5/10 bars - bigger preamp or taller mast?

TheEmrys · 2-Sep-2011, 4:03 AM

So, I picked up the Terk HDTVO. So for, the wife and I are pretty pleased. We picked up 50 channels and once we filtered through languages we don't speak and all the religious programming, we've got a pretty good mix of channels. That being said, everything is showing as 5/10 bars for both of our TV's.

Here is the situation:

1. The HDTVO comes with a 12 dB preamp, which really is necessary to work.
2. The antenna is mounted to our old Dish mast (took off the actual dish) ~13- 15 feet off the ground.
4. Every neighbor has a 10' mast near the peak of their roof for a near 30' height, but they've probably been there for 30 years.
5. We are pointed at 200 degrees where the majority of our channels are, but they are all between 23-60 miles away.

Here is my Signal Analysis:

http://www.tvfool.com/?option=com_wr...9e74f6d48df69b

What is my best option? Move to a more powerful preamp or get a taller mast? Given the distance and power numbers, would a 25+ dB preamp be excessive? 19? Let me know, because I'm loving this OTA thing.

GroundUrMast · 2-Sep-2011, 5:22 AM

Rule of thumb #1: If it's not broke, don't fix it.
Are you having problems with the picture pixelating or intermittent loss of signal? If not, then I'd suggest you not worry about the signal meter indication.

If you are seeing problems with reception, then...

Rule of thumb #2: One amplifier can be too many, two amplifiers are too many.

There is more to reliable reception than signal strength. Signal quality matters as much or more. Electronic noise, interference from other transmitters, multipath and a variety of other passive transmission impairments can all reduce the quality of both weak and strong signals. Two amplifiers in a row will add noise and quite likely overload, causing distortion that destroys the quality of the signals.

Most amplified antennas do not provide a way to remove the amplifier. As far as I can tell, the Terk HDTVo does not provide that option either. (Please, someone, correct me if I'm wrong.) If the amplifier can not be removed from the antenna, you're left with two options.

1. Verify the antenna is pointed toward the SW (it sounds like you've already done that) and there are no obstructions in front of it. Raise the antenna higher, if it's safe to do so.

2. Replace the antenna.

If this were a new install I would have recommended a non-amplified antenna such as the Winegard HD7694P. I would also suggest that you go with the consensus of your neighbors and mount high, in the clear - Using a roof mounted mast. Your signal prediction indicates little if any need for any amplifier. There is enough signal power in the air to easily drive two sets on all the stations not affected by adjacent or co-channel interference.

Rule of thumb #3: Antenna gain is better than amplifier gain.
Antenna gain will deliver more signal power and... improve the noise margin (one measure of signal quality - the antenna can add gain with out adding noise). Amplifier gain also increases signal power, but at the expense of reducing the noise margin (amplifiers add noise).

John Candle · 2-Sep-2011, 6:16 AM

GUM is correct on all points. Antenna gain is more important then amplifier gain. I recommend a bigger antenna , Winegard HD7697P antenna aimed at about 195 degree magnetic compass. No amplifier is needed. Also get the antenna higher , about 25 feet if you can.

TheEmrys · 2-Sep-2011, 8:35 PM

Quote:

Originally Posted by GroundUrMast

Rule of thumb #1: If it's not broke, don't fix it.
Are you having problems with the picture pixelating or intermittent loss of signal?.

It occaisionally happens. And there are a few channels that were picked up, but just get a black screen except late at night (after midnight).

Quote:

Originally Posted by GroundUrMast

Rule of thumb #2: One amplifier can be too many, two amplifiers are too many.

I would only replace the amp. The amp is completely seperate, and its in my basement (where my coax drop is) before my splitter. And the Terk HDTVO antenna has an external, 12 dB preamp.

Quote:

Originally Posted by GroundUrMast

Rule of thumb #3: Antenna gain is better than amplifier gain.
Antenna gain will deliver more signal power and... improve the noise margin (one measure of signal quality). Amplifier gain also increases signal power, but at the expense of reducing the noise margin.

I get what your saying. The only thing I'm unclear on is the noise margin. Can you explain a bit? Is it that it introduces a bit of interferences in the copper because of the amplification and that it then degrades the signal?

So, I guess I am better off leaving it as-is? A bigger pre-amp wouldn't clear up my minor issues? For what I've got going on, a bigger antenna isn't very appealling (to my wife). Our current antenna is small and its sitting on the back side of the house, out of sight.

If I'm better off as-is, I'll just delete the channels that aren't coming in and call it good.

TheEmrys · 7-Sep-2011, 12:51 AM

Quote:

Originally Posted by TheEmrys

I get what your saying. The only thing I'm unclear on is the noise margin. Can you explain a bit? Is it that it introduces a bit of interferences in the copper because of the amplification and that it then degrades the signal?

So, I guess I am better off leaving it as-is? A bigger pre-amp wouldn't clear up my minor issues? For what I've got going on, a bigger antenna isn't very appealling (to my wife). Our current antenna is small and its sitting on the back side of the house, out of sight.

If I'm better off as-is, I'll just delete the channels that aren't coming in and call it good.

I still want to understand noise margin. Anyone?

John Candle · 7-Sep-2011, 1:27 AM

Tv antennas receive the best when the tv antenna has a Clear View of the Tv Stations. Tv antennas work the best with elbow room.

TheEmrys · 7-Sep-2011, 2:33 AM

So noise margin is affected by elevation and the size of the antenna?

TheEmrys · 7-Sep-2011, 2:50 AM

I guess I'm trying to understand the relationship between nm(dB) and PWR(dBm). Do I add them together to get >0?

GroundUrMast · 7-Sep-2011, 3:34 AM

http://www.tvfool.com/index.php?opti...57#how_to_read

Quote:

This is the predicted Noise Margin (NM) of each channel "in the air" at your location, specified in dB. You must add/subtract any gains/losses you get from your antenna, building penetration, amps, cables, splitters, and other factors present in your situation. Hypothetically speaking, you need to end up with an NM value above 0 in order to pick up a station.

Every atom in the universe has electrons in motion, every atom produces a tiny amount of radio frequency energy. As a result there are always random radio waves 'in the air'. All of the parts of an antenna system and the TV connect to the antenna also generate small amounts of radio waves. As a desired signal gets extremely weak, the tiny amount of noise generated by atoms of the antenna will be more able to interfere with reliable reception. If the signal arriving at the antenna is too weak, the the noise generated by atoms in the antenna can be stronger than the signal received.

Noise margin as used here at TV Fool takes into account the expected level of 'background noise' produced in the antenna versus the minimum amount of desired signal needed to receive the TV signal.

If you see a listing on your TV Fool report that predicts a NM of 0.1 dB it means that an antenna with no gain or loss connected directly to an excellent tuner would in theory be on the extreme edge of just able to receive a usable signal. Any bit of noise or interference would interrupt the successful reception of the signal. In this scenario, installation of an antenna with gain would increase the chance of reliable reception.

Some TV tuners are better than others. One measure (that is very hard to find published) of tuner quality is it's noise figure. Lower NF is desired because it indicates less noise is generated by the tuner itself.

Preamplifiers and distribution amplifiers produce noise because like everything else in the universe, are made of atoms. One measure of the quality of an amplifier is it's noise figure (just like the tuner). Amplifiers amplify both desired and undesired signal. They can't tell the difference. In fact amplifiers amplify the noise generated inside the transistors and other parts of the amplifier. A good amplifier will have a low noise figure.

When you're dealing with weak signals (low NM) it would seem obvious that taking steps to increase the signal level would be a good thing. Not as obvious though, taking steps to avoid adding noise is just as valuable.

Antennas are able to provide gain when designed to be sensitive in one direction more than another. This gain does not change the tiny amount of noise generated in the antenna. This means that an antenna with gain, will deliver more desired signal to the coax, and if the antenna is directional, it will be less sensitive to interfering signals from directions other than the one it's aimed at. The result will be greater difference between the desired signal and noise, which is an improvement of noise margin. An antenna with 10 dB gain will increase the received signal 10 dB compared to an antenna with no gain (0 dB). The antenna noise does not increase, so, the signal to noise ratio and noise margin would increase by 10 dB also... more if the directional nature of the gain antenna resulted in less noise received.

An amplifier will increase the strength of a signal but will also amplify noise that comes in through the antenna. Then, in addition, the amplifier will add the noise produced by itself. The end result is less difference between the desired signal and undesired signal (noise). This results in a lower margin (difference) between desired signal and noise. In the case of a signal with a NM of 0.1 db in the air, using the zero gain antenna and a 10 dB amplifier with a noise figure of 3 dB would result in a signal power increase of 10 dB and a noise power increase of 13 dB (the 10 dB amplifier gain and the 3 dB additional noise produced by the amplifier). The resulting noise margin would then be -2.9 dB which would end any hope of seeing that signal.

Tower Guy · 7-Sep-2011, 3:21 PM

Quote:

Originally Posted by TheEmrys

I guess I'm trying to understand the relationship between nm(dB) and PWR(dBm). Do I add them together to get >0?

From the FAQ:

The Noise Margin (NM) of each channel "in the air" at your location is specified in dB. You must add/subtract any gains/losses you get from your antenna, building penetration, amps, cables, splitters, and other factors present in your situation. Hypothetically speaking, you need to end up with an NM value above 0 in order to pick up a station.

Simply put, ignore the receive power. Add your antenna gain to the NM value, subtract the noise figure of your preamp, and subtract distribution losses that add to the system noise figure. (The system losses are variable and use a process called the Friis equation, but that's a bit beyond the question asked.) The result must be greater than zero, but +10 db of margin is a good goal.

The use of a preamp depends on the total signal level available at your location. In your case, the signals are strong enough without a preamp, so your use of a preamp is compensating for some other losses, perhaps due to low antenna gain and/or trees and/or neighboring houses and/or multipath and/or distribution losses and/or cable problems and/or inadequate receiver performance.