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Author Topic: Counterpoise and radials  (Read 2368 times)
2E0ESW
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Posts: 27




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« on: June 16, 2005, 12:35:28 AM »

Dear Elmers,

I known the need, why and wherefore's of counterpoises and radials so this is more a definition, correct terminology and somewhat academic question as I am in the final preparations for my advanced (your extra)exams and the question came up in previous papers (multi choice aka "monkey puzzle" paper so you HAVE to choose the correct one). Elmer and google searches were no help at all as the 2 are used by some as synonyms.
Question; Am I correct, radials(ground or elevated) are the "other half" of the antenna (vertical or otherwise)if one works the antenna "against ground or groundplane" thus ground itself is the other half of the antenna, whereas a counterpoise is literally the other half of the antenna just draped on the floor or ground (or in the air) if one works the antenna "against it's other half literally and the ground forms NO PART of the antenna although , like with all antennas below a wavelenght above ground, the ground (according to it's conductivity, slope etc.etc. will always have some influence on the antenna. BUT with radials as above on it's efficiency and with counterpoise as above on it's radiation pattern(inclusive of angle of radiation).

Please is this correct? With the greatest of respect to all, please do not confuse the issue further, give me the correct theory or nomenclature not opnions or "as I see it", please.

Thank you in advance.
73
Etienne Swanepoel 2E0ESW
Bude
Cornwall
UK
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AA4PB
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« Reply #1 on: June 16, 2005, 04:00:23 AM »

I don't know that there is an "official" definition. The terms are often used interchangeably. Some of the old ARRL Handbooks define "radials" as a system of wires either laying directly on or burried in the Earth and a "counterpoise" as a system of wires above and insulated from the Earth but capacitivly coupled to it. Both "radials" and a "counterpoise" serve the exact same purpose, to effectivly be the other half of the antenna by providing a path for current flow.
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2E0ESW
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Posts: 27




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« Reply #2 on: June 16, 2005, 04:21:17 AM »

Just to illustrate the dilemma:
Question in paper of 2001: What effects the efficiency of an antenna
a.radials
b.power
c.modulation
d.couterpoise

Question paper 2003:
What effects the radiation pattern of an antenna
a. radials
b. power
c. counterpoise
d. modulation

Hope this ilustrates my need to understand, albeit a question of "questionable purpose" but examiners do want to have fun and stop anybody scoring 100% !!!

Regards
73
Etienne Swanepoel 2E0ESW
Bude
Cornwall
UK
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KZ1X
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Posts: 3229




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« Reply #3 on: June 16, 2005, 04:53:45 AM »

Here are your questions:

Question in paper of 2001: What effects the efficiency of an antenna
a.radials
b.power
c.modulation
d.couterpoise

Question paper 2003:
What effects the radiation pattern of an antenna
a. radials
b. power
c. counterpoise
d. modulation


Hard to answer:  what kind of antenna?  These aren't very well written questions.  You can't assume they mean anything other than what's written.  My yagi (no "radials") at 24 meters sees the earth as a counterpoise, which affects the gain and pattern, so the answer to both questions is "counterpoise."  If I lower the antenna, it sure affects the pattern!  That's why people have stacked arrays.

Right?

I only offer my own definitions, used when explaining the phenomena.  They work for me.

Radials are an active part of an antenna design which employs them (primary determinants of resonance and impedance) and a counterpoise is passively part of the antenna.  

The counterpoise is the mirror field of the EM from the radiating element(s), and in part determines efficiency and pattern.  Even without "radials" you always have a counterpoise, that's why there's NEC-2 (and -4 and etc.) because you can't ignore it.

Does this help?
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K5DVW
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« Reply #4 on: June 16, 2005, 05:18:30 AM »

Ummm, well, a counterpoise is just a single radial. Electrically, from the feedpoint of the antenna, it does the same thing as radials, provides the other "half" of the antenna. The main point to take away is that a true radial field is generally more efficient than a single counterpoise USED AT GROUND LEVEL, and a counterpoise being only a single wire can affect the radiation pattern. Radials are supposed to be symmetric, so their fields cancel and don't affect the radiation pattern.

I definately agree those are poorly written questions. But, I think the proper answer to Q1 is A, and the proper answer to Q2 is C. Do I pass?
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AA4PB
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« Reply #5 on: June 16, 2005, 06:11:05 AM »

Here's a definition from a U.S. Patent Office document.

ANTENNA COUNTERPOISE

Structure of conductive material most closely associated with ground but insulated from or capacitively coupled to the natural ground, and aiding in the function of the natural ground, particularly where variations or limitations of the characteristics of the natural ground interfere with its proper function, and such structure being connected to the terminal of the signal receiver or source opposing the active antenna terminal.


While hams often use a single insulated wire as a counterpoise, a counterpoise can and often does consist of a much more elaborate system than a single conductor. Some broadcast stations use a countepoise consisting of a system of multiple wires suspended on short poles at the base of the antenna but high enough that you can walk under them.

In regard to the specific questions, they sound like the person who made them up had his own idea of the differences between a counterpoise and radials unless he intended for there to be two correct answers to the questions.
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W9OY
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« Reply #6 on: June 16, 2005, 06:16:59 AM »

From Google

"counterpoise:
The second half of an antenna, which "balances" the radiating element to launch the RF wave. Although not always readily apparent, a counterpoise always exists and its suitability can greatly affect the efficiency of the antenna. A ground plane, as found on ground plane antennas, is one type of counterpoise, as is the outer case or circuit board of a receiver using an attached whip antenna."


Radials can effect the directionality of an antenna.  If you have for example 1 radial such that the antenna forms an inverted L, and measure the field strength of the antenna you will not get a circular plot of the field strength, but an oval.  This is why you try to make a radial system symetric and circular.  You can easily show this to your self in EZnec

73  W9OY

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W9OY
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« Reply #7 on: June 16, 2005, 06:25:16 AM »

this is the search term for google

define:counterpoise
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KA5N
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« Reply #8 on: June 16, 2005, 06:27:48 AM »

How about communicating with the RSGB and get definitions of "counterpoise" and "radials" and maybe the answers to your questions.  The Queen's English and USA English are often quite different (such as "earthed" and "screened" for grounded and shielded).
Allen
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N1OD
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Posts: 56




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« Reply #9 on: June 16, 2005, 06:53:20 AM »

Oh boy, here we go.

This is how I would answer the questions, and why.

I believe the answer they are looking for in the first question is "counterpoise".  The quality of the RF ground will affect the efficiency of the antenna, therefore the counterpoise, as the RF ground system, is the better answer.

In the second question, I think they are looking for "radials".  The positioning of individual radials will affect the pattern of the antenna.

The questions are worded somewhat ambiguously, but that isn't surprising.

I wouldn't manipulate a counterpoise system to change the pattern of an antenna.  The counterpoise is there to provide a low impedance RF ground.  You would install it to achieve the best ground possible.

Radials are likewise "the other half of the antenna", but they are more appropriately manipulated to change the antenna's pattern.

Thank you for allowing me to add to the confusion on this matter.

Paul
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K5DVW
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Posts: 2193




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« Reply #10 on: June 16, 2005, 07:15:59 AM »

Heh, interesting comments... I'll add some more indisputable antenna facts here...

I wouldnt expect any kind of radial manipulation to cause a change in antenna pattern, especially for ground mounted radials. Take the situation where you have an HF ground mounted vertical monopole, a classic case that needs radials. Look at the efficiency of using 4 radials vs 100 radials... huge difference. Massively huge difference, like 10 dB or so. Hence, radials, or rather the number or elevation of them can and do change efficiency. The pattern of the same antenna is the same whether you use 2 radials, or 200.

A counterpoise will change the pattern due to the fact that with a counterpoise it radiates along its length and doesnt have any symmetry to cancel out its radiation, like radials would. Move the counterpoise, you change the pattern, but not necessarily the efficiency.

The question should have included an example!

That's my story and I'm sticking to it. Smiley
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K5DVW
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« Reply #11 on: June 16, 2005, 07:17:01 AM »

By the way, one radial IS the same as a counterpoise.
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N1OD
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« Reply #12 on: June 16, 2005, 08:32:14 AM »

I'm developing a better appreciation for the dilemma here.

If you look at the marketing blurb for GAP antennas, the manufacturer makes a point of saying that they do not require radials.  Their antennas utilize "counterpoises", eliminating the need for radials.

Their antennas have three or four elements that they define as counterpoises for the purpose of providing a ground plane.  It is obvious that they define radials as a bunch of wires that you would lay out on the ground or bury.

However, plenty of vertical antennas come with elements that GAP would describe as "counterpoises", but the manufacturer instead refers to as radials.

At one time, Atlas mobile antennas came with the cautionary advice, "Be sure to provide a good counterpoise".  They considered the metal vehicle body as meeting this requirement.

When I answered the question I based my answers on the idea that "radials" referred to horizontal (or sloped) elements of a vertical antenna, and that they mounted well above ground level  (i.e. at the top of a mast, like the typical 10 meter vertical antenna).

I defined counterpoise as a system of wires used to provide the ground plane, or RF ground.

I did not consider "counterpoise" to refer to a single wire.  I do not believe that counterpoise must refer to a single wire.

If you remove one or two of the "radials" on the typical 10 meter vertical antenna referenced above, you will change the radiation pattern.  Antenna modeling programs verify this conclusively.  

Obviously, if you strictly define radials as elements of an antenna lying on, mounted close to, or buried beneath the ground, then my answer will be incorrect.

So we are back to the original question, "What does the person who created the test consider radials and a counterpoise to be?"

When you take the test, please let us know what the correct answer ended up being.

Thanks.

Paul N1OD
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2E0ESW
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« Reply #13 on: June 16, 2005, 08:42:12 AM »

Thanks all so far,

Some light does emerge from it all!. Thanks for the references and I like comment "one (1) radial is a counterpoise" -- Wo'nt help me in the monkey puzzle though.
73
Etienne Swanepoel 2E0ESW
Bude
Cornwall
UK
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K7UNZ
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Posts: 691




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« Reply #14 on: June 16, 2005, 10:02:53 AM »

Sounds like the question writers had some spare time on their hands (hi)....

Actually, there is another type of counterpoise, used "in" the shack, which would be the wire thrown across the living room floor that you mentioned.

This is simply a quarter wave length of wire, connected to your ground in the shack.  Usually the ground terminal of your antenna tuner, ground strip, whatever you use for common equipment ground.  This is used to tame stray RF within the shack, by making the system think it has "ground" right there at the end of the wire.  That's why it needs to a quarter wave at the frequency (band) you're getting the RF problem.

Now, a lot of people are going to tell you that a good, well installed, antenna will not cause RF in the shack, and they're right.  However, it does happen (i.e.-changes between wet and dry seasons) and when it does, a simple sounterpoise can make a big difference.

Frankly, with questions like you mentioned I'm glad I'm not the one taking the test....(hi)!

73, Jim/k7unz
 
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