Voltage/Current at the end of a dipole?
 

What kind of voltages and currents are present at the ends of dipoles
assuming 100W of RF and the antenna being close to resonant?

Just wondering,

Tom

Tom Sedlack wrote:
What kind of voltages and currents are present at the ends of dipoles
assuming 100W of RF and the antenna being close to resonant?

Just wondering,

Maximum current is in the feeding point, thus minimum at the ends - that's
pretty logical, as the current must flow between something of different
potentials. In case of voltage, zero in the middle of antenna, and
consequently max. at both ends. So, avoid touching the radiator at the ends
;-)

--
Pawe³ Stobiñski
Republic of Poland

"Tom Sedlack" wrote in message ...
What kind of voltages and currents are present at the ends of dipoles
assuming 100W of RF and the antenna being close to resonant?

Just wondering,

Tom

The current should be fairly low at the ends. The current max is at
the feedpoint. Just guessing on the voltage, probably about 700 volts
potential at the ends or so...x10 for a KW...Any kind of decent end
insulator should be ok for 100w. Same for a KW too, but you want to
stay away from green tree branches etc, if you run high power. I've
toasted some of those when the branches touched the ends of the
dipole. MK

Hi!
Maximum current is in the feeding point,
Not always.

Alexander

Tom Sedlack wrote:
What kind of voltages and currents are present at the ends of dipoles
assuming 100W of RF and the antenna being close to resonant?

Zero current. A voltage estimate would be V^2/600=100w, or V=245V RMS.
The net RMS voltage would be double that value so peak voltage might
be around 700 volts.
--
73, Cecil http://www.qsl.net/w5dxp



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Alexander Schewelew wrote:

Maximum current is in the feeding point,

Not always.

Always for a center-fed 1/2WL dipole.
--
73, Cecil http://www.qsl.net/w5dxp



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"Tom Sedlack" wrote What kind of voltages and currents are present at the
ends of dipoles
assuming 100W of RF and the antenna being close to resonant?

-----------------------------------------------

A half-wave dipole is a resonant L & C tuned circuit.

Its lumped equivalent is a series L and C circuit across the feedpoint.

It has a Q value like any other resonant circuit.

As is usual, Q = Inductive reactance of wire divided by resistance.

In the case of a dipole the resistance is the radiation resistance at its
centre, typically around 70 ohms.

Inductance gets smaller as wire diameter increases. So a 2-meter dipole
with 1" diameter tubes has a lower Q (about 4) than a 160-meter dipole made
with 18 gauge wire. (about 12). Q controls bandwidth.

A 40 meter dipole may have a Q around 9. If it is fed at its centre with
100 watts then the feeding voltage is 84 volts.

So the voltage difference between the ends of the dipole is Q times 84 = 756
volts.

Relative to ground, the voltage at one end of the dipole is half of this,
equal to 378 volts.

This would burn a nice little hole at the tip of your right forefinger if
you touched it. Electrical burns take a long time to heal because the
surrounding flesh is electrocuted. Do not confuse this with 'skin effect'.
----
Reg, G4FGQ

Cec, I'm not altogether happy about the current coming to a sudden stop at
the ends of a dipole.

What's that stuff which clearly flows between the plates of a wide-spaced
capacitor?

Reg, G4FGQ wrote:
"What`s that stuff which clearly flows between the plates of a
wide-spaced capacitor?"

It is displacement current as Reg and Cecil know very well. J.C.
Maxewell speculated long ago that displacement current is surrounded by
magnetic flux, same as conducting current is. That was the key to
electromagnetic radiation which explains how the radiated fields
replenish each other where there is no matter.

As there is displacement current between the tips of a horizontal dipole
and the earth, and the voltage vector is vertical in this voltage
stress, radiation from this source is vertically polarized. The main
(high intensity) radiation from a horizontal dipole is horizontally
polarized, especially so when the dipole is far from earth.

Best regards, Richard Harrison, KB5WZI

Reg Edwards wrote:
Cec, I'm not altogether happy about the current coming to a sudden stop at
the ends of a dipole.

It doesn't come to a sudden stop, Reg. It is reflected out of phase such
that the forward current vectorially added to the reflected current equals
zero, i.e. the net current is zero at the ends of the antenna. If appreciable
current flowed through the air, it would first have to ionize the air causing
an arc.

The reflected current from the ends of the dipole is what gives that standing-
wave dipole its low feedpoint impedance. If it weren't for the reflected current
from the ends of the dipole, the feedpoint impedance would be in the hundreds
of ohms and it would be a traveling-wave antenna.

What's that stuff which clearly flows between the plates of a wide-spaced
capacitor?

If current is clearly flowing between the plates, it is called an arc.
Appreciable current doesn't usually flow across the air gap between the
plates. Current flows in the rest of the circuit while charges are stored
on the capacitor plates. Current flowing directly between the plates means
that the air has been ionized and the cap is breaking down. I have one of
those in the horizontal drive section of my TV.
--
73, Cecil http://www.qsl.net/w5dxp



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