Newsgroups: rec.radio.amateur.antenna From: gary@ke4zv.atl.ga.us (Gary Coffman) Subject: Re: Distance/Path Loss etc.. Message-ID: <1994Apr6.104647.899@ke4zv.atl.ga.us> Reply-To: gary@ke4zv.atl.ga.us (Gary Coffman) Organization: Destructive Testing Systems References: <CnqvBz.1E2@chinet.chinet.com> Date: Wed, 6 Apr 1994 10:46:47 GMT Lines: 69 In article <CnqvBz.1E2@chinet.chinet.com> drx@chinet.chinet.com (Scott Whittle) writes: >I'm gonna be setting up a Yagi for 70cm at work to hit my friends >repeater about 30 miles away using a max of about 1 watt or so. >My question is what is the typically path loss in free air (line of sight)?? Free space path loss is Lfs(db) = 37 + 20*log(d) + 20*log(F) where d is distance in miles, and F is frequency in MHz. For 30 miles and 444 MHz, the free space path loss is 119.49 db. >Also, What is the calculation for figuring how much gain in the antenna I'll >need and beamwidth etc..... Well you need to know the required signal strength at the receiver to achieve a 20 db quieting. That's typically in the 0.5 microvolt range. Applying P=E^2/R, and assuming a system input impedance of 50 ohms, we have a required signal level at the receiver of -143 dbW. With a one watt transmitter, we're starting with 0 dbW, and must subtract path loss from that giving a signal of -119.49 dbW at the receiver with *no antenna gain*. That's a fade margin of 23.51 db with isotropic antennas at each end. So if you *really* have a free space path between the sites, simple 1/4-wave omni antennas will suffice. To determine the signal level at the receiver, you apply the following formula, Er = sqrt((10*10^(Txp + (Txant-TFeed) - PL + (Rxant-RFeed)))/50) Where Er is in volts, Txp is transmit power in dbW, Txant is transmit antenna gain in db, TFeed is feedline loss in db, PL is path loss in db, Rxant is receive antenna gain in db, and RFeed is feedline loss in db. Now you probably *don't* really have line of sight between the sites. 30 miles is near the radio horizon for an antenna 1000 feet HAAT. So you'll have additional loss. You have to figure loss from objects in the path, such as trees, houses etc. And if you really have terrain blockage, say by a ridgeline, then you'll be communicating by forward scatter or tropo. An equation that will give path loss in the forward scatter case is as follows, Lsc = 126 + 30*log(F) + 20*log(d) + 10*A - 0.2*N Where F is frequency in MHz, d is distance in miles, A is the scatter angle in degrees, and N is the radio refractive index of air over the scattering path. N is expressed as N = 77.6*P/T + 3.73*10^5*e/T^2 Where P is pressure in millibars, T is temperature in Kelvins, and e is water-vapor pressure in millibars. Typical sea level value for N is 324. Working your problem with a scatter angle of 1 degree, we get a path loss of 180.16 db. So you'll need combined antenna gains at the transmitter and receiver of 37 db to make the trip on 1 watt. Increasing transmit power to 45 watts (16.5 db) would help considerably, allowing antennas with 10 db over isotropic on each end. Tropo ducting will allow path loss to fall somewhere between the line of sight case and the pure forward scatter case. Gary -- Gary Coffman KE4ZV | You make it, | gatech!wa4mei!ke4zv!gary Destructive Testing Systems | we break it. | uunet!rsiatl!ke4zv!gary 534 Shannon Way | Guaranteed! | emory!kd4nc!ke4zv!gary Lawrenceville, GA 30244 | |