karn@allegra.UUCP (12/09/83)
Everyone who has heard W5LFL has remarked about his strong signal strength. Those who are familiar with satellite link calculations are not suprised; they are familiar with what can be done when you don't have the earth standing in your way! Here's a rough calculation to give you an idea of what to expect. W5LFL is running 4 watts out to an antenna with a gain (lets be charitable) of 0 db. Lets also assume you have a 0 db gain antenna on the ground (most verticals have 5-6 db over this, while HT rubber duckies may have a gain of -1 or -2 db, this info coming from memory and not from any authoritative reference). Of course, these numbers are affected by lots of things, so they are only approximations. At the horizon, the straight-line range to the shuttle, flying at an altitude of 250 km, is about 1800 km. The path loss is approximately equal to 22 + 20 * log10(distance in wavelengths) For 1800 km on 2 meters, this is about 141 db. Directly overhead (250 km), this is reduced to 124 db. For a transmit power of 4 watts, which is equivalent to +6 dbW (6 decibels over 1 watt), received signal strengths in omni antennas would be -135 and -118 dbW, respectively. This translates into levels of 1.3 and 8.9 microvolts into 50 ohms: capable of full-quieting signals into almost any modern 2m transceiver. Remember also that the link is reciprocal: if you can hear him when he runs 4 watts, he could also theoretically hear YOU running only 4 watts. Of course with the pileups he hears nothing but QRM unless one station is at least several db stronger than all the others, which naturally leads to an all-out power war. Phil Karn, KA9Q