[Antennas] Simple Question

Wed Nov 8 01:10:22 EST 2006

Hi all
First, I'd like to thank everyone here who has offered advice to me over 
the past month or so. I am relatively new to the area of antenna design 
so am still asking very basic questions. I've learnt a lot already just 
from being on this list, which has been great.
I have a quick question which somebody may be able to help me with. As 
you may recall, I'm trying to find the ideal antenna for 
vertically-polarised VHF signals on the airband from a transmitter 
located about 7km from me. An indoor antenna would be great. I'm going 
to try a Moxon, based on the advice of a few members on this list, but 
before I do I felt I should check one thing. I've been using a 1/2 wave 
vertical dipole (each element being 1/4 wave in length). Coax-wise I'm 
using RG58-C and am also using a 1/2 wave coax balun at the feedpoint.
So why do I get better performance out of my 'dipole' when I remove the 
bottom element and run it as a 1/4 wave vertical with no ground plane? 
With the antenna in a given location, a signal has a noticeable amount 
more background static when I connect the bottom dipole element than it 
does when disconnected. This behaviour is also experienced when I remove 
the balun, although this then turns the coax into an antenna to a 
certain extent.
I've noticed this in the past when I've built antennas too. It happens 
on both my scanners, so it's not the radio. It doesn't make any sense 
and I'd love for someone to tell me what I'm missing!
Thanks
Richard