First Ron you want to make sure it is YOUR problem not the other guys
receiver. A bad antenna pattern usually won't "garble" a transmitter. It
can reduce signal strength which would cause your signal to go into the
noise.
two questions how far away was the other aircraft and what was his
orientation to you?
Also what is the swr on your antenna?
My suggestion first is if you have a buddy with another plane (prefer two
seater with a helper installed in the second seat!). Have him monitor your
signal with a know good receiver/antenna and try different orientations and
distances to see it it is indeed your problem.

As far as antennas go my first impression would be an inverted 'V' type
antenna, which is basically a dipole with the feed end forming an angle
(between 45-90 degrees, not real critical) it isn't as directional as a
dipole is. Then make sure you have it matched properly and separate the
receive and transmit antennas by as much as possible to decrease likelyhood
of your transmitter feeding back into your receivers.

I hope this helps a bit!
John