> From: Eric Lemmon <[EMAIL PROTECTED]>
> Date: Thu, 07 Oct 2004 18:49:13 -0700
> Subject: Re: [Repeater-Builder] Re Circulator
>
> Andy,
>
> My understanding of British radio terminology is not complete, so I am a
> little confused by your statement about the "receive" port of the
> circulator.
>
> In circulator (isolator) applications I am familiar with, the input of
> the circulator connects directly to the transmitter output, the output
> of the circulator connects to the TX input cavity of the duplexer, and a
> 50 ohm load is connected to the "side" connector.
I learned that was an "isolator".
The product I work on in my dayside job has a four-port circulator. The
transmitter is connected to port one, which passes any signal
"clockwise" (for the sake of argument) to port two. The antenna is
connected to port two, the receiver to port three, and a load to port
four. Since all signals are passing clockwise, from port one to two,
port two to three, port three to four, and port four to one, the load on
port four is not visible to the transmitter. The receiver on port three is
not visible to the transmitter, assuming the antenna absorbs all the RF.
And any signals that the bandpass filter in the front end of the receiver
rejects do not go back out the antenna, as they would have to go
counterclockwise to get there. Ditto received signals and the transmitter;
the circulator won't allow them to go that way.
The same product also has an isolator in it at an earlier stage. What
goes in port one comes out port two; what comes in port two is turned
into heat in the load on port three. The energy coming in port two
can't go to port one because it's being steered clockwise by the
isolator.
Circulators are very common in microwave applications as they allow the
use of a single waveguide for both transmit and receive, minimizing the
problems inherent in running what is essentially a rectangular pipe from
the radio to the antenna and the problems of evenly illuminating a
reflector with two different feedhorns. I have casually wondered upon
occasion if a four-port circulator in a repeater would confer any benefit,
possibly increasing isolation and allowing the use of smaller, hopefully
less costly duplexers.
de kg7yy
[snip]
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