518kHz Navtex receiver

[polarized]

Hello,

I would need a simple antenna for 518kHz for Navtex reception. Are there any construction plans on the internet to make an antenna for this frequency? Or any suggestions and tips? The navtex signal is placed in a narow band around 518kHz, FSK modulation. My second question is: I have read that the navtex system uses an SSB transmitter, FSK modulation, where the baseband frequencies for "mark" are 1615 Hz and for "space" 1785Hz.The bitrate is 100 BPS. When an IF frequency in the receiver is produced by mixing the 518kHz band with 500kHz from the local oscillator a 18kHz band is produced. I am interested in the following: if the modulation index is low, are the 2 frequencies from the 518kHz(if we can disregard the upper harmonics(Bessel coeficients)) simply translated in a same manner into the 18kHz band and does the spectrum look very similar to an AM system? What would happen if the modulation index was not so small?Does the antenna used for the 518kHz reception depend on the filter circuitry on the receiver input? Because, there are different designs of receiver input filters.

Regards.

 

[flatulent]

At these low frequencies atmospheric noise is so high that you do not need an efficient antenna. Any random length of wire hanging from the ceiling of your room or a tree outside will work.

Your second question is about superheterodyne receivers. The frequency translation process is linear. That is the group of frequencies generated by the transmitter are shifted as a group to the IF of your receiver with the same differences in frequency and phase that they had at the transmitted frequenc.

 

[sergio mariotti]

I'm able to answer about antenna only.
I agree with flatulent about the atmospheric noise.
But the man made noise is very high too.
There is a way to reduce the |E| related noise simply using a loop antenna rather than a wire.
Bring pipes with wall thickness greather than penetration thickness (may be used pipes usad by electricians for explosion proof plants). Build a loop (square or round), the loop must be not "short ciucuited". The diameter should be 0.5 - 1 m.
Fill in 3 to 10 turn of copper wire.
(the resonance frequency should be at least 3 time greater than working freq.).

This antenna is sensitive to the |H| component of the EM radiation and it is relatively insensitive to most of man made noise as the atmospheric static discarge.
In the past, i've buid a larger loop for the ELF 10 KHz range with positive results.

 

[flatulent]

Another loop antenna to use is from the standard MF AM broadcast band radio receiver. You can copy the circuit of the first RF amplifier stage and raise the resonating capacitor which is usually a variable with 360 pF maximum capacitance. This new value should be in the region of 390 pF.