nuckollsr(at)cox.net
Guest
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 Posted: Sun Feb 04, 2007 9:55 pm Post subject: Sharing ship's COMM antenna with the hand-held |
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At 10:19 PM 2/4/2007 -0700, you wrote:
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One more comment... If you make a T in a transmission line and each leg
is of the same characteristic impedance there will be reflections
generated at the T. A device transmitting from one leg of the T will see
a section of the characteristic impedance line and then at the T the
impedance will appear to be half the characteristic impedance. The (any)
change in impedance in the transmission line will cause at least some
reflection. At a T each edge coming out of the transmitter will be
reflected at half the negative amplitude of the outbound signal. Needless
to say, this is likely to cause hi SWR and low transmitted power.
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T-connectors cannot be used for sharing a single antenna
with two transceivers. Under certain conditions, T-connector
are used to divide power between multiple antennas in an
array but they're also used in conjunction with some form
of impedance transformers (usually 1/4-wave chunks of
special coax) to keep all the SWR and power transfer
gremlins at bay.
The biggest problem with T-connectors for two transceivers
and one antenna is that transmitter energy from the speaking
transmitter is directly coupled to the input of the listening
receiver. Not only are there SWR problems but potential for
damage to the listening transceiver's front end.
There ARE techniques for having two transceivers share
a single antenna with a device called a diplexer. These
are generally very narrow bandwidth devices. I've built
diplexers that allowed a receiver bring in a 0.5 microvolt
signal from an antenna that is simultaneously radiating
100 watts of power from a transmitter in the same cabinet.
This is how repeaters are built . . . but they're fixed
frequency devices. For example, a repeater I built to
go up on the 1200-foot platform of KTVH . . .see
http://www.aeroelectric.com/Pictures/Misc/KTVH.gif
used a duplexer (duplexer/diplexer . . . these
terms are used interchangeably to describe a
variety of devices used to effect some form
of sharing for single antennas) that was about
as big as the repeater. Example can be seen
at:
http://www.utm.edu/staff/leeb/duplexer.pdf
It's not that two transceivers cannot share
a single antenna efficiently . . . but it's
done only under special circumstances and
in situations where the two transceivers don't
get very close to each other in operating
frequency.
Bob . . .
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