Kevin thank you for the fine explanation. Digging deeper into my junque box, I have come up with a Phelps-Dodge duplexer which was on a low power in-house repeater on 464 mc. This one is designed for TX low/RX high, and is the one that will be used for this project. I am tuning and evaluating it now. In the name of education, I will be comparing these two units as measured on the bench and noting the differences between them for future reference.
Thanks again Laryn K8TVZ --- In Repeater-Builder@yahoogroups.com, Kevin Custer <[EMAIL PROTECTED]> wrote: > Laryn Lohman wrote: > > >Tuning a TFE6030 duplexer. I am aware that the high/low arrangement > >of frequencies should normally remain the same. However this unit has > >three cavities (helicals?) on what was the TX (higher I believe) side, > >and only one cavity on the RX (lower) side. For the ham band the TX > >will be the lower frequency. What is the recommendation in this case? > > > > Not to use it on a repeater where the transmitter is the lower > frequency. Why? Read your question below first, then read on. > > >Also, those three cavities have an additional adjustment on the coax end of each. What is this for? > > > > To notch out transmitter side-band noise. (undesired energy in which > appears directly on the receiver frequency) > > You see, it generally much more important (or more of a requirement) to > eliminate transmitter side-band noise *on* the receiver frequency than > it is to eliminate transmitter carrier power from getting into the > receiver. This is the reason why there are more sections of filtering > on the transmitter side of this duplexer. > > Let's go into some math and theory. While this text represents a 2 > meter duplexer scenario, the basic theory is the same for any band. > > A duplexer must attenuate the transmitter carrier, this prevents > receiver overloading, which in turn would reduce receiver sensitivity. A > duplexer must also attenuate any noise or spurious energy from the > transmitter on or near the receive frequency. In addition, a duplexer > must provide a proper match between transmitter, antenna and receiver. > If you are having trouble visualizing these functions, take a look at > this link: > <http://www.repeater-builder.com/pix/dupfig13.jpg> > > Transmitter output on 146.94 MHz going from point C to D should not be > attenuated. However, the transmitter energy should be greatly > attenuated between points B and A. Duplexer section 2 should attenuate > any noise or signals that are on or near the receiver input frequency of > 146.34 MHz. For good reception, the noise and spurious signal level > must be less than -130 dB (0 dBm = 1 milliwatt into 50 ohms). Typical > transmitter noise 600 kHz from the carrier frequency is 80 dB below the > transmitter power output. For 60 watts of output (+48 dBm), the noise > is -32 dBm. The duplexer must make up the difference between - 32 and > -130 dBm, or -98 dBm. > > Now, let's talk about the received signal. First of all, the received > signal must go from point B to A with a minimum of attenuation. Section > 1 of the duplexer needs to provide enough attenuation of transmitted > energy to prevent receiver overload. For an average receiver, the > transmitter signal must be less than -30 dBm to meet this requirement. > It is the job of duplexer section 1 to make up the difference between > the transmitter output of + 48 dBm and the receiver overload point of > -30 dBm, or a reduction of about 78 dB. > > A typical duplexer, that has the same construction for both sides, will > generally give several dB of receiver overload headroom, but may *just > make* or *not even make* the transmitter side-band requirement for no > desense. > > My recommendation is not to use this Motorola TFE6030 duplexer in a > situation where the transmitter is on the side having only one cavity > because insufficient transmitter side-band noise reduction will result > and your system will have unresolvable desense. It will, however, work > fine on a repeater (or full duplex link) that transmits high and > receives low. > > Hope this helps... > Kevin Custer Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
