Uwe wrote:
CLIP..
The physical laws are the same, be it for a 6m antenna, for example, or a FM
broadcast receiving antenna. In fact I was thinking in stripping my
Cushcraft 13 element 2 m antenna for the purpose and just lengthen all the
elements.

It would probably be a poor idea to modify the 2M antenna. As someone else
mentioned, your spacing would not be correct.

A 15 foot long boom would support 1 Reflector, 1 Driven Element, and 5
Director elements. This would give you a estimated gain of almost 10 dBd.
Normally the bandwidth of these antennas is about 7% at the design frequency.
For your use this would be about 6.5 MHz. If you are only interested in a
small portion of the band, this will work fine.

Getting higher gains is a case of diminishing returns. Typically, to obtain
a 3 db increase in gain you need to double the number of director elements
and effectively double the boom length. So, going from 15 to 20 feet will
only gain you another 1 or 2 db. In cases of extreme fringe operation, this
may make a difference, but mostly you won't notice it. If you want to play
with boom lengths and element spacing try one of my web pages at:

http://www.k7mem.com/Electronic_Note.../yagi_vhf.html

Optimization is nice to do if you have the equipment, but with careful
construction, you should be able to build a antenna that is very close to
the intended specifications and not require any tedious tweaking.

For other antenna designs you might try Ian White's (G3SEK) web site at:

http://www.ifwtech.co.uk/g3sek/diy-yagi

If your intent is to cover the entire FM band, you may want to look for
information on Log-Periodic antennas. Gain will not be big, but the
bandwidth will be.

--
Martin E. Meserve - K7MEM
http://www.k7mem.com