The formula for a DIPOLE is 468/freq in Mhz = length in ft.....the formula
234/freq is for one leg OR usually quoted for a 1/4wave vertical but the
standard dipole formula is 1/2 wave (just to clarify that).

As for a good FM omni antenna, he should build a simple 1/4 ground plane or
coaxial sleeve antenna.....OR a turnstile (cross dipoles)..

FM stations USUALLY use Circular Polarization.....(similar to horiz and
vertical at the same time...)
so an antenna set for horiz or vertical will work (but lose 3db of signal
compared to a CP antenna!)

Chris
WB5ITT

"Buck" wrote in message
...
On 30 Jan 2005 17:14:42 -0800, wrote:

I don't know why you want FAT. It will give you lower gain. However,
you might consider building a dipole out of 1/2 copper tubing. A
piece of PVC T connector and two elements about 2.4 feet long each can
be connected to the coax, painted (if desired) and mounted inside the
attic or outside if desired.

The formula for a dipole is 234/frequency in Mhz = 1 leg of a dipole
or 1/4 wave length approximately. (Each dipole needs two legs.)

the copper pipe will give plenty of band width and do as well or
better than three inches.

You don't have to be worried about perfect dimensions as you aren't
worried about SWR so the elements I showed you will work very well.

Someone correct me, but don't FM stations transmit both vertical and
horizontal?

I hope this helps.


--
73 for now
Buck
N4PGW

"CWB" wrote
FM stations USUALLY use Circular Polarization.....
(similar to horiz and vertical at the same time...)
so an antenna set for horiz or vertical will work
(but lose 3db of signal compared to a CP antenna!)
________________

FM broadcast stations in the US _must_ radiate their licensed h-pol ERP
regardless of the ERP they radiate in other polarization planes. However
for most omni FM stations, v-pol RMS ERP cannot (legally) exceed h-pol RMS
ERP.

The gain of a true c-pol transmit antenna is 3dB lower than for the same
configuration of linearly polarized radiators. However in FM broadcast, the
transmitter power is increased by 3dB to make up for that, and h-pol ERP is
the same for either antenna configuration. Therefore the h-pol field
strength produced by either transmit antenna over the same path should be
the same.

Using a c-pol antenna to receive a c-pol FM station would add 3dB to the
signal seen by rx (over a free space path), as well as reduce the
self-interference caused by multipath.

RF

Visit http://rfry.org for FM transmission system papers.

CWB wrote:
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...

Here's a quiz question: Suppose the dipole could be made fat enough to
reduce the resonant length to 0.01 wavelength. How much would the gain
be reduced?

Part 2: Suppose the dipole remained the same diameter and was reduced in
length to 0.01 wavelength. If the conductor were perfect, how much would
the gain be reduced?

Hint for Part 2: This is easily answered using the free EZNEC demo
program, another modeling program of your choice, or a few minutes'
reading of just about any antenna textbook.

Roy Lewallen, W7EL