"Wimpie" wrote in message
el.net...
El 15-06-12 16:09, garyr escribió:
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

So you are saying that cases 1& 2 above are essentially equvalent if the
loops are balanced. In terms of noise rejection, there is no analogy to
be
drawn between a shielded loop and a shielded cable.





Hello Gary,

You are right, it is what I am saying.

There is no analogy between the shielding function of the braid in a
coaxial transmission line and the shield in your loop.

For the "shielded" loop, the received voltage is across the gap in the
shield. The shield is the actual single turn loop. The inner conductor is
just there to transport the received signal to the opposite side of the
gap where you can go down (with coaxial cable) to your receiver,
maintaining balance.

Theoretically a coaxial transmission line system is completely closed.
Water from the outside can't reach the inner conductor, source or load.
When cutting a gap in the screen, the coaxial transmission line system
will leak.

Shielded loop with more turns
If you run more turns within the shield of the loop, you pass the gap many
times. When you pass it 3 times, you will get three times the voltage,
hence the impedance and loop inductance increase.

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

Wim,

Thanks very much for your lucid explanation of how shielded loops function.
I've been using one with a receiver I made to monitor VLF signals. It works
quite well and I had thought it was because of all the local noise I was
avoiding. But then I stumbled across VK1OD's article...

Best regards,
Gary Richardson, AA7VM

PS
Thanks also to all the other respondents. This has been a very interesting
thread.