"Robert11" wrote in message
. ..
Hi,

Real newbie regarding antenna theory, but am trying to learn.
Have ordered the ARRL antenna book.

a. In my attic, right next to each other, I have a Scantenna and a
Discone
antenna.
Both have the same coax run to my study downstairs. Don't remember the
type
of coax, but it's the large diameter, "low loss" type for both.

The Scantenna feeds a new Uniden BCD996 Scanner.
The Discone feeds my JRC NRD 545 (with the VHF converter board).

When tuned to the Coast Guard station (156.8 MHz fm) about 25 miles away
(way beyond my line of sight, but they probably have a high tower) the 545
with the Disconne is vastly superior.

Of course the 545 is a much, much better radio than the the Uniden, but I
assumed that for this type of listening configuration, reception would be
mainly limited by the antenna ?

I also thought that the Discone would be most effective for aero., and
very
minimally effective for more or less horizontal signal paths like from the
Coast Guard Tower to me.

Any thoughts on this, and on Discones (for "horiz." propagated signals) in
general ?

b. Never thought about this before, but have started wondering.
Might be a good question for all you experts out there.

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

As mentioned, in the attic I have two VHF antennas, the Scantenna and a
Discone.
(do listening only)

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, and the Coast Guard station.

Probably have no effect for aero comm listening as these 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 the screens is probably about 20 feet or so.
This subtends a relatively small angle, but not that small. Haven't
measured the screen, but assume about the size of a typical house bedroom
window.

Any thoughts on this ?

As always, thanks,
Bob

Bob

I have always found the discone to be a very useful performer in horizontal
directions, limited only by the fact that it also picks up interference
equally well in all directions. In my experience, pickup from signals in a
vertical or near vertical direction is not as good as the horizontal
performance, but this is overcome to some extent by the source of these
transmissions being generally closer (unless you are trying to listen to the
ISS or Amsats).

Roofing material over the attic is probably attenuating signals just as much
as the mesh screens at the gable ends. Unless the mesh is connected to an
earth point in some way, it is not likely to cause too much reduction in
signal strength. The signal can be considered as being absorbed and
reradiated at a slightly lower level if there is no low impedence path to
earth.

Moving the discone antenna out of the attic and on top of the roof will
raise the height of the antenna and improve performance generally.
Alternatively, move the Scantenna outside and it will probably then match
the performance of the discone in the attic.

Too much gain in the antenna system will probably result in degrading the
performance of the Uniden. Strong relatively nearby signals will overload
the filtering and cause intermodulation interference blocking out the
signals you are trying to receive. Paging transmitters are notorious for
causing this type of problem in the UK when listening around aviation
frequencies and 2m amateur bands.

Directional antennas such as yagis and log periodic antennas are the most
frequently used solution when high gain and directivity are required but
have the added complication of requiring rotators to aim the antennas.

Regards

Mike G0ULI