On Fri, 26 Jun 2015 23:04:43 -0700, Robert Baer
wrote:

amdx wrote:
On 6/26/2015 7:24 AM, rickman wrote:
I read this post in an antenna group and I don't get how this guy is
coming up with a negative noise figure. Looks to me like he is
calculating the noise figure of a resistor, not the amplifier. Anyone
care to explain this to me?

The part that seems bogus is this...

The negative NF is defined as the amplifier noise being less than the
increase in noise due to the amplifier gain.

I thought noise figure was NF = SNRin / SNRout

Rick

I once ask Ratzlaff about the FSL antenna, in his comments he said,
"high Q or low Q has little relation to how the FSL will actually
perform for receiving weak signals.", but then went on to say, "Of
course using Litz is the only type of wire to use."
When I ask him about why Litz is important if Q isn't, he got ****ed off
and wrote back, "When you make sarcastic replies to the info I passed
along, and question what I say, then I write you off as just a
tire-kicker, not interested in possibly learning something, and I have
no interest in saying anything more."
Seems to me he said something incorrect and didn't like having someone
ask him to clarify it. I'll leave it to you to figure out which part was
incorrect.
I thought Q would be important, an FSL is a ferrite loaded tuned loop.
With magic mixed in! ;-)
If Q is not important, why use Litz?
I suspect you might have already tried to ask him, and now he's mad at
you too!
Mikek



** SNIPPED for brevity **
Even at DC frequencies (eg: broadcast FM or even AM) Litz wire gives
an edge for higher Q; that is a proven fact.
BUT....
If you do not give a rat about Q or gain losses or noise figure
(again, at DC frequencies), then bag Litz and pay the price on all of
them....

Litz is useful at VLF/LF/MF due to the high inductance and hence large
number of turns required. On a small coil former, the requirement is
to use the available copper cross section as effectively as possible.

At high HF and VHF, fitting a few turns into a coil former is
required, so using 0.5 mm solid thick wire to contain the RF within
the skin dept is not a problem.

For a single turn loop antenna, there are no such size restrictions. A
1-2 m loop antenna made of 10-20 mm copper tubing works quite well as
a transmitting antenna at 3.5 MHz with perhaps 1 % efficiency (-20 dB
gain).

I once made a loop antenna into a hoola hoop using a flat cable using
"off by one" connection to create a multiturn loop. The problem was
that the self capacitance between turns made it tunable only at VLF
frequencies.