Oct-2019 3:27 PM

Quote:

Originally Posted by BrooklinOTA (Post 62327)
My quess is that the RCA preamp causes an initial 2.6db of signal loss as the signal from the antenna goes into the unit. Is that accurate?

Not exactly.

The preamp does not cause a signal loss of 2.6 dB. What would be the point of using a preamp if it caused a signal loss of 2.6 dB?

It does, however, add its own internal noise, called the Noise Figure or NF, to the signal that reduces the SNR of the signal coming from the antenna terminals. The greatest affect of this added noise would be on the weakest UHF signals. The preamp also makes all signals stronger before they travel down the coax to the TV.

But, all is not lost, because the Noise Figure of the preamp is usually lower than the Noise Figure of the tuner in the TV that comes after the preamp, so there is a net improvement of the antenna System Noise Figure.

Since the preamp is just after the antenna, it is at the beginning of the antenna system. This means that it primarily determines the System Noise Figure according to the Friss Formula.
https://en.wikipedia.org/wiki/Friis_formulas_for_noise
Quote:

An important consequence of this formula is that the overall noise figure of a radio receiver is primarily established by the noise figure of its first amplifying stage. Subsequent stages have a diminishing effect on signal-to-noise ratio. For this reason, the first stage amplifier in a receiver is often called the low-noise amplifier (LNA).
Here is an example of with and without preamp that shows how adding a preamp might make it possible to receive weaker signals:

http://forum.tvfool.com/attachment.p...0&d=1470667859

BrooklinOTA 24-Oct-2019 8:43 PM

Quote:

Originally Posted by rabbit73 (Post 62328)
Not exactly.

The preamp does not cause a signal loss of 2.6 dB. What would be the point of using a preamp if it caused a signal loss of 2.6 dB?

It does, however, add its own internal noise, called the Noise Figure or NF, to the signal that reduces the SNR of the signal coming from the antenna terminals. The greatest affect of this added noise would be on the weakest UHF signals. The preamp also makes all signals stronger before they travel down the coax to the TV.

But, all is not lost, because the Noise Figure of the preamp is usually lower than the Noise Figure of the tuner in the TV that comes after the preamp, so there is a net improvement of the antenna System Noise Figure.

Since the preamp is just after the antenna, it is at the beginning of the antenna system. This means that it primarily determines the System Noise Figure according to the Friss Formula.
https://en.wikipedia.org/wiki/Friis_formulas_for_noise


Here is an example of with and without preamp that shows how adding a preamp might make it possible to receive weaker signals:

http://forum.tvfool.com/attachment.p...0&d=1470667859

Thanks! Colour me corrected! So that said, is it always good to introduce a preamp just after the antenna?

GroundUrMast 25-Oct-2019 8:16 AM

Quote:

... is it always good to introduce a preamp just after the antenna?
Yes... If there is truly need for amplification, placing the preamp as close as possible to the antenna will maximize the positive effects on the total system signal to noise ratio.

mikecandu 3-Nov-2019 5:46 PM

Hi,

I have a DB8e as well, but I'm in Hamilton, southwest of Toronto, on the top edge of the Niagara escarpment.

Here's a few recommendations I have.

1) If you can't get CFTO in Toronto get the "rabbit ears" (VHF) add on for your DB8e. This should allow you to get CFTO and maybe CKVR (CTV2) in Barrie. You might also be able to get CTV2 from it's Hamilton repeater, but it's UHF, not VHF.
2) Get a rotator. This will allow you to fine tune reception for the US stations. As already stated in this thread the buffalo station are actually pretty far apart, and the signal strength for me and you are quite weak. Sometimes less then a 10 degree movement is required to pick up CBS (WIVB) vs ABC (WKBW) for me. The new rotors work with a remote and allow you to pre-program each station (up to 9 different locations). You can pick one up for less then $200 CAD
3) Get a low noise pre-amp. Because the signals we are trying to get are so weak I'd recommend a low noise preamp. I had a Kitztech KT200 that recently died, so I went back to my old LNA200 from Winegard.

Another thing to keep in mind is that lately the repack has caused some issues. CBS used to be my best station and now it's much harder to get. Fox 29 was always problematic now it's nearly as strong as signals from the CN tower. Hopefully some of these weak signals are temporary and will get better as the repack process continues.


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