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Antenna Radials

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#1
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
 
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#2
If you have a vertical antenna and only one ground radial you will make an inverted V dipole on its side and both the vertical element and single ground radial will radiate equally. This will give you a radiation pattern that is slanted about 45 degrees from vertical and will be somewhat directional broadside to both elements.

When you have two or more equal length ground radials, the currents in the radials will be going in opposite directions and will cancel radiation, leaving only the vertical element to radiate. The amount of radials will determine how well the vertical omni directional pattern will be with more radials giving better efficiency and a more even pattern.
prcguy


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
 
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#3
Another consideration is impedance. Not having a sufficient quantity of ground radials will cause the antenna impedance to be higher than designed, which will raise SWR.

And driving a balanced antenna (a dipole) with an unbalanced feedline (any type of coax) without a balun/matching transformer will cause the shield of the coax to radiate, creating further SWR and antenna performance problems.
 
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#5
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 :)
 
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#6
Old textbooks describe a vertical element over a ground plane as creating a mirror image of the vertical and emulating a dipole, but its not true. The full length of a dipole radiates where only the vertical element of a ground plane radiates, so a ground plane has slightly less gain than a dipole.

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.
prcguy


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 :)
 
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#7
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.
prcguy
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.

Also check out this paragraph:
If the antenna is connected to a good earth ground, it is called a Marconi antenna. The ground structure serves as the other λ/4 half of the antenna. If the ground plane is adequately sized and conductive, the performance of the ground plane is equivalent to a vertically mounted dipole.
(source: Antennas, dipole, ground plane | Wireless content from Electronic Design)

Maybe they are wrong too, who knows.
But anyways, so what's the whole point of the radials then, what do they accomplish?
 
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#8
An antenna has to have 2 oppositely-charged conductors in order to create the electromagnetic field. With a 1/4-wave antenna, the vertical and the ground plane are the oppositely-charged conductors. If the ground plane is missing, the coax shield gets used instead, and you get radiation from the coax, high SWR, and crappy performance.

With a half-wave dipole, the upper and lower verticals are the oppositely-charged conductors.
 
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#9
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.
 
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#10
If a 1/4 wave monopole is mounted over a perfect conducting infinite ground plane (think 6" 1/4 wave UHF spike over acres of copper sheet metal), then the radiation pattern will be squished enough where the gain at the horizon might equal or surpass that of a 1/2 wave dipole.

For you or I making a 1/4 wave ground plane with three or four radials you will never have more gain than a 1/2 wave dipole. The radials on a ground plane do not radiate, so for a common 1/4 wave ground plane with a few radials, half the radiating antenna is missing compared to a dipole.
prcguy

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.
 
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#11
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.
 
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#12
If you have a ground plane with two or more equal length radials running opposite directions, the instantaneous currents in each radial will be going in opposite directions and will cancel radiation. This works the same for 3 or 4 or 20 radials as long as they are roughly equal length. The currents in each individual radial will go from the feed point to the tip of the radial and back toward the feed point and repeat continuously.

When you mount a VHF or UHF whip in the center of a car roof there are lots of RF currents running all around the surface of the metal roof but it doesn't radiate. Mount an HF whip at the extreme edge of a vehicle and you get something different where the vehicle body might radiate and act like the other half of a dipole depending on frequency and the vehicle size.
prcguy

A similar thing happens when you add a multi spoke capacity hat at the tip of a whip, the current in the whip will radiate but since the capacity hat has equal conductors running in opposite directions there is no radiation from the capacity hat even though it effectively lengthens the whip and modifies how currents run along the whip.
prcguy



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.
 
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#13
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 help 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.
 
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#14
Yes on a capacity hat pulling the current up higher on a shortened antenna and improving radiation, since the higher current part(s) of an antenna radiate more. Its much better to shorten a 1/4 wave antenna and make up with a capacity hat on top over a loading coil near the base where most of the current is.

For 1/4 wave long radials they would be high impedance at the tips where there would be higher voltage and little current.

Yes on a 1/4 wave monopole without radials not performing well and if the feed line is not decoupled, RF currents will flow on the feed line and it will radiate. In this case the length of the feed line will affect the overall performance and lack of ground plane will pull the pattern up off the horizon compared to the same monopole over a a good ground plane. The spacing between the ends of ground radials determines when you have enough to simulate a solid sheet metal ground and I believe 1/20 of a wavelength spacing is the accepted distance for that. That also means the longer you make the radials, the more you will need to achieve 1/20 of a wavelength spacing at the tips.

You can download a free version of Eznec and simulate most of this discussion and I've experienced all of this putting up various verticals over the years. On one 43ft vertical I first laid down four 33ft radials, took measurements and made contacts, then doubled and doubled, etc until I stopped at 30 something radials using up 1,000ft of wire.

I noticed the radiation resistance of the 43ft vertical on 80 and 160m went down as I added radials (as it should) and contacts got better, very noticeable going from 4 to 8 then to 16 but after that it was hard to notice much improvement. The stock 43ft vertical had a stupid 4:1 balun at the feed point and the auto tuner in my radio would easily tune the antenna on 160m with only 4 radials due to high ground loss raising the feed point impedance to something manageable, but after about 12 radials the internal could not handle the match.
prcguy


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.
 
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