I'd like to apologize for...
1. Taking so long to reply - the 4th is a very busy time for me.
And 2, for having a brain fart: My mention of excess amplifier gain being additive to the signal was erroneous - just couldn't see the forest thru the trees for the moment...
Quote:
Originally Posted by Small Engine
Lets go one step below that level and reduce it to the brain size of a Arkansas Razorback 
The antennas gain is 7.2 db then it travels thru a short cable to the booster box.. losing .2 db in its travel. The booster then doubles the gain to 14.0 db, now it sends the gain/signal thru 40' of good cable and loses another 3.0 db... at the converter box the 11.0 db enters in only to come out ( because of resistance) at 9.0 db...while traveling thru the cable from the converter box to the tv it looses another 1.5db...bringing the overall gain/db to 7.5... |
You're given an NM (Noise Margin) figure from TVFool. This represents how strong the signal is at your given location and antenna elevation, in dB, above (or below) the noise floor. You need to end up with an NM of 0dB or higher at the tuner to [theoretically] receive the signal. Figuring in a safety margin of 5dB or so to accommodate for losses unforeseen to TVFool and misc system losses such as SWR mismatches at connectors, Balun losses, etc. is a good idea. So you really want to end up with an NM of +5dB at the tuner.
The key is your antenna...
It is your antenna's job to gather more energy to raise the signal to acceptable levels above the noise floor. So you
add the gain of the antenna to the NM figure from TVFool, then
everything else in your system is a minus.
Three things to remember about an amp:
1. An amp is used to overcome
losses in your system. i.e. coax run, splitters, etc.
2. An amp cannot generate a signal; it merely amplifies what it is given - and that includes the noise.
3. An amp contributes its own noise into the system, and this noise figure must be
deducted from your overall NM figure.
As another example, lets say you're trying to receive a channel that TVFool gives you an NM figure of -2dB for your location and elevation. This channel corresponds to a frequency at which your antenna has a rating of +15dB of gain, thus your antenna has raised the NM to +13dB at the output of the antenna.
Now you want to use an amp to maintain that signal and overcome downstream losses in your system. lets say you estimate them to be -8dB. So an amp with +12dB of gain should more than overcome them. Our amp has a noise factor of 3dB which needs to be deducted from the +13dB coming out of the antenna, leaving you at +10dB NM out of the amp. So even though you have amplified the signal by +12dB of gain in the amp you have also amplified (& added) noise, so in effect you have lowered the NM. If you used an amp with +23dB of gain with a -3dB noise factor you still have the same NM of +10dB. You have more power, (and your signal will travel further thru more losses) but you have not increased the margin.
Now lets say you buy a cheap amp which has +30dB of gain but has a 10dB noise figure. Now you have +13dB out the antenna -10dB amp noise = +3dB NM. A signal with a lot of power, but not very far above the noise floor, IOW not as clean a signal.
To put it another way, you suffer the loss in NM of the amp's noise figure once - and then all downstream system losses come out of the amp's gain and don't lower your NM any further. (Provided you've estimated correctly, of course.
)
And don't sweat the tenths - this is partially why we build in a 5dB or so safety margin. IOW, if you end up with an NM of +5dB at the tuner you should be good.
Quote:
Originally Posted by Small Engine
Is keeping your conections clean and your cables as short as possible the answer to getting better tv reception from your equipment ?
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Properly installed connectors and using the shortest possible cable runs are definitely contributing factors towards a good performing system. Also remember to wrap outdoor connections with electrical tape to avoid moisture intrusion.
And as far as antenna gain of dBi vs dBd and the 2.15dB difference - may as well just base everything on dBd to be safe. IOW, if your antenna is rated as +17.5dBi subtract 2.15 to arrive at 15.35dBd and work from this number. (Worst that happens is you're pleasantly surprised with the final performance.
)