Keeping Your Signal Strength Up
August 21, 2008 5:50 PM
I would like to review some of the things that we have talked about on the different forums these last few months.
First of all, I want to encourage anyone who may need to depend on over-the-air television for any reason to go ahead and get set up for the digital transition now. We are in the waning days of summer, and a lot of you who live up in the northern reaches of the U.S. soon will be facing winter. I can’t think of anything more miserable than trying to put up an antenna, or add a signal boosting pre-amp, during February in Michigan, Maine, North Dakota (or South Dakota, for that matter), Montana or a lot of other places. And believe me, I know that you who are down here in the South are waiting for cooler weather to start your projects, but please don’t put them off any longer than you have to.
Signal strength is measured in units called decibels, abbreviated dBs. They usually are shown as a negative number. TVFool.com suggests a signal strength of at least -110 dBs at the antenna is necessary to be able to drive the tuner. But this is dependent on the sensitivity of the tuner. Unless your tuner is really sensitive in your digital equipment, that will not be nearly enough signal strength to get a picture.
I have found that a signal strength somewhere in the -90 dB range is more real-world signal strength. This is the signal strength at the antenna. For the tuner to be able to use it, it will need amplifying.
At the transmitting antenna, the signal strength would be designated as 0. The farther away you are from the transmitter, the more signal loss you have (-dBs). Signal strength also can be affected by terrain, buildings, trees and other obstacles between the transmitter and your antenna. Even the wall between the transmitter and that attic antenna will cause signal loss (5 dBs in some cases). That is why outside is better than inside, and the higher the antenna is over those obstacles, generally, the better.
If you need a pre-amplifier, be sure to pick one that is very low-noise. Pre-amps are mounted close to antenna. In an analog signal, noise looks like snow. For a digital tuner to lock on to the signal, it would have to have the equivalent of a clear, no-snow analog signal. Most Channel Master pre-amps are very low-noise. A noise rating of 0.5 dB is excellent and 2 dBs is very good.
Amplifiers should be linear. They should boost what is put into them and nothing more. If there is too much amplification, then they can become nonlinear and overload the tuner. The good news is that overload almost never causes damage to the tuner.
If you add an amp, and the reception gets worse, you may have a condition known as overload. This has happened to me. I know from experience.
The amplification side of the pre-amp also is measured in dBs. This is called gain. Antennas, too, always have a gain factor built into them. How much gain depends on their type and construction. Finding the gain rating of an antenna is not always an easy task. A lot of the time, an antenna will be merely listed as a medium-gain or high-gain antenna. The Channel Master model 3671 high-gain, deep fringe antenna has a gain rating of 5.6 dBs on channels 2-6 (VHF—low), 10.9 dBs on channels 7-13 (VHF—high) and 10 dBs on channels 14-69 (UHF).
When figuring the total gain of a system, this number for the antenna does not change. When figuring total gain, you would take the gain rating of the antenna and add the gain rating for the pre-amp, which for the CM 7777 pre-amp is 23 dBs on VHF (low and high) and 26 dBs on UHF. Then you would start deducting the loss of gain.
RG-6 is the preferred standard for coaxial cable. RG-6 has a gain loss of about 1dB for every 18 feet of length. Keep your coax runs as short as possible. RG-59 has a lot more loss of gain per foot, so stay away from it.
A one-to-two splitter will cost you 3 dBs. A balun, also called a transformer, or matching transformer will cost you about 1 to 2 dBs. Every 3 dB of gain loss equals half your signal strength.
For example, a station that puts a -80 dB signal at your Channel Master 3671 antenna will be increased by 10 dBs on UHF channel 20 (this is an approximation only). So you now have a signal strength of -70 dBs. Adding the CM 7777 pre-amp adds another 26 dBs. So we are now at -44 dBs. If you have a 54 foot RG-6 cable run down to the TV, then you would deduct 3 dBs loss for a total of -47 dBs. If you split that somewhere in the line, then you lose another 3 dBs, for an end signal strength of -50 dBs, and so on.
You can eliminate the loss from a splitter by using what is called a distribution amp, which typically comes in a one-into-two or a one-into-four version.
So there you have our review at the six-month mark before the transition. And just a reminder, there may be a pop quiz next week!
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