"...since the 51.5 is derived from 20*log(377)"
to be even more accurate, the 377 is the ratio of E/H, and for actual numbers
(if I get this right) permittivity / permeability in a vacuum or free space or
air since all are really close.
We have recently acquired an E-field and H-field all in one probe in order to
measure our near fields around an antenna for exposure requirements at low
frequencies. We have to figure out Instantaneous RMS units in this world.
Another convention !
We also have to check our radiated emissions, usually harmonics of the
fundamental, to the limits.
Since generically, the source impedance is not known in the near field, and it
is also changing very rapidly over short distances, certain assumptions,
conventions, are made in order to be consistent in the industry.
The approximate 51.5 db conversion is not accurate, but if everyone does it,
and the standards do as well, the results are the same.
ps. and the H-field does roll off by the 3rd order just as the physics seem to
imply.
The measurement uncertainty is rather large in this area, since the data seems
to have a wide range of values, but given enough points over a wide range of
measurements, and averaging, the roll off converges on the 3rd order, in the
near field.
And the environment has a noticeable effect on the results. Intuitive when
thought about. For low frequencies, the environment is integral to the
antenna. Dirt, wet or dry and in between, metal ground plane or something
else, like dirt, or concrete, or something else. Way different environmental
conductivities affecting the results.
I suspect that early low frequency wireless transmissions science did not
entirely carry forward for the rest of us to learn and understand. The early
digital transmissions were wired and gave us the telegraphers equations, which
are still valid.
But the rest seem to have faded into the past.
From: "Sundstrom, Mike" <[email protected]>
To: [email protected]
Sent: Tuesday, February 28, 2017 8:46 AM
Subject: Re: [PSES] CORRECTION (wrong page) Passive Loop Emissions [General
Use]
Not much different from any other in situ testing I've ever done. Guess I
looked like a mad scientist and didn't know it.
Thanks,
Michael Sundstrom
Garmin Compliance Engineer
2-2606
(913) 440-1540
KB5UKT
"We call it theory when we know much about something but nothing works,
and practice when everything works but nobody knows why." -- Albert
Einstein
-----Original Message-----
From: Cortland Richmond [mailto:[email protected]]
Sent: Monday, February 27, 2017 8:14 PM
To: [email protected]
Subject: Re: [PSES] CORRECTION (wrong page) Passive Loop Emissions [General Use]
On 2/27/2017 7:53 PM, Brent DeWitt wrote:
> I think Ken's rational makes sense to me, since the 51.5 is derived from
> 20*log(377).
>
>
Sure, but now we're back to how close we are -- wavelengths -- to the emitter.
20*log(??)
Low frequencies can be tricky, and I once had to double-check a test lab (not
yours) results at a vendor, dragging the EUT out to their parking lot then
wheeling a cart with a 6510 loop antenna, battery, AOR
AR5000 receiver and RMS voltmeter [all mine] away to see how fast the signal
dropped with distance compared to 3m.
A fun time was NOT had, but I probably came across as a mad scientist.
Again. At least no one ran out of a chamber...
Cortland Richmond KA5S
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