Communication Systems

What is the gain of the Hertzian dipole over isotropic antenna?
Why there is need for modulation?
The process of superimposing audio or video messages on r.f. carrier waves is called modulation.
The frequency deviation in a FM transmission is $$18.75\:KHz$$. If it broadcasts in $$88-108\:MHz$$ band, then the percent modulation is
If a given antenna of height h, then  maximum distance upto which it can receive signal is $$\sqrt{ahR}$$. (R is the radius of earth)
What is the value of $$a$$?