Subject: (Ozactas) MicroAlert review (fwd)
Date: Tue, 8 Dec 1998 172505 -0600 (CST)
From: "Roy L. Beavers" <rbeavers@llion.org>
To: emfguru <rbeavers@llion.org>
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---------- Forwarded message ----------
Date: Tue, 8 Dec 1998 20:34:50 +0300 (EET)
From: Haldun Ozaktas
To: rbeavers@llion.org
Subject: MicroAlert review
Hello,
I acquired the MicroAlert Alarm a while ago, and I thought
I'd share my thoughts in the nature of a "review" of the device.
(www.maui.net/~emf and www.trifield.com)
Such a device can go a long way towards crudely determining
field levels. The device is not a meter, but if it is indeed
able to clearly ascertain whether the field is less than a
small number of distinct levels, this would be quite
sufficient for general purposes.
Most desirable would be a flat frequency response. Since
this is not easy for a cheap meter to have, it is necessary to
know its frequency response/sensitivity curve. If such a curve
is available, the meter can be very useful since in most
practical instances, we actually may know the frequency:
an FM/TV broadcast, a cellular tower, etc. By pointing the
device in different directions and trying to shield it from
all but one source at a time, such a curve would also enable
one to untangle multiple sources to a certain extent.
My overall opinion is that as simple as it is, the device can
be potentially very useful for screening and prudent avoidance,
if it had better characterization and documentation.
Unfortunately, there where many questions left unanswered, which
I would like to share with you. Perhaps some of you will know
the answer to some of these.
1. The threshold of the alarm can be adjusted by the switch
and the screw. One needs a precise statement of what the threshold
is for different settings of the switch and the screw. At the very
least, one must know the threshold when a) The switch is HIGH and the
screw most clockwise, b) HIGH and most counterclockwise, c) STANDARD and
most clockwise, d) STANDARD and most counterclockwise.
(With threshold I mean the level at which an alarm will be heard.)
If one knows these 4 threshold levels, it can be possible to obtain
very useful measurements.
How are these numbers affected by "flipping the cover" and
"flipping the cover once more" as mentioned on the web page
www.maui.net/~emf/ What is exactly meant by flipping the cover?
2. At which incident direction to the device (towards which face or side)
are these nominal thresholds quoted?
3. At which frequency are these nominal thresholds quoted?
4. What is the frequency response curve (relative to frequency at
which nominal sensitivities are quoted). Even if a full curve is not
available, quoting them for a discrete set of frequencies such as
100 kHz, 333 KHz, 1 MHz, 3.3 MHz, 10 MHz, 33 MHz, 100 MHz, 333 MHz,
1 GHz, 3.3 GHz, etc. would be quite sufficient.
The information on various websites is confusing. The sensitivities are
quoted differently on different web pages, and even on the same web page!
1/3, 1, 3 uW/cm2 are the various numbers quoted for HIGH, and
1. 5, 10 uW/cm2 are the various numbers quoted for STANDARD sensitivity
as thresholds. Which is correct, and for which setting of the screw?
It is claimed that with a simple adjustment or two (easily made
by the owner of the device) you can measure down to .1 microwatt
per centimeter squared. Thresholds such as 0.1 microW/cm2 and
10 nanoW/cm2 are mentioned when the screw is turned. Which is
correct and under what circumstances?
How should one set the device to obtain these low thresholds?
I understand that such an inexpensive device may have variations within
20-50%, which is totally acceptable for most purposes.
Even a factor of 2-3 threshold variation can be acceptable
(i.e. it beeps above 2 uW/cm2 whereas the nominal value is 1 uW/cm2.)
But I assume the threshold is not uncertain within a factor of 10.
It is also not clear what the "center position" of the adjustment screw
is. Is it exactly midway between most clockwise and most counterclockwise?
Or something else?
Things become even more complicated when we consider the "coarse sensitivity"
control inside the box. How can I be sure what the threshold is for
a given setting, without having to get a signal generator and calibrate
it myself?
Since it is also of interest to measure FM levels, one would like
to obtain the frequency response curve which would enable
one to understand what he/she is measuring at, say 100 MHz? (It is said
that it has a "residual sensitivity" below this frequency, but how
much?) I understand the sensitivity will be less at this frequency, but
one would at least precisely know what he/she is measuring. At the least,
what is the "center" frequency at which the above minimum sensitivities
are quoted, and over what range is the frequency response flat,
say within 3dB.
In conclusion, I would like to say that the ability of this
device to give useful measurements depends greatly on the
availability of this type of information, without which
I cannot see how one can obtain meaningful results.
Given this information however, the device can go a long
way in providing a wealth of information on the RF/FM surroundings.
Haldun
________________________________________________________________________
Haldun M. Ozaktas (90) (312) 290 16 19
Bilkent University (90) (312) 266 43 07 (secretary)
Department of Electrical Engineering (90) (312) 266 41 26 (fax)
TR-06533 Bilkent, Ankara, Turkey haldun@ee.bilkent.edu.tr
www.ee.bilkent.edu.tr/~haldun haldun@alumni.stanford.org
Archive provided courtesy of WaveGuide, http://www.wave-guide.org
Reprinted with permission of Roy Beavers, http://www.feb.se/EMF-L/EMF-L.html