El 14-06-12 0:11, garyr escribió:
This site http://www.frontiernet.net/~jadale/Loop.htm states that: "A
properly designed Loop primarily responds to the magnetic component of the
radio wave. Note that noise resides primarily in the electrical
component..."

Whereas this site shows that that is not the case:
http://vk1od.net/antenna/shieldedloop.

So what is the advantage, if any, of a shielded loop antanna?

Consider three receivers:

1) Shielded loop antenna, receiver with differential input (center-tapped
transformer or instrumentation amp). The two ends of the inner conductor
of the antenna connected to the differential inputs and the shield
connected to ground.

2) Same as above but without the shield.

3) Unshielded loop antenna, receiver with single-ended input.
One end of the loop connected to the receiver input and the other to
ground.

Assuming equal gains and bandwidths, would there be any
difference in the sensitivity or noise level at the output of the three
receivers?


I agree with your second link (by VK1OD).

The time varying magnetic field generates an electric field and that
is received by the loop. When you screen it completely, it doesn't
work, you need the gap.

By placing the gap opposite to the feed point, you get a balanced loop
without the need of ferrite or other constructions. If you can get
balance via other means, you don't need the screen. Balancing the
loop reduces noise due to common mode issues. This isn't different
from using a balun between a coaxial cable and a symmetrical dipole.

Your option three may behave competently different, as the coaxial
cable, power supply cable, switch mode power supply, etc may
contribute to reception of signal and noise due to common mode to
differential mode conversion.

From my experience (reception) with electrically small well-balanced
indoor loops and indoor dipoles, I found some advantage of the loop
over the electric dipole at low frequencies (say below 3 MHz). I
contribute this mainly because of the nulling capability. Whether is
applies to your location depends on the field distribution of the
noise at your location.

At higher frequencies there is difference in S/N ratio, but not in
favor of one antenna. Sharp nulling wasn't possible. So to know what
option is best for you, you need to try it. Maybe install both options
and select the antenna that gives best results as this will depend on
frequency and the angle of arrival of the radiation you want to receive.

Other thing that may really help is to find your local source(s) of
noise, use lots of ferrites and try to find a sweet spot for best S/N
ratio.

--
Wim
PA3DJS
www.tetech.nl
Please remove abc first in case of PM