J-pole counterpose

[N4CR]

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

So, how did the zeppelins transmit radio signals after they left the ground? I don't believe they had radials or ground.

[N3OX]

So, how did the zeppelins transmit radio signals after they left the ground? I don't believe they had radials or ground.

Two comments.

1) If you attach your zepp to an actual zeppelin, the metal bits of the zeppelin are "ground".  Ground is not the earth, and it's not only radials.   We should probably stop using the word in certain circumstances.   But the fact of the matter is that antenna feedpoints have to shove charge on and off stuff, period.

2) It is possible to use a J-pole without any external connections at all if you have a tiny floating source.   Hook a dime sized oscillator with a button battery to a J-pole feedpoint, put the whole thing on a fiberglass pole, and there you go... good radiator with no stray currents.   A J-pole with a perfect common mode choke would work too.  If there's nowhere at ALL for it to go, charge will pile up on the stub.   Because of that, I can be convinced in a semantic sense that a "J-pole will work with no ground if you have a floating source."  In an operational sense, the stub is acting as the "ground" or "counterpoise."    It's a really high impedance one.

No real antenna has nothing attached to the stub, and almost anything would provide an easier path for current flow than piling up charge on the stub, including coax with a real-world choke.   Some current is going to flow on the thing you attach there.  That's the real issue.  It wouldn't actually be much of an issue on an airship.  Letting small stray currents go all over the place change the pattern, but it's all way up there in the clear.  That's a lot different from a VHF antenna where you're trying to get the thing up high on the mast to do all the radiating and a bunch of the coax is in places you don't want your antenna to be, like near the ground, or snaking behind the TV.

I think we all need to look very hard at what Tom said otherwise, especially #2 here:

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

I had a J-pole too when I was starting out.  One of the first antennas I built.    It worked "great".     But I might as well be saying "blah blah blah, blah blah blah blah blah" compared to what Tom is trying to say.  For all I know I got lucky with feedline length.  Maybe it would have worked even better if I'd done something differently.

Consistent, predictable operation is important.  That's why you might want radials on this type of antenna.  It is true that in a special artificial situation you could get it to "work" with no explicit connection to another metal thing, but that is inconsistent with how you actually need to hook the thing up.    To absolutely settle the matter of whether or not there's significant current on the coax in any particular J-pole setup, one could just go ahead and measure the current on the coax with a current meter.  Or you can assume there is and take some steps to reduce it.  Or you can ignore it because you really only use a couple of repeaters and they've got great voter receivers that render them practically telepathic.    But I see little point in arguing with someone giving good advice on how to improve a stub-matched half-wave antenna design.

[N4CR]

So far, all the support for 1/2 wave antennas needing a ground plane, the statement which I disagreed with, revolve around end fed or voltage fed antennas.

A half wave vertical dipole fed in the middle is an example of a 1/2 wave vertical antenna that doesn't need a ground plane.

Anyway, that was the point I was trying to make.

The blanket statement I disagreed with was:

...
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

The blanket statement that all half wave vertical antennas need radials is not true. Perhaps true if they are end fed, but certainly not true if they are center fed.

[AA4PB]

If I connect the negative end of a 1.5V battery to a ground rod and the positive end to a flashlight bulb and there is no other connection to the bulb, will the bulb light? No, because there is no complete path for current flow.

The same thing is true of an end-fed (1/2 wave or any other length) antenna. There must be someplace (even if coax shield, radio case, or the unterminated 1/4 wire feed of a Zepp) for the current to flow or the antenna will radiate zero signal.

[N4CR]

If I connect the negative end of a 1.5V battery to a ground rod and the positive end to a flashlight bulb and there is no other connection to the bulb, will the bulb light? No, because there is no complete path for current flow.

The same thing is true of an end-fed (1/2 wave or any other length) antenna. There must be someplace (even if coax shield, radio case, or the unterminated 1/4 wire feed of a Zepp) for the current to flow or the antenna will radiate zero signal.

Because you have created a capacitor, yes, some current does momentarily flow. It's just not enough to light a light bulb.

[W8JI]

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

So, how did the zeppelins transmit radio signals after they left the ground? I don't believe they had radials or ground.

They had a counterpoise.

Ironically some people want to dispense with the word "counterpoise", but here is a prime example of why we should use that word.

We always, without exception, must have the same current flowing into a ground or counterpoise of some sort that flows up into the antenna. That current can be transformed by standing waves to a much higher or much lower value elsewhere in the system. So even if the current exciting the shield of coax, or the top of a mast, is just a few milliamperes it can be hundreds of milliamperes elsewhere down the mast or coax.

Also a choking device cab be virtually impossible to build if it is placed at a voltage maximum and current minimum.

The radials or counterpoise simply established a low common mode impedance, and makes the system repeatable and predictable. Without is, even with the best choke in the world, common mode current can "leak" past the decoupling system.

Some multiple section antennas actually work by this principle. They have a 1/4 wave open sleeve skirt. Since the open end impedance of that skirt is finite due to losses and the voltage peaks at the open end of the skirt, it excites the next 1/4 wave skirt and 1/4 wave "radiator section" down. This continues downstream toward the transmitter until eventually some radials are installed to clamp the voltage steady at one point (or if you prefer, act like a current sink).

This is an age old problem, and an age old misunderstanding by casual antenna builders (and some professionals).

Here's another thing that is commonly misunderstood. People think the end-impedance or end-current of a half wave is zero. It is not, because the conductor is not infinitely thin. The open end still has surface. If it is end fed, the end toward the feedpoint has a even more limited impedance because of losses (from displacement currents, radiation, and ohmic losses) throughout the conductor. 

The feed impedance of a 1/2 wave two-meter antenna 1/2 inch in diameter is only 577 ohms. Current is about .4 amperes at 100 watts. I really can't understand why people cling to the misplaced notion that the end-impedance of a thick 1/2 wave resonant antenna is thousands of ohms, let alone infinite. It is no small wonder, when we get real, that end-fed antennas cause common mode current problems. 

The impedance would only be infinite if the antenna were lossless and infinitely thin. At which point it would not exist in the real world.

73 Tom

[N3OX]

The feed impedance of a 1/2 wave two-meter antenna 1/2 inch in diameter is only 577 ohms. Current is about .4 amperes at 100 watts. I really can't understand why people cling to the misplaced notion that the end-impedance of a thick 1/2 wave resonant antenna is thousands of ohms, let alone infinite.

I think part of it is because some people think about the ends of an actual half-wave dipole and know they must have zero conduction current... Maybe they think that's something particular to a half wave as opposed to something particular to an open end?

The blanket statement that all half wave vertical antennas need radials is not true. Perhaps true if they are end fed, but certainly not true if they are center fed.

The context of this thread was half wave end feds, was it not?
 

[N4CR]

The context of this thread was half wave end feds, was it not?

Sure was.

Now, how long will a new ham remember that association even if it was made? i guarantee that there are new hams reading this thread.

Blanket statements like that to a new antenna user tend to become what we call old wives tales if not corrected in real time when they happen. They need clarification because at face value and out of context, they are false. And at face value and out of context is where we will hear from them next year.

Ham knowledge is, to a great extent, passed on between us. It needs to be passed on in a way that doesn't easily fall down. That was really my point, not that something was stated that was so wrong in context. I certainly am guilty of not expressing well.

[KI4SDY]

The next time I go past an airport I will check with binoculars (and hope that I don't get arrested by the TSA) and see for sure, but I think the antenna that hams are seeing at the airports that look like J poles are Arrow Antennas made for the aircraft bands. They are not true J poles as the radiators are insulated from the rest of the antenna. They have been called ZEP antennas and ground plane antennas by our local "experts," but I will let the eHam.net "experts" argue about what it is. All I know is that it is rugged and works great for $39.00! But, so does my home made ground plane that cost me less that $5.00 to make and the SWR is so low I can't even measure it on the frequency I mainly use.  

[W0BTU]

The context of this thread was half wave end feds, was it not?

Sure was.
...

Half wave end feds were in the context of this thread. But the thread title is "J-Pole counterpoise", isn't it?

And thanks, Tom and Dan, for clearing this up.