Bob, the reason to want the high gain is as follows.

I live in Maine south of the Canadian border, close but not quite close
enough to receive Canadian FM stations. I can receive one station faintly
using a Radio Shack BC band antenna and wanted to expand on that with a
better antenna and possibly a change in IF filters in the tuner I use.

And I thought that a very high gain antenna with its narrow pattern would
also aide in adjacent channel rejection. I realize that it could be a
problem covering the entire BC band and I am not set on any particular
design, I first wanted to see what people here had to say.

Regards Uwe
KB1JOW

From: Bob Bob
Newsgroups: rec.radio.amateur.antenna
Date: Tue, 16 Nov 2004 19:18:13 +1100
Subject: Homebrew high gain FM broadcast antenna ??

Hi Uwe

I'd suspect that the optimisation you are talking about is not going to
make a lot of difference to your end use. You tend to be able to tune
yagi's for best forward gain or best front to back ratio but not both
together. You have the added complication that if you want to cover the
entire FM band a single yagi would be a major compromise and it might be
better looking into a different kind of design like a log periodic or
axial mode helix.

As a general rule too, the longer the yagi the narrower bandwidth its
operation is. My gut feel for a 20ft FM band yagi antenna would be only
for use over maybe 200kHz. It would of course still work further from
the design frequency but at reduced performance.

As an aside some of the optimisation will be due to slight material
differences and deviations from original mounting techniques (eg through
boom vs on top, proximity to buildings etc)

Would you care to share you reasons for the high gain design? Is it one
distant station you want or just for general DX use? Might you be trying
to reduce interference effects?

Cheers Bob VK2YQA



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.

This 'further optimization' might prove difficult and I wonder if anybody
here has experience with this or has built one of these antennas??

Uwe