BAREFOOT NAVTEX RECEIVER
(2023)


Will a simple ferrite rod bring the simple solution that I need for the simple Barefoot Navtex Receiver?
We're going to measure it! Barefoot on the way too cold tiled floor to avoid static electricity!

Barefoot Navtex Receiver
I want to make a very simple Navtex receiver, the Barefoot Navtex Receiver! Will this ferrite rod antenna bring the solution that I need to make that simple Barefoot Navtex Receiver?
I had already some exciting ideas! An 8 MHz very standard crystal divided by 16 will make 500 kHz for the mixer oscillator. That is easy with such a simple 74HC4060 oscillator - divider chip! But then 18 kHz audio has to be decoded for Navtex on 518 kHz and 10 kHz audio for Navtex on 490 kHz. Too high for the usual decoding programs, but my own simple NAVTEXsnoop decoder program can do that! The 18 kHz is just within the range of a good but not too expensive USB audio dongle.


The famous nanoVNA and an attenuator to attenuate the HF output signal for receiver measurements.

A ferrite rod antenna might be a simple solution!
I bought a cheap pack of five ferrite rod antennas. This might be the solution for my simple Barefoot Navtex Receiver! The bandwidth of the ferrite rod antenna has to be measured with the NanoVNA! With a Q of 50, the bandwidth is only 10 kHz. That is certainly a good bandfilter! And is the high audio frequency of 10 kHz and 18 kHz a disadvantage? It might be a huge advantage! The mirror frequencies of 510 kHz for 490 kHz Navtex and 482 kHz for 518 kHz Navtex are far away from the reception frequency and can perhaps be suppressed just enough by the bandwidth of the ferrite antenna! A little suppression of the mirror frequency is already enough to improve the sensitivity with at least 3 dB! No complex mixers with phase shifting networks required to suppress the mirror frequencies, the simple ferrite rod antenna will do it all!


A ferrite rod antenna is the simple solution!

Excellent test results!
A 470 ohm resistor is connected in series with the input of the NanoVNA to create a realistic impedance. That is why the reference level is -16 dB instead of 0 dB. And... The test results were exciting! A bandwidth of only 5 kHz! For 518 kHz, the suppression of the mirror frequency is 24 dB and it is 16 dB for 490 kHz! Much more than needed to suppress the noise on the mirror frequencies! And it will suppress the strong AM broadcast stations! I can even use the ferrite rod to receive strong Navtex stations. And a coupling winding of 2 windings to connect a long wire antenna for DX results! The ferrite rod antenna makes the simple Barefoot Navtex Receiver really simple!!! Other ferrite rod antenna's showed similar results. Let's have a look at the simple design!


Simple diagram!

A simple Barefoot Navtex Receiver thanks to the ferrite antenna!
What a nice, simple diagram! Only one chip, a very standard oscillator-divider chip. And for the rest only standard transistors and a FET as a mixer! This is the simple diagram that I want!!! Barefoot is the symbol for simplicity! A simple Barefoot Navtex Receiver thanks to the ferrite antenna!

Ferrite rod bandfilter (and antenna)
The ferrite rod antenna should not be placed close to metal objects and should also be kept away from sources of interference such as switching power supplies. So don't build the receiver in a metal box!!!
A coupling winding of 2 windings makes it possible to connect an external antenna. This coupling winding is completely isolated from the receiver. No conducted interference from the PC via the receiver can reach the antenna circuit! The ferrite antenna is tuned to the Navtex frequency of 518 kHz. For 490 kHz, an extra capacitance is connected in parallel by means of a switch. This is the only switch to change between the two Navtex frequencies! The capacitance for my ferrite rod is approximately 240 - 270 pF, but might be different for your ferrite rod.
The 10 nF capacitor creates a low-impedance tap for the HF transistor amplifier.

HF amplifier
One transistor is the HF amplifier, the gain is approximately 20 - 30 dB in combination with the ferrite rod bandfilter. Use a low-noise transistor like the BC550c or an older BC549c. The 470 ohm resistor with the 100 pF capacitor at the base of the transistor are a low-pass filter that suppresses signals from strong shortwave transmitters a little extra. The 1 nF capacitor with the 470 ohm resistor at the output are a high-pass filter that suppresses low frequency noise and signals (especially 10 and 18 kHz!) from the HF amplifier.

Mixer
The mixer is a BS170 FET. It is simple, it behaves like a switch that is switched on and off with a frequency of 500 kHz. It has a cut-off voltage of +3 volts or less and can be directly connected to the 74HC4060 oscillator with 5 volt TTL output. At the output there is a low pass filter of 2 capacitors of 10 nF and a resistor of 470 ohm. It suppresses frequencies above 18 kHz. What can I say more about such a simple mixer? Nothing!!!

Low Frequency amplifier
No op-amps, but ordinary transistors! The first transistor has a gain of (collector resistor / emittor resistor), that is 4700 / 47 = 100x or 40 dB. If you need less gain, increase the value of the 47 ohm resistor. The bias is obtained by a 1M resistor from the collector to the base. The exact collector voltage is not important, it is a low-level amplifier. The 1 nF capacitor at the collector is a low pass filter for extra attenuation of frequencies above 18 kHz.
The second transistor has a gain of 100k / 10k = 10x or 20 dB. Because of the feedback via the 100k resistor, it has a low-impedance output. Signals picked up by the LF output cable to the sound device are suppressed by this low impedance. The DC-collector voltage is approximately 0,7 x (100k + 47k) / 47k = 2,1 volt. Why not half the supply voltage? It is better suited for higher output levels in combination with low impedance loads. The 47pF and 1nF capacitor have a low pass filter function for extra attenuation of frequencies above 18 kHz.
The coupling capacitors of 1nF attenuate frequencies below 10 kHz so that the band pass of the LF amplifier is more or less limited between 10 kHz to 18 kHz.

500 kHz oscillator
A 8 MHz crystal and a 74HC(T)4060 oscillator-divider, very simple and reliable. If possible, take an HC type and not an HCT type. An HC type has a larger supply voltage range. Choose the value of Ra so that the supply voltage at pin A is 5 volt. For my version, Ra is 3900 ohm. But you can also buy small 500 kHz crystal oscillator blocks on the internet for a few euro's!

Low impedance eliminator, a very strange component!
The impedance of the receiver is very low at other frequencies than the reception frequency. And the coupling winding of the ferrite rod was shortened by the low impedance of the active antenna. It did not work as a good bandfilter anymore and the receiver was overloaded by strong AM broadcast stations. The mirror frequency was not suppressed anymore. Both problems were solved by the Low impedance eliminator resistor. It hardly affects the sensitivity of the receiver. When I connect the antenna, the noise increases, so the sensitivity is okay!

Various
In the power supply line you can find a 100 ohm resistor, 100uF and 0,1uF capacitors. They do suppress noise and interference of the supply voltages


The simple Barefoot Navtex Receiver, a simple construction!

The construction of the Barefoot Navtex Receiver
The receiver is constructed in a cheap but solid plastic box. Drilling holes is easy in the plastic box. The ferrite rod is mounted with two wooden clothespins. And the electronic circuit is soldered on a copper PCB, see the picture, that will explain it all. The construction of the components on an unetched piece of PCB copper plate is awesome! No much time waisted with the design of a PCB, no messing with chemicals! You don't have to keep turning the PCB over and over to solder at the backside. Components and soldering is only at te top side! And if you want to modify something, it is very easy to remove components and to replace them with the new design! PCBs are perfect for series production, not for a one-off production or for a prototype.
You can see that I did use two trimmers for each frequency for the tuning of the ferrite rod. That has a practical reason. I have many of those trimmers, bought once 50 or so, but they have a small value. That's why I connect two in parallel to get a larger value. And a better mechanical construction too.


The ferrite rod antenna is tuned by means of a SWR measurement with the NanoVNA!
Here without the Low impedance eliminator resistor of 47 ohm to have a better dip.

Tuning of the ferrite rod antenna
Tuning is easy with the NanoVNA! An awesome measuring device for less than 50 euro's!!!
I did remove the Low impedance eliminator resistor of 47 ohm to have a better dip and to measure the SWR of the receiver input.
Connect the antenna coupling winding to the S11 input and measure the SWR. You can tune the dip exactly to the right frequency of 518 kHz and test with the SWR if the impedance is more or less okay. First tune the ferrite rod to 518 kHz. Then close the switch and tune it to 490 kHz with the other trimmer for 490 kHz. As you can see on the diagram, my receiver has and SWR of 2:1, that is really perfect for a receiver!

NAVTEXsnoop software
The NAVTEXsnoop.py software is written in Python.
You can find the NAVTEXsnoop.py software by clicking the following link:

NAVTEXsnoop software

For 490 kHz Navtex reception, you have to modify the software to make it suitable for Lower Side Band reception.
The software will then select the 9500 - 10500 Hz frequency range in the last selection choice instead of 17500 - 18500 for 518 kHz Navtex reception.

Line 21: LowerSideBand = False
Change Line 21 to: LowerSideBand = True


Use a good USB audio device! The peak at the left is a good one,
the peak at the right a cheap, low quality one. I use the Behringer UFO202.

NAVTEX 518 kHz

The Barefoot Navtex Receiver is very simple, does it really work?
Click the link "NAVTEX 518 kHz" for the reception results, the simple Barefoot Navtex Receiver is a success! A very simple receiver, it needs only 12 mA! It works! The suppression of the mirror frequency is 24 dB for 518 kHz and 18 dB for 490 kHz! Many Navtex stations can be received! The results during one day of testing were: 81 messages with the Barefoot Navtex Receiver and 80 with the Shortwave receiver! But... it is very difficult to find an interference-free place in the workshop for the ferrite antenna... Indeed, simplicity has it's limits. You cannot walk barefoot to the North Pole!

ZIP file with NAVTEX reception results

Received Navtex stations 518 kHz near Valencia Spain with a PA0RDT miniwhip antenna during a few days:

Navarea 1 - North Atlantic, North Sea, Baltic Sea
E - Niton
G - Cullercoats
J - Gislovshammar
O - Portpatrick
P - Netherlands Coastguard
S - Pinneberg
T - V - Oostende

Navarea 2 - East Atlantic
D - Coruna
G - Tarifa
I - Las Palmas
R - Monsanto

Navarea 3 - Mediterranean Sea
H - Iraklio
K - Kerkyra
L - Limnos
O - Malta
P - Haifa
Q - Split
R - La Maddalena
T - Kelibia
U - Mondolfo
V - Sellia Marina
X - Cabo de la Nao

Index PA2OHH