On Mon, 15 Nov 2004 23:53:47 GMT, Uwe wrote:

I like to put up a big hi gain FM broadcast band antenna (by big I mean 15 -
20 feet).

I can get the formulas for the spacing of the reflectors and directors from
literature, but it always says further optimization will be necessary.

Uwe,

Check out www.cebik.com he has an article "some notes on FM BC
antennas". Or at least something like that. He does extensive
antenna modeling.

One of the problwms with FB broadcast antennas is that 88-108 (US FM
allocation) is a very wide band percentage wise to make a good yagi
for. Programs like YO, QY4, are likely inadaquate for wide
bandwidth antennas so you may end uo with EzNEC or one of the
NEC varient calculation programs for antenna design.

If you were to take a 13b2 and lengthen the elements they would be too
closely space to get the same performace as the origional. You have
to scale length, spacing (and boom length) and diameter by the same
factor to get the same antenna at a different frequency. IE:
everything would be about 1.45 larger to optimize for 100Mhz.

Good luck.
Allison