A question about a 20 meter amateur radio antenna?

unless you are a tuner, forget the long wire.

cut your dipole for 1/4 wave on each side of the center insulator.

Mount the antenna as high as you can get. If 30 feet is all, well, 30 feet will have to work.

I run a multi element dipole that is mounted on a side arm of my tower. My tower is only 40 foot tall, and I have a triband V/UHF vert.

I work a lot of 20M PSK31 and have never had a problem.

A resonant antenna is ALWAYS better than a non resonant antenna.

An end fed 1/2 wave would present a very high impedance to the feed line. Center fed would be close to the 50 ohms of the typical radio and coax. If you must end feed, use 450 ohm ladder line and a tuner. Keep it as far from the roof as possible, and with a simple wire antenna, center fed horizontal is probably best. Three rules of antennas, bigger, higher, farther away from other objects. Each one you violate makes the antenna less effective. Do the best you can with what is available to you.

The longwire will give you a higher VSWR than a dipole will if I remember correctly. I'd mount it vertically.