Learn about underground radials. You might be suprised to know this.
Simple SWR Theory.. Not difficult to understand.
Some Vert. antenna noise theory.. From W8JI's serious lab.
Skip the theory and go to the conclusions. You can just go to the answers.
Introduction to Vertical Antennas with buried Radials.In a perfect world, a vertical antenna would be exactly one quarter wavelength long and have a ground system that has NO resistance.
Natural laws have determined that this antenna would have 36 Ohms of "radiation" resistance, which means that the vertical conductor will act as a 36 Ohm resistor even though a DC resistance measurement reads practically zero Ohms.
The purpose of this discussion is to explain both "quarter wavelength" and "ground".
First, lets do "quarter wavelength".
If you are one of those who can not remember a darn thing about your high school algebra, fear not. I will do all the math and I will be happy if you just nod your head and say things like " That is what I thought.", or "I knew that", or "Sure, that makes sense." All you really need to do is to remember the conclusion of the discussion, and I will make that clear by using big or dark words.
The formula for finding the length of a quarter wave vertical wire (which is the vertical element of a vertical antenna) is...
Length = 234 / Frequency ( in MegaHertz)
An example for 40 Meters :
The center of the band is 7.150 MHz so that is the frequency to use in the formula.
Length = 234 / 7.150 = 32.7272 Feet.
OK, you say, but what do I do with the .7272 feet? How do I convert that to inches?
You can convert parts of a foot to inches by multiplying that part of a foot by 12, and then add the 32 feet later.
0.7272 times 12 = 8.72 inches. Adding the 32 feet back on gives an answer of 32 Feet and 8.72 Inches. ( For those who are wondering about the .72 inches, that is almost exactly 3/4ths of an inch) (That is about 0.2% of the length and can be ignored in real life.)
Another example: The 2 meter band.The center of the band is 146 MHz so that is the frequency to use in the formula.
Length = 234 / 146 = 1.602 Feet.
OK, you say, but what do I do with the .602 feet? How do I convert that to inches?
You can convert parts of a foot to inches by multiplying that part of a foot by 12, and then add the 1 foot later.
0.602 times 12 = 7.23 inches. Adding the 1 foot back on gives an answer of 1 Foot and 7.23 Inches. If you would like to convert that 1 foot into inches, you get a total of ( 12 inches + 7.23 inches) = 19.23 inches. ( For those who are wondering about the .23 inches, that is almost exactly 1/4th of an inch) (That is about 1.2% of the length and should not be ignored in real life.)
That is the end of the "quarter wavelength" part of this discussion. So far you should be able to use the formula to find the length of a wire that is one quarter wavelength long. This wire is the vertical element of your vertical antenna.
Next, lets talk about "ground".
There are two distinct different meanings of the word "ground". First, there is the connection at the base of the vertical element so electrons can flow into and out of the antenna. Second, there is the requirement for a large area under the vertical radiator that will hold the electro-magnetic field down near the earth. This is the area from the antenna to many wavelengths out from the antenna that is probably in your neighbors field or clear down the block. The conductivity of what is out there is what keeps the signal down along the surface of the earth.
First, lets start with the flow of electrons in to and out of the vertical radiator. It is true that an antenna will radiate a RF field with a very poor ground. That RF field will be far weaker than it needs to be. The thing that creates the RF field is the flow of electrons up and down the vertical radiator. The more electrons that flow, the stronger the field. In order to get the greatest number of electrons to flow up and down the vertical radiator, you need to have a very low resistance storage area full of electrons. When a rig provides a voltage at the antenna, the lower the resistance, the more electrons will flow.
The larger the storage area, the more electrons that can be forced up into and back down out of the vertical radiator.
Caution ..... Major controversy ahead.
Now I need to stop right here and warn you about the following information. The following information is one of the most controversial subjects ever discussed by amateur radio operators around the world. The only other subject that is full of incredible controversy is Standing Wave Ratio, or SWR, which I have written about later in this website, but not yet.
The controvery happens because amateurs have built vertical antennas with buried radials, and they have worked pretty well. The usual thought is that " since it works, I did it right." That is not the truth. It is possible that you could have done a much better job if you understood the theory of how a buried radial antenna works. I have a first hand example. Some years ago, I installed a buried radial vertical antenna for my friend Duane (SK). I used heavy copper braid for the radials and used two radials tuned for each band of operation. The antenna worked fine for years, even though I was unaware at that time that buried radials lose all their tuning when buried. I should have put down lots more and longer radials for a more efficient antenna.
So, why not use a large copper ground rod for the ground connection?
Ground rods work better than no ground at all, but even though they have a low resistance, they do not have much storage area for electrons. The ground rod is surrounded by dirt, which is a very poor conductor.Try this experiment at home. Measure the resistance of the dirt or sod under the grass at your house. I did it at my house, and found that by keeping the ohmmeter probes at one inch apart, and stabbing them into the ground at various places around the house, I got from 1 MegOhm to 100 KOhms of resistance. That was with the probes at one inch apart in the Pacific Northwest where it rains. Dirt seems to be a pretty good insulator, and not a conductor at all.Ground rods do not help control the shape of the electro-magnetic field that comes from your vertical antenna. You need a large conductive area under the antenna for that. Radial wires that leave the base of the vertical antenna are a wonderful way to provide a ground system. These radial wires will provide the needed electrons to move up and down the antenna, but they do not help hold the signal down along the surface of the earth. Radials are not long enough to do that.
The questions are ....
How many radials? ,
How long should they be?, and
Should they be underground or elevated?
These questions are very tricky and many amateurs have a poor understanding on what is needed. The internet is full of different information from different sources. Lots of emotion and arguments have resulted from the lack for good information.
Go to the Underground Radial section. Learn about underground radials. You might be suprised to know this.