<<< see embedded responses (w/VERN prefix) below >>> --- In Repeater-Builder@yahoogroups.com, "skipp025" <[EMAIL PROTECTED]> wrote: > > A quick reply... > > > "w6nct" <w6nct@> wrote: > > I am setting up my duplexer using four Phelps-Dodge cavities (all > > in series for Rx), and two Cell-wave cavities (in series for Tx). > > All of these are individual, cylindrical, PASS-REJECT cavities. > > Are the 5 or 8 inch... or some other size diameter cavities?
VERN: All are approximately 4" diameter by 8" long. > > At this point, I have a setup and test methodology to tune the > > PASS and REJECT adjustments; and have done so for the individual > > Phelps-Dodge cavities (mostly as a pre-tune and confidence > > building exercise). > > Nothing wrong with taking the time to learn... I do it all the time > and it really pays off. > > Undestand a single cavity adjusted location will be different from > a dual series cavity setup... and different from a triple cavity > setup. Broadly speaking you might see how additional cavities change > the final adjustments. So starting from scratch you'll get single > cavities in the ballpark and the final assembly will require a > second or third final adjustment. VERN: What I've noticed already is that if I use "bad" interconecting cables (i.e., lossy, or ones not near odd 1/4 wave in electrical length); then the combination of multiple cavities in series changes a lot. But the closer the interconnecting cables are to "good" ones, the less I need to re-tune once I connect the cavities in series. I've also noticed that I can work to tune up a duplexer system with "less than ideal" cables; but it's much more difficult and touchy, and doesn't seem to provide as good of an end-result. I expect that the duplexer system will work much better (i.e., more stable and and more efficient) if I invest the time into setting up the best interconnecting cables I can. I'm expecting that I may have a bit more of adjustment when I connect the Tx string to the Rx string, using the T-connector; since the two strings are tuned for different frequencies. We'll see... > > I have not re-tuned them in the full duplexer system configuration > > as I am still > trying to make up the "proper" length cables (see > > other related topic thread, "measuring coax for duplexer"). I'm > > still a little stuck in making these cables, due to the unknown > > length and velocity factor for the loop inside the cavities. If > > you have any thoughts on this topic, please add them to the > > associated thread. > > You didn't mention the type of coax you are using...? > Consider the physical loop/probe length to be added to > the coax length. The coax will have a velocity factor > of say... .66 to .93 typical and the internal loop/probe > length is most often considered free space, which is > also known as vel= 1.0 > > I don't march with the crowd on this one... every time > I try to follow the various posted cable length text the > results never seem to work out. So I use my own methods > and test gear, which gives me the nearly exact performance > of the specific network I'm working with. If you want > some rules of thumb I like to consider... I'd need to > know more about the cavity loops/probe assemblies. Email > me direct if you like. VERN: I'm using RG-400 (Vf=0.695), using N-connectors on Phelps-Dodge and BNC on Cell-wave cavities. Several from this repeater group have indicated that the loops (one per port) inside adds to the length; even though Hams local to me aren't convinced. I've already decided to believe the group's experts; but I am having a bit of problem determining "how much length it adds". I suspect that if I had a TDR I could tell; but I haven't found a way to get my MFJ to tell me. I hand-calculated a starting length for the cable alone; and using that as a test/reference cable, I have been trying to measure the internal loops of the cavities. The measurements I have taken (in trying to figure it out experimentally) have shown that the apparent/combined electrical length (i.e., reference cable plus single cavity loop) change dramatically if I change the cavities bandpass adjustment; and it is also dramatically different depending on what I do with the other port (e.g., open, 50-ohm termination, or 0-ohm termination). Thus far, I haven't figured it out; or decides what termination is appropriate. Perhaps I should buy an extra cavity and cut it open to see what's really inside (?). > > Beyond that, I have the Tx/Rx sections of the repeater > > operational; and am trying to figure out how to best > > interface my (NHRC-5) controller and (ComSpec TS32) PL > > decoder/encoder (see other related topic thread, > > "Controller connection to RPT-21"). This portion is > > still in progress, trying to figure things out. > > If you have any thoughts on this topic, please add > > them to the associated thread. > > An original Standard RPT-21 ..? VERN: Yes, it was still virgin when I got it; still tuned to a commercial frequency. It needed some cleanup and repair; but otherwise appears to work ok. > > I've figured out a lot about using the Wavetek 1080. > > It's a pretty amazing piece of equipment; but I still > > haven't gotten the manual(s) for it. > > Yes... the Wavetek gear is really nice... > > > QUESTION 1: Do you have the same model Wavetek (i.e., 1080)? > > If so, do you have the manual(s) for it? > > I have two models... but the one on the bench here right now is the > 1062. No I don't have the manual for it or any of the Wavetek units. > Manuals plus might have the book if you really want/need it. VERN: I've found a couple sites that say that they have it; but most want to charge an arm and a leg for a Nth-generation copy. I'm holding out a bit longer in hopes of finding an original at a reasonable price; or a good quality copy at a cheap price. > > QUESTION 2: I'm curious about the "Marker" input on the front. > > Do you know what it is for, and how it is used? > > Depends on which model Wavetek you have. The Markers on some models > are fixed at various intervals (1 MHz as one example, 30 & 60 MHz > on other models) and show up as blips on the display. We can and > do input an external rf generator on one of our setups to allow an > adjustable/agile marker at or near your project frequency. The MFJ > 269b antenna unit works as an excellent signal source. VERN: The 1080 has a built-in (adjustable) marker generator; which is why the indicated "Marker input" is a little confusing. I'm guessing that it might be for injecting another marker from an external source; but I haven't been able to confirm it yet. > > QUESTION 3: Do you know how to read the display in terms of the > > calibrated signal amplitude, given a specific setting of the > > "Output" (in dBm)? > > It's relative to how you have the various knobs adjusted. > In operation you set the display to represent various values > as desired. It's one of those instruments easily learned by > watching someone use it. Out of the starting gate it's a pain > to figure out the first few times. VERN: I understand; been there, doing that! The local Hams fall into two categories: ones that don't know how to sweep cavities without a multi-thousand dollar piece of test gear, or the ones that haven't a clue. So (sadly) there's no one around here that I can look over the shoulder of. At least none that actually knows how to use a sweep/marker generator with an oscilloscope (I think that maybe I'll teach a class around here when I'm done with the repeater). > > More specifically, a friend was saying something like "in order > > to read the actual bandwidth of the PASS and NOTCH, I needed to > > measure it a the 6 dB point". Does that make sense? > > Yes... kind of. Your friend sounds like he's reading from page 134 > of a Malvino Electronics Text Book. It would be kind of windy to > start talking bandwidths of cavities here. In very short simple > terms you get what you get related to the Q of the cavity and the > possible adjustments you might make/set. VERN: Off-group, could you explain a bit more; or point me to some on-line references I could read about it? I'll look to see if the "Malvino" is a text I might still have from college, it sounds familiar. > > Do you know how I would do it using the Wavetek 1080's display on > > an oscilloscope? I suspect that it's just a libration/ > > visualization thing that I am missing. > > > I appreciate any wisdom and experience you can offer; especially > > relating to the Wavetek 1080. > > <<< vern >>> > > Mostly visualization... > > Depends on how you have the wavetek set up. We get > different results doing different things with it. > Probably easier to talk to you about using it on the > phone some time. Email me direct if you want to > try and yack on the phone about it. > > cheers, > skipp VERN: Will do. Thanks. By the way, what geographic area are you in?
