Richard Fry wrote:
. . .There is no difference in gains between slender
and fat radiator designs in the broadcast industry.

Rest assured, there's no difference in any other industry either.

Roy Lewallen, W7EL

Why does everybody insist on OVER-COMPLICATING this simple problem?

The Q of a resonant 1/2-wave dipole is given by -

Q = Omega * L / 2 / R

Where L is the end-to-end wire inductance and R is the radiation resistance
of about 71 ohms.

Just the same 'formula', in fact, as any other tuned circuit or transmission
line. Resonant rise in voltage and current, and bandwidth, etc., all
follow.

It's so simple it doesn't occur to Terman and other 'beings' to mention it.
----
Reg

"Reg Edwards" wrote
Just the same 'formula', in fact, as any other tuned circuit or
transmission line. Resonant rise in voltage and current,
and bandwidth, etc., all follow.
______________

The impedance bandwidth of a fat dipole can be so large that an acceptable
input match is possible at frequencies where the dipole is no longer very
near a resonant 1/2 wavelength. In those cases and at a constant input
power, there is a redistribution of the current in the radiators, resulting
in a relatively modest change in the peak gain of the radiation pattern.

It is true that the Q of a fat radiator is less than a thin one, but that in
itself does not produce a change in gain. A gain change results from a
change in the radiation pattern of the antenna -- which is related only to
the length of the dipole elements with respect to the operating frequency;
independent of Q.

For example, a "short" dipole (fat or thin) has a gain of 1.50X and a 3dB
beamwidth of ~90°. A standard 1/2-wave dipole (fat or thin) has a gain of
1.64X and a 3dB beamwidth of ~78° [Kraus, 3rd Ed, Table 6-2].

Another example is that of the vertical radiators used in MW AM
broadcasting. There is no term for Q in the equations for their radiation
patterns. For a given set of installation conditions, a thin tower produces
the same elevation pattern/peak gain at the carrier frequency as a fat one.

RF

On Wed, 2 Feb 2005 08:57:29 -0600, "Richard Fry"
wrote:

"Reg Edwards" wrote
Just the same 'formula', in fact, as any other tuned circuit or
transmission line. Resonant rise in voltage and current,
and bandwidth, etc., all follow.
______________

The impedance bandwidth of a fat dipole can be so large that an acceptable
input match is possible at frequencies where the dipole is no longer very
near a resonant 1/2 wavelength. In those cases and at a constant input
power, there is a redistribution of the current in the radiators, resulting
in a relatively modest change in the peak gain of the radiation pattern.

It is true that the Q of a fat radiator is less than a thin one, but that in
itself does not produce a change in gain. A gain change results from a
change in the radiation pattern of the antenna -- which is related only to
the length of the dipole elements with respect to the operating frequency;
independent of Q.

For example, a "short" dipole (fat or thin) has a gain of 1.50X and a 3dB
beamwidth of ~90°. A standard 1/2-wave dipole (fat or thin) has a gain of
1.64X and a 3dB beamwidth of ~78° [Kraus, 3rd Ed, Table 6-2].

Another example is that of the vertical radiators used in MW AM
broadcasting. There is no term for Q in the equations for their radiation
patterns. For a given set of installation conditions, a thin tower produces
the same elevation pattern/peak gain at the carrier frequency as a fat one.

RF


How FAT would an FM broadcast dipole have to be to lose one db gain?

approximately
--
73 for now
Buck
N4PGW

"Buck" wrote
How FAT would an FM broadcast dipole have to be to
lose one db gain? approximately
______________________

Dipoles consisting of radiators of any practical diameter all will have the
same gain if they have the same electrical length at the operating
frequency. "Q" has nothing to do with it.

RF

Buck wrote:

How FAT would an FM broadcast dipole have to be to lose one db gain?

approximately

A perfect application for the free EZNEC demo program, from
http://eznec.com.

Roy Lewallen, W7EL

On Wed, 02 Feb 2005 14:44:07 -0800, Roy Lewallen
wrote:

Buck wrote:

How FAT would an FM broadcast dipole have to be to lose one db gain?

approximately

A perfect application for the free EZNEC demo program, from
http://eznec.com.

Roy Lewallen, W7EL


I was kinda thinking that the antenna would become a capacitor if the
elements get too fat.


--
73 for now
Buck
N4PGW

On Wed, 02 Feb 2005 20:26:39 -0500, Buck wrote:

I was kinda thinking that the antenna would become a capacitor if the
elements get too fat.

Hi Buck,

And the inductance goes down (think about the product and
proportionalities of the two.

73's
Richard Clark, KB7QHC

Reg, G4FGQ wrote:
"Its so simple it doesn`t occur to Terman and other "beings" to mention
it."

Well, Ed Laport had experience with many high-powered transmitters, so
it occurred to him to mention antenna Q in "Radio Antenna Engineering".

Ed assumes power in a dipole is all radiated and none is used to heat
the dipole. Radiation resistance of his horizontal dipole is a function
of its height above ground.

To calculate the Q of the antenna, he first calculates a factor "m"
which is the ratio of the attenuated voltage 180-degrees from the
generator on an infinite line to the generated voltage:
m = Zo-radiation resistance / Z0+radiation resistance

Then: Q = 1+m / 1-m

Radiation resistance = 276 log l/rho

where l is the total length of the dipole, and rho is the radius of the
conductor in the same units.

Laport is interested in antenna Q to make sure the antenna won`t plume
at maximum voltage.
Ed picks a wire size and computes maximum voltage at the antenna tips to
make sure that under assumed conditions the voltage is below the corona
firing potential. The calculation is on page 241 of "Radio Antenna
Engineering".
A trial wire size has a radius of 0.102 inch.

m ccomputes as 0.84

Q computes as 11.4

Balanced dipole feedpoint volts are picked as 690 rms from the
unmodulated applied power.
Volts at the ends of the dipole a
QV/2=3950 volts as rounded.

Corona is initiated on peaks and a further safety factor is added to
avoid pluming, but that`s the way a wire size might be checked.

Best regards, Richard Harrison, KB5WZI