in the image 354 wavelength correspond to 840 MHz.does wavelength decides height of antenna.if that so for low frequency signal.they have to build ery taller tower.is it possible to build such high antenna

 

in the image 354 wavelength correspond to 840 MHz.does wavelength decides height of antenna.if that so for low frequency signal.they have to build ery taller tower.is it possible to build such high antenna

You are wrong in your calculation. A wave length of 345 Meters corresponds to 870 kHz. The formula is:

300 / Wavelength(meters) = Frequency (MHz.)

300 / 345 ≈ 0.870 MHz. = 870 kHz.

An antenna can be any height above the ground and still radiate. Depending on the type of antenna, the optimal height might be a half wavelength or 5/8ths of a wavelength or a full wavelength. Antennas for AM radio stations are often in the neighborhood of 1000' feet or more AGL.

 

A vertical antenna would not be one wavelength high, it would probably be 1/4 wavelength. very few are 1/2 wavelength on the medium frequencies, but 1/4 wavelength is more popular.