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[SOLVED] how power will get effected if i shift from directional to omnidirectional antenna

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Full Member level 6
Sep 17, 2009
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I am having a directional antenna which is transmitting 10 watts and the angle is 30 degrees. (15 degree right and 15 degree left). The power which is received at 100 meter is 6 watts.

If i change my antenna from directional to omni directional antenna, then what power will i receive at 100 meter.
Is there any formula through which i can calculate this.

plz help me

Compare with a directional light beam. The area that is enlighten at 100 meter is 1000 times as big as at one meter. For picking up all power in that direction must antenna cover that area, or in your case 60% => more then 10 meter in diameter. Rather big antenna. Exactly how big RX antenna must be depend on some other factors also, such as frequency, antenna aperture, and assumes that most parts in the transmission must be almost without losses.
For a omnidirectional antenna with same efficiency as your directive antenna, it will radiate just as much power and same amount of power will be possible to receive at 100 meter distance. As it is a omnidirectional TX must receiving antenna cover all directions so RX antenna will looks like a 200 meter big ball.
Needed formulas for calculations are just simple algebra and trigonometry but there is one formula that is basic to at least knowing its existence: Friis transmission equation - Wikipedia, the free encyclopedia
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Thanks for ur reply.
The values which i wrote above are assumptions. Actually i dont have equipment and i want to solve through equation that what will be effect of omnidirectional antenna as compared to directional

can u kindly show me that exact equation

For what do you need a formula? If same amount of power is radiated in both cases, will also maximum receivable power have same values. Directive source or not have no impact. Cant be much simpler.
Not even distance is of interest as long as you not specify detailed parameters for both of your antennas, as 100 meter air can be assumed as almost whiteout losses.
If you know a bit more about your antennas performance is Friis formula a simple and good enough equation to get a bit more exact answer.

If same amount of power is radiated in both cases, will also maximum receivable power have same values.

So u want to say that if we use omnidirectional or directional antenna, we will receive almost same power at certain distance.
it is very strange .....

what i think that as we shift to omnidirection, the power will reduce by a factor of 12 at a specific point. (assuming directional was having 30 degree beam)

Same amount of power, yes. The difference is that in omnidirectional case will that amount of power be spread evenly on whole sphere. As long as we not know anything about what sphere that is covered by RX antenna, its efficiency and frequency, RX and TX gain... is also distance and omnidirectional or not without any value.
If you have a pool filled with water and you say that you can throw 60% of it in a certain direction and distance, how much can you then throw away omnidirectional? Same amount I guess.

Power in a distant specific point, does not exist. Fields strength as a vector, crossing a specific point maybe? Can be calculated without need for an RX antenna but then you need to know more about radiation source.
How about these missing 4 Watt, what kind of losses is it?
A beam-width of 30 degree, how is it calculated? As you can recover more then 50% of total radiated power must RX antenna cover bigger area then what TX beam-width corresponds to as beam-width normally is defined as 3 dB points. Given numbers results in transfer efficiency that seems less realistic and requires an unlikely big RX antenna compared to wavelength if we are in fare field distance at 100 meter. Too much confusion.
If it is an schoolbook exercise, let me know the whole question.
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sir it is not school book questions, these are all assumptions. not real value.

i just want to know effect of omnidirectional antenna as compared to directional

From ur above stated details, it seems omni and directional have same effect on receiver.

Then why people use directional antennas. I read in a book that directional antenna concentrate power to specific region due to which range increases

is it right.

Thanks for sparing ur time.... ( i understand that same amount of power will be transmitted by both antenna. i want to know power effect at certain distance. u can assume ideal conditions.) i hope i m making u clear now

what is the gain of your directional antenna in dBi or dBd ?

and what is the frequency ?


This is written in wikipedia

Gain is a parameter which measures the degree of directivity of the antenna's radiation pattern. A high-gain antenna will preferentially radiate in a particular direction. Specifically, the antenna gain, or power gain of an antenna is defined as the ratio of the intensity (power per unit surface) radiated by the antenna in the direction of its maximum output, at an arbitrary distance, divided by the intensity radiated at the same distance by a hypothetical isotropic antenna.
The gain of an antenna is a passive phenomenon - power is not added by the antenna, but simply redistributed to provide more radiated power in a certain direction than would be transmitted by an isotropic antenna. An antenna designer must take into account the application for the antenna when determining the gain. High-gain antennas have the advantage of longer range and better signal quality, but must be aimed carefully in a particular direction

So I got my answer, If we will use directional antenna then range will increase and if we use omni range will decrease.
If we keep all parameters contant and only change gain of antenna to change from directional to omni, is there any equation which can give me received signal strength at certain distance for both scenarios.

In my opinion, the previous discussion has been confused by your unreasonable receiver specification. It would be rather recommended to refer to a reference antenna, e.g. a dipole and calculate the received power by Friis equation, as E.Kafeman mentioned. Friis transmission equation - Wikipedia, the free encyclopedia

You can however easily estimate the gain of a directional antenna with a given beam angle. Simply apply rules of sphere geometry. You get about 22 dB gain for 15 degree half angle compared to a (theoretical) isotropic antenna respectively 20 dB to an omnidirectional dipole.

If we keep all parameters contant and only change gain of antenna to change from directional to omni, is there any equation which can give me received signal strength at certain distance for both scenarios.
The "change" is given by the antenna gain, the absolute signal strength by Friis equation.
Yes Wikipedia says "and better signal quality" as a directive antenna resulted in some kind of signal purification. A bit amusing. Author was probably thinking about an improved snr for a whole system. Also remaining part of text assumes that reader have a certain level of antenna system knowledge or else can much be misunderstood in that text.
A isotropic antenna is a radiating point in free space. Its radiation will not result in any power-level that can be calculated for at a certain distance without taking account how receiving antenna perform and what angel of total sphere they both cover. Much the same as a collector for energy from Sun. It must have a certain area to be able to collect any energy at all. A point is not enough as definition of a power collector. Both transmitter (Sun) and receiver can be directive which effect amount of transferable power.
A single antenna in free space is like screaming in space, without listener will no one hear you, no power will be transferred in any direction if it not exist a defined listener. Directive antenna or not doesn't matter. No power will be transferred in a single antenna system, calculating transfer of power requires a defined receiving system.
Your question "i want to know power effect" is without a defined receiver not possible to answer. I want answer in Watt from you, and you will get 3 Ampere as only clue from me? Some important parameters are missing.
Maybe was your intention of the question more like "in a sphere of 1000 square-meters, is it better to enlighten one or all square-meters from a limited power source, resulting in a certain field strength (not power level!!) and what will the difference in signal strength (Volt/meter) be?"
If you concentrate a light source with aid of a reflector will light be spread more in one direction then another, and can enlighten more distant object, but only if there is a object in that direction that can reflect enough light so it can be detected.
If it is nothing reflective in that direction, you will not even be able to know that it is in that direction your light source is radiating. Same as for antennas.
FvM did wrote: The "change" is given by the antenna gain, the absolute signal strength by Friis equation. ,was probably the most concise and correct answer.
Thanks to him too.

Soory i forgot to say thanks to FVM.
Thanks to all who helped me

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