On Sat, 30 Nov 2013 21:42:38 -0600, amdx wrote:

I have a low power FM transmitter that I use for in my house and yard.
I want to put a 1/4 wave vertical on the roof of my home. The 1/4 wave
vertical will be made as many have seen from a UHF panel mount connector
with the vertical on the center pin and the four radials soldered to the
holes for the screws.
Like this,
https://encrypted-tbn1.gstatic.com/i...M00RrltFyCH-LA
Only bigger :-)

Unless you use stiff elements, they will sag. At 100MHz, 1/4
wavelength is about 75 cm (2.5 ft) long.

Looking at the pattern of a 1/4 wave vertical, I think I could best
cover my yard with the pattern upside down.

Think again please.

Can I mount the antenna upside down?

Yes.

Is this feasible?

Yes. It's commonly done with UHF antennas on mountain top sites.

If I did turn it upside down, what would the feedline do to the pattern?

The feed line will mangle the pattern.

Is there a better physical layout to avoid pattern distortion caused
by the feedline?

Yes. Use a vertical dipole on a tower or a coaxial antenna on a vent
pipe.

The alleged problem with a ground plane antenna is that there is a
slight vertical uptilt of the beam. It varies with the height above
the rooftop ground, but my guess(tm) is maybe 5 to 10 degrees uptilt.
I just ran a simple ground plane simulation using 4NEC2 and found that
the uptilt is small when the vertical beamwidth of the ground plane is
about 90 degrees. In other words, inverting the antenna isn't going
to do much good at delivering the signal towards the ground. You're
better off with an antenna that puts the main lobes where your
receiver is located or perhaps has some gain and/or downtilt. Without
a description of your house and yard, I can't offer any suggestions.
Numbers please?

You might find it useful to look at what the LPFM people are doing for
antennas:
https://www.google.com/search?q=lpfm+antenna&tbm=isch
Remember, the stranger it looks, the better it works.

Ummm... what problem are you trying to solve?

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558