[RFI] [BULK] Re: Ambient Noise Levels
w8ji at contesting.com
Fri Aug 11 21:41:36 EDT 2006
> I would believe you, maybe, if it wasn't for evidence and
> measurements and observations like this made with
> test equipment:
Don't shoot the messenger! If you don't want to believe
everyone from Faraday and Maxwell on down the food chain
there probably isn't much I can do about it.
> Relative strengths of two alternative l.f. antennas:
> o E-field antenna the RF Systems LF-520 and an
> o H-field antenna the Wellbrook LFL1010.
> I then went trawling for a slightly stronger signal and
> found Waterford (callsign WTD) on 368kHz as seen in
> Fig.2(a) with the Wellbrook and Fig.2(b) from the RF
> Once again you can see the noise level from the LFA-520
> [E-field active antenna] is some 30dB higher than that
> the Wellbrook [the active/amplified H-field shielded
> but of course you can also see that the signal level of
> Waterford Beacon is 28dB higher from the LFA-520.
What does that have to do with anything?
All the author is saying is one antenna picked up more noise
than another. Just because he wrongly thinks the loop does
that by virtue of a shield filtering out the electric field
doesn't negate how things actually work.
Nobody is saying there can't be a difference between two
vastly different antennas. The E-field antenna probably has
terrible common mode decoupling and almost certainly uses
the feedline shield and everything connected to it, even
more than what they call the antenna, as the actual antenna.
Most voltage probes are built that way and the poor
construction reinforces all this voodoo about magnetic
fields and electric shields.
> However, if you compare the signal level above noise in
> both cases,
> H-field - the S/N ratio of the Wellbrook is about 26dB
> E-field - the LFA-520 is about 16dB,
> so the Wellbrook [H-field] provides a better signal to
> noise ratio
> and a quieter background.
I don't see how that proves Maxwell, Faraday, and even
formulas used in field strength measurements are wrong. Does
it mean every broadcast FS meter is wrong? Virtually every
field strength meter used to measure FS is calibrated in dB
V despite using a "magnetic loop" to measure intensity. How
can that be if the shield blocks the time-varying electric
Is the ARRL Handbook wrong when it says the outside of the
shield of a coaxial cable is in effect a third conductor
isolated from the inside by skin effect? How do shields on
radios work if the magnetic field passes? What can we put
two IF transformers with aluminum cases next to each other
and not have magnetic field coupling?
My inclination is that Faraday and Maxwell and almost every
engineer and scientist dealing with electromagnetics
throughout history has the edge here. Too bad so many people
perpetuate the shield myth, but maybe one person at a time
we will eventually grow out of it.
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