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

Thanks for the overload of info guys. I may have resolved his issue by
purchasing a Marantz ST-17 tuner, which has provision for two antennas.
The ST-2 that he has is fine for all stations he listens to except
one. I'll have him get the Radio Shack Yagi abd some good coax and aim
it for best reception on that station.

However, I need to make a good omnidirectional antenna to go in my
attic. I did not get an answer on if the ARRL handbook has the
directions for makinng the fat dipole.

I live outside of Atlanta, GA (hilly) and cannot have an outside
antenna, and have very little room horizonally in attic - a directional
is out, but do have an area for a tal vertical antenna.
What would be your recommendations?

What suggestions

On 2 Feb 2005 16:09:02 -0800, wrote:

but do have an area for a tal vertical antenna.
What would be your recommendations?

Hi Brad,

Go back to radio shack and buy their discone antenna.

73's
Richard Clark, KB7QHC

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

Brad wrote:
"What would be your recommendations?"
Brad also wrote:
"---but do have an area for a tall vertical antenna."

There are often obstructions in an otherwise line-of-sight path. Antenna
gain is usually not enough to overcome an obstructed path. Antenna
height can overcome the obstruction.

Flagpoles are usually acceptable where antennas are banned. In Terman`s
1955 edition of "Electronis and Radio Engineering" on page 902 are found
"Flagpole Antennas". These are balanced vertical dipoles, transformed to
a coax feed through the bottom of the dipole.

If the flagpole is tall enough, you receive FM broadcasts.

Q = f/BW means your dipoe needs a Q of less than 4.8 to span 88 MHz to
108 MHz. The antenna would center upon 97.5 MHz, the geometric mean or
center of the band.

Such a bandwidth is likely impractical. You can settle for less or
center the response on the part of the band you would most use.

Multiply the lowest frequency of high interest by the highest frequency
of high interest and take the square root of the product to find the
frequency the dipole should be cut for. The fatter the dipole is, the
lower its Q will be, and the more even its frequency response will be.
You can probably do quite well with uneven response in your antenna is
high enough.

Best regards, Richard Harrison, KB5WZI

I dont know where YOU got your antenna info from....but just because a
dipole has wide b/w (low Q) does not lower its gain unless the diameter of
the dipole causes the resonant length to be much shorter than 1/2
wavelength...but in practical terms, that wont happen (unless he makes the
dipole out of 4 ft pipe for instance! ;)

A wide b/w (fat) dipole made for 80m (a cage dipole) does not have any less
signal strength than a single wire dipole (2.15 dbi).....sorry.

Chris
WB5ITT
PG-9-5322 FCC Commercial
Telecom/Broadcast engineer for 30 years

"Richard Harrison" wrote in message
...
Roy, W7EL wrote:
"That`s interesting.(I don`t know why you want fat. It will give you
lower gain.) How much lower? Why?"

It`s a fact. Fat antennas have more bandwidth, and that is inversely
proportional to Q. Teducing antenna Q, by fattening the antenna, reduces
the antenna potential by about the same factor.

Best regards, Richard Harrison, KB5WZ