J-pole counterpose

[W0BTU]

...
Explanation:
J-Poles are end fed vertical antennas, vertical antennas NEED radials.
Without them, RF will run down the coax. ...

Exactly, 100% Correct.

73, Mike
www.w0btu.com

No, not exactly 100% correct.

1/4 wave verticals need radials.

Have wave verticals DO NOT need radials.

Why do you say that? MW and SW installations using a tower taller than 1/4 wave require an even more extensive ground system than a 1/4 wave vertical. Why should the laws of physics be different at VHF?

73, Mike
www.w0btu.com

[N4CR]

I'd like to see your example of this or some modeling that supports your premise. Do you put radials on your horizontal dipole?

[WB6BYU]

There are several different reasons for using radials that are getting confused here.

First, the use of radials TO COMPLETE THE ANTENNA, as is the case with a quarter wave ground plane.
A J-pole doesn't need those because it is a half wave antenna, even if it is end fed.

Second, radials TO REDUCE GROUND LOSSES.  This is important for ground mounted antennas, especially
those less than half a wave long.    But these are rarely used when a self-contained vertical is mounted
3 wavelengths or more above ground, as would be the case of a 2m J-pole on a 20' mast.

For the two cases above, adding radials to a J-pole would be the same as using them with a elevated
vertical center-fed dipole.  Would you normally do that?


Third, radials TO REDUCE FEEDLINE RADIATION.  This would be the only reason to add radials to a
J-pole.  The problem is that there is no appropriate location on many J-pole designs to put them.
The best place would be 1/4 down the coax from the feedpoint - this forces a high impedance
looking back down the coax, which reduces current flow.  Often a second set is used 1/4 wave
below that for improved decoupling.  Such radials have much less current flowing on them than
you would see with a quarter wave ground plane - their purpose is to change the impedance
at the feedpoint to minimize common mode current.

The nominal "ground" point on a J-pole SHOULD be the center of the bottom shorting bar.  If
the shield of your coax is grounded at that point, adding some radials could reduce the common
mode currents - but that configuration is less prone to them than most designs that hams
build.  Connecting the shield an inch or two up on one side of the stub means that either the
stub isn't balanced, or the shield is going to be hot with RF.  Adding radials at the point where
the shield connects to the antenna with such a design may reduce the current on the coax
(or it might not), but likely will also make the balance WORSE for the matching stub and may
increase currents on the support mast instead.

There is no advantage from a radiation perspective with adding radials to a J-pole.  If you are
doing it to reduce common mode current, you'll likely have better results with less effort just
rebuilding the whole antenna in a more balanced manner.

And certainly before you argue that radials are necessary, you should be clear on WHY you
think so, WHAT you expect them to accomplish, and HOW adding them would actually
achieve the desired result (and what the resulting current distribution on the antenna would
be.)

[W0BTU]

I'd like to see your example of this or some modeling that supports your premise. ...

It's a common misconception that half-wave verticals do not need radials (and it's easy to understand why). Here's an answer, for starters, from ON4UN's book, 4th edition, page 9-50.

4.3. The Radial System for a Half-Wave Vertical
The results of the experiment are shown in Fig 9-61. Let us analyze those results.
    Here comes another surprise. A terrible misconception about voltage-fed [1/2 wave] verticals is that they do not require either a good ground or an extensive radial system.

4.3.1. The near field
     If you measure the current going into the ground at the base of a λ/2 vertical, the current will be very low (theoreti­cally zero). With λ/4 and shorter verticals, the current in the radials increases in value as you get closer to the base of the vertical. That’s why, for a given amount of radial wire, it is better to use many short radials than just a few long ones.
     With voltage-fed antennas [such as a half-wave vertical], however, the earth current will increase as you move away from the vertical. Brown (Ref 7997) calculated that the highest current density exists at approximately 0.35 λ from the base of the voltage-fed λ/2 vertical. Therefore it is even more important to have a good radial system with a voltage-fed antenna such as the voltage­ fed T or a λ/2 vertical. These verticals require longer radials to do their job efficiently compared to current-fed verticals.

This is by no means a complete answer, but I hope it helps clear it up.

[N4CR]

So you extrapolated from ON4UN that all 1/2 wave verticals need radials? We're talking J-Poles here and you've now brought in a scenario that a J-Pole would virtually never be used in.

So I'll revise what I stated.

Elevated 1/2 wave verticals don't need radials.

[W0BTU]

So you extrapolated from ON4UN that all 1/2 wave verticals need radials? We're talking J-Poles here and you've now brought in a scenario that a J-Pole would virtually never be used in.

So I'll revise what I stated.

Elevated 1/2 wave verticals don't need radials.

I agree that we can make contacts using elevated half-wave verticals without radials. I'm not trying to pick a fight with you, but please check out these pages, and then let us know what you think.

http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm
http://www.w8ji.com/end-fed_vertical.htm

Respectfully,
Mike

[W8JI]

So you extrapolated from ON4UN that all 1/2 wave verticals need radials? We're talking J-Poles here and you've now brought in a scenario that a J-Pole would virtually never be used in.

So I'll revise what I stated.

Elevated 1/2 wave verticals don't need radials.

You can go on record saying that, but it is an absolutely incorrect statement. That statement implies an antenna of a certain specific length precludes ever having common mode current in the feed system, regardless of the feed system arrangement.

Hand-in-hand with ideas like that are the misplaced notions that half-wave dipoles fed with unbalanced feedlines don't ever require baluns because they are "balanced antennas".

I've seen some otherwise very competent engineers fall into traps like that, because they never took time to think about the feed system of various antennas and how the feed system really works. Even Sevik, in Baluns and Ununs, obviously misunderstood the feed system of a dipole and why a balun might be needed.

This is a misunderstanding that pops up more frequently than it should. Worse yet when the problem pops up, it seems to appear with undeserved final authority.

Here are some detailed descriptions on how it really works, and some pages that include the "models" requested:

http://www.w8ji.com/end-fed_vertical.htm

http://www.w8ji.com/end-fed_vertical_j-pole_and_horizontal_zepp.htm

http://www.w8ji.com/end-fed_1_2_wave_matching_system_end%20feed.htm

http://www.w8ji.com/ground_plane_verticals.htm

http://www.w8ji.com/verticals_and_baluns.htm

http://www.w8ji.com/balun_single_core_41_analysis.htm


73 Tom

[W8JI]

There are several different reasons for using radials that are getting confused here.

First, the use of radials TO COMPLETE THE ANTENNA, as is the case with a quarter wave ground plane.
A J-pole doesn't need those because it is a half wave antenna, even if it is end fed.

Second, radials TO REDUCE GROUND LOSSES.  This is important for ground mounted antennas, especially
those less than half a wave long.    But these are rarely used when a self-contained vertical is mounted
3 wavelengths or more above ground, as would be the case of a 2m J-pole on a 20' mast.

The problem with that statement is there are MORE than two reasons to have radials. One additional reason is to establish a reference point of a low or modest impedance to "push against" or act as an electrical charge reserve, where charges can be pushed or pulled into or out of with minimal electric field gradient.

For the two cases above, adding radials to a J-pole would be the same as using them with a elevated
vertical center-fed dipole.  Would you normally do that?

Because of the other reason I just listed.

Third, radials TO REDUCE FEEDLINE RADIATION.  This would be the only reason to add radials to a
J-pole.  The problem is that there is no appropriate location on many J-pole designs to put them.
The best place would be 1/4 down the coax from the feedpoint - this forces a high impedance
looking back down the coax, which reduces current flow.  Often a second set is used 1/4 wave
below that for improved decoupling.  Such radials have much less current flowing on them than
you would see with a quarter wave ground plane - their purpose is to change the impedance
at the feedpoint to minimize common mode current.

Cushcraft got smart and did that with the Ringo and Ringo Ranger, curing an old problem that caused inconsistent performance with the Ringo. The Ringo was an end-fed half-wave.

The nominal "ground" point on a J-pole SHOULD be the center of the bottom shorting bar.  If
the shield of your coax is grounded at that point, adding some radials could reduce the common
mode currents - but that configuration is less prone to them than most designs that hams
build.  Connecting the shield an inch or two up on one side of the stub means that either the
stub isn't balanced, or the shield is going to be hot with RF.  Adding radials at the point where
the shield connects to the antenna with such a design may reduce the current on the coax
(or it might not), but likely will also make the balance WORSE for the matching stub and may
increase currents on the support mast instead.

There are multiple problems with the J-pole design:

First, it end feeds an antenna without a suitable counterpoise or "ground reference" in the system.

Second, it has two improperly treated balanced to unbalanced transitions. An unbalanced antenna is fed from a balanced stub, and the balanced stub is fed from an unbalanced feedline.

It takes some planning and thought to fix the system, because leaving any single problem uncorrected can undo a necessary correction added in another place.

There is no advantage from a radiation perspective with adding radials to a J-pole.  If you are
doing it to reduce common mode current, you'll likely have better results with less effort just
rebuilding the whole antenna in a more balanced manner.

I agree there is minimal radiation advantage, but there can be a large advantage is reducing common mode on the mast or feedline. As a matter of fact with certain coax connection methods used in some J-pole designs, radials will pretty much cure all of the problem. We have to be specific which J-pole feed system we are talking about.

It is very unlikely we can cure the common mode problems of a J-pole with a feedline decoupling device. The major problem is anytime we have an end-fed antenna we need a counterpoise of some sort to allow us to push and pull charges into and out of the end of the antenna without creating common mode currents on the mast or feedline.

Unless we cure that, we will still have a system sensitive to mast and feedline common mode impedance. The only way to cure that is to provide a counterpoise with a stable low common mode impedance. By definition that would be a 1/4 wave open ended decoupling sleeve around the mast and feed system or radial system. Since the sleeve radiation field phase would oppose the antenna field at low angles, it would be better to use the radials. Otherwise we suppress the low angle signal.

These technical shortfalls of the J-pole are why it never fell into favor commercially, and why it's life has primarily been confined to amateur use for a simple homebrew antenna. In amateur use being able to make a contact and having a low SWR is more important than having a consistent near-optimum results.

Study the history of the Ringo antenna by Cushcraft, and you can see they learned their lesson a little late in life.....but they learned it. They added radials.

73 Tom

[N1LO]

Hi Jeff,

To finally address your masting question, PVC is a poor choice. It's heavy and bends far too easily for its size and weight.

Two chain link fence top rail sections make excellent portable masting for light duty.

For a drive-on mast base,  cut a 24" length of 2x10 lumber and attach a 1-1/4" pipe deck flange with 3/8 lag screws near one end. Screw in a 12" long, 1-1/4" pipe nipple and park your car on the works. Use wood shims for levelling.

20' of top rail and a small antenna are easy to stand up and will drop right in to the base, with just a little slop.

You can also lash a telescoping fishing pole to that. It'd be real easy to make a ground plane antenna right at the top of the pole. Sure would be a lot lighter and higher than a J-pole, especially a copper one!

For a lot more money, you can buy 33' or taller telescoping fiberglass masts. These will not support a jpole at the top, but will easily support a ground plane antenna made from 16 ga solid wire.


Good Luck,

- - · · ·  M A R K · N 1 L O · · · - -

[N3OX]

The current is zero at the antenna end of the matching section and it is zero at the end of the antenna, the feed point.  (As an aside, how hard do you have to push to push nothing into nothing?  Not very much.)  If the current at the end of the matching section is zero, and it is, why would it make any difference how you connected the center conductor of the coax?  The current is still zero regardless of how you hook the coax up.  

If the current is ZERO at the feedpoint then the power delivered to the antenna is zero....

The current at the feed of a J-pole may be milliamps, but it's not zero.

Those millamps have to flow into something else, and if that "something else" is near a high voltage, low current point of a multi-wavelength long feedline, that will set up strong currents further away.

The conduction current at the open end of an antenna element is necessarily zero but that doesn't mean the same is true when you make a direct connection to that point on the antenna using it as a feedpoint.  I don't really want to get started on displacement currents from the changing electric field and how they come into play near the ends of antennas, but you could have common mode current problems with a feed system that doesn't even touch the bottom end of the half wave element, but just capacitively couples to it.

This is basic physics.  To quote Richard Feynman (he won the Nobel Prize in physics):  "If you don't like it, you have to go to a universe where the rules are different."

 

[WB6BYU]

Study the history of the Ringo antenna by Cushcraft, and you can see they learned their lesson a little late in life.....but they learned it. They added radials.

(Technically they added radials to the ARX-2B Ringo Ranger II, but not to the AR-2 Ringo or the
ARX-2 Ringo Ranger, at least according to the manuals available on their web site.)

Indeed, after AEA demonstrated the problems with common mode currents on an end-fed design and
introduced their IsoPole, Cushcraft added a set of radials, but NOT at the feedpoint.  The radials are
connected to the feedline and mast 50" below the feedpoint, or about 5/8 wavelengths.  I presume
this distance was chosen so that the coax/mast down to the radials acts as an additional radiator
section, with current in phase with that from the top portion of the radiator.  It could, however,
be simply the length they found gave the best decoupling.


In the end I still think that, when common mode currents are a problem, the best solution is not
to add radials to some point on a J-pole, but to replace the J-pole with an antenna that is not as
prone to common mode currents in the first place.  That has a better chance of maximizing low
angle radiation while minimizing common mode currents.

[W8JI]

(Technically they added radials to the ARX-2B Ringo Ranger II, but not to the AR-2 Ringo or the
ARX-2 Ringo Ranger, at least according to the manuals available on their web site.)

Indeed, after AEA demonstrated the problems with common mode currents on an end-fed design and
introduced their IsoPole, Cushcraft added a set of radials, but NOT at the feedpoint.  The radials are
connected to the feedline and mast 50" below the feedpoint, or about 5/8 wavelengths.  I presume
this distance was chosen so that the coax/mast down to the radials acts as an additional radiator
section, with current in phase with that from the top portion of the radiator.  It could, however,
be simply the length they found gave the best decoupling.

It was to add gain and decouple the mast currents.

In the end I still think that, when common mode currents are a problem, the best solution is not
to add radials to some point on a J-pole, but to replace the J-pole with an antenna that is not as
prone to common mode currents in the first place.  That has a better chance of maximizing low
angle radiation while minimizing common mode currents.

Then what you are saying is the J-pole should never be used.

What I am saying is to fix the J-pole so it does not excessively excite the mast or feedline, we have to add radials or some other low impedance counterpoise. If we do not provide something to push against so we can force current into the several hundred ohms of end-impedance a thick 1/2 wave element would have, we will have common mode problems.

73 Tom

[AA4PB]

As OX said, P = I * E so if I = 0 then P must = 0 regardless of how much E is present. You can't have power without both voltage and current. Therefore, an end-fed 1/2 wave antenna ALWAYS has some amount of current flowing in the ground side. If you don't provide radials or some other type of counterpoise then it flows down the outside of the coax shield, the mast, etc.

That doesn't mean that an end-fed 1/2 without radials doesn't work. It does mean that the coax shield, metal mounting mast, and any other conductor attached to it are acting as part of the antenna. Since these items are generally uncontrolled, it is often better to use a counterpoise designed for the purpose like a set of 1/4 wave radials.

For VHF/UHF there are better antenna designs around than the J-Pole, in my opinion. An ordinary ground plane with a 1/4 wave vertical radiator and at least four 1/4 wave radials comes to mind.

[WB6BYU]

Then what you are saying is the J-pole should never be used.

Not necessarily - common mode currents aren't a problem for many operators.  This may be because they
happened to use coax and/or mast lengths that discourage them, or perhaps they just don't realize that
they have common mode currents, or don't worry about it if they do.  I've known many hams who put a
less-than-optimum antenna on a 25' mast and it is so much better than a rubber duck that they leave it
up for years, assuming it must be working properly, even if it isn't.  If the antenna works well enough for
their needs, fine.

But if one sets a goal of minimizing common mode currents, the common "Copper Cactus" style J-pole
with the shield connected above the shorting bar (so it is not at the same RF voltage as the mast)
is probably not the best place to start.  That is where I'd suggest a good first step is to replace
the J-pole.  I'm sure it is possible to build a J-pole with minimal common mode currents and good
decoupling for the mast and coax, but, as you have stated, it requires an understanding of the
issues involved, and the more one studies the actual behavior and current flow, the less attractive
the antenna becomes (except as a challenge, perhaps.)

[W8JI]

Then what you are saying is the J-pole should never be used.

Not necessarily - common mode currents aren't a problem for many operators.  This may be because they
happened to use coax and/or mast lengths that discourage them, or perhaps they just don't realize that
they have common mode currents, or don't worry about it if they do.  I've known many hams who put a
less-than-optimum antenna on a 25' mast and it is so much better than a rubber duck that they leave it
up for years, assuming it must be working properly, even if it isn't.  If the antenna works well enough for
their needs, fine.

But if one sets a goal of minimizing common mode currents, the common "Copper Cactus" style J-pole
with the shield connected above the shorting bar (so it is not at the same RF voltage as the mast)
is probably not the best place to start.  That is where I'd suggest a good first step is to replace
the J-pole.  I'm sure it is possible to build a J-pole with minimal common mode currents and good
decoupling for the mast and coax, but, as you have stated, it requires an understanding of the
issues involved, and the more one studies the actual behavior and current flow, the less attractive
the antenna becomes (except as a challenge, perhaps.)

We all know things that don't work as well as they should or could still make people happy. Some pretty crummy systems make people grin from ear to ear so long as the SWR is low and they "get out".

I'm only pointing out how to fix the antenna so it:

1.) Has more gain than a 1/4 wave groundplane

2.) Is consistent in perfromace regardless of mast and feedline length

Personally, I would never feed a J-pole by tapping the coax up on the stub. I would feed with the shield connected to the short element and the center connected to the long element, and I would add some decoupling radials.  To adjust impedance I would vary element spacing. To adjust resonant frequency, the element lengths.

The most important thing though is to dispell the silly myth that an end-fed half-wave antenna needs no ground. If it really had NO ground, it would have no signal. It just uses the feedline or mast as a ground if one is not provided at the antenna feedpoint.

73 Tom