Actually, Gary, you are 180 degrees out. On a pass cavity, off frequency
signals see a very high impedence path, an open not a short. If your version
were true you could never use pass cans as a duplexer since both sets of cans
together would show a "short" to EVERYTHING.
The T connector is just an impedence bump to the radio equipment, nothing more.
It is not an active device, like a preamp would be, that makes the rest of the
feedline disappear.
He can use the T connector and any random length of cable to connect, as long
as the whole feedline doesn't show up as a resonant length.
Dan N8DJP
Posted
by: "Gary Schafer" gascha...@comcast.net
k4fmx
Date: Wed Mar 10, 2010 10:47 am ((PST))
Well yes
the T is sort of a magical device that makes the OTHER SIDE of the
T
disappear electrically. Actually it is not the T itself that does the
job
(that is just where IT happens) but it is the quarter wave length
cables
that perform the magic!
Without the quarter
wave length cables between the T and each set of
cavities the
duplexer would not work! That is what provides the 50 ohm
isolation
between tx and rx cans so the feed line still sees 50 ohms.
The
quarter wave cable effectively "disconnects" the transmitter from the
feed
line at the T (at the receive frequency).
The quarter wave cable
on the receive side of the T effectively disconnects
the receive
side from the feed line (at the transmit frequency).
Without
doing this each would load the other down and there would not be 50
ohms
at the antenna port of the T.
Once you are on the other
side of the T (the antenna port) the feed line
length has no effect
on the duplexer operation. All that the quarter wave
lines do on the
duplexer side of the T are to give isolation to the opposite
side
(tx-rx) so each does not short out the feed line.
A
similar thing happens between can cables in a duplexer but rather than
using
them for isolation they are used to enhance the notch of each can by
presenting
a high impedance at each cans T from the previous cavity. Working
with
a high impedance is easier to notch out than a low impedance.
The
notch in the first cavity presents a short (low impedance) at the
unwanted
frequency and 50 ohms at the wanted frequency. By coupling the
next
cavity with a quarter wave length cable (at the unwanted frequency)
that
short is transformed to a quite high impedance at the next cavity while
at
the same time the wanted signal being at 50 ohms is passed to the next
cavity
where it sees 50 ohms and goes on its way unatenuated. But we are
left
with the high impedance at the unwanted frequency that was transformed
by
the quarter wave cable. The second cavity notch is also tuned to the
unwanted
frequency which it pulls down to a short (low impedance) to give
further
attenuation.
When I say the notch presents a "short" it
is not really a short but a very
low impedance of say a few ohms.
But by having the unwanted source impedance
high rather than at 50
ohms it is much easier to pull the high impedance
down with the "few
ohms" short circuit than it would be if we were working
at 50 ohms
for the unwanted.
It works like a voltage divider between the two
impedances. The higher the
source is (from previous cavity) to the
short the more loss there will be
which is just what we are looking
for.
In the case of the quarter wave cable to the T on
the output of the duplexer
we want to transform the low impedance up
to a very high impedance at the T
so that it does not load the
circuit at that point on that frequency.
73
Gary
K4FMX
