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How Far Can I Talk on 2
Meters? Ai2c: Hello Paul, I was looking for a Web Site that would explain 'smooth earth' as you used it in your article. Maybe you could give me something to add about this? Ai2c: I stumbled across the UK site on radio horizon which I thought was interesting. Ai2c: Anyway, when we profiled microwave shots in the Army, we plotted them on 4/3rd earth paper. I was trying to figure out how that related to your numbers. Phbjr: 4/3 earth is the classic propagation concept, but you'd need to see the charts I mentioned in the article to fully understand the numbers. They were developed empirically over a period of years using actual transmissions to record the path loss through the troposphere at 50, 144, 220, and 432 MHz. There is no "formula" because the distance is taken directly from the chart based on the path loss that two identical stations can overcome using the equipment,SNR, fading loss, etc. as mentioned. Ai2c: OK - Understand - Very Cool! Phbjr: The only "formulas" used are those for calculating the amount of loss that can be overcome with a particular setup, i.e., a summation of the gains and losses. There are some nomographs with the charts that also are helpful in determining how much to allow. Ai2c: OK fine - I was just trying to understand why you assumed 'smooth earth' and what that means Phbjr: I'll get you a copy of the charts - in fact, I have a sort of rough tutorial I use whenever I have to refer back to the methodology to make sure I don't accidentally overlook something in the calculations. Phbjr: Well, my meaning was that there are no natural obstructions that would impede normal tropospheric propagation - like a mountain range, for example. The mountains west of me SIGNIFICANTLY reduce my signal in that direction!. Ai2c: OK - so it is still a curved surface just no big obstructions which would influence propagation at these frequencies Phbjr: Exactly. But again, the whole concept of "operating range" is based on empirically-collected data of how the troposphere behaves at those frequencies, not just the simple 4/3 earth assumption. Ai2c: Roger - Got it. Ai2c: If you don't mind, I'll post this IM Exchange as a subpage note to your article? Phbjr: Here's another notion for you to consider, one that many hams don't understand: If I build two identical two-way stations for 6 and 2 meters (power, antenna gain, noise figure, mode, bandwidth, etc.) will they have the same maximum working range? The answer is no, because the 2-meter antennas have a smaller "capture area" on receive. Phbjr: So, you either need bigger antennas for the higher band (more metal), or more transmit power at the sending end, or a lower NF at the receiving end. I'll show you the path loss formula sometime that shows this phenomenon. Phbjr: And yes, any exchange we have can be used, edited as you see fit, if you think it will help explain the concepts. Ai2c: OK - Cool! They also say 'bigger is better' and also say if it hasn't fallen down yet then it is not big enough. Ai2c: Well, I'll just copy this IM into a HTML page and link it back to your article. Phbjr: You got it! BTW, look at the photos on page 92 of the August issue of QST for a "modest VHF/UHF array"..... Ai2c: I'll look it up - Thanks NNNN |