BAREFOOT NAVTEX RECEIVER BY...
THE BAREFOOT NAVTEX NERD!
(2023)

Never walk barefoot on the ice when it is colder than -3C to -4C because of the risk of frostbite on your toes!
Barefoot Sailor! Long exciting barefoot walks with suffering way too cold toes
on the ice during the winter! And... Time to play with Navtex!!!

Barefoot Navtex Receiver
The barefoot Navtex Nerd promised George to make a very simple Navtex receiver, the Barefoot Navtex Receiver!
But it is winter and the Nerd has to go barefoot on the ice cold floor of his workshop! Shoes, socks and carpets are forbidden because of static electricity! Only 8C in the workshop and an ice cold tiled concrete floor! Impossible!!! Nobody can walk barefoot on such a cold floor! He looks at his ice cold suffering toes but still wants to start with this nice simple project. That ice cold floor is just as cold as when he had to walk barefoot on ice! Will a ferrite rod antenna bring the solution that the barefoot Navtex Nerd needs to make that simple Barefoot Navtex Receiver?


Barefoot Navtex Nerd with suffering way too cold toes on the way too cold tiled floor to prevent static electricity!
Will a simple ferrite rod bring the simple solution that he needs for the simple Barefoot Navtex Receiver?

He has 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 the 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!
The barefoot Navtex Nerd bought a cheap pack of five ferrite rod antennas. This might be the solution for the 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 on the far too cold tiled floor of his workshop! No problems with static electricity anymore! Barefoot on such an ice cold tiled floor is very challenging! 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 this 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 this 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 barefoot Navtex Nerd has terribly cold purple-red toes that have to suffer way too much on that ice cold floor of his workshop!
But he does not have problems anymore with static electricity and tries to continue with this exciting Navtex project!

The construction of the Barefoot Navtex Receiver
The barefoot Navtex Nerd looks at his horribly cold bare feet on the much too cold tiled concrete floor. His toes are absolutely not resistant to the extreme cold and have scary purple-red colors. Unbelievably, he still hasn't suffered any damaged toe joints!
After a few weeks, the Nerd has a great design! Oh, you can guess it, his first idea was also the best! A new challenge! Also the construction has to be simple! The Barefoot Navtex Nerd complains about his ice cold toes. But George encourages the Barefoot Navtex Nerd to continue with the exciting Barefoot Navtex Receiver project! George also wants to see how long the ice cold suffering toes of the barefoot Navtex Nerd will be able to survive the horrible cold! The Barefoot Navtex Nerd finally agrees... Despite his suffering cold toes with scary purple-red colors, he's going to make that simple "thing". After all, he promised that!


The simple Barefoot Navtex Receiver, a simple construction! The barefoot Navtex Nerd was able to finish this project,
despite his terribly cold toes that had to suffer way too much on that ice cold floor of his workshop!

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.
Two trimmers have been used for each frequency for the tuning of the ferrite rod. That has a practical reason. The barefoot Navtex Nerd has many of those trimmers, bought once 50 or so, but they have a small value. That's why two are connected 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!!! Everyone should have one. And barefoot, the Navtex Nerd can't destroy it with static electricity!
The Low impedance eliminator resistor of 47 ohm is removed ruing the tuning procedure 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, this receiver has and SWR of 2:1, that is really perfect for a receiver!

Again!
Yes, the Barefoot Navtex Nerd did it again! A really simple circuit! The Barefoot Navtex receiver! Barefoot for weeks on the incredibly cold stone floor! Amazing, it was far too cold for his toes, yet his horribly cold toes barely survived! And then there is that exciting moment, finally the simple Barefoot Navtex receiver receives the Navtex messages


A Spartan-living, poor, barefoot sailor with an old boat and no Navtex!
Barefoot on ice with ice-cold toes and an old compass, a very simple life!

NAVTEXsnoop software
The NAVTEXsnoop.py software is written in Python. You only need a simple Raspberry Pi PC to execute it!
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. The good one is 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


The miniwhip antenna and ice cold bare feet with suffering cold toes on that way to cold tiled floor!

Comparison with the standard ATS-20+ receiver used here for Navtex!
A simple test, the Barefoot Navtex receiver and the standard ATS-20+ receiver were connected to the same antenna. Which receiver does receive the most Navtex messages?
Barefoot Navtex rx: 1184 and the ATS-20+ rx: 1124. So the winner is the Barefoot Navtex rx! Is it really better or has the long suffering of the barefoot Navtex Nerd his way too cold toes been rewarded with extra positive energy from the Universe?

The Barefoot Navtex Receiver project is a success!
Barefoot on the ice cold tiled floor of his workshop to prevent static electricity was really way too difficult and too dangerous for the vulnerable toes of the barefoot Navtex Nerd! His suffering toes barely survived the horrific cold! But... the suffering of his far too cold toes is rewarded with a huge success with that simple Barefoot Navtex Receiver and the miniwhip antenna! He's already looking forward to the next challenging Navtex project, barefoot again on that far too cold stone floor!

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