What would be some cons to using 1 ground radial instead the standard 4. If 1 ground radials works perfect, why would I need to use 4?
This will be on a VHF antenna
Here is a thought.
Imagine the radials and imagine the electric field that forms from the antenna to those radials. Well those fields look exactly the same as if instead of the radials you had the other (non-existing) side of the antenna flipped down (so a dipole really). Because the fields look the same whether you use the radials or you use both legs of the antenna (dipole), it means that essentially the radials form a mirror.
If you imagine the radials as an optical mirror and you were to look at your antenna, you would see it's reflection and it would look just like a dipole.
It's just like if you put a shoe box against a mirror, you'll basically see 2 shoe boxes.
Well here is the trick, if you are looking at it from a flat angle, for this to work, the mirror needs to be really large (kind of like looking over a lake and seeing all the trees reflect on the other side). But if the mirror is too small, you won't be able to see the whole thing reflect, only a portion of it.
Then again, if you had this smaller mirror, but instead of looking at it from a flat angle, you looked at it from an elevated angle, you may see both sides again. This is why by itself your monopole may be radiating (and receiving) more up into the sky but not so much at ground level. Well the radials form this mirror and it makes your radiation pattern more flat to the surface of the earth. In fact the way I understand it is that the radio waves do literally reflect off the radials.
For this reason, I think it's good to have many radials and to make them long.
Disclaimer: I could be wrong, but this is how I understand it at this time
I think it's really about the fields that form around the antenna. The ground plane gives those fields a way to look "good", without it, the fields may not have anywhere to form a good pattern. In any case, I do believe what you say, and it is totally possible that I'm wrong, but in this case you'll have to explain it to me.There might be an imaginary mirror image of the vertical element in a ground plane but the radiation from this mirror image is also imaginary.
Btw. according to wikipedia the monopole with mirror image has twice the gain than if it was replaced with an actual dipole:
Because it radiates only into the space above the ground plane, or half the space of a dipole antenna, a monopole antenna will have a gain of twice (3 dB greater than) the gain of a similar dipole antenna, and a radiation resistance half that of a dipole. Since a half-wave dipole has a gain of 2.19 dBi and a radiation resistance of 73 ohms, a quarter-wave monopole, the most common type, will have a gain of 2.19 + 3 = 5.19 dBi and a radiation resistance of about 36.8 ohms if it is mounted above a good ground plane.
Well then let me ask you this. The transmission line has current going through it in the inner conductor and the shield, although they are in opposite polarity. In an ideal world there is no reflection and all power is radiated. Looking at the dipole, yes both elements are radiating and it's pretty clear where the currents go from the transmission line. However when we look at a 1/4 wave with radials, now you're telling me the radials do not radiate. Since the shield of the transmission line goes to the radials, where does the energy go then? Do you see how this would create a total imbalance between the energy from the center conductor versus what is on the shield?
Let's take one snapshot in time, the tip of the antenna is now positively charged, for example let's say it's +10V. The field lines have to go somewhere right? If there are no radials, where will they go?
Let's say there is some neutrally charged object that they go to. So the field is from +10V to 0V.
Let's add radials and say they are now charged at -10V. Essentially the field strength will be double. While the radials themselves may not radiate in a traditional sense, it is my believe that they still contribute to the radiation energy from the vertical element.
I also think the mirror image theory proofs this. The fields above the ground plane / radials will look identical than if instead of the ground plane one were to use a dipole, or am I wrong?
Let's also bear in mind that we're talking theory. Only because it is not feasible to have 5000 radials that are 100 feet long each, doesn't disprove the concept. We can also be talking about 900MHz were the wavelength is pretty small.
I am not an expert in this field, didn't go to school for it, and it is quite possible that I'm wrong.
That's a nice point you make about the currents canceling out. When we talk about current though, the tip of the antenna doesn't have current (it has voltage). So perhaps adding a capacitive hat, moves the tip out further, so that even though the currents in the hat cancel out, it will let the current reach up further in the antenna. In that sense, the capacitive hat wouldn't radiate, but it would indirectly allow a little more of the antenna to radiate right?
I think the same is true with radials. The current may cancel in the radials but somehow someway it must helprn current in the vertical element. Also the current, even though it cancels, it should produce voltage spikes at the tips of the radials that become part of the electric field to the top of the vertical element.
I believe that a 1/4 monopole without radials or other ground plane does not perform well, at least for transmitting. I believe that radials are very important for this, and that even if they don't radiate on their own, they do contribute to the radiation of the antenna as a whole. I think especially for the 1/4 wave monopole, that each radial lowers the beam in that direction, and that without it, the antenna will radiate (and receive) much more up into the sky. If one only has 4 radials, I believe that those 4 directions will be lowered while everything else may still point more upwards. Adding more radials will make it more uniform but I also think there is a diminishing return. I bet as few as 12 radials will already be very close to as if the ground plane was made solid and of the same diameter. Doubling the radials to 24 in my opinion will have very little effect.