no-hertz
01-31-2011, 13:44
This is a blog article I am writing for a friend of mine, I know some of these things have been mentioned throughout here previously. I wanted to put this up for some of the guys to read, critique, and point out errors I may have made before I publish it elsewhere.
Thanks
This is just a quick explanation of a few different things related to Decibels, commonly misunderstood, and are very useful to know. Decibels are the choice means for rating an antenna's ability to send and transmit signals, output power is sometimes rated in decibels, so on. It is a commonly used term, although many people are unaware of it's proper use.
To start, The decibel system is a logarithmic function, very simple where +3 decibels gain is 2x the previous value. -3 Decibels is a loss of half the value. +6 is 4 times the base value. They system is quite easy to understand and calculate once you get the hang of it. You can also calculate the difference in decibels between to values using this formula;
Number of dBs = 10 log (P1/P2) where P1 and P2 are the two powers/values being compared and where the log is to the base 10
Where the situation comes in (in my opinion) is the improper use of different types of decibels. If 3 decibels higher than x equals 2x, we can only qualify 2x if we know the value of x. When someone advertises an antenna as having 6dB of gain, that really means nothing to me unless I know what is used as the base value for the comparison.
A few types of Decibels.
dBm where 0dB is equal to 1 Milliwatt. 30dBm is equal to 1 Watt
dBW where 0dBW is equal to 1 Watt
dBi is a reference to Decibels when compared to an Isotropic antenna. These don't exist, but are a good reference because the are theoretical and always the same. An Isoptropic antenna is one that transmits signal at the same power in every direction on every axis. It has no lobes.
dBd is in reference to gain when compared to a reference Dipole antenna. If a particular antenna has 6dBd gain, it has a gain equal to 4 times a reference dipole antenna.
A reference Dipole antenna has a constant gain of 2.14 dB over an Isotropic antenna. This is a useful tidbit when comparing antenna's that may be marketed under one scheme or the other.
So now that we have obtained a working knowledge of the Decibel system, and it's different applications I have one thing left.
Lobes. Lobes are the area's of concentrated signal radiating from an antenna. On an Isotropic antenna, there are none. It is a sphere of equal power on all axis. As we move from the Isotropic antenna into Dipoles and then even more concentrated antennas, you will see gain number rise to much higher levels.
This comes at the expense of your lobes becoming smaller in size, and more powerful in depth. An example of this would be a multi element Yagi antenna. These exhibit high gain, but are very directional. These are antenna's that can transmit the same amount of power a farther distance, as long as the antenna is pointed in the right direction.
Something can't come from nothing, and as of now in the antenna world, more gain comes at the expense of smaller lobes.
Erik
Thanks
This is just a quick explanation of a few different things related to Decibels, commonly misunderstood, and are very useful to know. Decibels are the choice means for rating an antenna's ability to send and transmit signals, output power is sometimes rated in decibels, so on. It is a commonly used term, although many people are unaware of it's proper use.
To start, The decibel system is a logarithmic function, very simple where +3 decibels gain is 2x the previous value. -3 Decibels is a loss of half the value. +6 is 4 times the base value. They system is quite easy to understand and calculate once you get the hang of it. You can also calculate the difference in decibels between to values using this formula;
Number of dBs = 10 log (P1/P2) where P1 and P2 are the two powers/values being compared and where the log is to the base 10
Where the situation comes in (in my opinion) is the improper use of different types of decibels. If 3 decibels higher than x equals 2x, we can only qualify 2x if we know the value of x. When someone advertises an antenna as having 6dB of gain, that really means nothing to me unless I know what is used as the base value for the comparison.
A few types of Decibels.
dBm where 0dB is equal to 1 Milliwatt. 30dBm is equal to 1 Watt
dBW where 0dBW is equal to 1 Watt
dBi is a reference to Decibels when compared to an Isotropic antenna. These don't exist, but are a good reference because the are theoretical and always the same. An Isoptropic antenna is one that transmits signal at the same power in every direction on every axis. It has no lobes.
dBd is in reference to gain when compared to a reference Dipole antenna. If a particular antenna has 6dBd gain, it has a gain equal to 4 times a reference dipole antenna.
A reference Dipole antenna has a constant gain of 2.14 dB over an Isotropic antenna. This is a useful tidbit when comparing antenna's that may be marketed under one scheme or the other.
So now that we have obtained a working knowledge of the Decibel system, and it's different applications I have one thing left.
Lobes. Lobes are the area's of concentrated signal radiating from an antenna. On an Isotropic antenna, there are none. It is a sphere of equal power on all axis. As we move from the Isotropic antenna into Dipoles and then even more concentrated antennas, you will see gain number rise to much higher levels.
This comes at the expense of your lobes becoming smaller in size, and more powerful in depth. An example of this would be a multi element Yagi antenna. These exhibit high gain, but are very directional. These are antenna's that can transmit the same amount of power a farther distance, as long as the antenna is pointed in the right direction.
Something can't come from nothing, and as of now in the antenna world, more gain comes at the expense of smaller lobes.
Erik