|
How Far Can I Talk on 2
Meters? Recently, there has appeared on this forum some discussion regarding the "working range" of the VHF/UHF bands. This prompted me to develop a set of tables for the 2-meter band which demonstrate how different types of station setups can be expected to perform. The ranges given here are *estimates* based on *smooth earth*, and in the interest of not misleading anyone I have tried to play the game conservatively. The actual distances were taken from a "path loss versus distance" graph which was first discussed by D.W. Bray, K2LMG, in 1961 and re-published by Ed Tilton, W1HDQ, in all three editions of "The Radio Amateur's V.H.F. Manual." If you don't have a copy of the old V.H.F. Manual and want to understand path loss at VHF/UHF a little better, I highly recommend looking for one at a hamfest. Even with conservative estimates of performance, however, caution should be the watchword. Some locations just "seem to work better" for VHF than others, so remember that *your* mileage may vary. Variances aside, the tables should help newcomers understand something about the characteristics of VHF path loss and develop an appreciation of the necessity for carefully evaluating each planned improvement *before* shelling out a lot of dough. There are four tables below: two for FM, and two for SSB. The tables are based on two *identical* stations, i.e., the distances given presuppose that equipment performance at both ends of the path is in all respects identical. The first table for each mode lists communications ranges for identically-equipped stations for 99% reliability, while the second lists ranges for 50%. To understand why the numbers look the way they do you'd have to actually see the path curves themselves, because path loss increases steeply out to 50 miles (at 50% reliability) or 100 miles (at 99% reliability), then flattens noticeably out to about 250 miles, then steepens again (but not as much as at the shorter distances). This means that below 100 miles (or 50 miles at 50% reliability) it takes quite a few dB of improvement to gain greater distance, but once over the "hump" in either curve small improvements can mean large increases in effective working range. At the extremes of the flat portions where the curves steepen again (about 210 dBw path loss, representing ranges of 285 and 315 miles, respectively, for 99% and 50% reliability) the two curves run nearly parallel with about 30 to 40 miles difference between the ranges for any given path loss, and a range increase of about 5 miles/dB out to beyond 500 miles. Here's an example of the significance of the flat portions after the "hump:" For 99% reliability, there is approximately a 21 dB path loss difference between 50 and 100 miles, but only a 10 dB difference between 100 miles and 250 miles. Assuming you were at the 100-mile "hump" in the path loss curve (which is actually a path loss of about 195 dBw), by increasing transmitter power, reducing receiver noise figure, replacing the antenna with one having higher gain, raising the antenna higher, or some combination you could make a significant improvement in your working range. A word about the "50%" and "99%" nature of the tables: "99%" means that any time you turn on your rig you should expect to have the working range shown, under the stated conditions. "50%" means that about *half the time* you may work out this far, but half of the time you *won't*, either; and it *doesn't* mean 50% of each hour, or day, or week, it means 50% of the time over a long period (months, certainly; probably over a year is more like it). Also, the tables do *not* consider any of the more esoteric long-distance modes such as sporadic E or F2 layer skip, aurora, meteor scatter, or extreme tropo ducting caused by inversions or unusual air-mass boundary conditions, any of which can give working ranges of many hundreds or even thousands of miles. The tables only apply to the routine tropospheric propagation we all know and love. ;-) The following assumptions were made in calculating the data contained in the tables: 1. Receiver noise figure was assumed to be 5 dB without a preamp, and 2 dB with a preamp (preamp located at the rig, not at the antenna; for example, an "integral" preamp common in commercial amplifier "bricks"). 2. Receiver bandwidth was assumed to be 2.5 kHz for SSB and 12 kHz for FM. 3. Transmission line loss was assumed to be 1.5 dB, and was added to the receiver noise figures listed above and subtracted from transmitter output power. 4. Antenna height gain for 30-foot antenna height is 0 dB, and for 60-foot height is 4 dB. The tables assume antennas are at the same height on both ends of the path. 5. Required SNR was assumed to be 3 dB. This may seem low for FM, but in fact a signal 3 dB above the "capture" level can be easily copied. What may happen, however, is that if the signal strength fluctuates near the capture point the signal may drop in and out continuously, making copy impossible. A SSB signal, on the other hand, will fade in and out more gracefully with at least partial copy even down close to the noise floor, making an exchange of grid squares, signal report, and callsign possible even under poor conditions. This is one reason why SSB is preferred over FM for weak signal voice work (another being the better sensitivity on SSB due to the narrower bandwidth and subsequently lower receiver noise floor). 6. Antenna gain was assumed to be the same at both ends of the path. 7. Ground reflection gain was assumed to be 3 dB (combined). 8. A factor of 7 dB was subtracted for fading loss in all cases. I should note that the antenna gains shown were chosen to represent typical antenna configurations used on these modes: a 5/8 ground plane, omni collinear, and small and medium yagis for FM; two-element quad and small, medium and medium-large yagis for SSB. Finally, as a "sanity check" on the numbers shown, I can vouch for the ranges shown for SSB stations with 25 watts output and a 12 dB yagi at 30 feet, and with 80 watts plus preamp and the same antenna. In fact, with 25 watts I've had QSOs out to 290 miles with better-equipped stations without any super-unusual ducting, just some good tropo path enhancement (but it's *definitely* in the "50% or less reliability" category). So, without further ado, here are the estimated working ranges of identically-equipped FM and SSB stations for 99% and 50% reliability at 144 MHz.
If you have questions about the tables above, please feel free to drop me an e-mail. If there are inaccuracies or inconsistencies in the information the fault is mine, and if you find any please e-mail me and I'll post corrections and/or additional info as necessary, with due credit to the finder(s). Finally, if you'd like a photocopy of the 2-meter path loss chart send me a *legal-size* SASE and I'll shoot one your way.
Mailing address:
Paul H. Bock, Jr.
K4MSG **************************************************************** Paul H. Bock, Jr. K4MSG FM19ee "Imagination is more important than knowledge." - A. Einstein **************************************************************** WebMaster Note: Also See Some Further Notes On This Article NNNN | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||