Electrical Engineering - VHF fm Antenna & House Metal Screen Question

Author

VHF fm Antenna & House Metal Screen Question

Robert11

2007-06-24, 9:25 am

Hello,

Never thought about this before, but have started wondering.
Good question for all you experts out there.

Live in a typical New England Colonial with a Louvre in the attic on each
gable end that are screened to keep the bugs out. Screens are metal.

In the attic I have two VHF antennas for my scanner. A Scantenna and a
Discone.
Do listening only at around 156 MHz fm

Was wondering if the metal screens might have any effect on what I can pick
up in the more or less horizontal directions ? e.g., P.D.'s from our or a
neighboring town.

Probably have no effect for aero comm listening as the received signals are
pretty much from a vertical, or near vertical direction.

Should I go to the trouble of replacing them with plastic screens ?

Distance from antennas to either screen is probably about 20 feet or so.

Any thoughts on this ?

Thanks,

B.

Benj

2007-06-24, 9:25 pm

Robert11 wrote:

> Was wondering if the metal screens might have any effect on what I can pick
> up in the more or less horizontal directions ? e.g., P.D.'s from our or a
> neighboring town.
>
> Probably have no effect for aero comm listening as the received signals are
> pretty much from a vertical, or near vertical direction.
>
> Should I go to the trouble of replacing them with plastic screens ?

No. First off the I presume the louvers are metal and typically flap
closed so the screen is doing little compared to the rest of the
metal. If the louvers are plastic then plastic screen would probably
be a good thing.

Since the antennas are not totally enclosed in screen and the angle
subtended by the louvers is relatively small, and the louvers are not
grounded, All they probably do is simply modify the reception pattern
of your antennas. This can make signals stronger (if they happen to
be in phase and the metal acts like a "director" or weaker if it is
interfering with the received signal. Since you are receiving in all
directions around you, you probably won't notice this pattern
modification at all even though for certain angles it probably is
having some kind of effect. A real problem would be large areas of
metal such as a copper roof. In that case you'd move the antennas
outside and use the roof as a ground plane and it'd probably work very
nice.

LinkBot