TV Fool has long been the go-to site for seeing what TV signals you can pick up in your area. I went into how to read a tv fool report the easy way in an earlier post. For this post, we’ll get into some of the details if what a report is telling you. Specifically, we’ll take a look at noise margins, gains, and what can cause a loss of signal. (Spoiler Alert: EVERYTHING!)
Noise Margin: NM (dB)
Looking at your tv fool report (see the earlier post on how to generate one), in the table on the right is a column labeled NM (db). NM is short for Noise Margin and it is a measure of how much the signal can be degraded before it is no longer usable. The dB in parenthesis (dB) tells you that it is measured in decibels.
Stations with a higher noise margin are stronger and are easier to pick up. Stations with a low or negative noise margin are harder to get a reliable signal. Technically, anything above 0 means that you should receive that TV station. in reality, you really want a signal at of at least 10 dB.
How To Use The Noise Margin Column
Hopefully, you’ll be getting a lot of stations. But we are going to focus on the one that is a “must have” and that has the lowest noise margin. If we can pick up the weakest signal, then the rest should be picked up also. The example below is a report I generated for my personal location. As you can see, the lowest “must-have” station for me is WBBM, the local CBS affiliate.
WBBM has a noise margin of 22.8 dB. Remember that number. We’ll be returning to that it shortly.
Antenna gain is how well an antenna focuses or concentrates a tv signal. Directional antennas have a higher gain as they are focused in one direction. for our purposes, you can think of gain as how much the antenna amplifies the signal. Antenna gain can be measured in either dB or dBi.
dB vs dBi
Getting a little technical here, but a decibel (dB) is a ratio between input power and output power. It’s a reference. The reference for dB is a half-wave dipole antenna. If the antenna gain is listed as dBi, the reference is to what is called a theoretical isotropic radiator, which is a fancy term that simply means a transmitter that radiates equally in all directions. Problem is, a transmitter that radiates equally in all directions can’t be manufactured.
Ok, enough with the technical talk. Let’s simplify this. The reference for dBi is theoretical and doesn’t exist in the real world. We need to use dB. If only there was some way to turn dBi into dB. Turns out that there is and its simple. dB = dBi – 2.15.
Antennas will have their gain printed on the package, on their website, or on a spec sheet. If the gain is measured in dBi subtract 2.15 to get the gain in dB. My antenna is a Clearstream DB8e antenna which has a gain of 17.4 dBi. 17.4 – 2.15 = 15.25.
A Note About The Antenna
My antenna won’t be able to pick up the WBBM signal at that channel. The antenna that I use is UHF only while that particular channel is VHF-Hi. I DO pick up WBBM on virtual channel 48.3, a UHF frequency. For our illustration, we are going to assume this antenna would pick it up.
Adding Up The Losses
A loss is a degradation of the signal and, like gain, it is measured in decibels (dB). Everything causes a loss of signal. Some signal loss occurs before the signal even gets to you. We’ll take a look at those first.
Losses That Occur Before The Antenna
Distance & Terrain: The farther a signal travels, the weaker it gets. You don’t really have to worry about this since TV Fool does a good job of estimating this in the report. It’s also pretty good at taking the terrain into consideration when it generates the report.
Adjacent Houses/Buildings: If you have a house or a building next to you and it casts a shadow on your antenna you can expect a loss of between 10-21 dB. This is a huge loss. There are only two ways to get around this. You could try raising the antenna to where the shadow isn’t falling on the antenna. If that fails, you have to find another spot to place the antenna. In my case, there is a two story house next door, but the shadow doesn’t fall on my antenna so I won’t consider this loss.
Trees: Trees can cause up to 10dB of signal loss. Again, the solution is to either raise the antenna or find a different location to place the antenna. In some rare cases, actually lowering the antenna so that it is underneath the tree canopy can help. The street in front of my house is lined with trees so I am going to subtract 10 dB from WBBM’s noise margin which leaves me with 12.8 dB. 22.8 – 10 = 12.8.
Indoor antennas have another loss that outdoor antennas don’t have to take into consideration. Building materials, what your house is made from, can account for up to 14 dB of loss. If you have an indoor flat antenna, tape it to a window that faces the broadcast tower to get better reception.
Finding The Real Noise Margin
We have to take losses before the antenna first. Add up all the before antenna losses and subtract it from the TV Fool noise margin. In my case the only loss were the trees in front of my house so I subtracted 10 from 22.8 which gives me 12.8.
Next add the gain from the antenna. Remember to subtract 2.15 from the antenna gain if it is measured in dBi. My antenna gain is 17.4 – 2.15 = 15.25. So my noise margin is 12.8 + 15.25 = 28.05. To keep the numbers simple we’ll say 28.
After Antenna Losses
As I’ve said before, everything causes signal loss. We’ll take a look at my set-up and hopefully you’ll understand by the time we get done. I’m going to start at the TV and work my way back to the antenna.
TV: The TV causes a signal loss of about 3 dB.
Short cable run: I have a short 3-foot RG-6 cable going from a splitter with one end going to the main TV in the living room. The loss is negligible, but to be conservative we’ll call it 1 dB.
TV splitter: I have a two-port splitter with one end going to the main TV. The other goes to a bedroom TV which we won’t consider in this post. Two-way splitters cause a 3.5 dB loss on each port. A three-port splitter will lose 3.5 dB on one port and 7 db loss on the other two. A four-port splitter loses 7 dB on each port.
Short cable run: From the splitter to a power inserter. The inserter is part of a pre-amp which we’ll get to shortly. Again, the loss is negligible, but we’ll call it 1 dB to be safe.
Long Cable Run: I have 100 feet of RG-6 co-ax cable running form the power inserter to the pre-amp on the antenna mast. You’ll see two types of cables that are used in antenna TV installations. RG-59 and RG-6. Use RG-6 as it has a lower loss. The actual loss depends on the channel that you are trying to pick up. Lower frequencies have lower losses while higher frequencies have higher losses. RG-6 co-ax cable loses about 5.6 dB per 100 ft at the highest TV frequency. We’ll use that and to make the numbers simple round it up to 6 dB.
Cable connectors: If you use a cable connector, it causes a .5 dB signal loss.
A good rule of thumb is to add another 5-8 dB of loss. I’m going to add another 10 dB of loss, just to be on the safe side and to make the numbers simpler.
Re-capping the losses I have:
- TV 3 dB
- Short cable run 1 dB
- TV splitter 3.5 dB
- Another short cable run 1 dB
- Long cable run 6 dB
- Misc. losses 10 dB
In total, I have 24.5 dB in losses. My noise margin (from above) was 28. Subtracting the losses from the gain gives me 3.5 dB. That is not enough to get the signal reliably. This is where the pre-amp comes in.
Pre-amps DO NOT amplify the signal! You should never add the advertised gain of a pre-amp to your antenna gain or the noise margin. In fact, using a pre-amp actually adds another loss into the system. That loss is called “noise figure”. You should be able to find noise figure of a pre-amp on the package or the manufacturers web site.
A pre-amp is used not to amplify the signal, but to cancel out any losses that occur after the antenna. I have a Channel Master CM-7777 pre-amp. It has a noise figure of <2dB (we’ll just call it 2) and a gain of 30dB, which is more than enough to take care of the 24.5 dB after antenna loss that I have. Subtracting the noise figure (loss) from my noise margin I get 28 – 2 = 26 dB, plenty of signal to get that channel reliably.