Understanding Gain aka db

Small Engine

Jim,
Thats what my yagi is doing, this morning at 7am KTAL has a signal strength of 80% but as the day goes on the signal starts to break-up or becomes not available. I can run a antenna scan and sometimes find other channels. digital signals seem to be very contrary and how they find there way out here to the fringe is a mystery to me ? my gut says the 91-XG is the way to go but for the sake of helping others I might go with the 4228 and post the results here.

__________________
Antennas are like Real Estate.. its all about location.. location.
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jim5506

Variation within the day is caused by atmospheric changes, if the signal is not pulsing in 1-10 second intervals, then you will get the best result from the 91-XG.

The daily fluctuation is a wide area loss of signal during the daytime not a very localized oscillation.

Go with the 91-XG.

__________________
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aka.Hooper

Reducing things to their most basic level, from TV Fool:
"NM (dB) - 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."

So you add the gain of the antenna to the NM figure given, and then everything downstream of your antenna itself incurs some loss, and must be subtracted from the NM figure. This would include coax cable, splitters, and the noise factor of your tuner in an un-amplified system.

When you add a pre-amp the gain of the pre-amp usually cancels out the losses you suffer beyond it. But you must deduct the noise factor of the pre-amp itself from the NM first. (This assumes you're not splitting the signal 4, or 6 ways to many tv's in which case you may need to factor in a secondary distribution amp to make up for these losses.)

So lets examine a sample case...
Say the worst station you're trying to receive is a high freq UHF channel (50) and has a NM of -10dB
Your antenna has a gain of say, 16dB at this frequency, so -10 +16= 6dB NM. So far, so good. Now if you're running say 50' of RG-6 cable and one 2-way splitter (for 2 TV's) you'd be screwed without an amp: Roughly -2.9dB cable loss and -3dB for a two way plus insertion loss of maybe another -0.5dB. So we have 6dB -6.4dB = -0.4dB in a perfect world. (Actual losses may be higher due to mismatches at connectors, etc. Plus this doesn't take into account the noise factor of the tuner.)

But if you add a pre-amp that has say 12dB of gain you more than cancel out the losses of these items. The tradeoff is you have to factor in the noise of the amp, lets say it's 3dB. Now you have -10 +16 -3dB = 3dB above the zero NM floor required to get a signal. And from what I understand this is about the minimum safety factor to incorporate to cover for things unforeseen to TVFool. i.e. trees, hills, buildings, between your location and the transmitter, misc system losses such as connectors, atmospheric fluctuations etc. Ending up with a +5dB NM would be better still.

This is why the amp is put as close to the antenna as possible: You're trying to amplify the signal received before it's drops into noise thru losses incurred in cable, etc. You can't amplify a signal that isn't there.

I am still a bit fuzzy on one point however: When adding the gain of an antenna are we using the gain in dBi or in dBd? As there's a 2.15dB difference, this can either work in our favor, or against us.

Another question for greater minds than mine: Does excess gain from an amp (that which is above and beyond system losses) then count as additive to overall system gain? IOW, in the example above the amp has +12dB of gain, system losses to be overcome are only -6.4dB, so does the excess +2.6dB (After deducting the amp's 3dB noise) then add to the equation? Thus giving you a final NM of +5.6dB? (This would explain the use of such powerful pre-amps, otherwise 30dB amps make no sense to me...)

JB Antennaman

l

An amplifier is an amplifier.

I would recommend using only one amplifier.

The best is to only use a amplifier as close to the antenna as possible
with the highest gain in the area that you are interested in receiving
(UHF) with the lowest possible noise and both at a reasonable cost.

The KEY is that you want a relative "good" level of input at your receiver (TV) with the minimum noise.
A mast mounted amplifier (doesn't matter if it is called a amplifier or
preamplifier) is the best when you are in a remote area.
In theory it is best if you amplify a "good" signal to a higher level than a "poor noisy" signal to a higher level.

All cable has loss and the higher up that you go in frequency (VHF to UHF) the more loss that the cable has.
If you start with a low level signal at the antenna and then butt it into
coax cable you are going to have less come out at the end than went into it from the antenna. The amount of loss depends upon a number of factors, cable type and cable length are the two most important.

Cable is rated in db loss per 100' at X MHz (frequency). RG-6 cable is the cable most used in TV use.
Like all things there is good RG6 and not so good.

Belden 1829AC Coax - Series 6 has a loss of 4db/100 feet at 500 MHz (TV Channel 18),
Channel 32 is 580 MHz
Channel 52 is 700 MHz a 5 db loss

At TV channel 2 the cable would have a loss of 1.4db.
So at channel 18 you loose more than 1/2 the power in 100' of cable between the antenna and the TV.

In the analog days there were two effectively transmitters for the TV
station.

A transmitter for the video and a transmitter for the audio.
The video transmitter was Amplitude Modulation.

The audio was FM at a much lower power.

The reason that the video was a higher power is that AM is more susceptible to noise requiring a stronger signal at the receive and a higher power output of the video transmitter

"In North America, full-power stations on band I (channels 2 to 6) are generally limited to 100 kW analog video (VSB) and 10 kW analog audio (FM), or 20 kW digital (8VSB) ERP.

Stations on band III (channels 7 to 13) can go up by 5dB(W) to 316 kW video, 31.6 kW audio, or 63.2 kW digital. Low-VHF stations are often subject to long-distance reception just as with FM.

UHF, by comparison, has a much shorter wavelength, and thus requires a shorter antenna, but also higher power.

North American stations can go up to 5000 kW ERP for video and 500 kW audio, or 1000 kW digital.

Low channels travel further than high ones at the same power, but UHF does not suffer from as much electromagnetic interference and background "noise" as VHF, making it much more desirable for TV.

Despite this, in the U.S., the FCC is taking another large portion of this band (channels 52 to 69) away, in contrast to the rest of the world, which has been taking VHF instead.
This means that some stations left on VHF will be harder to receive after the analog shutdown.

UHF communications are more "line of sight" communications than lower frequency VHF.

It is sort a like having a sound vs a light.
If you make sound it radiates in all directions, around buildings, through walls, down into holes(valley).
Shining a light does not go around corners or through walls and if it is a pinpoint light it doesn't go down into the valleys.

The earth is round and eventually the beam of light, UHF TV, the beam will no longer touch the earth but go up into the sky.

The height of the transmitting antenna is factored into the power output of the TV station.

The power output for TV & FM is rated in ERP (Effective Radiated Power) - There are a number of factors the go into this calculation.

Several Key items are:
Height of Antenna. listed both as HAAT (Height Above Average Terrain) and AMSL (Height Above Mean Sea Level)

However all
amplifiers amplify everything both the good signal that you want to
hear/see and bad noise from power lines, ignition systems, electric fences, motor brush noise etc.

Amplifiers also generate their own noise.

So a amplifier with a high gain is great but the internally generated noise must be at the minimum.

You should always look for a amplifier that has the best gain and the lowest noise figure.

The best one I have found commercially so far is the 'Winegard AP 8275".

• Input: 82 CH 75 Ohm
• Output: 75 Ohm
• Gain: VHF 29 dB / UHF 28 dB
• Noise: VHF 2.9 dB / UHF 2.8 dB
• Variable FM Trap
• Durable construction, mounts directly to TV antenna mast
• Max Total Input µV VHF: 29,000
• Max Total Input µV UHF: 30,000

It is also the cheapest of all the long range amplifiers.
http://www.solidsignal.com/prod_disp...p?PROD=SAP8275
Price: $ 38.89 - Standard shipping fee is $7.95

JB Antennaman

MAXIMUM TOTAL INPUT# (MICROVOLTS)

• Max Total Input µV UHF: 30,000

• Max Total Input µV UHF: 30,000

http://www.convertunits.com/from/volt/to/microvolt

How many volt in 1 microvolt?

1 volt is equal to 1000000 microvolts.

volt - is defined as the difference of electric potential existing across the ends of a conductor carrying a constant current of 1 ampere when the power dissipated is 1 watt.

The kilovolt (1,000 V), the millivolt (0.001 V), and the microvolt (0.000001 V) are units derived from the volt.

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...

Is keeping your conections clean and your cables as short as possible the answer to getting better tv reception from your equipment ?

__________________
Antennas are like Real Estate.. its all about location.. location.
http://www.tvfool.com/?option=com_wr...bc2786867f8eb2

aka.Hooper

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...

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.

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.)