Related Pages:
Can we end-feed a half wave without any ground?
The answer depends on what we want to call a "ground". A simple rule that (that cannot be broken) applies to all end-fed antennas. The rule is:
Current flowing into the antenna's end must be equaled at that same point by the same amount of current flowing into a ground or counterpoise of some type.
An end-fed antenna must have the outside of the coax shield, the feedline, or something attached to the feedpoint that carries the same current away from the feedpoint as the current flowing into the antenna! This is true no matter how many series traps or choking devices we add at the feedpoint. If we add chokes at the feedpoint, voltage across the choking device increases until the same end-current flows.
This drawing does NOT include differential or normal transmission line currents on the feedline. It is the common mode or radiating currents only. Elsewhere in the system antenna and transmission line currents can change to other values, but right at the feedpoint they must always be equal.
The analogy I
like to use is this:
Assume we are trying to push and pull a car. The car's resistance to being pushed is the antenna impedance. The speed the car moves is the voltage, and how far the car moves in the direction of push or pull is the current.
Even if we push a very light car (high impedance) end, we still have must have a countering force in our foothold. Unless we are "heavy" or have very good footing compared to the mass of the car, we will move the opposite direction (voltage) that the car moves at the point of contact. The distance our hand moves is ALWAYS the same as the distance the car moves because they are connected, and the force pushing backwards on us is always the same on us as it is on the car.
The end-impedance of a half wave, even at exact resonance, is never infinite. The antenna end impedance is always some finite value between a few hundred ohms (for very thick antennas like large cross-section towers) and perhaps as high as several thousand ohms for very thin wire antennas that are up high and in the clear. Because the end-impedance is some finite value, some form of counterpoise is always needed. A feedpoint simply cannot force current into something without equal current flowing into something else (to push against) at the feedpoint.
Do We Notice Problems? Does this mean an end-fed won't "work"?
The fact we always have ground currents does not mean we will always notice those currents. It does not mean there will always be problems caused by undesired common mode currents. This just means common mode or radiating currents that flow into a counterpoise or feed cable must exist or we could never apply power to the antenna. The purpose of this page is to point out the effects of common mode currents.
Why Does The "Equal Current" Rule Exist?
Don't blame that rule on me! Kirchoff's Law has been around a long time. Everyone worth his salt in electronics knows it as a perfect rule, just like Ohm's law. That's why we call rules like this "laws" and not something like Kirchoff's Suggestion or Ohm's Wild Guess.
- We cannot force current or power into a single terminal device. There has to be a return path. Power sources or "generators" must always be two (or more) terminal devices. If the second terminal doesn't have a return path via displacement (electric field) currents or conduction currents we cannot transfer energy from the generator to a load. The path can even be displacement currents through a stray capacitance, but it MUST always occur.
- The end-impedance cannot be infinite. The end-impedance goes through a relatively broad peak that reaches maximum when an antenna is approximately resonant as a half-wave, but the peak dissipative or radiating part of that impedance is limited to a few thousand ohms for very thin conductors having only air as a perfect insulation. Anything else has LESS impedance at the open end. Finite voltage and current is required to apply power to or extract energy from the antenna, and the return currents must flow into a "current-sink" of some type at the feedpoint.
- The antenna must terminate through displacement currents of some type. If it didn't, it would have no current at the ends. It also would not have standing waves on the antenna, and would not have a current maxima. In a normal "push-pull" center fed fed dipole, displacement currents largely (but not totally) flow from one side to the other. This establishes the electric field, and allows current to flow outwards into the wires hanging "open" in space. With an end-fed, current flowing into the end of the radiator must be returned to the OPPOSITE terminal of the generator. This returned current (called displacement current) always equals the common mode current flowing into the antenna end.
One of the most common arguments we hear is a half-wave has infinite or near infinite impedance.
That argument is easily seen to be false. If impedance was infinite we could not match into the antenna at all! If impedance was extremely high we could not efficiently transfer energy into the antenna. The end impedance ranges from a few hundred to several thousand ohms, depending on conductor diameter.
Let's look at a few typical matching systems.....
The Open Stub
Most of us know a 1/4 wave stub or Q-section can transform impedances. The formula is Zi = Zt squared over Zl. Where Zi is the input impedance seen on the rig side, Zl is the load or antenna impedance, and Zt is the Q-section transmission line impedance. Let's try it with a 1/4 wl 75 ohm line used to match a 100 ohm quad....
Zi = 75 squared divided by 100. That's 5625/100 or 56.25 ohms. See how that works? This is why a 75 ohm line 1/4 wl long can be used to match a 100 ohm quad to a 50 ohm line.
Let me give you an easy way to calculate the impedance transformation in your head...use the SWR! The SWR on the 75 ohm Q-section (Q is quarter wave) is about 1.3:1. The input SWR has to be 1.3:1 on that 75 ohm section, except being 1/4 wl away it is inverted. A high impedance becomes a low one. 75 ohms with a 1.3:1 SWR would be about 57 ohms! We can see the impedance transformation actually relates to SWR on the Q section!
Let's assume we have an end-fed 1/2 wl and match it perfectly with 1/4 wl of 450-ohm line. We know the SWR has to be 450/50 = 9:1. From the above exercise we can calculate the TOTAL end impedance seen by the Q section at the antenna, and that impedance will tell us the SUM of counterpoise and antenna impedances. Since the SWR along the Q section is 9:1, the sum of antenna and counterpoise impedances is 450 * 9 (SWR) = 4050 ohms! (4050/450=9:1 SWR)
The fact we can end-feed a half-wave through a 1/4 wl Q-section PROVES the antenna end-impedance is some value less than 4050 ohms. What did I say earlier? I said the maximum end-impedance of a typical end-fed thin wire antenna is a few thousand ohms at resonance. Where is the counterpoise? It is the common mode currents on the Q section, and that Q section typically has a common mode (counterpoise) impedance of a few thousand ohms. The antenna and the Q-section EACH have end-impedances of a few thousand ohms (they are in series and the total of both is about 4000 ohms), the antenna and Q-section each have equal common mode currents, and they each share in radiating your signal and receiving signals!
The Tuned Circuit
A second matching method is a resonant circuit with a link, or a modification of that circuit. The impedance ratio of such a circuit, assuming perfect coupling, is equal to the square root of the turns ratio between the secondary and primary.
A 2500-ohm antenna worked against a 2000-ohm impedance counterpoise would have a differential feedpoint impedance of 4500 ohms. Assuming lossless coupling, the tank would require a turns ratio equal to the square root of 4500/50, or sqrt 90 = 9.5:1 turns ratio.
A 28 turn winding tapped at approximately 3 turns would work, but this assumes no flux leakage (100% primary to secondary coupling) and infinite unloaded Q in the resonant circuit.
For an example of a system like this consider the feed system at AA5TB website. You'll see Steve uses a 4.7k tuning resistor and a 3:28 turns ratio transformer. In an ideal transformer, that turns ratio would produce a 1:87 step up. 50*87 = 4350 ohms. Steve used a 4700 ohm load resistor to substitute for the end-fed-half-wave (EFHW) he was using. Obviously the end impedance of the end-fed half wave Steve matched is about 4700 ohms, NOT infinity.
The 4700 ohm
impedance proves we
have ground current
flowing. It
proves the antenna
impedance of a thin
wire is very high,
but not infinite.
A thick end fed
antenna compared to
the length like a
broadcast tower on
the AM band or
aluminum tubing on 2
meters can have an
end-impedance in the
hundreds of ohms.
The same is true for
a thin end-fed wire
near lossy media.
End-fed Myths
There are many incorrect ideas and claims surrounding end-fed vertical antennas (like the I-Max 2000), end fed horizontal antennas, Zepp, and J-pole antennas. All of the above antennas are actually quite similar. The J-pole is closer to the original Zepp antenna (Zeppelin antenna) than the bent antenna we commonly call a Zepp.
Some myths have been perpetuated or re-enforced by use of incomplete models. The most common modeling mistake is omitting the feedline or, if included, the coupling device (tuning or matching unit) properties are omitted. The major problem often isn't feedline radiation, it is common mode currents from the feedline. This article will explore why those currents occur, and give worse and best case examples of things affecting common mode currents.
I'll examine the most common and troublesome myth or false claim, that such antennas do not require a ground or will not have troublesome ground currents flowing over conductors into the station ground.
Origination of J-pole and Zepp Antenna
This form of antenna originated as a practical solution to feeding a wire antenna trailing behind a dirigible. This type of antenna is a Marconi antenna, and it requires a counterpoise of some form.
(Marconi, by definition, means an antenna that is excited against a counterpoise or ground. An end-fed antenna is really a Marconi antenna, since it is impossible to force current into the end of an antenna without equal current flowing in some form of counterpoise.)
One way to create an effective ground system or counterpoise would be to use metal structural parts of the airship. This could create a problem if an RF-induced arc occurred where a mixture of hydrogen and oxygen was present. Running high RF current levels around inside a large bag of hydrogen with a pod of people hanging below was considered poor engineering.
The solution was use of a trailing counterpoise wire. With a second trailing wire was a counterpoise, the antenna system cold be floated from the airship frame. By floating the antenna and ground from the structure, most of the RF currents would be confined to the trailing wire and trailing counterpoise. The initial trailing wire antenna was 1/2 wavelength long, and the counterpoise 1/4 wl long. This set up a high impedance antenna feed working against a low-impedance counterpoise. Most of the current was in the antenna, as seen in this model:
Notice the current curves in this model. Maximum current in the counterpoise wire is much less than the maximum current in the 1/2 wave section. Antenna to feedline junction currents are equal. Currents at 500 watts are:
- .71a into the 1/2 wl wire
- .71a into the 1/4 wave counterpoise wire that parallels the 1/2 wave wire
- 2.32a maximum at the middle of the 1/2 wl wire
(Pattern is similar to a 1/2 wl dipole at the same height.)
Notice how currents fit the rule I mentioned at the start of this article. At the feedpoint the counterpoise wire, because there is no other path, carries exactly the same current as the antenna wire.
The above model, like the models almost everyone uses, has an omission. This omission makes things look better in the model than actually occurs in real life systems using tuners.
The flaw in the model above is the same modeling omission used with many Zepp antenna models. Nearly all models omit a real-world antenna coupling or feedline system! Unless we add some other ground or wires representing the feedline and devices connected to the feedline, the model isn't real.
Current sources in models do NOT have capacitance to earth or other conductors. Current or voltage sources in models feed the antenna with a perfect ground independent current source. The perfect source has no path to ground or to the station wiring. The tuner sitting on your desk is not a source with infinite impedance to ground! Models let us have perfect feed systems, the real world does not.
When the model includes an imperfect source, we see things change dramatically. Here is what happens when a worse-case ground path is added to the original Zepp model:
In this case, I added an additional 1/4 wl wire. We all know a 1/4 wl wire open at the far end presents a low impedance at the near end. The wire I added presents a low-impedance common mode path from the counterpoise side of the antenna feedpoint connection. Let's assume this wire is a two-wire or coaxial transmission line that connects to the feedpoint. We can see ground currents flow down this wire towards the shack. These currents almost equal the maximum antenna current. The new path, which might be a 1/4 wave long feedline connected to a voltage balun on a tuner or a push-pull tuned circuit, radiates.
500-watt system currents, when a typical maximum common-mode impedance of a GOOD tuner is added, now become:
- .71a flows into the end of the antenna
- .61a into antenna counterpoise.
- .1a into ground at the tuner
It is the .1a that causes all the problems. If you do not have a good low-impedance RF ground on your tuner, the significant 100 mA of unwanted RF current will flow into you, the power line, the power supply, the microphone, and everything else connected to the radio and tuner! This is why RF problems are commonly with Zepps even though models show them to be perfect.
In reality, most antennas will not be fed this poorly. A typical GOOD tuner has a few thousand ohms common-mode impedance. (There isn't any way around this, it is an impedance limitation inherent in the devices. You especially can't get around the problem by moving a balun to the input of a tuner! )
The fact is, if an antenna is fed properly there is absolutely no need for a station RF ground! The station RF ground is a band-aid for poor antenna system design.
If an antenna is modeled properly to include feedlines and tuners, you will see problems that idea-source models overlook! Conclusions about system behavior change when tuners and transmission lines are included in models.
The Next Zepp Step
One of the big problems with models is the omission of real-world effects like a tuner or coupling device. There have been a few popular articles and web sites that state a Zepp has no substantial feedline radiation. This may be true so far as pattern is concerned, but it is not true so far as affecting things near or connected to the feedline. Let's look at a "double Zepp", or a balanced Zepp design. I can show you specifically what the other models omit. Before we go further, we should understand balance in feedlines.
Feedline Balance
Many people think a balanced feeder has exactly equal currents in each leg. That's correct, but it does NOT go far enough! The correct and complete rules for a balanced feeder are:
- Both conductors must have equal currents
- Each conductor must have currents 180 degrees out-of-phase with the other conductor at the same point
- Each conductor must have equal and opposite voltages with reference to ground or the spatial area around the conductors. As with current, the voltages must be 180 degrees from each other.
Unless all of these conditions are met, the feedline could be a source of unwanted energy leading to RF in the shack or noise ingress into the antenna.
The Double Zepp
This is a poor name. The double Zepp is really just a doublet or dipole fed with a balanced line, generally high impedance. Looking at the feeder allows us to understand the problems with a regular amateur radio Zepp antenna, so let's ignore semantics and look at the "Double Zepp".
Here is the wire table of the double Zepp:
EZNEC+ ver. 4.0
Double Zepp 4/29/2007 3:51:32 PM
--------------- WIRES ---------------
No. End 1 Coord. (ft) End 2 Coord. (ft) Dia (in) Segs Insulation
Conn. X Y Z Conn. X Y Z Diel C Thk(in)
1 W2E1 0, 0.3, 1 W3E1 0, 0, 1 #14 1 1 0
2 W5E2 0, 0.3, 1 W6E1 0, 0.3, 65 #14 65 1 0
3 W1E2 0, 0, 1 W4E1 0, 0, 65 #14 65 1 0
4 W3E2 0, 0, 65 130, 0, 65 #14 130 1 0
5 GND 0, 0.15, 0 W1E1 0, 0.3, 1 #14 5 1 0
6 W2E2 0, 0.3, 65 -130, 0, 65 #14 130 1 0
Wire 4 Wire 6
2 3
1
5 moves to test balance
Wire 1 allows insertion of the source. It is a jumper across the feedline.Wires 2 and 3 are the balanced feedline.
Wires 4 and 6 are 130-foot, forming a 260 foot long "dipole".
Wire 5 allows us to test balance
Let's look at the sources:
Source 1 is in the middle of wire 1, at the normal feedpoint
Source 2 is in the middle of wire 5. In this case wire 5 is connected to the junction of wire 2 and 1.
The second source is set for zero amperes! This means the voltage and phase would be the voltage and phase
required to produce zero amperes.
Frequency = 3.75 MHz
Source 1 Voltage = 361.1 V. at 33.07 deg.
Current = 4.957 A. at 0.0 deg.
Impedance = 61.04 + J 39.74 ohms
Power = 1500 watts
SWR (50 ohm system) = 2.076 (200 ohm system) = 3.418
Source 2 Voltage = 171.3 V. at -147.94 deg.
Current = 0 A. at 0.0 deg.
Impedance is infinite
Power = 0 watts
SWR (50 ohm system) > 100 (200 ohm system) > 100
Total applied power = 1500 watts
Now watch source 2 as we move wire 5 to the other feeder terminal:
Frequency = 3.75 MHz
Source 1 Voltage = 361.1 V. at 33.08 deg.
Current = 4.957 A. at 0.0 deg.
Impedance = 61.05 + J 39.76 ohms
Power = 1500 watts
SWR (50 ohm system) = 2.076 (200 ohm system) = 3.418
Source 2 Voltage = 171.1 V. at 31.79 deg.
Current = 0 A. at 0.0 deg.
Impedance is infinite
Power = 0 watts
SWR (50 ohm system) > 100 (200 ohm system) > 100
Total applied power = 1500 watts
Notice the nearly perfect balance. The voltage difference is only 0.2 volts out of 171 volts. The phase difference is 179.73 degrees. Almost perfectly 180 degrees!!
This is a well balanced feeder. It will not radiate significantly, and the voltage on each terminal of a tuner would be nearly equal to ground for perfectly balanced currents in each leg. The 171 volts is very manageable from a tuner, and it won't matter much if it is a good voltage balun or a good current balun. This is a simple system that would be unlikely to produce much RF in the shack.
If we do nearfield electric field table around the feedline, we find the electric field out several feet from the transmission line in the house is fairly low. This is with a "perfect" tuner.
The Zepp
Now
let's delete one of
the wires so we have
a traditional
amateur Zepp and see
what happens. The
antenna model looks
like this:
4
2 3
1
5
The wire table is:
EZNEC+ ver. 4.0
Zepp 4/29/2007 11:10:47 PM
--------------- WIRES ---------------
No. End 1 Coord. (ft) End 2 Coord. (ft) Dia (in) Segs Insulation
Conn. X Y Z Conn. X Y Z Diel C Thk(in)
1 W2E1 0, 0.3, 1 W3E1 0, 0, 1 #14 1 1 0
2 W5E2 0, 0.3, 1 0, 0.3, 65 #14 65 1 0
3 W1E2 0, 0, 1 W4E1 0, 0, 65 #14 65 1 0
4 W3E2 0, 0, 65 130, 0, 65 #14 130 1 0
5 GND 0, 0.15, 0 W1E1 0, 0.3, 1 #14 3 1 0
The sources are:
Frequency = 3.75 MHz
Source 1 Voltage = 175.7 V. at 1.02 deg.
Current = 8.537 A. at 0.0 deg.
Impedance = 20.58 + J 0.3663 ohms
Power = 1500 watts
SWR (50 ohm system) = 2.430 (200 ohm system) = 9.717
Source 2 Voltage = 131.5 V. at -8.8 deg.
Current = 0 A. at 0.0 deg.
Impedance is infinite
Power = 0 watts
SWR (50 ohm system) > 100 (200 ohm system) > 100
Total applied power = 1500 watts
and on the other terminal:
Frequency = 3.75 MHz
Source 1 Voltage = 199.7 V. at -0.35 deg.
Current = 7.51 A. at 0.0 deg.
Impedance = 26.6 - J 0.1616 ohms
Power = 1500 watts
SWR (50 ohm system) = 1.880 (200 ohm system) = 7.520
Source 2 Voltage = 285.6 V. at -5.45 deg.
Current = 0 A. at 0.0 deg.
Impedance is infinite
Power = 0 watts
SWR (50 ohm system) > 100 (200 ohm system) > 100
Total applied power = 1500 watts
We see now the phase and amplitude of voltages at each source terminal are not equal. The voltage difference is 154.1 volts while the phase difference is only 3.35 degrees.
Despite the fact current appears equal in both feeder wires near the sources, the large unbalance in voltage and lack of 180 degree phase difference represents terrible common mode problems. A regular voltage balun would no longer work well. Even a good current balun would be taxed.
This is why people have problems with RF in the shack from Zepps, despite what incorrect models might claim.
By the way if you are uncomfortable using a zero ampere source to measure voltage, you could substitute a very high impedance load in wire 5.
Another way to determine unbalance, and remember VOLTAGE or phase unbalance is just as harmful as current unbalance to devices near or connected to the feedline, is to run a Near Field table of the electric field. If we do that we will find the end fed Zepp has from a few times to a dozen times or more the electric field levels near the feedline as a center fed double Zepp with about the same feedline impedance and the same power level! In the tabular data I examined the end fed Zepp had many places where the electric field 5 feet from the feedline was ten times or more higher than a center fed balanced antenna, both with perfect tuners of course.
The claim by some sources that an end fed Zepp has minimal radiation and good feedline balance is clearly false. The authors made the fatal mistake of not modeling the near field levels for the same power level and same feedline impedance (conductor to conductor voltage). They also did not check voltage balance, which is just as important for preventing near field problems like RF in the shack. They only looked at one parameter and concluded if current was balanced the feedline was tame.
The problem isn't far field pattern, the problem is feedline balance and RFI in the near field.
More "Zepps" and end feds
Let's look at the next evolution of the Zepp (this applies to end-fed verticals like the I-max 2000 although I will cover end-fed 5/8th wave verticals in detail at a later time). In this case we will model a 1/2 wl end-fed antenna fed with a 1/4 wl transmission line (a conventional year-2000 Zepp antenna), and see what happens when the 1/4 wl feedline is tied to ground through the common-mode impedance of a typical better-grade antenna tuner.
The ideal case for Zepp or J-pole (they are the same except for the bend in the Zepp at the feed) is when the radiator is very thin, exactly 1/2 wl long, and fed with a parallel conductor stub that is exactly 1/4 wl long.
Here is the antenna view:
Ideal antenna shows no pattern distortion:
Although high feedline currents appear, they are closely balanced and out-of-phase. This is why many authors conclude feedline radiation is not a problem, and I agree for this specific perfect case! The specific case is the tuner on your desk must have infinite common mode impedance, it must be a perfect ground independent source.
The real world adds a few variables, however. Real-world sources are not ground independent, real-world antennas are not always 1/2 wl long, and real-world feedlines are not always 1/4 wl long on every band.
Let's explore what happens when the real world enters the perfect model!
Adding a worse-case ground path to a perfect antenna, we have:
Now we see a slight flattening of the bottom of the pattern. While the pattern does not change much, we have the following currents at 500 watts:
- 6.1 amperes feedline wire 1
- 5.35 amperes feedline wire 2
- .75 amperes ground path
Even with the small pattern error, we now has a very significant .75 amperes flowing to earth through the station equipment and perhaps eventually the operator. This is with an IDEAL antenna, and a normal worse-case antenna feed.
The J-Pole
What is a worse case feed? The common J-pole antenna fed with coax! .75 amperes would represent the undesired shield currents and supporting pipe or mast current total when a J-pole is fed with coax.
This problem gets worse when the Zepp feedline is not a 1/4 wl long, and the antenna is not 1/2 wl long. The I-MAX 2000 CB vertical is one example. The antenna has a random feedline (your coaxial feedline and mast) as a counterpoise, and is not a 1/2 wl end-fed vertical! This is why so many people complain about RF feedback with Zepp and end-fed 1/2 wl antennas.
If time permits I'll add examples of worse length combinations, but this example of a best length proves common-mode currents are an issue with a perfect Zepp connected to a real tuner. This is why people with Zepp are generally people who believe good RF grounds are required. They are correct in that belief. if you have a poor antenna design and run any power you need a good RF ground at the station! None of us have tuners that provide a perfect ground isolated source like those used in models.
Ground Plane and J-Pole Problems and Patterns
The J-pole is a worse-case situation for examining unwanted feedline radiation. In the case of the J-pole without radials, all of the common mode current created by the poor feed system flows over the coax shield and mast. The mast and coax is generally vertical and as high and clear as the actual thing we think is the antenna, and so unwanted radiation from the feedline is much more apparent than in the low-band Zepp where both feedline and antenna are near earth.
We will use this as a reference. Here is the pattern of a 1/4 wl groundplane antenna without proper feedline decoupling:
In the above model common mode current obviously exists on the feedline. The amount of current is actually significant, almost 1 ampere compared to 2.5 amperes in the antenna (500 watts applied)! A 1/4 wave groundplane with four radials actually needs a balun or some other form of feedline decoupling.
The pattern in freespace of this antenna is:
The pattern distortion is caused by feedline common-mode current. If you think this is bad, imagine what happens with an end-fed antenna (even a 1/2 wave antenna) and NO radials! In that case all of the antenna current flows over the feedline shield! Many antenna designs actually use the feedline and mast radiation that others dismiss as "insignificant" to increase antenna gain. In some cases, the antenna designers really don't even understand what they did to create a "magic" antenna.
Here is the same 1/4 wl groundplane with a feedpoint balun with a common-mode impedance of 3000 j3000:
In this case feedline currents are under .4 amperes, and antenna current increases to 3 amperes. pattern is:
See how much cleaner the pattern is? The pattern is much cleaner and more like what we expect from a groundplane far above earth. We still don't have a good system, because the balun needs a ground on the "unbalanced" side to be effective. (This is why I recommend grounding the unbalanced side of baluns to antenna booms of Yagi antennas. That coil of wire hanging in the air might not be enough without a ground!)
With a perfect balun and optimum de-coupling (a few extra radials below the balun or an extra-ordinary balun), the pattern looks like this:
This is the ideal pattern of a 1/4 wl groundplane, and it takes extraordinary care to obtain this pattern. Many people think they have this pattern, because they model antennas without the feedline attached!
To obtain this pattern I had to:
- Use a good balun just below the radial tip level
- Ground the feedline shortly below the balun to a small groundplane
- Slope the radials downwards more, to a 75 degree angle.
It is easily possible to do this in the real world. Instead, we have numerous articles that ignore the feedline and assume we don't have a problem with common mode current upsetting antenna pattern.
Worse yet, some articles claim the groundplane antenna only needs one or two radials!! Why does this happen? Because the person who made that assumption never modeled the mast or feedline. They eliminated the real-world problem through an omission in the model.
If an antenna model does not include the feedline, losses in the feedline, or imperfections in the source it much more likely than not is a flawed model. The real-world antenna often will be much different if the model ignores feedline loss and feed system source or matching system problems. Short dipoles are one example.
The J-pole and other end-fed Hertz antennas as prime examples of antenna with severe common mode current problems. The coax shield has to be at zero volts potential and have exactly equal and opposite currents flowing into and out of the load and source, otherwise the feedline radiates. Feedline length or weather changes that affect feedline moisture between the outer jacket and the support for the feedline will often change SWR. The severe common-mode feedline problems of end-fed 1/2 wave antennas is why some people swear by them, and other people swear at them.
Here is a zoom of a correct model of a J-pole with a vertical feedline and/or mast attached.
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 with the feedpoint, and the shorter leg connected to the shield. The shield can be connected to any supporting mast with any change in results. Here is the resulting pattern:
The gain is now 2.37 dB at 4 degrees elevation compared to 2.69 dB for the 1/4wl groundplane. This isn't the worse feed arrangement....it is actually the best for the J-pole! Here is the pattern with the feedpoint reversed, the shield is connected to the longer element, and the center conductor to the short element:
Notice the low-angle gain dropped about 5dB with just a simple reverse of feedline connections! If I didn't model the feedline, the model would never show this problem. In ALL cases, the SWR stays near 1:1, yet gain at low angles changes 5dB!
The following model is an I-Max 2000 vertical with a vertical feedline. In this case I picked the worse feedline length:
Feedline current is 100% of antenna current. This illustrates why so many people complain about SWR problems and RF in the shack with end-fed verticals like the I-MAX 2000!
Here is the pattern of an antenna that copies the I-MAX dimensions and feed system:
This is a NEGATIVE gain antenna at low angles. A 1/4wl groundplane would seriously out-talk the I-MAX 2000 or any other 5/8th wl antenna that does not have a large groundplane.
Even if we use the optimum feedline and mast length, here is the very best the end-fed antenna will do:
In this case we now have 2.67 dBi, which is actually a little less than a 1/4wl groundplane will do! The severe common-mode mast and feedline currents make "no-radial" verticals extremely sensitive to mounting height, mounting structure, feedline length, and grounding. This is NOT normal for antennas, it is a sign of a design problem.
PAR Antenna and other End-fed Half-waves
Many antenna designers and experimenters now seem to think we can end-feed a half-wave without having feedline common-mode current. The antenna view below is what a coaxial feedline would do if grounded 3/4 wl away from the end-fed 1/2 wl antenna:
Current tables are shown below. I fixed the power level at 100 watts. I deleted all but minimum and maximums:
EZNEC+ ver. 4.0
end-fed wire
(similar to PAR)
10/18/04 2:45:47 PM
---------------
CURRENT DATA
---------------
Frequency = 28 MHz
Wire No. 1
(antenna):
Segment Conn
Magnitude (A.) Phase
(Deg.)
1 Open .04076 -100.1
20 .75765 -94.07
40 W2E1 .25843 0.00
Wire No. 2 (feedline
shield):
Segment Conn
Magnitude (A.) Phase
(Deg.)
1 W1E2 .18476 -3.66
28 1.0201 -170.1
54 .03185 162.86
80 Ground 1.0156
9.60
Antenna current maximum is .758 amperes, while the coax common mode current maximum will be 1.02 amperes! It is actually possible to have MORE radiating current at the current maximum in the feedline than in the antenna itself.
This is because standing waves on the OUTSIDE of the cable transforms voltage and current. (This has nothing to do with standing waves INSIDE the feedline that an SWR meter measures)
That was worse case. Now here is best case.
EZNEC+ ver. 4.0
end-fed wire
(similar to PAR)
10/18/04 3:04:00 PM
---------------
CURRENT DATA
---------------
Frequency = 28 MHz
Wire No. 1:
Segment Conn
Magnitude (A.) Phase
(Deg.)
1 Open .06048 -95.10
20 1.1232 -90.98
40 W2E1 .18904 0.00
Wire No. 2:
Segment Conn
Magnitude (A.) Phase
(Deg.)
7 .19736 -24.42
31 .02775 -114.8
58 .21915 139.41
80 Open .01282
133.71
Now we see the current maximum in the antenna is 1.12 Amperes, while the radiation current maximum in the feeder is .219 amperes. Clearly the antenna now radiates considerably more power more than the feedline. This is a good thing.
What did I do to "fix" the common mode problem? I opened the ground connection path at the transmitter!
This problem doesn't mean the antenna won't work or is a bad antenna. It does mean that the feedline is a part of the radiating system, and how the feeder is routed and grounded will affect performance.
It is electrically impossible to end-feed a 1/2 wl antenna without a good counterpoise and have the feedline be "cold". A ten-turn choke or current balun at the feedpoint certainly won't be a cure!
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.
Some manufacturers have wised-up.
Cushcraft, in their Ringo-Ranger, eventually added a separate additional groundplane below the antenna to tame the wild common-mode currents of the Ringo. Even that solution is barely acceptable.
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 conventional 1/4-wave groundplane! Instead concentrating your signal towards the neighbor's TV set or an airplane flying overhead. They cause your entire 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!
If you find this page interesting, please pass it along. It's time to get the feedline and tuner into models!
This page has
hits
since creation on
July 31, 2004