End-fed Vertical and J-pole
Because the J-pole and Zepp are electrically identical in function, and are similar to all other end-fed antennas in problems, pages on J-poles, Zepps, and end-fed verticals overlap.
The J-pole and other end-fed Hertz antennas are prime examples of antenna that can have severe feedline common mode current problems. The coax shield has to be at zero volts potential and have exactly equal and opposite currents to those flowing into and out of the center conductor at the load and source, otherwise the feedline radiates.
When we allow the feedline shield to be part of the radiating system, due to poor feed system design or construction, the system can be unstable. With improper feedline and mast decoupling, feedline and mast length and grounding can affect SWR. Weather changes can affect feedline moisture between the outer jacket and the support for the feedline, and this can change SWR with rain or snow. Even if SWR does not change, pattern can change significantly. For example, just reversing the shield and center on a J-pole feedpoint can change low angle field strength several dB, without affecting SWR!
Potentially severe common-mode feedline problems of end-fed 1/2 wave antennas vary with feedline length and feedline routing. This is why some people swear by end-fed antennas, while other people swear at end-fed antennas.
The J-pole is a good example of a poorly implemented feed system, because it mixes balanced and unbalanced systems. In the J-pole, an unbalanced end-fed half wave radiator is fed by a balanced 1/4 wave stub. The balanced stub is fed by unbalanced coaxial cable. This creates two improperly treated balanced-to-unbalanced junctions. Additionally, a metal support is often connected to the J-pole antenna, adding a third variable.
Here is a zoom of the feedpoint in a correct model of a bottom-fed J-pole. Notice the model includes the coaxial feedline and/or mast attached to the "grounded point" of the J-pole.
Wire 2 is the long vertical element
Wire 4 is the short vertical element
Wire 3 (obscured) is the horizontal bend (red circle the source)
Wire 5 is the mast and coax shield
You'll see the feedline or mast grounds directly to what everyone assumes is a "zero voltage" point. This is the electrical equivalent of any J-pole with the coax connected in series at the feedpoint, and the longer J-pole leg connected to the shield. The shield can be connected to any supporting mast with much change in system performance. The feedline in this case is relatively cold.
Wire 5 can be moved to either side of the base. The side where wire five attaches represents the side the mast and shield connects to.
Here is the resulting pattern of the shield (wire 5) to short leg wire 4. This is with a split base feed, NOT with the coax tapped up on the "J" :
The gain is 2.37 dBi at 4 degrees elevation (compared this to 2.69 dBi for a 1/4wl groundplane). This is actually the best feed system for the J-pole! The shield is connected to the bottom of the short element of the J-pole, with the center conductor connected to the bottom of the longer element of the J-pole.
This antenna model is in freespace, so earth reflection gain is not a factor. It is essentially equal to a vertical dipole in the same environment.
There is some distortion of pattern cause by the imperfect feed, even though it is the best feed.
Here is the pattern with the feedpoint connections reversed. The shield is connected to the longer element (wire 2 in my model) and the center conductor to the short element (wire 4):
Low-angle gain dropped about 5dB with just a simple reversal of feedline connections! If the model did not include the feedline, the model would never show this problem. In both cases, the SWR stayed near 1:1, yet low angle gain was reduced 5dB by reversing the shield and center conductor positions on the antenna!
J-pole With Coax Tapped up on Stub
This is the model without coaxial feedline. The feedline shield would attach to the open end of wire 8.
1 is the 3/4 wave element (1/4 wave plus 1/2 wave element)
6 is the stub
7 is the feed tap
With feedline of poor length choice:
Peak gain is now below the horizon.
None of this means the J-pole won't work, have a low SWR, and make contacts. It simply shows the pattern is unpredictable because the feedline, mast, and grounding significantly affects performance.
In contrast, a properly decoupled 1/4-wave groundplane pattern:
This antenna is significantly shorter in height, has more gain along the horizon, a smoother pattern, and is less sensitive to mast and feedline changes.
This is why commercial two-way radio manufacturers avoid J-poles. For hobbyists, in particular with portable antennas, the J-pole is considerably easier to build and it works OK.
Things Affecting J-pole Pattern and Gain
Affecting J-pole gain and pattern, but not included in models, are:
The following model is an I-Max 2000 5/8th wave vertical with a vertical feedline or mast connected to the antenna base, and no radials. In this case I picked one of many worse-case feedline or mast lengths:
Feedline shield current is 100% of antenna current. This illustrates why some users complain about SWR problems and RF in the shack with end-fed verticals like the I-MAX 2000, while other people do not complain and seem to love the antenna. This is because some people pick a lucky mast height or feedline length, while others are not so lucky. Unlucky people happened to choose a mast height, feedline length, or grounding system length that enhanced common mode problems.
Here is the pattern of an antenna that copies the I-MAX dimensions and feed system:
Most of the radiation is up in the sky at a high angle. The angle is so high, it is even useless for skywave.
This is a NEGATIVE gain antenna at low angles. A 1/4wl groundplane would seriously outperform the I-MAX 2000 or any other 1/2 or 5/8th wl antenna that does not have a large groundplane.
This pattern is over real earth, where a conventional dipole has about 8 to 8.5 dBi gain. This antenna about -2 dBd gain maximum. It has negative gain over a dipole. The gain over a dipole at most useful angles for DX is about -10 dB....significant negative gain.
Optimum feedline length and antenna mast height:
Even if we use the optimum feedline and mast length, here is the very best the end-fed vertical antenna will do is this pattern.
In this case we now have 2.67 dBi at 8 degrees elevation. This is actually an amount that is unnoticeably less than a perfect 1/4wl groundplane will produce! These severe common-mode mast and feedline currents make "no-radial" verticals extremely sensitive to mounting height, mounting structure, feedline length, and grounding. CB'ers for example often talk about grounding coax or changing coax length to match an antenna. If changes in mast length or feedline length or grounding affect the antenna pattern or SWR, it is an antenna design problem.
The gain over a dipole is now a few db at some really low angles, so it can be better than a dipole. At slightly higher angles for shorter skip, the dipole takes over and can be several dB better than the vertical.
This change is entirely the result of altering height and feedline/mast length!!! No antenna changes were made!
Summary End-feds Without Grounds
ANY END-FED ANTENNA REQUIRES A LARGE GROUNDPLANE OR OTHER EXTRAORDINARY ISOLATION METHOD OR METHODS TO PREVENT FEEDLINE OR MAST COMMON MODE CURRENTS!
This is true for 5/8th waves, Zepp antennas, R7's, R5's, or even common J-poles. End-feeding antennas is bad news unless you have a large well-established ground at the feedpoint. Even 1/4wl groundplanes have common mode problems. When I designed a commercial 1/4 wave groundplane with four 1/4 wave long radials, I had to insulate the radials from the mast and isolate the coax shield from the mast and radials with a 1/4 wave stub that formed a choke balun. Without the decoupling, I could change SWR simply by changing mast or feedline grounding.
Some manufacturers have wised-up.
Cushcraft, in their Ringo-Ranger, eventually added a separate additional groundplane below the antenna to tame the significant common-mode currents of the Ringo. Even that solution is barely acceptable, still leaving some mast currents.
The Isopole antenna used multiple sleeve sections to decouple the feedline, and it probably was one of the best antennas available for immunity to feedline coupling problems.
This problem gets worse when the element is 5/8th wave long. Think of that when you read claims of "no-radial" CB antennas with "3dB gain" and a low wave angle. They actually have negative gain at desired DX angles over a properly constructed conventional 1/4-wave groundplane! Instead of focusing the signal at useful DX or groundwave angles, long end-fed antennas without radials concentrate the signal toward a neighbor's TV set or toward an airplane flying overhead. These unwanted common mode currents cause the antenna system to be critical for feedline grounding, routing and length and even allow moisture on the feedline jacket to change performance of the system!
To read more about end-fed antennas of various types, follow the end-of-page link: