[Antennas] Simple Question

Wed Nov 8 04:13:04 EST 2006

Richard Greaney wrote:
> 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
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As a side note, I should probably add that when working in a 'proper' 
signal zone (eg outside) adding the ground plane has the opposite effect 
(eg it strengthens the signal). I'm thinking maybe the indoor phenomenon 
is due to some sort of phase cancellation.