"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

Richard Clark wrote:
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

Then, the next step is to think about why having less inductance and
more capacitance should reduce the gain.

Does it cause the pattern to change?
Does it reduce the efficiency?

Those are the only ways to change the gain.

Roy Lewallen, W7EL