NASA Clipper depth transducer cable extension. Is it just TV co-ax?

[st599]

And just where did this (most unusual interesting) piece of information originate??

Vic

Nearly remembered it correctly.

77 Ohms characteristic impedance gives the minimum attenuation per unit length. (75 when rounded)
30 Ohms characteristic impedance gives the highest power handling capacity.
50 Ohms is a compromise between the two.

Lee T. H., 'Planar Microwave Engineering', Cambridge University Press, Cambridge 2004. pp 71-73.

The book gives the full mathematical workings to get these results.

 

[VicMallows]

Lee T. H., 'Planar Microwave Engineering', Cambridge University Press, Cambridge 2004. pp 71-73.

Thanks. Does sound an interesting book, though from the title and synopsis wonder how relevant at lower frequencies (and especially at 150kHz ).

Bit pricey for me though, even at £51 on Amazon.

Vic

 

[PabloPicasso]

my nasa depth sounder with all oe cable is not a reiable performer often needing the reset feature. so perhaps they used TV cable instead of the proper stuff??

 

[VicMallows]

my nasa depth sounder with all oe cable is not a reiable performer often needing the reset feature. so perhaps they used TV cable instead of the proper stuff??

The discussion about different cable impedances (50, 75, etc) was a bit of an academic diversion.

When it comes to the echo-sounder, as far as all typical yacht equipment is concerned, the transducer+coax form a tuned circuit at the designed resonant frequency (typically around 150kHz).. It is the overall capacitance of whatever cable is used that is relevant, hence the general advice not to shorten or lengthen the originally supplied cable. Signal loss will be insignificant at the frequency.

Sounds like you have some other problem like poor contacts?

Vic

 

[nigelmercier]

the transducer+coax form a tuned circuit at the designed resonant frequency (typically around 150kHz)...

And by using the wrong impedance co-ax, this tuning is messed up. 150KHz is in the LW AM radio band, so we are still talking RF here.

 

[VicMallows]

And by using the wrong impedance co-ax, this tuning is messed up. 150KHz is in the LW AM radio band, so we are still talking RF here.

Agreed. But my argument is that the echo-sounder will still be messed up if you extend the coax (or reduce it) even with the correct identical cable and perfect connections due to the changed overall capacitance. I see this as quite different to the more usual situation where the primary aim is to maintain a constant impedance along a transmission line of indeterminate length.

However interesting new research comes along all the time, as SJBs post shows, so always open to persuasion.

Vic