Amateur Radio Wiki - User contributions [en]

Vertical Antenna efficiency

2011-08-29T23:00:26Z

Ke4uyp:

'''Efficiency'''

by Lou Rummel KE4UYP

Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mounted radial wires and a 1/4 wavelength tall vertical radiator you can achieve the highest possible efficiency for a vertical on 80m.

Unfortunately for the average amateur radio operator this is not practical considering the size of the average back yard. On 80 m you would need a clear area in your backyard of 135 feet square and the vertical radiator would be approximately 66 feet tall. For decades antenna manufacturers have attempted to miniaturized the classic monopole vertical, but here is the first problem, if you reduce the amount of radials from 120 down to say eight or less the ground loss goes up dramatically.

Ground loss is the No. 1 contributor to low efficiency of low band antennas. The closer any antenna is to the ground the higher the loss regardless of whether it is vertically or horizontally polarized. The second biggest contributor to efficiency deficit is when the vertical radiator is less than 1/4 wavelength long, and of course most if not all
commercially manufactured 80m verticals fit into this category.

=== Radiation Resistance ===

On the opposite end of the spectrum from ground loss is radiation resistance, unlike ground loss the more radiation resistance you have the better off you are. A textbook perfect vertical would have no ground lost or resistive loss just radiation resistance. The length of the antenna determines the amount of radiation resistance it will have. The classic monopole vertical only develops radiation from the vertical element. The ground mounted radial wires contribute no radiation in fact, they do not even contribute to the radiation resistance. For any part of the antenna to contribute to radiation resistance it must produce RF radiation.

{{antennas}}

Vertical Antenna efficiency

2008-02-08T13:12:01Z

Ke4uyp:

'''Efficiency'''

by Lou Rummel KE4UYP

Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mounted radial wires and a 1/4 wavelength tall vertical radiator you can achieve the highest possible efficiency for a vertical on 80m.

Unfortunately for the average amateur radio operator this is not practical considering the size of the average back yard. On 80 m you would need a clear area in your backyard of 135 feet square and the vertical radiator would be approximately 66 feet tall. For decades antenna manufacturers have attempted to miniaturized the classic monopole vertical, but here is the first problem, if you reduce the amount of radials from 120 down to say eight or less the ground loss goes up dramatically.

Ground loss is the No. 1 contributor to low efficiency of low band antennas. The closer any antenna is to the ground the higher the loss regardless of whether it is vertically or horizontally polarized. The second biggest contributor to efficiency deficit is when the vertical radiator is less than 1/4 wavelength long, and of course most if not all
commercially manufactured 80m verticals fit into this category.

'''Radiation Resistance'''

On the opposite end of the spectrum from ground loss is radiation resistance, unlike ground loss the more you have the better off you are. A textbook perfect vertical would have no ground lost or resistive loss just radiation resistance. The length of the antenna determines the amount of radiation resistance it will have. The classic monopole vertical only develops radiation from the vertical element. The ground mounted radial wires contribute no radiation in fact, they do not even contribute to the radiation resistance. For any part of the antenna to contribute to radiation resistance it must produce RF radiation.

Capacity hat

2008-02-08T13:07:14Z

Ke4uyp:

'''Capacity Hats'''

by Lou Rummel KE4UYP

Capacity hats will reduce the amount of inductance necessary to resonate the antenna, and increase bandwidth. But contrary to popular belief they add nothing to the radiation resistance. For a component of an antenna to increase radiation resistance it must itself radiate.

It is true if you put a Capacity hat on top of a short mobile antenna it will change the antennas efficiency but what happens is the current on the radiating element moves further up to the top.

Because the current is now further away from ground this lowers ground lost. So the ratio of radiation resistance to ground lost, and Omni Loss resistance, which is the reduction in the amount of inductance needed for the loading coil, goes down.

Although it is true that the input impedance of antennas using Capacity Hats is higher than coil loaded antennas this is not an indication of an increase in radiation resistance, for the same reason that a folded dipole with 300 ohms impedance has the same radiation resistance as a standard 73 ohm dipole .

Capacity hat

2008-02-08T13:03:59Z

Ke4uyp: Removing all content from page

Capacity hat

2008-02-08T12:57:35Z

Ke4uyp:

Wire Antenna

2008-02-08T12:52:31Z

Ke4uyp:

Many amateur radio antenna systems use a simple wire to carry the RF current in such a way as to radiate. One of the simplest is the dipole. When a dipole oscillates current, in sync with the radio's RF output during a transmission, the magnetic field generated around the wire expands and contracts very quickly, in most cases millions of times per second. It is the outer most part on the field that is radiated away.

The shape of a dipole resembles the letter "T". The middle leg connects the radio to the center of the upper, horizontal legs. There are several ways to make the connection. The simplest has the coax shield connected to one side and the center conductor connected to the other side. This works but there are losses at the connection due to impedance mismatch.

The feedpoint impedance various dramatically depending on the electrical height above ground.

For example an 80m horizontal dipole at 66ft. equal to 1/4 wavelength above ground has a feedpoint impedance of 84 ohms. This would create a mismatch of 1.68:1 VSWR. Now take the same antenna and install it only 18ft above ground, equal to 7 percent of a wavelength, and it now has an impedance of 45 ohms with a mismatch of only 1.1:1 VSWR.

A better way is to make or purchase a "balun". A balun matches the impedance of the radio to the impedance of the dipole more closely. This increases the power actually transferred to the upper legs of the dipole. (Balun stands for BALanced to UNbalanced.)

Mobile antenna

2008-02-08T12:18:35Z

Ke4uyp: Mobile moved to Capacity Hats

#REDIRECT [[Capacity Hats]]

Capacity hat

2008-02-08T12:18:35Z

Ke4uyp: Mobile moved to Capacity Hats

'''Capacity Hats'''

by Lou Rummel KE4UYP

Capacity hats will reduce the amount of inductance necessary to resonate the antenna, and increase bandwidth. But contrary to popular belief they add nothing to the radiation resistance. For a component of an antenna to increase radiation resistance it must itself radiate. It is true if you put a Capacity hat on top of a short mobile antenna it will change the antennas efficiency.

What happens is the current on the radiating element moves further up to the top. Because the current is now further away from ground this lowers ground lost. So the ratio of radiation resistance to ground lost, and Omni Loss resistance, which is the reduction in the amount of inductance needed for the loading coil, goes down.

Although it is true that the input impedance of antennas using Capacity Hats is higher than coil loaded antennas this is not an indication of an increase in radiation resistance, for the same reason that a folded dipole with 300 ohms impedance has the same radiation resistance as a standard 73 ohm dipole .

Vertical Antenna efficiency

2008-02-08T12:16:22Z

Ke4uyp:

'''Efficiency'''

by Lou Rummel KE4UYP

Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mounted radial wires and a 1/4 wavelength tall vertical radiator you can achieve the highest possible efficiency for a vertical on 80m.

Unfortunately for the average amateur radio operator this is not practical considering the size of the average back yard. On 80 m you would need a clear area in your backyard of 135 feet square and the vertical radiator would be approximately 66 feet tall. For decades antenna manufacturers have attempted to miniaturized the classic monopole vertical, but here is the first problem, if you reduce the amount of radials from 120 down to say eight or less the ground loss goes up dramatically.

Ground loss is the No. 1 contributor to low efficiency of low band antennas. The closer any antenna is to the ground the higher the loss regardless of whether it is vertically or horizontally polarized. The second biggest contributor to efficiency deficit is when the vertical radiator is less than 1/4 wavelength long, and of course most if not all
commercially manufactured 80m verticals fit into this category.

'''Radiation Resistance'''

On the opposite end of the spectrum there is radiation resistance, unlike ground loss the more you have the better off you are. A textbook perfect vertical would have no ground lost or resistive loss just radiation resistance. The length of the antenna determines the amount of radiation resistance it will have. The classic monopole vertical only develops radiation from the vertical element. The ground mounted radial wires contribute no radiation in fact, they do not even contribute to the radiation resistance. For any part of the antenna to contribute to radiation resistance it must produce RF radiation.

Capacity hat

2008-02-08T12:15:50Z

Ke4uyp:

'''Capacity Hats'''

by Lou Rummel KE4UYP

Capacity hats will reduce the amount of inductance necessary to resonate the antenna, and increase bandwidth. But contrary to popular belief they add nothing to the radiation resistance. For a component of an antenna to increase radiation resistance it must itself radiate. It is true if you put a Capacity hat on top of a short mobile antenna it will change the antennas efficiency.

What happens is the current on the radiating element moves further up to the top. Because the current is now further away from ground this lowers ground lost. So the ratio of radiation resistance to ground lost, and Omni Loss resistance, which is the reduction in the amount of inductance needed for the loading coil, goes down.

Although it is true that the input impedance of antennas using Capacity Hats is higher than coil loaded antennas this is not an indication of an increase in radiation resistance, for the same reason that a folded dipole with 300 ohms impedance has the same radiation resistance as a standard 73 ohm dipole .

Capacity hat

2008-02-08T12:14:06Z

Ke4uyp:

'''Capacity Hats'''
by Lou Rummel KE4UYP

Capacity hats will reduce the amount of inductance necessary to resonate the antenna, and increase bandwidth. But contrary to popular belief they add nothing to the radiation resistance. For a component of an antenna to increase radiation resistance it must itself radiate. It is true if you put a Capacity hat on top of a short mobile antenna it will change the antennas efficiency.

What happens is the current on the radiating element moves further up to the top. Because the current is now further away from ground this lowers ground lost. So the ratio of radiation resistance to ground lost, and Omni Loss resistance, which is the reduction in the amount of inductance needed for the loading coil, goes down.

Although it is true that the input impedance of antennas using Capacity Hats is higher than coil loaded antennas this is not an indication of an increase in radiation resistance, for the same reason that a folded dipole with 300 ohms impedance has the same radiation resistance as a standard 73 ohm dipole .

Vertical Antenna efficiency

2008-02-08T12:07:39Z

Ke4uyp: New page: '''Efficiency''' by Lou Rummel KE4UYP Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mo...

'''Efficiency'''
by Lou Rummel KE4UYP

Traditional monopole verticals rely heavily on having large quantities of ground mounted radial wires to achieve high efficiency. With 120 ground mounted radial wires and a 1/4 wavelength tall vertical radiator you can achieve the highest possible efficiency for a vertical on 80m.

Unfortunately for the average amateur radio operator this is not practical considering the size of the average back yard. On 80 m you would need a clear area in your backyard of 135 feet square and the vertical radiator would be approximately 66 feet tall. For decades antenna manufacturers have attempted to miniaturized the classic monopole vertical, but here is the first problem, if you reduce the amount of radials from 120 down to say eight or less the ground loss goes up dramatically.

Ground loss is the No. 1 contributor to low efficiency of low band antennas. The closer any antenna is to the ground the higher the loss regardless of whether it is vertically or horizontally polarized. The second biggest contributor to efficiency deficit is when the vertical radiator is less than 1/4 wavelength long, and of course most if not all
commercially manufactured 80m verticals fit into this category.

'''Radiation Resistance'''

On the opposite end of the spectrum there is radiation resistance, unlike ground loss the more you have the better off you are. A textbook perfect vertical would have no ground lost or resistive loss just radiation resistance. The length of the antenna determines the amount of radiation resistance it will have. The classic monopole vertical only develops radiation from the vertical element. The ground mounted radial wires contribute no radiation in fact, they do not even contribute to the radiation resistance. For any part of the antenna to contribute to radiation resistance it must produce RF radiation.

Coaxial Cable

2008-02-07T11:45:31Z

Ke4uyp:

'''Common Mode Current''' what is it, and how can you deal with it?
by Lou Rummel KE4UYP

First lets talk about differential current and hopefully I can make this a little less confusing. This type of current is ideally what you would want flowing inside your coax.

It is called differential because it is balanced, what this means is, the amplitude of the current flowing up the center conductor of the coax is identical in amplitude and 180 degrees out of phase, with the current flowing down the inside diameter of the braided shield surrounding the center conductor.

Unfortunately this rarely happens in the real world. You will almost always have an imbalance between these two currents.

This imbalance is what will cause, Common Mode Current. The obvious question is what caused this imbalance well there are actually three possibilities.

'''The first scenario:'''
If the antenna is a balanced antenna and you connect coax to it directly then technically speaking you will always have to some degree an imbalance with this differential current. You may ask how much of an imbalance can I expect well, that depends on several different factors.

For example if the coax is perpendicular to a dipole as it is going back to the transmitter the imbalance could be negligible. On the other hand it could be excessive if the length of the coax is a particular length. I will talk more about this a little later.

'''The second scenario:'''
When you connect coax to an unbalanced antenna, the situation only gets worse. The antenna or more technically speaking the load has more to do with creating this current imbalance than practically anything else. The more the load is unbalanced, the more current imbalance you will see on the coax.

'''The third scenario:'''
There are lengths of coax that will develop the highest level of common mode current. These lengths are odd multiples of one quarter wavelength for example 1/4, 3/4 and 1 1/4 wavelengths. When you have coax of these lengths, they will develop a low impedance path to ground, this in turn will cause excessive amounts of this common mode current to develop on the outside of the braided shield of the coax.

Because common mode current is located only on the outside perimeter of the braided shield unlike the differential current that is located on the inside diameter of the braided shield. Its velocity factor is not the same as the center conductor of the coax. For example RG-8X has a velocity factor of .78 to .82%. But common mode current has a velocity factor of .95%.

So the $52 million dollar question is how do you eliminate this problem. One of the best solutions to this problem is to install either a current balun or an RF choke sometimes called a (Line Isolator), at the feed-point of the antenna.

'''My Second Choice:'''
As I mentioned before there are lengths of coax that develop the highest level of common mode current. There are also optimum lengths of coax that develop the lowest level of common mode current. These lengths are odd multiples of 1/8 wavelength.

For example 1/8, 5/8, 1 1/8 and 1 5/8 wavelengths when you have coax that are these lengths, they develop an extremely high impedance path to ground which works as efficient as a high-quality RF choke or (current balun) Because we are dealing with electrical lengths that are odd multiples of 1/8 of a wavelength precise measurements do count.

A ½ wavelength dipole for example 468/ 3.5mhz=133.7ft. for 3.6mhz it would =130ft. Notice that there is almost four feet difference in their length. But at 1/8 wavelength the difference in these two frequencies is only 0.9ft. Here are the optimum coax lengths for 1/8 wavelength on the 80m and 60m band.

80m 60m

3.5 Mhz=33.4ft. 5.373Mhz=21.75ft.

3.6 Mhz=32.5ft.

3.7 Mhz=31.6ft.

3.8 Mhz=30.78ft.

3.9 Mhz=30.0ft.

Unfortunately the next coax length would be to excessive, for most people. For example 5/8 wavelength at 3.5Mhz=167ft. This is why using this technique is so limited on 80 or 60m but on VHF frequencies like 2m where one wavelength equals only 6ft. 5" it is quite manageable.

Coaxial Cable

2008-02-07T11:44:24Z

Ke4uyp:

Coaxial Cable

2008-02-07T11:43:41Z

Ke4uyp:

'''Common Mode Current''' what is it, and how can you deal with it?
by Lou Rummel KE4UYP

First lets talk about differential current and hopefully I can make this a little less confusing. This type of current is ideally what you would want flowing inside your coax.

It is called differential because it is balanced, what this means is, the amplitude of the current flowing up the center conductor of the coax is identical in amplitude and 180 degrees out of phase, with the current flowing down the inside diameter of the braided shield surrounding the center conductor.

Unfortunately this rarely happens in the real world. You will almost always have an imbalance between these two currents.

This imbalance is what will cause, Common Mode Current. The obvious question is what caused this imbalance well there are actually three possibilities.

'''The first scenario:'''
If the antenna is a balanced antenna and you connect coax to it directly then technically speaking you will always have to some degree an imbalance with this differential current. You may ask how much of an imbalance can I expect well, that depends on several different factors.

For example if the coax is perpendicular to a dipole as it is going back to the transmitter the imbalance could be negligible. On the other hand it could be excessive if the length of the coax is a particular length. I will talk more about this a little later.

'''The second scenario:'''
When you connect coax to an unbalanced antenna, the situation only gets worse. The antenna or more technically speaking the load has more to do with creating this current imbalance than practically anything else. The more the load is unbalanced, the more current imbalance you will see on the coax.

'''The third scenario:'''
There are lengths of coax that will develop the highest level of common mode current. These lengths are odd multiples of one quarter wavelength for example 1/4, 3/4 and 1 1/4 wavelengths. When you have coax of these lengths, they will develop a low impedance path to ground, this in turn will cause excessive amounts of this common mode current to develop on the outside of the braided shield of the coax.

Because common mode current is located only on the outside perimeter of the braided shield unlike the differential current that is located on the inside diameter of the braided shield. Its velocity factor is not the same as the center conductor of the coax. For example RG-8X has a velocity factor of .78 to .82%. But common mode current has a velocity factor of .95%.

So the $52 million dollar question is how do you eliminate this problem. One of the best solutions to this problem is to install either a current balun or an RF choke sometimes called a (Line Isolator), at the feed-point of the antenna.

'''My Second Choice:'''
As I mentioned before there are lengths of coax that develop the highest level of common mode current. There are also optimum lengths of coax that develop the lowest level of common mode current. These lengths are odd multiples of 1/8 wavelength.

For example 1/8, 5/8, 1 1/8 and 1 5/8 wavelengths when you have coax that are these lengths, they develop an extremely high impedance path to ground which works as efficient as a high-quality RF choke or (current balun) Because we are dealing with electrical lengths that are odd multiples of 1/8 of a wavelength precise measurements do count.

A ½ wavelength dipole for example 468/ 3.5mhz=133.7ft. for 3.6mhz it would =130ft. Notice that there is almost four feet difference in their length. But at 1/8 wavelength the difference in these two frequencies is only 0.9ft. Here are the optimum coax lengths for 1/8 wavelength on the 80m and 60m band.

80m 60m
3.5 Mhz=33.4ft. 5.373Mhz=21.75ft.

3.6 Mhz=32.5ft.

3.7 Mhz=31.6ft.

3.8 Mhz=30.78ft.

3.9 Mhz=30.0ft.

Unfortunately the next coax length would be to excessive, for most people. For example 5/8 wavelength at 3.5Mhz=167ft. This is why using this technique is so limited on 80 or 60m but on VHF frequencies like 2m where one wavelength equals only 6ft. 5" it is quite manageable.

Coaxial Cable

2008-02-07T11:40:37Z

Ke4uyp: New page: '''Common Mode Current''' what is it, and how can you deal with it? by Lou Rummel KE4UYP First lets talk about differential current and ...

'''Common Mode Current''' what is it, and how can you deal with it?
by Lou Rummel KE4UYP

First lets talk about differential current and hopefully I can make this a little less confusing. This type of current is ideally what you would want flowing inside your coax.

It is called differential because it is balanced, what this means is, the amplitude of the current flowing up the center conductor of the coax is identical in amplitude and 180 degrees out of phase, with the current flowing down the inside diameter of the braided shield surrounding the center conductor.

Unfortunately this rarely happens in the real world. You will almost always have an imbalance between these two currents.

This imbalance is what will cause, Common Mode Current. The obvious question is what caused this imbalance well there are actually three possibilities.

'''The first scenario:'''
If the antenna is a balanced antenna and you connect coax to it directly then technically speaking you will always have to some degree an imbalance with this differential current. You may ask how much of an imbalance can I expect well, that depends on several different factors.

For example if the coax is perpendicular to a dipole as it is going back to the transmitter the imbalance could be negligible. On the other hand it could be excessive if the length of the coax is a particular length. I will talk more about this a little later.

'''The second scenario:'''
When you connect coax to an unbalanced antenna, the situation only gets worse. The antenna or more technically speaking the load has more to do with creating this current imbalance than practically anything else. The more the load is unbalanced, the more current imbalance you will see on the coax.

'''The third scenario:'''
There are lengths of coax that will develop the highest level of common mode current. These lengths are odd multiples of one quarter wavelength for example 1/4, 3/4 and 1 1/4 wavelengths. When you have coax of these lengths, they will develop a low impedance path to ground, this in turn will cause excessive amounts of this common mode current to develop on the outside of the braided shield of the coax.

Because common mode current is located only on the outside perimeter of the braided shield unlike the differential current that is located on the inside diameter of the braided shield. Its velocity factor is not the same as the center conductor of the coax. For example RG-8X has a velocity factor of .78 to .82%. But common mode current has a velocity factor of .95%.

So the $52 million dollar question is how do you eliminate this problem. One of the best solutions to this problem is to install either a current balun or an RF choke sometimes called a (Line Isolator), at the feed-point of the antenna.

'''My Second Choice:'''
As I mentioned before there are lengths of coax that develop the highest level of common mode current. There are also optimum lengths of coax that develop the lowest level of common mode current. These lengths are odd multiples of 1/8 wavelength.

For example 1/8, 5/8, 1 1/8 and 1 5/8 wavelengths when you have coax that are these lengths, they develop an extremely high impedance path to ground which works as efficient as a high-quality RF choke or (current balun) Because we are dealing with electrical lengths that are odd multiples of 1/8 of a wavelength precise measurements do count.

A ½ wavelength dipole for example 468/ 3.5mhz=133.7ft. for 3.6mhz it would =130ft. Notice that there is almost four feet difference in their length. But at 1/8 wavelength the difference in these two frequencies is only 0.9ft. Here are the optimum coax lengths for 1/8 wavelength on the 80m and 60m band.

80m 60m
3.5 Mhz=33.4ft. 5.373Mhz=21.75ft.
3.6 Mhz=32.5ft.
3.7 Mhz=31.6ft.
3.8 Mhz=30.78ft.
3.9 Mhz=30.0ft.

Unfortunately the next coax length would be to excessive, for most people. For example 5/8 wavelength at 3.5Mhz=167ft. This is why using this technique is so limited on 80 or 60m but on VHF frequencies like 2m where one wavelength equals only 6ft. 5" it is quite manageable.