That's just what I was thinking about, concerning the transformers. I
understand the concept of the input stage determining the system
NF..............for a MW loopstick, this doesn't seem to be an issue,
especially with a tuned circuit that has a loopstick with a measured Q of
400............I am using ferrite material that has a permeability of 900.
Anyway, a source follower had a voltage gain of approximately 0.95, so there
shouldn't really be any noise of any magnitude inserted into the system.
There is, however quite a bit of current gain; this is good when driving a
lower impedance load such as a MMIC. This link should provide some insight:
http://www.qrp.pops.net/probe1.htm
This person has some example schematics.
This is also a good link (PDF file): www.analogzone.com/hft_0930.pdf
Since I am using a very low value source resistor, about 62 ohms, this
should be the main contributing factor as far as noise voltage is concerned.
The MMBF5486 has a 3dB to begin with, so once again, I don't believe noise
to be an issue here.
I can make some NF measurements to verify this, but as you say, the
differential mode seems to be the way to go, especially with a good JFET
pair. I do have some Siliconix E431 and U430 dual JFETs on hand, but since I
have less than 30 of each it wouldn't make any sense to build anything
unless it was a one-of custom project. Ulrich Rohde has a good design
example in his communications receiver design book.

wrote in message
oups.com...
The trick with the Wellbrook loop is that the loop isn't tuned. Tuned
loops make sense if your radio itself has poor filtering, but anyone
who forked out money for a Wellbrook already has a decent radio. You
have to admit the Wellbrook marketing is clever. They don't have to
compete with Kiwa, Quantum, etc.

In a good amplifier, you set (or blow) the noise figure at the input
stage. [You can do worse of course if you aren't careful.] Thus
starting with a source follower means you are inserting a noise source
right that the front end.


Since noise is "input referred", starting
with a source follower (i.e. no gain) means that you don't have the
gain of the transistor to reduce the noise.

The only noise that there should be here is antenna noise.

Thus you should start with
a gain stage. Now this is where I don't know what Wellbrook does. For
low noise, you want a single ended input stage (one less elemen to
contribute to the noise). For the loop to work, it has to float about
ground as you don't want the antenna to be the loop relative to ground,
i.e. you want the loop to be the antenna. Thus Wellbrook puts in a
differential to single ended converter, aka the transformer.

This is how they achieve their very high IP2 and IP3 specifications. A good
way to go, but after a certain input level, the ferrite material will become
saturated. Now, if they are using the type of material that is used in the
broadband transformers in transmitter output stages, that is a different
story.


Now what I
don't know is the turns ratio of the transformer. Are they picking the
turns ratio fo impedance matching? Is the goal voltage gain?

I am not sure about this myself. Dale, W4OP would understand this one.

I don't think the Wellbrook is complicated in terms of the number of
components, but it requires good planning to make a low noise amp that
works with the loop.

I have seen a couple of designs that use an op-amp at the front end. I have
thought about using an AD797 as the input stage. With a 30 volt bipolar
supply, dynamic range could be very good since you would have approximately
28 volts of swing at the output stage.

You may want to investigate North Hills transformers. They often show
up at swap meets or on ebay. They are very wide band.


Pete