Radiation Resistance
 

I am not trolling.

What I want to know is the radiation resistance, referred to the base,
of a short vertical wire above a perfect ground, the current in the
wire being assumed uniformly distributed.

The radiation resistance at the base is in the form of -

C * Square( Length / Lambda )

where Length is the physical length or height of the wire and Lambda
is the free-space wavelength.

What is the value of the constant C ?

Thank you.
----
Reg.

Radiation Resistance
 

C = 160 * pi^2 ~ 1579.

This is exactly 4 times the radiation resistance of a short dipole with
linear current distribution (i.e., one without a top hat), since the
average current is twice the amount for the same radiated power.

Of course, this assumes an infinitely thin wire. Any real wire will have
a higher radiation resistance than this.

Roy Lewallen, W7EL

Reg Edwards wrote:
I am not trolling.

What I want to know is the radiation resistance, referred to the base,
of a short vertical wire above a perfect ground, the current in the
wire being assumed uniformly distributed.

The radiation resistance at the base is in the form of -

C * Square( Length / Lambda )

where Length is the physical length or height of the wire and Lambda
is the free-space wavelength.

What is the value of the constant C ?

Thank you.
----
Reg.

Radiation Resistance
 

Reg Edwards wrote:
I am not trolling.

What I want to know is the radiation resistance, referred to the base,
of a short vertical wire above a perfect ground, the current in the
wire being assumed uniformly distributed.

The radiation resistance at the base is in the form of -

C * Square( Length / Lambda )

where Length is the physical length or height of the wire and Lambda
is the free-space wavelength.

What is the value of the constant C ?

Reg, I believe it would be 10*pi^2 = 98.7, half of the
value of a small dipole. Balanis gives a dipole a very
thorough treatment and then says the monopole is half
of those values. His constant in the value of radiation
resistance for a short dipole is 20*pi^2. Kraus rounds
that constant off to 200. That value assumes the short
dipole is not infinitessimal and has a linear standing
wave current distribution. That constant doesn't seem
to need to be a very exact value.
--
73, Cecil http://www.qsl.net/w5dxp

Radiation Resistance
 

Roy Lewallen wrote:

C = 160 * pi^2 ~ 1579.

This is exactly 4 times the radiation resistance of a short dipole with
linear current distribution (i.e., one without a top hat), since the
average current is twice the amount for the same radiated power.

Since the resistance is inversely proportional to the current,
shouldn't you have divided by 4 instead of multiplying by 4?
--
73, Cecil http://www.qsl.net/w5dxp

Radiation Resistance
 

Roy Lewallen wrote:
C = 160 * pi^2 ~ 1579.

This is exactly 4 times the radiation resistance of a short dipole with
linear current distribution (i.e., one without a top hat), since the
average current is twice the amount for the same radiated power.

Roy's formula above is correct. It is approximatey 1580 times the
square of effective height over the wavelength.

http://www.w8ji.com/radiat1.gif

http://www.w8ji.com/radiation_resistance.htm


Cecil's answer is not correct, but I'm sure you figured that out on
your own.

73 Tom

Radiation Resistance
 

wrote:
Cecil's answer is not correct, but I'm sure you figured that out on
your own.

Silly me, I was assuming the length of the monopole was
assumed to be 1/2 the length of the dipole.
--
73, Cecil http://www.qsl.net/w5dxp

Radiation Resistance
 

Reg wrote:
"What is the value of the constant C?"

395

It is found on page 137 of Kraus` 1950 edition of "Antennas".

Best regards, Richard Harrison, KB5WZI

Radiation Resistance
 

Richard Harrison wrote:
Reg wrote:
"What is the value of the constant C?"
395
It is found on page 137 of Kraus` 1950 edition of "Antennas".
Best regards, Richard Harrison, KB5WZI

Richard,

You didn't read something correctly.

Reg asked for "C" for a small vertical with uniform current over a
perfect groundplane.

You are off by nearly a factor of 4 times.

For a monopole with uniform current, C=1580
For a monopole with triangular current C= 395

Radiation resistance is four times greater when the antenna has uniform
current.

73 Tom

Radiation Resistance
 

Cecil Moore wrote:
Reg, I believe it would be 10*pi^2 = 98.7, half of the
value of a small dipole.

My bad. I falsely assumed that the length in the
monopole equation was 1/2 the length in the dipole
equation. Another senior moment.
--
73, Cecil http://www.qsl.net/w5dxp

Radiation Resistance
 

wrote:
For a monopole with uniform current, C=1580
For a monopole with triangular current C= 395

Radiation resistance is four times greater when the antenna has uniform
current.

Reg didn't say "uniform current". He said "uniformly distributed
current". A triangular current wave is uniformly distributed,
i.e. it has a linear taper. Seem to me we need to find out
what Reg meant by "uniformly distributed current". I thought
he meant uniformly triangularly distributed. I think Richard
thought the same thing.
--
73, Cecil http://www.qsl.net/w5dxp