[SOLVED] Antenna problem. need help

[gudduarnav]

A lossless antenna with directive gain of 8 dBi is fed with a 2 W power. The total radiated power of the antenna will be:
i. 16 W
ii. 8 W
iii. 4 W
iv. 2 W

 

[davenn]

HI there

OK before I or anyone else gives you the answer directly
have a go at which one you think is correct and why

Hint every 3dB you double the power

cheers
Dave

 

[vfone]

http://www.csgnetwork.com/antennaecalc.html

 

[gudduarnav]

http://www.csgnetwork.com/antennaecalc.html

Need some working formula.... can't get the relations used in the site....

 

[davenn]

is this a study question ? it looks like it

if so, surely you have had some teaching on the subject in class ?

look again at my hint and do some basic maths

Dave

 

[gudduarnav]

GOT 1 hint....
dbi = dbd + 2.15
dbd = dbi - 2.15 = 8 - 2.15 = 5.85
so power gain of antenna = 10^(0.585) = 3.85
total radiation pwr = 2 W * 3.85 =7.7 W ~ 8 W....
it was study qstn......

 

[E Kafeman]

Free energy? :roll:

 

[D.A.(Tony)Stewart]

The total radiated power in all directions is the same, but the peak power is 8dB higher in one direction.

the source power is 3dBW.

 

[E Kafeman]

Knowing about thermodynamical laws and conservation of energy would be enough to find correct answer.

The total radiated power in all directions

Total radiated power wouldn't be total if excluding some directions, adding "all directions" once more can cost unclear further assumptions.

peak power is 8dB higher in one direction

Given relative directive gain in one direction is 8 dBi. It is not a power gain expressed in dB. Omitting (i) is no good idea. It is not either possible to assume a power ratio ("db higher in one direction") relative any other directions as we not have information about these other directions, or better expressed as a radiation sphere surface area, where size of current 8dBi area compared to any else part of the sphere area is unknown (gain&area).

As the antenna is assumed lossless do it not consume any power. If feed by 1 Watt will it also radiate same amount of power.
Amount of radiated power will not increase if concentrated in a narrow beam.
If power increases due to a narrower antenna radiation pattern would result in a system delivering more power then it is feed with -> Nobel-prize for the one that succeeds.

Compare with headlight of a car. A reflector add directive gain and can increase light intensity at a certain distance and direction, at cost of radiation intensity in other directions. Amount of radiated power will however remain constant with or without reflector.

Maybe did original study question include other questions such as difference between TRP and EIRP, which would make given answer alternatives a bit more educational.