There are many good points being offered in this thread. One extra point that needs to be made is that regardless of the "quality" of the antenna, if it is designed to be a ground-plane antenna, it needs that roof area to provide the physical ground plane that narrows the vertical angle of reception/transmission. Even in this context, ground-plane has two meanings: (1)the description of the directional preference of the antenna, and (2) the literal phyical flat surface on which the antenna is mounted, which is actually an electrical part of what makes the antenna work that way. An antenna focuses its senstivity in one direction at the expense of sensitivity in another direction (plane). Ground plane gain means that maximum sensitivity is formed perpendicular to the vertical element. You are not trying to hear airplanes, so why waste the antenna performance pointing it at the sky? Most receiver front ends have clipping diodes to prevent massive overloads and even damage from nearby transmitters. If you are concerned about mixing products from unwanted signals, try buying an inline filter (must be tuned with proper equipment or preselected by the manufacturer). By keeping the antenna signal input mostly desired frequencies, there will be much less objectionable noise, interference and mixing products generated from the front end protection diodes. There are even line amps with filters, but any active electronics need a power supply. Generally, there will be a world of improvement with just the natural gain of a GP antenna with a suitable filter. With a better quality frequency-cut antenna, you will be at pro performance and noise levels. It is truly amazing how well even a "cheap" scanner will work if you optimize what goes into the antenna port. It still amazes me how many railfans are also into electronics and RF.