Richard Harrison wrote:
Tom, W8JI wrote:
"You didn`t read something correctly."

OK, here is the arithmetic.

Radiation Resistance of a Short Electric Dipole:

RR = 80 pi squared (L/lambda)squared

Constant = 80 (8.97) = 790

But a short monopole has 1/2 the resistance of a short dipole.

790 / 2 = 395

All Reg asked for was the constant.


If you'll read more in the chapter of Kraus you're quoting, you'll
notice that L is the length of the dipole, not the length of a monopole.
Do the proper substitution and you'll get the correct answer.

Roy Lewallen, W7EL

Roy, W7EL wrote:
"Do the proper substitutionn and you`ll get the correct answer."

Yes. The warning also appears on page 137:
"In developing the field expressions for the short dipole, which were
used in obtaining (5-56), (5-56) is the value of radiation resistance,
the restriction was made that lambda is much larger than the length of
the dipole L." No problem there, Reg specified a short monopole.

Kraus does a sample calculation for a short dipole. I used Kraus` data
and got the same answer when duplicating his calculation.

But Reg was not asking for an answer to a specific problem. Reg was
asking for the value of the constant in a formula of the same form.
Kraus gives it as 80 pi squared for a dipole.. This is 790.

We know that a monopole has half the resistance of a dipole. Example: 73
ohms and 36.5 ohms. 790 / 2 = 395. That`s not a resistance, it is only
the value of a constant which must be multiplied by (L/lambda) squared
to give the radiation resistance of a very short monopole.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:
We know that a monopole has half the resistance of a dipole. Example: 73
ohms and 36.5 ohms. 790 / 2 = 395. That`s not a resistance, it is only
the value of a constant which must be multiplied by (L/lambda) squared
to give the radiation resistance of a very short monopole.

Does it matter that for a vertical that is 1/2 of the length
of the dipole, (L/lamda)^2 is different by a factor of 4?
--
73, Cecil http://www.qsl.net/w5dxp

Cecil, W5DXP wrote:
"Does it matter that for a vertical that the length of a dipole
(L/lambda)squared is different by a factor of 4?"

It doesn`t make a ratio different than two to one in the ratio of
resistances of the 1/2-wave dipole to the 1/4-wave monopole. We are not
comparing a monopole that is the the length of a dipole with the dipole.
We are comparing a monopole that is 1//2 the length of a dipole to the
dipole when we make the resistance ratio.

The small dipole is working against a perfect ground in Reg`s
specification. It would see its reflection in that perfect ground, so
its equivalent length is doubled. Kraus` dipole is presumed to be in
free space.

Best regards, Richard Harrison, KB5WZI

Richard Harrison wrote:

Cecil, W5DXP wrote:
"Does it matter that for a vertical that the length of a dipole
(L/lambda)squared is different by a factor of 4?"

It doesn`t make a ratio different than two to one in the ratio of
resistances of the 1/2-wave dipole to the 1/4-wave monopole. We are not
comparing a monopole that is the the length of a dipole with the dipole.
We are comparing a monopole that is 1//2 the length of a dipole to the
dipole when we make the resistance ratio.

Richard, Balanis doesn't say that the 'L' in the monopole
formula is 1/2 the 'L' in the dipole formula. Does Kraus?
--
73, Cecil http://www.qsl.net/w5dxp

On Sun, 12 Mar 2006 20:56:05 -0600, (Richard
Harrison) wrote:

Roy, W7EL wrote:
"Do the proper substitutionn and you`ll get the correct answer."

Yes. The warning also appears on page 137:
"In developing the field expressions for the short dipole, which were
used in obtaining (5-56), (5-56) is the value of radiation resistance,
the restriction was made that lambda is much larger than the length of
the dipole L." No problem there, Reg specified a short monopole.

Kraus does a sample calculation for a short dipole. I used Kraus` data
and got the same answer when duplicating his calculation.

But Reg was not asking for an answer to a specific problem. Reg was
asking for the value of the constant in a formula of the same form.
Kraus gives it as 80 pi squared for a dipole.. This is 790.

We know that a monopole has half the resistance of a dipole. Example: 73
ohms and 36.5 ohms. 790 / 2 = 395. That`s not a resistance, it is only
the value of a constant which must be multiplied by (L/lambda) squared
to give the radiation resistance of a very short monopole.


Is all that to mean that you used the formula given by Kraus for a
short thin dipole and applied your own rule to halve the coefficient.

In your original response you said "395 It is found on page 137 of
Kraus` 1950 edition of "Antennas"."

Is that correct, or did you make the number 395 up according to your
own rules and then attribute it to Kraus?

Owen
--

Owen Duffy wrote:
"Is that correct---?"

No, I don`t think so.

Kraus` formula is:
Radiation resistance = 80 pi squared L squared

L is the fraction of a WL made by a tiny dipole.

For the same wavelength, a monopole is only 0.5 the length of a dipole
and it has 0.5 the radiation resistance.

If we use its length in the formula abbove, the radiation resistance
would calculate as only 1/4 that of a dipole because the constant is the
same and L squared is 0.5 squared.

I speculrte from the resistance ratio of a normal dipole to a normal
monopole that the answer should be 0.5. So I erred by halving the
constant. I should have doubled it to offset the quartered answer an
unchanged constant would produces when L = 0.5.

My new and improved answer to what the value of C is:

1580

Best regards, Richard Harrison, KB5WZI