Steve,
As others have said it will probably work much better to
separate the lines at the antenna port of the duplexer.
Leave the transmitter and receiver connected as they are
now, but remove the T at the duplexer output and feed the
TX antenna from the TX side of the duplexer and the RX
antenna from the RX side of the duplexer.
Why?
All transmitters, no matter how good they are, have some
amount of noise (often referred to as sideband noise)
extending out a few MHz on either side of the carrier.
A duplexer serves two vital functions insofar as preventing
your repeater from desensing itself. First, it must
attenuate the transmitter fundamental enough to prevent
desense from receiver overload; the notches on the receive
side of the duplexer take care of that. Second, it must
attenuate sideband noise from the transmitter on the
receive frequency (otherwise this noise raises the receiver
noise floor and covers up weaker signals); the notches on
the transmit side of the duplexer handle that.
With close spacing of the antennas I don't think you will
have enough isolation to drop the sideband noise below your
receiver noise floor without the transmit side of the
duplexer to clean it up before it reaches the antenna.
Good luck troubleshooting your duplex noise problem!
The duplexer tuning might be slightly affected by splitting
the lines at the output. If you have the means it would be
worthwhile to check the tuning (at least the notches) after
making this change.
Paul N1BUG
On Sunday 01 May 2005 10:34 pm, k3phl wrote:
> Hello all,
>
> I am considering running split antennas at my 220 MHz
> site in Philadelphia as a test to increase sensitivity
> and troubleshoot a duplex noise issue. The machine is
> currently a modifed Motorola Micor using a TX/RX 4 can
> duplexer, DCI bandpass filter in the RX leg, and a Comet
> Super 22 (220) base antenna.
>
> I am considering running separate Super 22 antennas 12
> feet apart horizontally. One feedline run will go from
> the transmitter direct to the TX antenna. The other
> feedline run will run from the receiver to RX port of
> duplexer, common port of duplexer to 220 bandpass filter
> and then to the RX antenna. The TX port of the duplexer
> will remain unconnected.
>
> I expect the duplexer to act as a 2 can deep notch,
> eliminating the transmitter from the receiver with the TX
> port open. I assume the TX rejection in the receiver
> line should improve and the resulting RX sensitivity
> should improve since the transmitter has been reduced to
> a strong near field instead of RF on the same line. Is
> this the correct theory of using a duplexer in a split
> antenna site and if so, should the TX port be left
> unconnected or capped with a 50 ohm load?
>
> Thanks in advance,
>
> Steve
> K3PHL
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