Aperture is exactly the point I was going to get at.........this is the
reason that the receivers I have built that use the Mini-Circuits mixers as
the first stage have worked very well with an 8 foot diameter untuned loop.
No IM products anywhere in the LW/MW bands.
wrote in message
oups.com...
Differential pairs have more noise than a single ended design, which is
why the transformer is the way to go. An ideal transformer has no
noise. I real life transformer has the noise of the resistance in the
windings. This is why the transformer choice isn't trivial.

I understand this, but take a look at the AD797, and other op-amps that are
used at very low input signal levels. This device is designed for ultrasound
transducer applications, strain gauge amplifier service, etc. There are
quite a few very low noise op-amps that are suitable for RF service.

As far as transformers, I don't think there is such a thing as an ideal
transformer. In my experience, the input Xl must be at least ten times the
impedance of the expected driving source's impedance at the lowest frequency
of interest.There is going to be a certain amount of DC resistance in the
primary winding if we are going to be able to achieve the required
inductance to provide the proper load. If we are talking about a single
turn loop that has a relatively low reactance in this application, that
would be ok.

If you buffer (source follow) then use an amplifier, the noise of the
source follower gets amplified. This is why you wouldn't normally do
any buffering before amplification if low noise was the issue. However,
in the case of a tuned loop, you need the high impedance to keep the Q
high.

Exactly my point.....................in the LW/MW ranges, this is not an
issue. When you are using a high Q loopstick, the output voltage out of the
secondary winding is relatively high. This is observed from my own
experience. In direct comparison with the Palomar active loopstick, my own
units have better performance in the areas of output voltage, and small
signal pickup is slightly better because of the higher selectivity of my
implementation.
For higher frequencies where the ambient noise is lower, a different
technique would be required.

Wellbrook gets around that problem by using a loop that is not
tuned. The assumption is the purchaser has a real radio and doesn't
need front end filtering. So cource having a high dynamic range like
the 7030 is a plus.

That isn't such a good deal......................anybody can build that type
of antenna for relatively cheap................the clincher is the amplifier
stage that Wellbrook uses. I would like to take a look at a schematic of
their circuit and see how they do it.

Loops using ferrite will not work as well as a Wellbrook IF the
Wellbrook uses a large enough loop. What you are gaining with the
Wellbrook is aperture. It soaks up lots of RF due to it's size