Field Probe Pre-amplifier
This project is for a small handheld
field measuring device, particularly for locating, identifying and
measuring noise levels in conjunction with a TinySA or (especially) a TinySA
"Ultra" hand-held spectrum analyzer. It is intended to help
identify and understand a particular candidate antenna site for
amateur radio reception.
It is a member of a class of receive antenna systems using
non-resonant and therefore broadband antennas. Included in this are
the Hybrid Antenna System which uses a similar PreampA electronics and
the Loop-over-Earth and Beverage "Traveling Wave" antenna types as
well as short dipoles.
High Impedance Field Probe pre-amplifier Board.
Here's a portable arrangement using the TInySA "Ultra"portable
spectrum analyzer with a short dipole
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.
Calibration is based on a theoretical model for a short dipole,
length L in meters, providing impedance Z = Ra + jX
Z = 20*((pi*L/lambda)^2) - i(120/(pi*L/lambda))*(ln(L/(2*a))-1)
Understanding Ra to be a source of thermal power kTB = -174 dBm/Hz at
K=290 Kelvin in B = 1 Hz bandwidth.
This source is modeled as input to the FP preamplifier which results
in the output power shown in silkscreen on the FP PCB for reference.
An assumption is that the dipole elements are set to equal length for
a total dipole L of 1 meter and held vertically polarized.
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.
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
- marker noise selected.
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.
Assembly
Since there is no separate enclosure, once the assembled board arrives solder the 9V battery connector/pigtail to Battery +/- wire holes on the board. The only assembly required is to TyWrap a standard 9V battery to the back/bottom side and attach the two telescoping monopole antennas. The calibration curve will apply only to a specified total dipole length and vertical polarization when the board is held about 1m above the earth.