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RE: [PSUBS-MAILIST] retractable buoy
Jens,
VHF radio is ~144 MHz. and there is a significant signal loss with an
impedance miss match which occurs when the insulative material in a typical
radio antenna coax compresses with depth. Harder core material can be used
but then you have issues with the minimum acceptable bend radius and it will
not coil on a small drum suitable for most PSUBs.
Yes I did receive your message and have to apologize for not replying
sooner...have been swamped since my return from 3 weeks of vacation. Will
mail the materials out at the first opportunity.
R/Jay
KF4DBI
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jens Laland
Sent: Thursday, October 29, 2009 4:14 PM
To: personal_submersibles@psubs.org
Subject: RE: [PSUBS-MAILIST] retractable buoy
Jay
In order for a cable's characteristic impedance to make any difference in
the way the signal passes through it, the length of the cable must be closer
to a wavelength long for the particular frequency it is carrying.
Most wires will have a speed of travel for AC current of 60 to 70 percent of
the speed of light, or about 640 million feet per second. A narrow audio
frequency band between 300 Hz and 3000 Hz would therefore represent
wavelengths ranging from 213,000 ft and up to 2,132,000 feet - so a cable
would have to be extremely long (and expensive) before it even began to have
an effect on the frequency of a good old-fashion, analog telephone
conversation.
That's why the characteristic impedance of audio interconnect cables is not
something for us to worry about.
Normal video signal rarely exceed 10 MHz. That's just a few inches more than
65 feet for a wavelength. Those frequencies are getting close to being high
enough for the characteristic impedance to be a factor. High resolution
computer video signals and fast digital signals easily exceed 100 MHz so the
proper impedance matching is needed even in shorter cable runs.
Best regards,
Jens Laland
PS. Did you ever receive my off-list email?
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