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I don't know why foliage penetrating radar is temperature dependent.It doesn't work at temperatures below 20 degree C,is it because low frequency radio waves are scattered more due to increase in intensity.

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The principal obstacle to foliage penetration is absorption of microwaves by the water in vegetation. I had not heard that FoPen radars work poorly at low temperatures, but I am not very surprised. The dielectric “constant” of water is both temperature- and frequency-dependent, and complex as well: $\varepsilon \approx 5+75/(1-i\omega \tau )=5+75/(1-if/{{f}_{0}})$, where $\omega $ denotes angular frequency, and $\tau $ dielectric relaxation time, which is found to be roughly proportional to viscosity. The attenuation coefficient in a complex dielectric medium is $4\pi \operatorname{Im}(n)/\lambda $, where $n=\sqrt{\varepsilon }$. Considered as a function of frequency, $\operatorname{Im}(\varepsilon )$ is greatest when $f={{f}_{0}}=1/2\pi \tau $, roughly 10 GHz in water at 20°C but roughly 5 GHz at 0°C, whereas $\operatorname{Im}(n)$ is greatest at $f\approx 2.2{{f}_{0}}$. (Caveat: Don’t expect the dielectric properties of organic materials to be all that close to those of pure water.) FoPen radars use much lower frequencies in the VHF band, but you will find that $\operatorname{Im}(n)\propto f/{{f}_{0}}$ when $f\ll {{f}_{0}}$.

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