In the early part of the twentieth century a trailing wire antenna was developed for shortwave communications ,for use in dirigibles or zeppelin airships. It consists of a one wave length antenna fed at a current loop ,the loop folded into itself to forms a parallel (balanced ) transmission line, the equal and opposite currents canceling out the radiation fields. Essentially what has been established is a 1/4 wave length of open air transmission line (see sketch) ,end feeding a 1/2 wave length radiator. Because of the antennas original application, it has become known as the zepp or zeppelin antenna. This antenna configuration has been adopted for use in the amateur community due to its ease of construction and due to the simple feed which requires no knowledge of the antenna feed point impedance (more on the universal stub later}.
Along comes the need for a simply constructed vertical antenna for VHF and UHF fm use in the amateur community, the frequencies are high enough (shorter wave lengths) to construct the antenna from rigid elements made from tubing, pipe and rod, hence the 'J' Pole. Because of the configuration of this antenna the entire antenna and transmission line can be connected to DC ground which helps to protect against lightening strikes and static discharge. Common copper plumbing pipe has become the material of choice to construct the 'J' pole, due to the ease with which copper can be soldered and the plenteous availability of copper pipe and fittings, and also the fact that copper is one of the best metals for conductivity.
The antenna can be fed with balanced line (parallel identical conductors) or coaxial cable, the coaxial cable being an unbalanced line requires a transformer ( from unbalanced to balanced ) called a balun. The proper impedance sweet spot can be obtained by sliding the feeder up from the short across the bottom of the 'J' section and reading the results with the use of an SWR meter. The mistake that most people make when constructing this antenna is to omit a balun and instead using a choke made up of several turns of the feeding coaxial cable which in many instances is mistakenly called a balun.. Many examples can be found on the internet that promote this method .It is like the blind leading the blind and the reason success is never achieved in removing all of the SWR. fluctuations ,and thereby giving this antenna the reputation that it is not stable or reproducible.
Baluns can be constructed from coaxial cable, such as a 1/4 wave 1 to 1 coaxial sleeve balun, or a 1/2 wave 4 to 1 coaxial balun or a balun constructed using electrical circuits and components. Instructions for constructing coaxial baluns can be obtained from the ARRL antenna book or the ARRL hand book
A coaxial cable is inherently a pair of conductors in an unbalanced state ,due to the conductors being of different configuration and size. The 'J' pole is composed of a 1/2 or 5/8 radiator and a feeder composed of two parallel conductors, which if they are equal in diameter are in a balanced state. To feed this antenna properly with coaxial cable a transformation from unbalanced to balanced is required. This is accomplished with a transforming device called a balun. Baluns are used to transform impedance and to provide isolation isolating a load from a grounded source} Note that a random length of coax wound into a coil is not a balun but rather an attempt at choking current on the shield, which does not provide impedance transformation nor provide isolation .This current on the shield of the coax causes fluctuating S.W.R. in the line when a proper transformation from unbalanced to balanced is not used.
Contrary to opinion the 'J' pole is a very stable and reproducible antenna when fed properly
!/2 wave 'J' pole fed with a 4:1 TV balun
1/2 over 1/2 two meter 'J' pole with the upper half consisting of 1/2 over 1/2 ,440 'J' pole
440 antenna composed of (8) , stacked 5/8 elements
VE3XKV station 2 meter antenna consisting of (8) stacked 5/8 wave elements with 1/4 wave spacing between bays each bay consisting of two 5/8 elements separated by 1/4 wave) total length of approx. 43 ft. Each bay fed with a universal feeder and 4:1 coaxial balun
Universal feeder, 5/8 over 5/8 stacked
2 meter elements (notice round Plexiglas insulator between elements)
The ideal condition and operation of a transmission line is to have equal and opposite currents flowing throughout its length. This condition does not necessarily occur even though the antenna is impedance matched to the transmission line. When RF is fed to an antenna , current flows and voltage is induced in the transmission line and the antenna elements. Any current flowing in the transmission line is equal and opposite and cancels out any radiation from the line itself . Not so in the radiating element of the antenna. In the radiating element of an antenna current flows in one direction, and there is no parallel element with equal and opposite current present and hence radiation occurs. All other conductors in the radiation field of the antenna are influenced by said radiation and coupled to it by induction, that is to say that when current flows in the antenna element all other conductors in its field of influence will have a voltage induced on them, and consequently current will flow in those conductors. The transmission line feeding the antenna is a conductor which is inherently in the antennas field of influence. The only way to reduce the influence of antenna current is to place the transmission line at 90 degrees for a distance of 1/2 wave length, on a center fed dipole, but this does not work on an end fed antenna. Current can be induced on any type of line be it balanced or unbalanced, however if the feed line is coaxial the antenna current flows only on the outside of the shield not the center conductor. Because the center conductor in coaxial cable is shielded from the antennas radiation field by the outer conductor, no current is induced from said radiation field on to the center conductor. Any antenna current induced onto the shield that flows the same direction as the current already flowing on the shield will amplify that shield current. This will cause the shield to become a radiator, because there is no current of equal magnitude on the center conductor to cancel radiation.
The following statement is taken from The ARRL antenna book: When there is an antenna current of appreciable amplitude on the line it will be found that not only are the line currents unbalanced but the apparent SWR is different in each conductor, and that the loops and nodes of current in one wire do not occur at corresponding points in the other wire. Under these conditions it is impossible to measure the true SWR.
The following statement is also taken from the ARRL antenna book: Stated broadly, the unbalance with coaxial line is caused by the fact that the outside of the outer conductor is not coupled to the antenna in the same way as the inner conductor and the inside of the outer conductor. The overall result is that current will flow on the outside of the outer conductor.
What this is referring to is that current does not flow through the material of a conductor but on the surface of a conductor and the outer shield of coaxial cable has an inner and outer surface.
It is also further stated that this type of coupling is not the same as coupling caused by antenna current ,but it is in addition to coupling caused by antenna current. Luckily the remedy is the same for both cases of coupling with a device called a balun.
Note: an antenna should be in the clear when being tuned and operated as conductors in its field of influence can cause the antenna to become unbalanced as well ,even the human body can cause unbalance.
All electromagnetic waves ,which include visible and invisible light and radio waves travel through a vacuum at the same speed, which is 300 million meters per second. Vacuum is the medium used as a bench mark because it poses no resistance to wave propagation. The place that resembles a perfect vacuum most is outer space, but it is not a perfect vacuum. There are influences that can effect the propagation of electromagnetic waves even in outer space, such as black holes which bend and swallow light, concentrations of certain gasses and dust particles that can absorb reflect and redirect. The understanding of the propagation of radio frequency waves in the earths atmosphere is a study unto itself. The purpose of this short discourse is just to illustrate that there are influences and conditions that can resist or changes how an EM wave propagates
It is common practice to use the position of the electrical field of an EM wave to describe said EM wave.. If the electrical field of an EM wave is used to describe said EM wave then the laws of electricity are applied as well.
Remember how vacuum has the least resistance to wave propagation and that it is common practice to use the electrical field of an EM wave to describe said EM wave and that the electrical field is governed by the laws of electricity. Having said all this ,how an EM wave propagates or travels is governed by resistance, The resistance afforded by the make up of the medium that the EM wave travels through. The same rule applies how RF current travels on a conductor. An excellent example of how RF current is effected by the medium it travels on or in,is to look at the velocity factors of all the different transmission cables available today. The velocity factor of a transmission line determines the wave length of an RF current applied to the transmission line. As with transmission lines the wave length of an RF current applied to a radiating antenna element is affected by certain parameters which determine the resonant length of the radiating element.
The ability of a conductor( half wave antenna element) to conduct an RF current flow depends on the following factors:
1- Specific resistance of the material
2- Diameter of the conductor( this has the greatest effect on antenna element length) The resistance decreases as the diameter increases and the result is an antenna element that can be significantly shorter than a free space half wavelength
3- Temperature
Items 1and 3 can be largely ignored when constructing an antenna, but item 2 cannot.
There are a number of formula available to determine the lengths of the "J" Pole antenna elements and the methods used and the figures obtained are as numerous, all granted in the so called ballpark, but not accurate. There are also charts and formula available for the current velocity factors for copper and aluminum of varying element diameters, also K factor charts to determine the resonant length of the elements all difficult to to some degree to use, and the end result not necessarily accurate
The Following Are Some Simple Rules To Remember and Follow When Constructing A "J" Pole Antenna So That The Previous Conditions Discussed ,Can Be Ignored.
1-Construct both elements of the antenna out of the same material ,be it copper ,aluminum or stainless steel.
2- Construct both elements and connecting components of the antenna using the same diameter material so that the velocity factor of the current will be uniform in all components of the antenna
3- Remember that a "J" Pole is a half wave radiator end fed by a one quarter wave open wire transmission line, the open wire transmission line should be made of the same material and diameter as the radiating element. (this ensures that the material specific resistance ,and the resistance change caused by various diameters will not be present to interfere with tuning the antenna to resonance). The velocity factor of the current will be the same in all components of the antenna.
4- Keep the two elements as close together constructionally as possible, this maintains the efficiency of the antenna. less radiation from the 1/4 wave open wire transmission line section where the two elements are parallel)
5- Use the free space or vacuum formula to cipher the proper length of the two elements (Wave length equals 300 divided by frequency}
A-long element cut to exactly 3/4 of a wavelength---3/4 of a wave length equals 225 divided by frequency in MHz
B- Short element cut to exactly 1/4 wavelength ---1/4 of a wave length equals 75 divided by frequency in MHz.
Note: Cutting the elements to the free space length is the most accurate starting point to commence from. When construction is finished and tuning begins antenna elements will have to be trimmed to resonance to the frequency desired. It is important to trim only exactly 1/3 from the shorter element of the length that is trimmed from the longer element as it is only 1/3 the length of the longer. When the antenna is finally tuned to resonance the shorter element will be exactly 1/3 the length of the longer. In other words if during the tuning stage 3 inches is trimmed from the longer element then 1 inch must be trimmed from the shorter element. While tuning and trimming the antenna elements always maintain this 3 to 1 ratio.
6- Always tune the antenna in the clear ,at least 1/2 wave length away from other objects. Stay well clear of electrical power lines, far enough so that if the antenna were to fall it would not hit the power lines. When the antenna is initially checked for resonance and a low SWR it will tune to a lower frequency than desired but as it is trimmed the antenna will resonate at the proper desired frequency. Once the initial frequency has been established without any trimming ,a simple formula can be used to cut the antenna closer to the desired frequency. The formula to obtain the desired element length is as follows: The measured length of the element, divided by the desired frequency for the antenna, multiplied by the measured frequency of the antenna. Do not apply the formula nor do the trimming in large steps. Whether the use of the formula is opted, or if the trial and progression method is used, work towards the desired length in small steps to see how the results works out. The formula can be used to trim both elements, keeping in mind the note in rule number 5 as the progression develops. The formula can be used to get into the ballpark but the fine tuning of progressively smaller trims will get the final results desired.
A half wave over half wave antenna with a center feed can be constructed using the same method. This can be done by rotating the radiating half wave element at 90 degrees to the 1/4 wave feeder and attaching another half wave element to the open section of the 1/4 wave feeder . The result is a 1/4 wave section of open wire transmission line end feeding two half over half in phase stacked elements . This is an ideal antenna for mounting on the side of a tower. This antenna is much easier to construct if a simple half wave "J" Pole is first constructed and tuned ,because that will determine all the resonant lengths and a second radiating element already cut to resonance can be attached to the open end of the 1/4 wave feeder. The antenna would be attached to the tower from behind the short across the 1/4 wave feeder.