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A monopole antenna vs a dipole antenna, what is the difference? Their actual connection to the circuit

sram

Member
Okay so monopole should have only one pole and the dipole should have two, right? The monopole antenna is a quarter-wave antenna as it uses ground as a reflector to construct the other quarter wave. The dipole is however a half-wave antenna because it doesn't connect to ground and has two radiating elements. I have a question, where do the leads of each antenna connect exactly? Assuming that one lead of the monopole antenna connects to the signal generator and the other connects to ground, where does the leads of the dipole antenna connect exactly?

Now if I have an HF radio that can work with different types of antennas, you should connect the coaxial cable connector of the antenna ( a whip monopole antenna for example) into the radio antenna connector, right? If you get another antenna that is a dipole, should its coaxial cable connector connect to the radio antenna connector also? That wouldn't make sense because one lead will connect to ground and that shouldn't be the case with dipole antennas as they are half-wave antenna, right? If right, so how do you connect them? Can you please clarify this for me?


Dipole antenna connection:

Thanks.
 

RadioRon

Well-Known Member
Yes, it is like they say in that website. For a monopole, the connection at the antenna is that the center conductor is connected to the base of the radiating element, and the outer shield of the cable is connected to the ground plane. For a dipole, the center conductor is connected to the base of one of the two radiating elements, while the outer shield of the cable is connected to the base of the other element.

It might be confusing if you think that the shield of your coaxial cable is a ground connection. Many beginners think of "ground" as a place where there is no voltage and no resistance, a place where all the current goes to disappear. At radio frequencies and especially on transmission lines like coax, there is just as much current flowing on the ground wire (the shield if we talk about coax) as there is in the center wire. The shield isn't really a "ground" at all, its just the second conductor in a two conductor cable. In the same sense, the outer shell of your radio transceiver is not an ideal ground either. It may be connected to the ground wire in the wall through its power cord, but that doesn't mean that the high frequency voltages and currents are zero at the radio chassis.

There is a kind of magic in coax cable. Its not really magic, but until you understand the physics of it all, it does seem that way. You see, the electrical energy is flowing inside the coax, but not on the outside (in the ideal case). Where the coax connects to your radio, the outside of the radio case has no high frequency voltages or currents on it at all, so you can touch it and nothing will happen. But inside the metal case the high frequency currents and voltages exist on the inner wall of the metal housing. When you connect a coax cable to this box, the energy inside the metal housing can flow to the antenna by conducting on the outside of the coax cable center conductor, and on the inside of the coax cable shield. In the ideal case, it stays inside the cable and does not leak out. This remarkable behavior is the result of the skin effect, which is something you can look up and study. The skin effect is what we call the behavior of high frequency energy to stay on the surface and not penetrate through solid metal. Anyways, the same thing happens with signals picked up on the antenna. They conduct into the inside surfaces of the coax cable and down to the radio chassis, staying always on the inside (in the ideal case).

The connection between coax cable and a monopole is almost ideal because the current flowing on the inside of the coax shield doesn't have to go through any tight spots to flow out onto the ground plane, because the shield touches the ground plane all the way around the base of the radiating element. The dipole, on the other hand is not an ideal connection to coax cable. When you strip the end of the coax to prepare for connection to the antenna, the shield must be reduced to a wire and then connected to one element. This is a compromise connection, and not ideal. We call this an unbalanced to balanced connection (another thing to study later on) and it is a problem for those trying to make a more ideal connection between cable and dipole. This is why the "balun" was invented, to correct this non-ideal connection.

When I started in ham radio, I strung a wire between some trees with a break in the middle with a piece of plastic holding the two pieces of wire. I fed the center in half wave dipole fashion by stripping the coax back and separating it into a center conductor wire and a shield wire (by twisting the shield braid together) and then soldering these two to the two sides of my dipole. It worked fine.
 

sram

Member
Yes, it is like they say in that website. For a monopole, the connection at the antenna is that the center conductor is connected to the base of the radiating element, and the outer shield of the cable is connected to the ground plane. For a dipole, the center conductor is connected to the base of one of the two radiating elements, while the outer shield of the cable is connected to the base of the other element.

It might be confusing if you think that the shield of your coaxial cable is a ground connection. Many beginners think of "ground" as a place where there is no voltage and no resistance, a place where all the current goes to disappear. At radio frequencies and especially on transmission lines like coax, there is just as much current flowing on the ground wire (the shield if we talk about coax) as there is in the center wire. The shield isn't really a "ground" at all, its just the second conductor in a two conductor cable. In the same sense, the outer shell of your radio transceiver is not an ideal ground either. It may be connected to the ground wire in the wall through its power cord, but that doesn't mean that the high frequency voltages and currents are zero at the radio chassis.

There is a kind of magic in coax cable. Its not really magic, but until you understand the physics of it all, it does seem that way. You see, the electrical energy is flowing inside the coax, but not on the outside (in the ideal case). Where the coax connects to your radio, the outside of the radio case has no high frequency voltages or currents on it at all, so you can touch it and nothing will happen. But inside the metal case the high frequency currents and voltages exist on the inner wall of the metal housing. When you connect a coax cable to this box, the energy inside the metal housing can flow to the antenna by conducting on the outside of the coax cable center conductor, and on the inside of the coax cable shield. In the ideal case, it stays inside the cable and does not leak out. This remarkable behavior is the result of the skin effect, which is something you can look up and study. The skin effect is what we call the behavior of high frequency energy to stay on the surface and not penetrate through solid metal. Anyways, the same thing happens with signals picked up on the antenna. They conduct into the inside surfaces of the coax cable and down to the radio chassis, staying always on the inside (in the ideal case).

The connection between coax cable and a monopole is almost ideal because the current flowing on the inside of the coax shield doesn't have to go through any tight spots to flow out onto the ground plane, because the shield touches the ground plane all the way around the base of the radiating element. The dipole, on the other hand is not an ideal connection to coax cable. When you strip the end of the coax to prepare for connection to the antenna, the shield must be reduced to a wire and then connected to one element. This is a compromise connection, and not ideal. We call this an unbalanced to balanced connection (another thing to study later on) and it is a problem for those trying to make a more ideal connection between cable and dipole. This is why the "balun" was invented, to correct this non-ideal connection.

When I started in ham radio, I strung a wire between some trees with a break in the middle with a piece of plastic holding the two pieces of wire. I fed the center in half wave dipole fashion by stripping the coax back and separating it into a center conductor wire and a shield wire (by twisting the shield braid together) and then soldering these two to the two sides of my dipole. It worked fine.
Hey buddy. I'm at your debt. I can't possibly get an explanation better than this. Many thanks.
 

JimB

Super Moderator
Most Helpful Member
When I started in ham radio, I strung a wire between some trees with a break in the middle with a piece of plastic holding the two pieces of wire. I fed the center in half wave dipole fashion by stripping the coax back and separating it into a center conductor wire and a shield wire (by twisting the shield braid together) and then soldering these two to the two sides of my dipole. It worked fine.
I agree 100%, I do it all the time.

But, to add a bit of detail...
A coax cable is an unbalanced transmission line, it is ideally suited for connecting to a monopole antenna, which is considered to be unbalanced.

A dipole on the other hand is a balanced antenna, and when connected directly to a coax cable (unbalanced), there can be what are known as common mode currents flowing on the outer of the coax.
To overcome this problem (if it is a problem) it is usual to use a BALUN between the coax and the dipole.
The word balun is a contraction of BALanced to UNbalanced transformer, and is often used as part of the construction of a dipole centre insulator.

JimB
 

sram

Member
On a 2nd thought and looking at the photo in the website I linked, wouldn't they both be considered dipole antennas, because in the monopole case, the shield conductor can be looked at as the 2nd radiating element. It is just shorter....
 
Last edited:

JimB

Super Moderator
Most Helpful Member
On a 2nd thought and looking at the photo in the website I linked, wouldn't they both be considered dipole antennas,
An insightful thought.

It is true that all antennas form a circuit in which current flows, even when the conductors are separated by air (an insulator) currents flow in the antenna.
Hence all antennas effectively need two connections, in the case of the monopole, one connection is to the antenna and the other connection is to the groundplane.
In the case of a dipole (ie two quarter wavelength wires end to end) there is a connection to each of the two wires.

If any one connection is broken, the effectiveness of the antenna system becomes very poor.

JimB
 

unclejed613

Well-Known Member
Most Helpful Member
I agree 100%, I do it all the time.

But, to add a bit of detail...
A coax cable is an unbalanced transmission line, it is ideally suited for connecting to a monopole antenna, which is considered to be unbalanced.

A dipole on the other hand is a balanced antenna, and when connected directly to a coax cable (unbalanced), there can be what are known as common mode currents flowing on the outer of the coax.
To overcome this problem (if it is a problem) it is usual to use a BALUN between the coax and the dipole.
The word balun is a contraction of BALanced to UNbalanced transformer, and is often used as part of the construction of a dipole centre insulator.

JimB
one of the simplest ways of isolating the shield is to wrap the coax several times through a ferrite toroid very close to the connection to the antenna. this acts as a 1:1 balun of sorts, and isolates the common mode currents.
 

ellymina

New Member
In essence, the difference between a monopole and dipole antenna is that a dipole antenna uses an additional radiator to generate a synthetic ground plane between the symmetric radiator elements, where a monopole antenna requires a physical ground plane.
 

unclejed613

Well-Known Member
Most Helpful Member
correct.... and the monopole can have it's efficiency reduced by poorly conducting soil, which is why commercial radio stations bury a huge counterpoise system underground.
 

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