Use
When used with the TinySA Ultra BW is set to 300 kHz for a 0-30 MHz
span resulting in a 10*log10(3e5) = 55 dB offset such that kTB will
display as -174 + 55 = -119 dBm. The TinySA can be set to display
Marker noise power directly in dBm/Hz.
A starting point for settings might be:
- 300 kHz RBW,
- LNA off,
- manual attenuation set to 0,
- external gain set to 10 dB to adjust for the gain inside
the FP
A first purpose of this probe is to identify areas for siting a
permanent receive antenna system with regard to minimizing local
noise sources. This involves surveying potential candidate
mounting locations for minimum unwanted, often near-field, noise
sources. To the degree that a user has any degree of freedom
locations that appear better or worse may be identified. As a
reference methodology, setting the telescoping monopoles to .5m each
and holding the FP vertically polarized 2m above the earth seems a
reasonable reference orientation.
Once a likely antenna site is selected on the basis of minimum
noise within the sub-spectrum of interest between approximately 1
kHz and 200 MHz that site may be further examined by seeking to
identify sensitivity of the noise level measured to polarization and
azimuth. This may best be done by focusing on the signal-to-noise
ratio (SNR) of a propagated signal, e.g. a NIST Time/Frequency
standard station or other easily visible signal coming and changing
the FP orientation to maximize SNR.
Because the FP has high axial ratio, because it predominantly
linearly polarized along the axis of the monopoles, and because
other sources of signal ingress have been eliminated by the high
CMRR of the design, it is possible to get a high degree of nulling
of a linearly polarized interferer which compromises the SNR of the
desired signal. In this way, seeking to find a polarization
and azimuth the optimizes not signal strength but SNR a better
understanding of preferred orientation for a permanent installation
may be discovered. This type of "noise nulling" can produce
impressive improvements in SNR in situations where there is only a
single interferer. In some situations this is many 10's of
dB.
Sources of elliptically polarized interference may need to be
mitigated with a more complex system such as the Polarimeter
described elsewhere. If there are multiple interferers presenting
similar magnitudes even though neither may be eliminated a best
compromise for communications SNR may be found.
Antenna Factor & Calibration
When used with a properly calibrated receiver or spectrum
analyzer it can be a useful tool for measuring local receive
system noise temperature and identifying some of the contributors
that raise it. A 50 ohm calibrated VNA S21
measurement over the frequency range from 50 kHz to 200 MHz
for the active circuits of the probe measuring from one monopole
input, with the other input terminated and both inputs AC coupled,
to the SMA output is shown below.
Although this probe can be very useful without providing absolute
electric field level measurement, through modeling and
measurement it is possible to relate measurements made using it to
absolute field strength levels over a broad frequency
range. To provide this it is necessary to know its
Antenna Factor which is way of connecting signal power or voltage
levels measured by an SDR or Spectrum Analyzer connected to its
output back to the electric field that created those outputs.
For more discussion of Antenna Factor, see the
SAS Deployment page where similar hardware is described.
Antenna Factor for this probe is developed in a similar manner.
Internally this probe has slightly more gain than the SAS to
compensates in part for the smaller dipole dimension. While a
useful tool it does not provide as low noise floor as the SAS. For
comparison with the SAS, the plot below provides S21 measured
in a 50 ohm environment.
That Short Dipole model is most accurate at frequencies
below where the dipole is one tenth wavelength. In this case that
is 30 MHz . A label has been placed at that point on a plot below
which shows FP output under those conditions up to 100 MHz.
[Placeholder for 1m FP Antenna Factor Plot]
As a handheld tool it is very useful to examine SNR as
polarization and azimuth are varied. The device has high
axial ratio, that is, an incoming vertically polarized wave can
easily be nulled by going to horizontal polarization.
Sometimes local and near-field noise results in highest SNR of DX
signals occurring at neither of these polarizations, even
recognizing that there is often significant decrease in levels due
to earth absorption when oriented horizontally which also tends to
push the arrival angle overhead.
The FP PCB has a graph of the ITU "Quiet Rural" noise
level so that measured noise may be read directly from the
TinySA in comparison to both ITU and thermal noise levels as
location is changed.
