Higher frequencies are absorbed by organic substances such as clay, wood, rock, meat, flesh. This is also how micrpwave ovens work, and why different foods in a microwave cook at different rates, based on their absorption rates. For example, 40 MHz frequencies will work well in wooded areas, and hilly areas, but will work poorly in high steel environments (IE: Bridge, elevator, etc. UHF (400 + MHz) will work very well in high steel areas, but poorly in wooded climes. Google RF absorption rates for the actual formula, which inputs frequencies, distances and environments (as well as antenna types) and give general distance of usabilty and reception qualities.