A free demo NEC antenna simulation program can be downoaded from the Internet. The demo program has enough capability to deal with most Part 15 AM vertical antennas. A program with full NEC-2 capability can be purchased for about $100.
Some people are confused by the elevation radiation pattern produced when an antenna over "real" ground is simulated. The pattern shows radiation directed upward, but no radiation at all in the horizontal direction. When the antenna is simulated to be over a "perfect" ground, the peak radiation is in the horizontal direction, as expected. I never saw an explanation for this paradox anywhere.
Here is the explanation: Over a "real" ground, antenna gain decreases in the horizontal direction because the ground wave is absorbed more and more compared to the skywave as the distance from the antenna increases. What seems to be little known is that the shape of the NEC radiation pattern is computed at an infinite distance from the antenna. At an infinite distance, the groundwave is completely absorbed compared to the skywave, and there is no radiation at all in the horizontal direction.
Ermi you are absolutely right about confusion. In this modern time every state now has its own confusion center, so maybe we should let them know about whatever it is.
I comprehend how the ground absorbs horizontal antenna gain, but I don't know where the infinite distance is located.
It is implied that plenty of radiation goes up, so what if everything was tipped on its side so you'd have a radiation spill in the right direction.
What if we removed the real ground?
It just seems like there should be a better way.
The infinite distance is located at the edge of the infinite, homogeneous, flat earth that NEC assumes. The infinite flat earth was assumed by the developers of NEC to simplify the computation. Computational techniques exist for ground waves beyond the horizon, but they are too complex to be included in the program.
Ermi, thanks for the explanation, but I've got a few more questions as a result.
The main one is this: has anyone ever made measurements to confirm the NEC simulation? Do they in fact verify that most of the radiation is at high angles and that there is almost none on the horizon? I have felt for a long time that this might be one of the reasons that Part 15 stations do not get out as well as would be expected, since in most cases that one can think of, your listeners would be near the ground.
On the other hand, there must be some radiation at ground level, or you wouldn't hear anything. I'm wondering if it isn't so much that most of the radiation goes out at a high angle, as the fact that a big "bite" of the signal is missing at zero degrees. I'd appreciate your thoughts on that.
No matter how much radiation occurs at high angles, it's nowhere near enough for a Part 15 station to produce a sky wave since the D layer is 60 miles high. You're lucky to have a usable signal at 1 mile.
Yes, there have been tests of the propagation of medium frequency radio waves using field strength measurements of actual AM broadcast stations to confirm the theoretical predictions, dating back to the 1930s, or maybe earlier. The calculations have been found to be at least approximately correct, but there is a lot of scatter in the results.
The problem is that the material of the earth is not homogeneous, and the surface is not smooth, except for still water. The conductivity and dielectric constant of the earth nornally varies with depth, and is different in different surface areas. So the measured results are different in different regions, even if the nominal electrical constants of the soil are numerically the same. Something that is usually ignored is that the electrical constants actually vary with frequency. It is common practice to consider the electrical constants to be truly constant instead of variable, which they actually are.
Ermi, I am guessing that the variable nature of earth composition is not only variable from place to place, but also variable in a particular location, where moisture conditions naturally vary.
Are there other variables besides moisture that add to the shifting landscape?
This is as much as I read of a personal e-mail I received in response to one of my posts in this thread. Give me a break! Sometimes my posts are a little tongue-in-cheek. Not everything needs to be nit-picked.
I haven't tried the NEC calculator, but I can say one thing for absolute sure: My antenna being on a boat in a marina, grounded to the sea, gets out great compared to any I've experimented with at my studio with poor ground.
Some additional notes are that literally everything in the intended listening area is above the antenna, and except for the streets, is virtually all buildings ... then also, sometimes the signal travels out into the valley quite a ways, and has been heard up to 3 miles away. With hills and buildings in the way at 1/2 mile or better, the signal gets noisy, but as soon as the valley opens up, it gets better then fades out sort of slowly.
A few blocks from my studio on a street that angles out away from town, there is some interference over an actual rise, but as I go back down on the other side, which is of course further away, the signal gets better. On another road that turns off, fairly close to the water, but yet with a hill in the way, the signal is quite good. I can't come up with a terrain explanation, so maybe there is some sort of strange counterpoise thing, perhaps with the proximity to open water.
Overall, I'm saying the signal does very well close to the ground, i.e., in the horizontal, even with buildings in the way (driving up an alley seems to make little difference), with the exception that getting close to one with a lot of florescent or neon lights does me in. Vertical height did make some difference ... I added 4 ft. I tried with another 8 ft. mast extension (which required re-tuning), but it didn't do nearly as well as the in-between one, which is lower.
Of course, as night falls, everything gets worse until sometime around midnight, when for some reason it begins to get a little better.
Antennas and propagation is indeed physics, but there are apparently so many variables that it's nearly impossible to factor everything in, and so their real-world operation becomes extremely critical at these puny power levels. I find my signal varies in some small way each day, but I'd really just be guessing as to what's causing it.
It still comes down to a lot of experimentation ... which is what we do, yes? There's some sort of inner glow of excitement (endorphins?) that come when some crazy experiment pays off. I can't explain that either, but It seems to have me hooked 😉
Hooked? Yes, it's that adiction I've referred to in more than one post here. When that signal pops up at some new place you "feel the rush".
With such strong testimony about the benefit of water to the propagation of RF, it brings curiosity over the potential of adding water features to the landscape for those on dry ground. The gardening shows on TV often give instruction for adding ponds to outdoor settings.
Maybe ground plane arrays could include a few water hoses filled with water under pressure and containing a ground wire inside the hose (?)
It did seem that our signal at 1680 went noticeably farther with three inches of snow and ice on the ground.
Perhaps one could spread salt over one's lawn to raise the conductivity. Cheap and no more weeds to tend or grass to mow.
Neil
And how! The FCC has mapped just how "not homogeneous" the earth is here in the good old USA.
I could have sworn this map was already noted on this site but I can't find it now - must have been lost in the shuffle.
Naturally, a good RF ground system will depend, in part, on the natural phenomenon called "ground conductivity." This refers to the electrical properties of minerals in the local soil - the "not homogeneous" part.
Yep, I'm jealous of anyone in the 15 - 30 range, I'm sittin' on a 4 🙁 One of our contributors lived on a 15 for a while - pretty remarkable range there.
Note the conductivity of salt water (5000!) relative to that of earth - no wonder Ken is experiencing such great results up there in the harbor!
Read more at the FCC web site page titled "Map of Effective Ground Conductivity in the USA"
Well maybe there is a chance for a part 15er living in a "poor conductivity" area. The typical yard isn't so large a space, therefore perhaps there are safe additives which could significantly improve the electrical properties without killing wanted vegetation.
As to salt, knowing little about chemistry, can anyone explain why it does so much to improve grounding?
Carl mentioned variable moisture in a particular area causing changes in ground conductivity. This is certainly true for the first few inches of surface soil, but there is less effect to ground conductivity due to precipitation than might be supposed. This is because the RF penetrates quite deep into the soil, and the water from brief rainfall may not penetrate very deep.
Please don't "salt the earth" in your back yard to get better ground conductivity, so that nothing will grow near the antenna. Use metal, like ground radials, chicken fencing, and the like. Salt provides electrolytic conductivity, and electrolytic conductivity is thousands of times less than metallic conductivity. A metallic ground screen near the antenna will improve antenna efficiency.
The conductivity of the earth a long distance from the antenna affects the propagation of your signal, but there is nothing you can do about the conductivity of the soil beyond your yard.
Other prisoner asks, "Why are you in here?"
We reply, "I was caught laying down chicken wire in my neighbors yard at 2 in the morning."