The Difference in Decibels.

[no-hertz]

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

[glebo]

Makes sense, good info.

It may also help if folks understand what gain is..."The apparent increase in signal strength, without the increase of transmit power".

When using antenna charts, plotting the major lobes of radiation, and understanding the gain involved on the plotted chart, making it more useful on antenna orientation to the distant station.

Or something like that

[no-hertz]

yea that sounds like a good explanation to get into, tied into how antenna construction and placement affects lobes, near field coupling and stuff.

thanks

[StRaTeGy_]

http://www.youtube.com/watch?v=mLMfUi2yVu8
http://www.youtube.com/watch?v=Faxiu...eature=related

[Paragrouper]

I'm not sure what the topic is where you will post, but you make some critical points about gain versus directivity that cannot be stressed enough when evaluating an antennas performance, especially when the antenna data is provided by a manufacturer (full disclosure; I work for one).
Antenna gain is primarily impacted by directivity across a stated frequency range. It is often expressed in a manner that can be misleading. When evaluating the stated performance it is important to understand the application that the antenna would be used for—more gain is not always better. A common example, a high-gain antenna used to point at a satellite from a fixed point on the ground would not be suitable for use on a vehicle that is traversing over uneven terrain, where you would want a more hemispherical radiation pattern to support communications on the move. A lot of the vehicular TACSAT antennas (crossed dipoles, a.k.a. X-blades) on the market provide you with ~5-6 dBic (or more) gain, measured at zenith, which is a respectable value. What this means in practical terms is that if you have a high look angle to the satellite, the antenna works okay. However, once your look angle falls below 35-40 degrees you will lose link. Those who have used these types of antennas in areas with low look angles to the satellite or in extremely uneven terrain will know what I’m talking about. If the gain is stated higher than 6dBic the problem will be worse. Other designs, such as helix antennas, will provide a more hemispherical pattern even though the stated gain is usually less than 3dBic.

[no-hertz]

Paragrouper -

I have worked with most of the SATCOM antenna's you mentioned, know exactly where you come from. The dBic label was one I am not familiar with, glad you brought it up so I can go track that one down and understand it in the scheme of things.

glebo-

I did a small write up on gain and lobes as per earlier suggestion in this thread. These articles are not really for any established site, more so a Blog / Forum type thing a echo buddy and I are trying to get off the ground as a knowledge center of sorts for the community and other interested parties. Not sure what direction it will go, I find myself wondering and then not posting a lot of things I work on due to simple concern on revealing TTP's. same thing here.

Hard to gauge where the line is on some things like that, and when I question it, I choose to err on the side of safety.

[Paragrouper]

Originally Posted by no-hertz

Paragrouper -

The dBic label was one I am not familiar with, glad you brought it up so I can go track that one down and understand it in the scheme of things.

Dbic is the the directive gain of a circularly polarized antenna, expressed as the ratio (as discussed in this thread), in decibels, of the antenna's directivity to that of an isotropic antenna with the same polarization characteristic. It is derived from decibels over isotropic. In the case of UHF TACSAT, the antennas are right-hand circularly polarized (RHCP).

I'll throw this out, and not to distract, but, consider also a small paragraph regarding atmospheric pressure, salt, water, dry air, wet conditions, soil composition, ground water tables, etc.

The antenna design scenario is a compromise that others have mentioned, but changes to include signal strength to signal clarity, to direction, to any other factor, the antennas seeks balance to freq., sometimes to the loss of understanding of the operator, - "Effin Magic".