> -----Original Message----- > From: [email protected] [mailto:Repeater- > [email protected]] On Behalf Of Jeff DePolo > Sent: Saturday, August 14, 2010 10:45 PM > To: [email protected] > Subject: RE: [Repeater-Builder] Coax length, etc. > > > Jeff, you aren't stepping on my toes at all. Glad to see your > > comments. > > OK, good. Since you've never met me, I can assure you, you definately DO > NOT want me stepping on your toes, it would be painful. > > > I do have to agree with Kevin that most duplexer > > manufacturers recommend different cable length trials between > > the transmitter and the duplexer when full power can not be > > reached into the duplexer. > > Ah, but the crux of the matter is that we're not changing the performance > of > the duplexer, we're just getting the transmitter to transfer more power > into > the line.
Yes! I fully agree. > > > Over the years I have been a manufacturers rep for TX-RX, > > Sinclair and Telewave. All of them recommend the same thing. > > Again, it's a CYA measure as Kevin pointed out. PA won't make power? > Don't > blame us, try mucking with the cable length, see if that helps. But it is not necessarily the duplexer's problem. > > > I am not a transmitter expert but it is my understanding that > > the problem is not one of the duplexer not presenting 50 ohms > > at the wanted frequency but the impedance that it presents > > off frequency to the transmitter finals. Some solid state > > devices do not like to see high reactance, even off > > frequency. > > But why? If all of the power (or, let's hope, at least 99.999999% of it) > is > on-channel, *should* a properly-designed and properly-functioning > transmitter misbehave due to the poor match a duplexer presents at > frequencies far removed from the channel center? Well yes, properly designed transmitter. But how much do you want to pay for it? A built in isolator will solve all of those problems as an example. > > > For one thing the reactance causes them to draw > > more current than normal. > > Again, why? Not sure why. I have been told by device engineers that is a characteristics of some devices. > > > This may be why you find that > > tuning for minimum pa current and maximum power out don't > > exactly agree with one another. > > I can promise you they almost never do, but that's not any great mystery. > > > You are probably finding a > > balance between the off frequency reactance and the on > > frequency wanted load that the finals see. > > No, that's not it. The off-frequency Z issue is a totally separate topic > from the efficiency vs maximum output subject. Let's keep those two > topics > separate for the sake of this discussion. If what you find in tuning happens directly into a 50 ohm load I agree. > > > If you have the duplexer properly tuned to provide 50 ohms at > > its input port, the transmitter may still not be happy > > because of the off frequency reactance presented by the duplexer. > > I disagree. I would accept the notion that the transmitter may not be > "happy" (and I put that in quotes not to mock you, but becuase I can't > come > up with a better word either) because it is not *properly matched* when > looking into a 50+j0 load. This indicates a deficiency in the amplifier; > if > it were designed and working right, it *should* make rated power when > terminated in a 50 ohm load on-channel. Yes it would be a transmitter problem. Maybe as designed. > > > Changing the cable length in this case really does nothing > > for the on frequency load between the duplexer and > > transmitter, when the duplexer is presenting 50 ohms, but it > > can change the off frequency impedance transformation that > > the transmitter sees. > > Yes, but again, I argue that this all points back to a PA problem. Or the > input Z of the duplexer really isn't 50 ohms and the line is acting as a > transformer. Again I agree. In this instance I was describing a duplexer that "did" present 50 ohms at the operating frequency and still the transmitter was not "happy". Because of the off frequency impedance being transformed to something that the transmitter does not like. It is almost impossible for a high Q cavity to not present some reactance away from the tuned frequency. If it didn't then it would not have any selectivity. The random length cable of course transforms that reactance to something that the transmitter may or may not be comfortable with as discussed above. > > > Detuning the duplexer and or changing > > cable length to get the transmitter power up is the wrong way > > to go here. First the transmitter should be optimized into a > > 50 ohm load. Then optimize the duplexer input for 50 ohms input. Of course I am talking about when the duplexer is presenting a good 50 ohm input impedance at the operating frequency. > > Yes, yes, yes, amen! > > > Someone asked about a "rule of thumb" for transmitter to > > duplexer cable length. There is none! > > Yes there is. You take out a tape measure and the distance from the > transmitter to the duplexer. You make the cable at least that length. And if the transmitter balks,, you place an isolator on it. :>) 73 Gary K4FMX > > > The cable length between multiple cavities is predictable. As > > an example between two notch cavities; the first notch > > presents a very low impedance. With a quarter wave line to > > the next cavity that low impedance is transformed to a high > > impedance at the input to the next cavity. That high > > impedance is then presented with a very low impedance of the > > second cavity. This critical length cable increases the > > ultimate notch depth because the high impedance that the > > cable presents and the low impedance of the cavity form a > > voltage divider. The greater the ratio the better the rejection. > > > 'zactly. When done right, you can pick up close to 6 dB additional net > notch depth when cascading notch (or pass/notch) cavities when the > intra-cavity cables are cut this way. > > ---- Jeff WN3A > > > > ------------------------------------ > > > > Yahoo! Groups Links > > >
