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Author Topic: Why high voltage at antenna ends?  (Read 19464 times)
KJ4RQV
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« on: February 20, 2010, 03:13:19 PM »

I really don't know how to frame this question so it makes sense, I'm afraid. As I study for my General ticket and as I read about antennas for HF, I have come to understand that the ends of a dipole can/will have a high voltage and are dangerous.  I have read the ARRL Wire Antenna book and this is emphasized that even with a low transmitting wattage there is a potential problem. How does the high voltage get to the ends? Is the danger as great at the center of a dipole? If not, what happens as the energy travels along the wire? I am missing some knowledge and I would really like to understand this subject.
Any help will be appreciated, Don
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N3OX
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« Reply #1 on: February 20, 2010, 03:49:28 PM »

How does the high voltage get to the ends?

In short, the electromagnetic field set up around the antenna tend to shove a high density of electrons to one end or the other at different parts of the RF cycle. 

In the middle of the antenna, the electrons can easily travel further down the antenna, but when they reach the antenna ends and the electric field starts getting strong, they pile up there. 

As far as the DANGER goes, something you need to know is that RF tends to travel on the outside of your body when you come in contact with it.  The conductivity of your body tends to exclude currents from traveling straight through you.

This is called skin effect, but it's named for the effect in other conductors, not the effect on people :-)  Electromagnetic fields can't penetrate far into a good conductor.  A perfect conductor would exclude them entirely.  So the current density in a conductor decays exponentially as you go in from the outside.

This means that bridging hundreds of volts of RF is much less serious (i.e. not fatal), where bridging several hundred volts of 60Hz or DC can kill you dead, while the same RF voltage will probably just hurt and maybe give you a nasty black spot where you get a "RF burn."  I read some cases of people who came into contact with quite high voltage RF and got a serious full body RF burn with some lingering health effects... but there don't seem to be many cases of fatal RF electrocution even from very high power transmitters.  You don't want to mess around but it's not like you're playing with 1kVDC...

Quote
Is the danger as great at the center of a dipole?

Nope, not even close.  I know it's weird for the voltage between the two dipole halves to depend on where you are on the dipole, but it's because of the way the electromagnetic field is set up around the antenna.  Near the middle, there's a lot of magnetic field encircling the wire.  At the ends, there's a strong electric field that "starts" on one end and goes to the other, if you like thinking about field lines.

Of course, at EVERY point on the dipole there's some electric field ending on the wire and some magnetic field encircling it, but at the middle, there's a lot more magnetic field from a high current flowing in the wire, and much weaker electric field.  So the voltage from one dipole half to the other at the feedpoint is low compared to the ends... a hundred volts peak (70.7V RMS) at 100W into a 50 ohm load.   

It is true that coming into contact with antenna tips even at low power isn't nice.  I got my first RF burn when I was a ham... running 4W of CB power into a half square I built.  I touched the end of that and it really stung.

The nastiest RF burn I got was off a 10m matching network on a random wire I was adjusting with 30-40W or so.  That sucker made a tiny hard black scorched spot on my skin  that lasted for several weeks.  The problem with high voltage RF or lower frequency, of course, is that it easily ARCS to you as you pull away.  And at high power arcing to trees or whatever can pretty easily start fires.

So don't mess around, but also don't think that you can casually kill yourself with a low power transmitter's output and the voltage step up inherent in a dipole antenna.  It's very high voltage, but it's not like working inside an amplifier or contacting high voltage power lines, just because the current can't get inside your body easily.

73
Dan


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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
AA4PB
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« Reply #2 on: February 20, 2010, 04:43:58 PM »

I like to think of it in terms of impedance. The center of a 1/2 wave dipole is its lowest impedance point so for a given power (P = I * E) it will have the lowest voltage but the highest current. The ends of a dipole are its highest impedance point so for the same power it will have the highest voltage and the lowest current.
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Bob  AA4PB
Garrisonville, VA
W8JI
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« Reply #3 on: February 21, 2010, 01:37:15 PM »

There are several equally good ways to look at this.

The ends have nothing terminating them. At the open ends, antenna current has nothing to flow into except a very small amount of capacitance. Since the antenna is also a single wire transmission line, the surge impedance of the antenna transforms the very high impedance at the open ends down to a low impedance 1/4 wave away from the ends. In other words the antenna acts like an open ended single wire transmission line.

Another more esoteric way to look at it is with standing waves. An open circuit inverts current 180 degrees. An open circuit does not invert voltage. Because current is inverted 180 degrees, the reflection from the open end "cancels" the current. Because the voltage is not inverted at the open ends, the voltage goes very high. It is only limited by the surge impedance of the single wire trnamssion line making up the antenna and the capacitance and electric field boundary at the open end. 1/4 wave away the voltage is out of phase, and the current is in phase. This makes current high 1/4 wave in from the open end.

It is this voltage distribution that sets up the induction fields around the antenna. The electric field is very high at the ends because of the standing waves on the antenna, and because there is a very sharp boundary between the antenna end and space around the antenna. The magnetic field is stronger near high current areas. Radiation, a totally different effect, is caused by the charges accelerating and so is more intense near the high current areas of the antenna where there are many more charges "wiggling".

The fields are actually created by what is going on in the wire, not the other way around.

The most simple concept is that the open circuit ends cannot support much current because the impedance is very high, and for a given power that means voltage is very high compared to current. Since the "load" is the open end of the antenna, and since it acts like a transmission line, the impedance is progressively lower until 1/4 wave back from the open end.

If it is a closed loop like a dipole, you simply look at what happens exactly opposite the feedpoint. Exactly opposite the feedpoint is a short, and the short between the ends means current is maximum. 1/4 wave back from that short is a voltage maximum and current minimum, again because of transmission line effects. 1/4 wave more and the high current area maximum repeats.

73 Tom
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WB2WIK
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« Reply #4 on: February 21, 2010, 02:54:18 PM »

I like Tom's answer best, although a bit technical.

It doesn't matter if the wire antenna is a dipole, or if it's 1/2-wavelength.  The voltage is maximum at the end of any wire antenna, irrespective of its length or where it's fed.  If it happens to be a 1/2-wave end-fed, or a 1-wavelength center-fed (examples) a similar high voltage also appears at the feedpoint.  If the (each) wire is not an increment of 1/2-wavelength long, it doesn't.

When fed at its lowest impedance point like a center-fed 1/2-wavelength dipole, the antenna is called *current-fed* because maximum current occurs there.  If fed at its highest impedance point like a center-fed 1-wavelength doublet, it's called *voltage-fed* because maximum voltage occurs there (just as it does at the element tips).

Under the latter condition, indeed dangerous voltage occurs right at the feedpoint of the antenna and special precautions are in order.

Current-fed antennas are much easier to deal with!
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KJ4RQV
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« Reply #5 on: February 21, 2010, 06:51:40 PM »

Thanks to all of you! I think that I now understand what I was asking about. This is almost as good as having a bunch of real life elmers to call on!.
Don
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KL7AJ
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« Reply #6 on: February 22, 2010, 02:35:47 PM »

Hi Tom:

  Actually, I think the the concept of a single wire transmission line is more esoteric than your latter example.  In fact, I'm not convinced a single wire transmission line can exist in free space.  However, over a ground surface...of course it can.  I actually have a TRUE Windom that relies on this...probably one of a handful of hams still breathing who actually knows what a true Windom is! Smiley

   Fortunately, all the analogies presented in this thread are useful in explaining this voltage distribuition in one way or another.  Even more significantly (and curiously), the concept also applies to DC!  If you take a Van DeGraff generator for example, the highest voltage will exist on the SURFACE of the sphere...whether it's a hollow ball or a solid sphere of brass!  This is because the charges all repel, and are all forced to the surface.  When you reach the "end of the road" there's nowhere to go, so the charge HAS to accumulate there. Smiley

eric
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W8JI
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« Reply #7 on: February 22, 2010, 08:44:03 PM »

Hi Tom:

  Actually, I think the the concept of a single wire transmission line is more esoteric than your latter example.  In fact, I'm not convinced a single wire transmission line can exist in free space.  However, over a ground surface...of course it can.  I actually have a TRUE Windom that relies on this...probably one of a handful of hams still breathing who actually knows what a true Windom is! Smiley

   Fortunately, all the analogies presented in this thread are useful in explaining this voltage distribuition in one way or another.  Even more significantly (and curiously), the concept also applies to DC!  If you take a Van DeGraff generator for example, the highest voltage will exist on the SURFACE of the sphere...whether it's a hollow ball or a solid sphere of brass!  This is because the charges all repel, and are all forced to the surface.  When you reach the "end of the road" there's nowhere to go, so the charge HAS to accumulate there. Smiley

eric

A single wire transmission line can certainly exist is freespace. It has an electric field meaning it has capacitance distributed along its length, and it has series inductance.

Some microwave antennas launch a wave along a single wire transmission line. No return path required. Just a cone at each end.
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KL7AJ
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« Reply #8 on: February 23, 2010, 07:53:38 AM »

I've always wondered exactly where a G-line fits.  I've seen the analogy of a G-line being more like a really really long wire or a Beverage (since they're way up there in the microwave bands)...they're actually an antenna, but the radiation lobe is essentially parallel with the line.  G-lines can also be made with dielectric material, like Teflon rope, which wouldn't seem to have much distributed inductance...or any inductance at all. Smiley

There may not be much of a distinction between a transmission line and an antenna in these cases...it's all a matter of degree...how much energy gets transported versus radiated.  I know my Windom feedline does a good bit of both. Smiley

Incidentally, I see the latest ARRL antenna book makes some mention of the single wire feedline...they had dropped it for a few editions...but I guess it's back by popular demand.

73!

Eric
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N3OX
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« Reply #9 on: February 23, 2010, 12:59:31 PM »

G line exploits a surface wave on the wire, and requires an excitation that excites that mode and not the one that couples well to free space waves.

73
Dan



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73,
Dan
http://www.n3ox.net

Monkey/silicon cyborg, beeping at rocks since 1995.
KH6AQ
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« Reply #10 on: February 23, 2010, 02:15:48 PM »

The question is not why? It is why not?
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KL7AJ
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« Reply #11 on: February 23, 2010, 07:35:47 PM »

That would require that the wave is perpendicular to the surface of the wire....in other words, vertically polarized in all directions. Smiley  

I guess the conical launchers sort of do that.
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VK1OD
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« Reply #12 on: February 28, 2010, 04:30:39 PM »

I really don't know how to frame this question so it makes sense, I'm afraid. As I study for my General ticket and as I read about antennas for HF, I have come to understand that the ends of a dipole can/will have a high voltage and are dangerous.  I have read the ARRL Wire Antenna book and this is emphasized that even with a low transmitting wattage there is a potential problem. How does the high voltage get to the ends? Is the danger as great at the center of a dipole? If not, what happens as the energy travels along the wire? I am missing some knowledge and I would really like to understand this subject.
Any help will be appreciated, Don

Don, discussing "voltage on the end of a dipole" can be somewhat confusing, especially when you think about voltage with respect to what? For a dipole in free space, what is the voltage on the end? What do you think is plotted in the classic graphs of voltage and current on a half wave dipole.

A better model is perhaps to think of radiation as being caused by accelerating charge, and remember that current is the rate of flow of charge.

With that model, thinking about a half wave dipole, as charge oscillates, the ends are regions of accumulation of higher alternating charge, and the middle is a region where the rate of movement of charge (current) is higher.

There is nothing wrong with plotting charge and current distribution on a half wave dipole, and it is probably what people mean when they draw some other diagrams.

Owen
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KL7AJ
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« Reply #13 on: March 04, 2010, 03:41:57 PM »

Voltage with respect to the other end of the dipole is useful too!

However, most of us folks who've worked with high power know that you can have corona off the end of a dipole or other antenna with NO nearby "reference."  This  generally means you have a surplus or deficit of electrons relative to the surrounding air.

Eric
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VK1OD
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« Reply #14 on: March 04, 2010, 09:23:37 PM »

Voltage with respect to the other end of the dipole is useful too!

However, most of us folks who've worked with high power know that you can have corona off the end of a dipole or other antenna with NO nearby "reference."  This  generally means you have a surplus or deficit of electrons relative to the surrounding air.


Corona may occur where the electric field strength is very high, electric field strength is expressed in Volts/metre. Electric field strength is not  simply Voltage as in potential difference. To illustrate the difference, two electrodes, one with a needle point end, and the other a spherical end could be raised to the same voltage and only one experiences significant corona. It can't be explained simply by their identical scalar voltage, the electric field strength (a three dimensional vector) is different around each electrode.

A "surplus electrons" is charge, not Voltage.

This is basic electricity... no rocket science.

Owen
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