APT Weather Station Reception on 137MHz with a Patch Antenna

APT Weather Station Reception on 137MHz with a Patch Antenna
APT Weather Station Reception on 137MHz with a Patch Antenna
and DVB-T Stick
By Gunthard Kraus, DG8GB
First published in the German UKW Berichte journal 4/2014
This idea arose while browsing The Internet and finding a homepage with weather images
received from a satellite. So I thought how I could do that with existing equipment - and what
would need to be developed. This article is the result of a revised and expanded lecture given at the
2014 VHF conference in Weinheim
1 Introduction
Such projects always start with an extensive planning phase to find the shortest route to the
destination. It soon became clear that the following modules, already available from completed
developments, could be used:
a. A commercially available DVB-T Stick as measuring receiver [1] and [2]
b. A very low noise preamplifier with sufficient gain described in [3].
New ground was broken for the antenna used for this project. The author has already implemented
numerous patch antenna projects successfully and has repeatedly published the corresponding
development procedures. This kind of antenna offers a circular polarised design with its spherical
radiation pattern. For this purpose, the back (in this case: the underside!) forms a perfect shield
against the ground and you can even place the antenna directly flat on the ground. The "half
wavelength", approximately 1 metre edge length in the 2m band, required for this project presents a
big hurdle and is discussed later.
The concept finally implemented is shown in Fig 1 and shows what items need to be developed. The
patch antenna and a narrow band 137MHz bandpass filter need to be developed. The fact that various
computer programs also have to be used successfully is aptly described by the biblical quotation
"every day has its own plague".
Fig 1: From now on probably the most basic receivers will look like this: an analog front end
followed by digital signal processing feeding a USB port
2 The components of the receiver
2.1. The low noise preamplifier
The development of the preamplifier for 137MHz can be read in
[3]. The operation is explained
quickly here with the help of Fig
2. The MMIC contains a GaAspHEMT cascode amplifier with a
"bias" circuit with its voltage fed
to pin 1 via L1 and thus to the
gate of the first pHEMT. The
other inductance L2 on pin 7
forms the input resistance of the
second stage. This preamplifier
is operated with a supply voltage
Fig 2: Technical development has not stopped even for LNAs. of +5V. A major problem of the
This is the circuit that stands out at 137MHz with a noise figure pHEMT construction is stability
at low frequencies, which is their
of 0.35dB and an amplification of +25dB
tendency to oscillate. This is
solved with a simple trick: with decreasing frequency the resistor R1 of approximately 50Ω at the
input pin 2 effectively prevents the oscillation. It is decoupled by capacitor C3. A special feature is
the transformer Tr1 (bandwidth = 500kHz to 1GHz) at the output. It brings the output reflectivity
S22 between 130MHz and 200MHz to values better than -20dB. The details of its development and
behavior can be found in [3]. The
amplification of the arrangement
in this frequency range amounts
to approximately +25dB with a
noise figure of around 0.35dB.
A view of the housing with the
lid removed is shown in Fig 3.
The 2: 1 transformer is easy to
recognise. The MMIC looks
quite harmless but it has 4 connections on each side and a small
ground strip on the bottom with
the whole thing only 2mm x
2mm, which represents a real
Fig 3: The design - everything is small. The main improvement challenge to solder. For this
reason friendly professionals
comes from line transformer at the output
were enlisted who are dealing
with such tiny components every
day.
2.2. The 137MHz bandpass filter
This should protect the input of the DVB-T Stick from strong "out-of-band" signals, which lead to
overdriving or intermodulation. The Stick inputs are not exactly famous for extremely high intermodulation strength, even when heavily limited.
As usual this presents a dilemma: a higher filter degree gives steeper flanks and greater selectivity,
but more components need more space, and the limited space leads to considerable attenuation. The
same milled aluminium housing as used for the LNA (PCB size: 30mm x 50mm) is used for the
bandpass filter. On this board, a maximum of 4 shielded filter coils (NEOSID type 10.1) as well as 4
SMD trimming capacitors must be accommodated for fine adjustment. Therefore, the
"Coupled Resonator Type" was used as the
basic circuit because:
a. It allows small bandwidths and
steep filter edges with realistic
component values.
b. A design can be carried out easily
with the filter calculator provided
free of charge in the ANSOFT
DESIGNER SV software.
The design screen for the Chebyshef bandpass filter selected is shown in Fig 4 with
Fig 4: The ANSOFT DESIGNER SV is still a strong the filter degree N = 4, a ripple of 0.3dB
horse; The integrated filter calculator used for the and a bandwidth of 3MHz for a centre
137MHz bandpass filter
frequency of 137MHz.
Note the attenuation:
At 130MHz it is already over 60dB, at 150MHz it is already 75dB!
For this type of filter, the inductance must be specified (the same for all 4 circuits). NEOSID type
10.1 coils in silver screening cans were used. From preliminary tests it was found that the 73nH
version with a removable brass core not only increased the inductance to 78nH, but also the quality
up to Q = 150. These values were used in the simulation and this resulted in the circuit shown in Fig
5:
Fig 5: This is how fast you get to the
desired circuit with its component
values
Up to three 0603 NPO SMD capacitors
with a trimmer (1.4 ... .3.5pF) were connected in parallel as circuit capacitors.
Each "coupling capacitor" of 2.49pF consists of a parallel connection of 1.5pF and
1pF in SMD capacitors soldered together
"piggy back". The actual coupling capacitors consist of finger-shaped printed "interdigital capacitors" whose design was carried out with the help of the ANSOFT
Fig 6: This ideas must be used: The mechanical data for DESIGNER SV and the circuit principle
the interdigital capacitor can only be determined with shown in Fig 6:
this "half-bridge" technique (see text)
A half-bridge is used, often found in crystal filters, and the dimensions of the interdigital capacitor
are changed until an S21 value below -70dB is obtained at 137MHz. The capacitance value of
0.2846pF (or 0.2346pF for the middle coupling capacitor) is correct.
The following considerations were made for the mechanical dimensions of this structure:
a. The distances between the fingers should not be a headache for the PCB manufacturer, so
0.25 mm was chosen.
b. In order not to make the finished capacitor too large, a finger width of 0.5mm was used.
c. The complete arrangement should be approximately a square. This finally yields the
required number of fingers (here: N = 4) and the length of the fingers calculated for this
purpose.
Fig 7: This list of capacitor dimensions is needed later on for the circuit board layout
This leads to the property table shown in Fig 7. The mechanical dimensions for the circuit board
layout of the finished interdigital capacitor can
be derived from this.
Now it is exciting, because in the simulation
circuit the fixed capacitors are now replaced by
these interdigital versions. This is followed by a
consolidation phase. Each interdigital capacitor
has an additional capacitance to ground show in
the equivalent circuit diagram as a pi-circuit at
each end. These parallel capacitors de-tune the
resonant circuits so you have to correct and simulate again until you finally have the result
shown in Fig 8. The final circuit is shown in Fig
Fig 8: This is the dream result. Everything should 9 and the finished circuit board layout in Fig 10.
end up like this!
Fig 9: The final corrected circuit diagram associated with the simulation of Fig 8. Interdigital
capacitors and parallel capacitors have been added to the circuit
After setup and the adjustment the measured bandwidth was identical to the result of the simulation but
the attenuation was not less than 10.5dB. It was
considered what might be improved, e.g. replace the
NPO capacitors with high quality microwaves ATC
capacitors (size 0603 as well). The attenuation decreased to approximately 9dB. Further improvement
possibilities were the trimming capacitors (quality:
also NP0) as well as the circuit board. The circuit
board manufacturer did not use the desired gold
plating but had only tinned the tracks. The board
Fig 10: The board and its details; For people
material itself (ROGERS 4350B) is usable up to over
who want to make a copy (not to scale)
10GHz.
A photograph of the finished building block including the housing is shown in Fig 11. The interdigital capacitors as well as the
coils and trimming capacitors are
easy to recognise. The remaining
0603 size components are virtually invisible.
Fig 11: The finished bandpass filter in its case - and it is already
in use in receiver
3 The DVB-T Stick as a Software Defined Receiver
These Sticks have been getting smaller and cheaper (under €20 when ordered via eBay from China)
lately. Of course you have to pay attention to which IC is used for the tuner:
The E4000 goes up to 2.26Hz, but has a reception gap form 1100MHz to 1235MHz. In addition, its
reception starts at 50MHz. However, since it is slowly replaced by the "R820T", it is now more
difficult to obtain and its price has increased massively.
Therefore the version with the R820T was used, which starts at 25MHz and works very reliably up to
1400MHz without a gap. In addition, the amplification can be changed by hand by almost 50dB
which is more than the E4000 (this gives only
42dB). The IQ decoder type RTL2832U, which
is mostly used for this purpose, must also be
installed on the board - Please check carefully
before buying!
Of course, there was still a lot of work before
commissioning; The Stick has been removed
from its plastic housing, built into a milled aluminum housing and the antenna input changed
from MCX to SMA as shown in Fig 12.
Fig 12: The tiny PCB from the DVB-T Stick. The
milled aluminium housing is clearly too big!
4 Now things get moving
4.1. SDR software
A good program is required to make a good measuring receiver. The proven and free software,
"SDR#" (pronounced SDR sharp) is available from The Internet. It is continually improved and
therefore you should update this software on your computer once a month.
4.2. The complete receiver
Fig 13: This is the complete experimental setup. It just waiting for the antenna cable, the USB
cable for outputting the data and the +5V power supply for the LNA
The modules that form the receiving system are shown in Fig 13. With a 3dB SMA attenuator
following the bandpass (gives the filter a better load than the Stick alone) results in a chain
approximately 25cm long. At the LNA input there is a transition from SMA to BNC for the antenna
cable connection. The USB cabling is missing in the picture.
Now for the reception and sensitivity tests. It was carried out with an HP8640B precision measuring
oscillator producing the "SDR#" screen shown
in Fig 14 when feeding with an AM signal with
a carrier level of -120dBm = 0.22µV. You can
only say: "Everything is good"
5 The patch antenna
In this area, the frequency range has been shifted
to a new level, because no one has ever tried this
before. The advantages are obvious: a simple
design, right handed circular polarisation suitable for the satellite, excellent radiation characteFig 14: The screen with the measurement results ristics with a directional diagram (with circular
is a real thanks for the effort
polarisation), theoretically a perfect sphere.
Indeed:
The active radiator is about half a wavelength with air as the dielectric. The edge length is more than
1m for a frequency of 137MHz. In addition, the underside must be a metal plate which also gives
mechanical strength. But the antenna design is still difficult.
The design is based on a previous collection of smaller patch antenna designs (see various
publications in VHF Communications Magazine as well as the SONNET tutorial on the author’s
homepage). But unfortunately the development of this antenna was the hardest!
The first test specimen was glued together from 20mm thick Styrofoam plates and covered with
0.05mm copper foil. This showed the principle but:
a. The dielectric constant for the Styrofoam at 137MHz appeared to be significantly smaller
than expected, so an edge length of 1m was not sufficient. The resulting resonance was thus
over 150MHz.
b. The lightweight construction was too unstable and flexible. The copper foil was damaged
when touching the structure.
The next design stage was 40mm thick polystyrene
(cheap) covered on both sides with 0.6mm thick Cu
sheet from the plumbers merchant (expensive, approximately €100). The whole assembly was held together
by 6mm thick plastic screws and an N socket on the
bottom for the feed.
This specimen with an area of one square meter can be
seen in Fig 15 and the comparison of the size with the
road bike is impressive. The diagonally chamfered
corners affect the circular polarisation and the position
of the feed ensures correct adjustment.
Fig 15: Compared to the racing bike, the Unfortunately, due to the fact that at 137MHz the
size of the antenna is impressive
Styrofoam has a Q of greater than 50 but a very low
dielectric constant of only 1.05 meant that the previously selected radiator dimensions were still too
small and the resonant frequency was still at 152MHz instead of 137MHz!
For the third design a polyethylene plate with a surface area of 1m2 and a thickness of 15mm was
purchased from The Internet (price: approximately €100, weight over 10 kg) complete with electrical
data. The total antenna weight (for plate and copper on both sides) has unfortunately increased to
about 20kg.
The simulation data was:
Polyethylene with a thickness of 15mm
Dielectric constant = 2.4
Loss factor = 0.005
It was simulated in a box with a base area of 9m x 9m (comes from the SONNET requirement
"distance of the structure from the box wall everywhere about 2 wavelengths”). Above the antenna
there is an air cushion with the height "Half wavelength = 1.1 metre". The box lid is set to "Free
Space". The patch itself is 730mm x 730mm, the diagonal corners are trimmed by 75mm (to achieve
the right hand circular polarisation). An N type connector on the lower "ground" level was provided
as feed connection. A pin made on the lathe extends through the polyethylene plate and was soldered
to the socket and the patch. This is taken into account using a "Via" with a diameter of 5mm in the
SONNET simulation.
The corresponding SONNET editor screen and
the feed point used are shown in Fig 16: (Feed =
355mm in horizontal and 180mm in vertical
direction from the lower left corner).
The simulated antenna structure (Fig 17) begins
with the bottom of the box as an infinitely good
conducting ground plane. Followed by the polyethylene layer with a thickness of 15mm. A
1mm thick air layer was inserted between the
lowest "ground" layer and the polyethylene
because the copper sheets did not lay flat everyFig 16: Something from "SONNET Lite". Inforwhere - especially when the thing curled by its
mation about the design sequence. This is the
own weight. It quickly became apparent that the
input screen with the antenna array
free, but limited SONNET Lite version has its
Fig 17: From the "Dielectric Layers"
menu (see text)
Fig 18: The S11 curve is very satisfactory. Fig 19: The cartesian diagram also shows the
Shown in Smith chart
S11 behavior, this time in dB
limitations: the lower ground plane of the antenna is not
infinite in size; the size of the polyethylene used for the
simulation should only be as large as the area of the
antenna; the "space" contained lots of plastic screws; etc.
The simulation was carried out with a "cell size” of 5mm
x 5mm.
The very pleasing S11 simulation result is shown as a
Smith Chart in Fig 18 and as a Cartesian diagram in Fig
19. These almost match the measured results of the completed antenna shown in Fig 20. However, the author must
confess that "fine tuning" was necessary. The patch dimeFig 20: The S11 measurement on the nsions were quite accurate because the resonance was
finished antenna is satisfying, but only exactly at 137 MHz BUT: the cut-off at the corners was
too much and as with a clearly over critically bandpass
after the corresponding fine tuning
filter, the S11 bandwidth was much too large combined
with a very deep "hole" at the centre frequency.
So, "fine tuning" was performed with sheet metal pieces brazed to the corners to give a cut off of
40mm instead of 75mm. This resulted in the S11 curve shown in Fig 20. It was measured at the input
of the antenna feed line (5 metre long RG58U
cable) with an HP8410 network analyser
6 Finally it is finished!
Now the time had come: the antenna was placed
in the garden on two plastic buckets. An RG58
cable approximately 5m long was pulled through the basement window into the basement
workshop (Fig 21) and connected to the receiFig 21: Now it's serious! The antenna is installed ver. The "SDR#” software was started on the
in the garden in front of the cellar workshop, the notebook and tuned to 137MHz.
antenna cable (5m RG58) disappears through the
cellar window
And what was to be seen?
Only an insane noise level, which was at least 30dB higher than the one described in chapter 3.2.
Individual carriers appeared briefly that were recorded as AM signals and on the basis of the short
messages, e.g. "Hello Lufthansa Flight 309 ..." they could be identified as an air traffic announcements from the airport tower at Friedrichshafen. After a lot of experiments there was only one
possibility: try again either very late in the evening or very early in the morning and hope that no
"Man Made Noise" obliterates all interesting signals.
A search at 5 o’clock the next morning was successful: suddenly on 137MHz there was a clear FM
signal seen on the spectrum display with approximately 35kHz bandwidth. The chirping and
whistling used for APT transmissions was heard loud and clear from the PC loudspeaker. So it was
quickly recorded as a “WAV file” using the "SDR#" "Recording" and "Audio" options. 10 minutes
later the energy saving lamps belonging to a neighbour went on and the noise level rose by 15dB.
Therefore, a more practical solution is still required e.g. antenna mounted at the highest point of the
house roof. One possibility is directly under the roof and just above the desktop computer in the
study on the upper floor.
7 The evaluation, the return for all the work
The received signals were
evaluated later using the
free software "WXtolmg"
downloaded from The Internet.
If reception is always perfect you should download
another program (e.g. from
"www.VBCABLE.com"),
which serves as a "virtual
audio cable" and directly
connects the "SDR#" and
"WXtolmg". This allows
the weather image to be disFig 22: If you have this picture on the computer screen, you should be played directly on the PC
screen without delay during
happy (see text)
reception.
The WAV file was tried with the "WXtolmg" software and a result was achieved very quickly. Fig
22 shows the representation "MCIR map colour IR (NOAA)":
It is not very difficult to understand the picture:
In the middle there is Iceland, on the left Greenland, right below that is Ireland and Scotland and on
the right is Norway. There are no clouds to be seen - after all it is stock photograph taken at night!
The remaining details can be found either in the image or in the file name:
The satellite is NOAA19, which transmits on 137.079MHz at 3:55UTC. It’s position is 14 degrees
west (almost exactly above Iceland).
It was recorded on 14 June 2014 with the recording ending at UTC = 3 o'clock 57 minutes and 45
seconds. The reception
level was just under 0.2µV.
After a calculation the distance between my QTH and
the satellite is more than
2000km, which is amazing.
For fun you can select a
second setting in “WXtolmg” called "Contrast enhance only" (NOAA channel
A only) and you get Fig 23.
Normally this is used for
Fig 23: The same again, but now the land is only shown as a contour pictures with lots of cloud
(see text)
that are recorded during the
day when the islands or
land masses under the cloud cover have partly or completely disappeared.
At this point the author gives a heartfelt thank you to the hard working software programmers!
The goal was achieved with a lot of work, a lot of brain teasing, a lot of mechanics and much sweat
... but in the end, much pleasure in the received picture.
8 Literature
[1] Gunthard Kraus, DG8GB, "A DVB-T Stick With An E4000 Tuner As A Measuring Receiver”,
UKW Berichte 2 / 2013, pages 131-148
[2] Gunthard Kraus, DG8GB: “The Never Ending Story Of The SDR Continues; Examination of a
DVB-T Stick with an R820T tuner and RTL2832U decoder as a receiver”, UKW Berichte 1/2014;
pages 3- 14
[3] Gunthard Kraus, DG8GB, “A Low Noise Preamplifier With Improved Output Reflection For
The 2m Band” 412013, UKW Berichte 4/2013 pages 203 - 222
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