It's the current which matters.

The bird has a capacitance to the rest of the world which depends on
its size. The current which flows up the bird's legs is the voltage on
the antenna divided by the capacitive reactance of the bird's body.
For small birds the capacitance is only a few picofarads and so the
current is extremely small. The bird continues to sing its happy
song.

Whereas, a human body not only has a much larger capacitance it may
also complete the circuit by having the other hand in contact with
grounded metal work. Or the victim might be standing in his bare feet.
The current due to capacitance of the human body alone is enough to
cause burns. The pain is sufficient to stop the human from singing
and begin using bad language.
----
Reg.

Reg Edwards wrote:
It's the current which matters.

The bird has a capacitance to the rest of the world which depends on
its size. The current which flows up the bird's legs is the voltage on
the antenna divided by the capacitive reactance of the bird's body.
For small birds the capacitance is only a few picofarads and so the
current is extremely small. The bird continues to sing its happy
song.

I did surprise an Owl one night sitting astride my Inv-V
center insulator. He was apparently touching both dipole
wires when I went key down with 100 watts.
--
73, Cecil http://www.qsl.net/w5dxp

Ralph Mowery wrote:
"Cecil Moore" wrote in message
.com...

Frank Alforo wrote:

If I touch the coil in my tuner while transmitting I get a painful rf
burn.
But a bird can perch on my antenna wire while I transmit 100 watts and it
stays there seemingly without a care in the world. How come?

You could probably hang by one hand from a 20 kV power
line and be OK as long as you didn't complete a current
path to another wire or ground.
--
73, Cecil http://www.qsl.net/w5dxp


You should have seen the TV educational channel where in some countries they
have a basket type trolley that men are put in and hung by helicopter on the
multi megavolt lines. They do wear some kind of suit that is conductive so
the charge will not affect their body so much.


They ;do that in this country. Mesh suit of some metal, can't remember
which metal but not important as long as it's conductive I guess.
Great job I bet, big bucks....

"Dave" wrote in message
. ..
even if you aren't touching something you have a much larger surface area
to
act as a capacitive path to ground for the rf.

snip

Yes. One of the standard tools used during RADHAZ inspectiions at RF
transmitters sites is an RF Burn Gun. (It's shaped like a gun for
convenience of holding and use.) It measures the RF potential on metallic
structures near a transmit antenna and the tech is part of the circuit,
coupling to ground. Current limiting in the RF Burn Gun keeps the tech from
harm while he reads the potential off a meter in the "gun."

It strikes me that the measurement accuracy varies with operating frequency
and size of the tech (via collective X-sub-C). With my big butt and short
legs, I might offer more coupling to ground than you scrawny people.

There's a great difference between working on antenna conductors and
60 Hz power lines.

Body capacitance is all-important at HF. At 60 Hz it is the
conduction path which matters. Workmen can be enclosed in Faraday
screens.

Frank Alforo wrote:
If I touch the coil in my tuner while transmitting I get a painful rf burn.
But a bird can perch on my antenna wire while I transmit 100 watts and it
stays there seemingly without a care in the world. How come?

What Muppet Labs would like to know is why you touch the coil in your
tuner while you are transmitting. Don't do that! Also avoid things
like putting your fingers across live mains conductors and placing your
hand on hot stoves.

Bunsen

Frank Alforo wrote:
"But a bird can perch on my antenna wire while I transmit 100 watts and
it stays there seemingly without a care in the world."

Frank isn`t using enough power. I worked in a shortwave broadcasting
plant filled with 50 KW and 100 KW DSB fully modulated transmitters.
Our relatively flat transmission lines were of the 2-wire unshielded
600-ohm variety. These lines must have attracted countless tired
migratory birds to alight or try to alight. When a bird would try to
grasp a wire, it would instantly be consumed in a puff of smoke and
steam. No carcass fell to the earth. Poof! it was gone. Many times their
claws were left behind gripping the wire leaving us a record of the
bird`s visit. Many wires were covered with bird claws. RF can kill more
birds than cats can.

Best regards, Richard Harrison, KB5WZI

On 2006-06-26, Richard Harrison wrote:
bird`s visit. Many wires were covered with bird claws. RF can kill more
birds than cats can.

Maybe we can use this to distract people who claim wind farms are bird
killers.

--
Ben Jackson

http://www.ben.com/

Okay silly question Richard

Did the birdless claws have any noticable effect on the match/tuning of
the antenna?

grin

Cheers Bob

Richard Harrison wrote:
steam. No carcass fell to the earth. Poof! it was gone. Many times their
claws were left behind gripping the wire leaving us a record of the
bird`s visit. Many wires were covered with bird claws. RF can kill more
birds than cats can.

"Frank Alforo" wrote in
:

If I touch the coil in my tuner while transmitting I get a painful rf
burn. But a bird can perch on my antenna wire while I transmit 100
watts and it stays there seemingly without a care in the world. How
come?

The bird is (usually) small compared to the wavelength. Much smaller than
you are. And, except at the ends of antenna elements, the voltage is not
particularly high. Whereas there is a high voltage across your coil and
you are a much larger portion of a wavelength than the bird, so significant
current will flow into you if you touch the coil.


--
Dave Oldridge+
ICQ 1800667