vertical antenna problem

[KE4QCO]

I have a A-99 for 10 meters, works well. I went to 80m with it, ( just recieve, no TX) done great utill I started to thread the pl 259, soon as the barrel touched the rigs so- 239, recive went to 25% of what it was with just the center of the pl 259. what caused this?? thanks don. ke4qco

[WB2WIK]

The antenna is effectively a short circuit on 80m, by design.

When using only the center pin of the coax, you're using all of the coax plus all of the antenna as a long wire antenna. As soon as you connect the outer conductor (connector shell), that "feature" goes away and the antenna is essentially a short circuit on the end of the coax.

It was never designed to be used on 80m. Actually it was never designed to be used anywhere except 25-30 MHz.

[W9GB]

Wavelength of the Radio Waves matter !!

3.5 - 4.0 MHz or 80 meters .... the wavelength is EIGHT times (Longer) that of 28.0 - 30.0 MHz or 10 meters.
The Audio analogy is :
Trying to have a Soprano voice (A99, for 11/10 meter) sing the Bass musical line (80 meters) :-).
===
Don -

Best 80 meter antenna (covers full 500 kHz) is 1/2 dipole with parallel elements 2-3 feet apart (fat dipole) .... that antenna is ~ 135 feet in length (a real estate problem for many).

N3UJJ Antenna Project
http://n3ujj.com/the_N3UJJ_antenna_project.html

Shortened antennas for 80 meters (Verticals or Horizontal Dipoles) suffer from narrow bandwidths.