Understanding Gain aka db

Small Engine

I am currently veiwing the new digital signal with a antenna that has a average gain of 7.2 db. Because I dont have LOS with the transmitting tower I guess I am veiwing most/all my digital signals from refraction from the clouds. currently my area is under a high pressure ridge aka no clouds in the sky and I am getting signal interuption or no signal messages. I am currently looking for a new antenna and there are so many styles/shapes and sizes that its very frustrating to choose witch one to purchase... one is a 8 bay or bow tie antenna that says it has a gain of 15.8db and the other is a Yagi style and is a bit smaller in size but says its gain is 28db. I'm just guessing but I would think the higher the gain the better the reception would be ?... but if I dont have any signal to recieve I could have a 100db gain antenna and still not receive anything ..Right ?

__________________
Antennas are like Real Estate.. its all about location.. location.
My TvFool
XG-91 DB8 Wg-4800 Ac-10G212

JB Antennaman

Gain - is simply a term that explains how to measure a signal.

http://www.hdtvprimer.com/ANTENNAS/types.html

http://www.hdtvprimer.com/ANTENNAS/G...ml#capacitance

http://www.hdtvprimer.com/ANTENNAS/basics.html 20 dB = gain factor of 100

10 dB = gain factor of 10

3 dB = gain factor of 2 (actually 1.995)

0 dB = no gain or loss

-1 dB = a 20% loss of signal

-3 dB = a 50% loss of signal

-10 dB = a 90% loss of signal

http://en.wikipedia.org/wiki/Antenna_gain

http://www.tech-faq.com/television-antenna.shtml

JB Antennaman

Unit conversions

Zero dBm equals one milliwatt. A 3 dB increase represents roughly doubling the power, which means that 3 dBm equals roughly 2 mW. For a 3 dB decrease, the power is reduced by about one half, making −3 dBm equal to about 0.5 milliwatt. To express an arbitrary power P as x dBm, or go in the other direction, the following equations may be used:

x = 10 \log_{10}(1000P)\, or, x = 10 \log_{10}P + 30\,

and

P = 10^{(x/10)}/1000 \,

where P is the power in W and x is the power ratio in dBm. Below is a table summarizing useful cases:

Tower Guy

Be careful with antenna gain numbers in advertisements. There is pure antenna gain and there is antenna gain plus amplifier gain if the two are packaged together. There is gain over an isotrope antenna, gain over a half wave dipole in free space, and gain over an isotropic antenna with 6 db of ground gain added.

Advertising agencies call it "puffery".

What can't be easily inflated is beamwidth. If you compare the antenna patterns of two antennas, the one with a narrow beamwidth has more gain than an antenna with a wider beamwidth.

If you can find a real antenna with 28 db gain, let me know, I want one. If you stacked multiple 91xg's to get 28 db gain, you'd need about 24 of them.

JB Antennaman

A: The units of dBm and milliwatts are used quite a bit in communications and wireless. Both are units of RF power. There seems to be a lot of confusion between the two. Watts (or milliwatts) is a linear unit, while dBm is a logarithmic unit. Confusing the two can really make for interesting results. Recently, a WRT54G wireless router was described as outputting 84 dBm. Obviously, mW was the desired term, since 84 dBm equals 250 Kilowatts! One should really avoid being anywhere near a 250KW RF transmitter! Here is simple description of the two terms.

0 dBm in the RF world is arbitrarily defined as 1 mW of power (.001W). From this, we get:

dBm = 10 * log(mW) (Note: power is in mW, not Watts!)
mW = 10 ^ (dBm/10)

Both of these can easily be calculated using the Windows Calculator in Scientific mode. Or, for the lazy, try this site: »www.aerialix.com/calculators/dbm···ion.html

Here is a simple cross reference, including some commonly found values for wireless, and all 1 dB increments from 20 to 30 dBm. Remember, for all practical purposes, every 3 dB is a doubling/halving of power, and every 10 dB is an order of magnitude difference.

-10 dBm = 0.1 mW
0 dBm = 1.0 mW
10 dBm = 10 mW
17 dBm = 50 mW
19 dBm = 79 mW

20 dBm = 100 mW
21 dBm = 125 mW
22 dBm = 158 mW
23 dBm = 200 mW
24 dBm = 250 mW
25 dBm = 316 mW
26 dBm = 400 mW
27 dBm = 500 mW
28 dBm = 631 mW
29 dBm = 794 mw

30 dBm = 1 Watt
40 dBm = 10 Watts
50 dBm = 100 Watts
60 dBm = 1000 Watts ...etc.

Some of the above numbers are approximated for clarity; 26 dBm actually equals 398.107 mW.

Hint: to quickly find some other unlisted value, say 12 dBm, look up the wattage for 22 dBm and divide by 10 (pwr = 15.8mw). Likewise, for 32 dBm, one would multiply the wattage by 10 (pwr = 1.58W).

Use the term "dBm" when referring to any absolute power level; never just "dB." e.g. "This router has 23 dBm of output power."

The phase, "This outputs 25 dB" is technically meaningless, although most will interpret this to mean 25 dBm.

The term "dB" is a relative term only; such as, "This router has 5 dB more output power than that Access Point." This is somewhat analogous to percent, where one does not say, ”This sweater cost me 70%”, but you can say, “I saved 70% on this sweater.”

JB Antennaman

You can only amplify a signal if a signal is there. A amplifier cannot produce a signal when none is present.

You have to remember that there is always signals around you, just that very few of those signals are television.

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 see
http://en.wikipedia.org/wiki/NTSC

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

http://en.wikipedia.org/wiki/Television_station
"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.

There are no stations on channel 1.

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.
Since at least 1974, there are no stations on channel 37 in North America for radioastronomy purposes."

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.
Here is some information concerning line of sight and how it effects TV
http://www.softwright.com/faq/suppor...re_values.html

http://www.ian-ko.com/ET_Surface/Use...Discussion.htm

http://en.wikipedia.org/wiki/Horizon

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)


Gain of antenna: Just like your receiving antenna has gain in db so does the transmitter.
Remember 3db gain is equivalent to doubling the output power. or 1/2 the electric consumption expense.

JB Antennaman

http://www.hdtvprimer.com/ANTENNAS/types.html

http://www.hdtvprimer.com/ANTENNAS/G...ml#capacitance

http://www.hdtvprimer.com/ANTENNAS/basics.html

20 dB = gain factor of 100

10 dB = gain factor of 10

3 dB = gain factor of 2 (actually 1.995)

0 dB = no gain or loss

-1 dB = a 20% loss of signal

-3 dB = a 50% loss of signal

-10 dB = a 90% loss of signal

http://www.geocities.com/toddemslie/UHF-TV-DX.html

http://www.terrestrial-digital.com/91XG.html

JB Antennaman

Just for an example, let's say that you live in Russelville Ark and you wanted to watch WARK

Callsign: KARK-TV Service: DT Community: LITTLE ROCK, AR, US

DTV Type:

Licensee: NEXSTAR BROADCASTING, INC.

Channel: 32 Offset:

Specified Channel: 32 Max HAAT: 520 m

File No.: BPCDT-19990614KE Cutoff Date: Dom. Status: CP

Facility ID: 33440 Application ID: 286175 ASRN: 1019242


Latitude: N 34° 47 ' 57 "
Longitude: W 92° 29 ' 59 "

ERP: 989 kw
HAAT: 474 m
RCAMSL: 602 m
RCAGL: 288 m

Now let's say that The average estimated signal level from WARK in your area is -41db.
That calculates to .00008 millawatts (000.000.08 watts) of power per meter of antenna receive (radiator) element size.
Not the whole antenna size.

There are three parts to a antenna.
1. Director
2. Radiator (the element/s that the wire connect to)
3. Reflector

The Directors and Reflectors are generally grounded either directly or
through a tuning wire.

With a -41db input and a receiver that can pick up a signal to -55db (most are in this range, but manufactures do not publish any specifications), you have 14db of safety factor for all atmospheric signal reduction, trees and other factors.
You should get a good picture (analog).

So what you want to do is find the best antenna for your situation with the most amount of gain and the broadest width or reception since your area might have all UHF channels from digital channel 7 - PBS - KETS to digital channel 44 - KARZ

If you did not wish to receive channel 7 and channel 13 - KEMV
You could buy a UHF only antenna such as a Channel Master CM 4228 and a good pre amplifier and be able to receive as many channels as possible on the UHF in your area, as long as your antenna was higher than the highest thing in your neighborhood and there was no buildings more than 4 stories high, trees, hills, mountains etc. in your area between your antenna and the transmitter.

electrictroy

My switch says it incurs a 1 dB insertion loss. Is that a 66% loss?

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My internet cost == $15/month. My cellphone cost == $5/month. My television cost = Free!

aka.Hooper

Nooooo, did you not read JB's informative (if somewhat irrelevant to the OP's question) post's?

A 3dB loss would = HALF your power out the other side, thus giving you a 50% power drop.

dB=10*log (P in/P out)

A 1.0dB insertion loss would be roughly a 20% loss of power.

aka.Hooper

As to the OP's question, gain is referred to as gain as compared to an isotropic receptor/radiator (dBi) or over a dipole structure (dBd). A dipole structure has 2.15 dB gain over an isotropic receiver/transmitter. Thus when comparing antenna's listed in both ways take this into account. i.e. an antenna listed as having an 11.5 dBi gain is comparable to an antenna having a gain of 9.35 dBd.

Your current antenna with 7.2 dB of gain doesn't sound very impressive - especially if it's rated as dBi. Run a TVfool report for your exact address and antenna height and see what you're actually dealing with first. Chances are you have some signal out there...

electrictroy

Alright. 1dB == 20% loss.

And my converter box's "analog passthru" incurs a 1.5 dB penalty, or about 33% loss.

__________________


My internet cost == $15/month. My cellphone cost == $5/month. My television cost = Free!

aka.Hooper

Close... @ 1.5 dB I got a 29.2% power loss.

It's logarithmic, not linear...

Small Engine

I'm not sure how to read my Tvfool results as far as my antenna selection/recomendations, here is my long and lat

Long -94.47296 Lat 33.95730

I'm trying to get a constant/good/dependable signals so I can watch KTAL, KTBS and KFXK amongst other stations all broadcasting in the same area. KTAL is the closest at 77.6 miles. Trying to understand gain db and the calculations that are involved are very confusing to me. It seems obvious that a directional antenna like a XG 91 pointed at 165% would work the best but a CM 4228 antenna has more mass to collect signals thus providing less interuptions and loss of signals ? my lil 11 element yagi style antenna and a 30 db booster has been a work horse sense April 2009 ....I am just trying to move up to the next level of quality. So should I go Direstional or have more mass to receive better tv signals in my area.

__________________
Antennas are like Real Estate.. its all about location.. location.
My TvFool
XG-91 DB8 Wg-4800 Ac-10G212

jim5506

At 50 ft, KTAL-DT has a NM of 4.4dB and KTBS is -8.7dB.

Signal strengths below that is almost impossible.

On a 4228 if the signal is oscillating around some of the bow-ties will have signal others will not, but you will never get the full advantage of the signal.

On the other hand, a yagi like the 91-XG will get the full signal at times and will drop off at times.

Do you find your yagi getting pulses of signal where the picture fluctuates a lot? If so you might try the 4228 and see if it overall collects enough signal to lock. On the other hand if it is rather stable, you'll be better off with the 91-XG (which really is no all that small overall with the reflector being 22" high and 20 " wide).

It would be interesting to set up two antenna systems side-by-side in a fringe to deep fringe area, one with the 4228 and one with the 91-XG over open flat territory and see how each performed, then set up both in a hilly situation and see which was best there.

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