RE: [Repeater-Builder] Coax length between added cavities
So my take is that there are critical cable lengths involved for adding a pure pass cavity to a BP BR duplexer, but I would be interested to hear from anyone aboard who has the necessary hardware kicking around to repeat that experiment and either replicate or refute the results I got. As Bob and others have said, the cable length is not critical from a Z-matching standpoint if both cavities are tuned for 50 ohms at the pass frequency, but the cable length will affect the reject response. As I said, my sole cavity experience has been with Wacoms, but I find it difficult to believe that this parameter is OEM specific. I'm doing some RD work today and have the VNA (Agilent E5070B) lit up, so I'll do some real-world tests and post the results. Taking a quick look around here, I see some Wacom 900 MHz pass/reject and pass-only cavities, so those will be my test subjects. --- Jeff
Re: [Repeater-Builder] Coax length between added cavities
To All: Over the past 50 years I have worked with most all Cavity mfg's on special and standard product applications. Let me assure you that the interconnecting cables are critical. Some applications more so than others. Msg for Nate; Before you try shot-gunning the symptoms, please spend the needed time to identify the problem, then a cure can be better established. Yes, this will take more time, but it will be well worth it in the end! Fred W5VAY (Retired Engineer) - Original Message - From: [EMAIL PROTECTED] To: Repeater-Builder@yahoogroups.com Sent: Friday, July 27, 2007 12:03 AM Subject: [Repeater-Builder] Coax length between added cavities Can we wind our way back to addressing the original query which asked if there is a critical length for the interconnect between a BP BR duplexer and added pure pass cavities? I can't speak for all bottle manufacturers, but I own 8 Wacom BP BR duplexer + pass cavity arrays some of which go back to the early eighties. Lloyd Alcorn was kind enough and patient enough to give me a pretty good nuts and bolts education on cavity characteristics. He said in no uncertain terms that there were optimum cable lengths for both the interconnects between the duplexer cavities and also an optimum length for any pure pass cavities added to the chain. For the added pure pass cavity, the optimum interconnect length would ensure that the pure pass curve would superimpose over the duplexer curve. When I did some experimenting with the pure pass cable length, it validated his point. If I significantly lengthened or shortened this cable, the tracking generator would indicate that the pass curve was no longer superimposed on the duplexer curve. It would either lead or lag the duplexer curve producing two results: 1) the composite curve began to show some distortion and 2) the total attenuation at the desired frequency was higher than when the optimum cable length (supplied by Lloyd) was used. So my take is that there are critical cable lengths involved for adding a pure pass cavity to a BP BR duplexer, but I would be interested to hear from anyone aboard who has the necessary hardware kicking around to repeat that experiment and either replicate or refute the results I got. As I said, my sole cavity experience has been with Wacoms, but I find it difficult to believe that this parameter is OEM specific. K7IJ -- Get a sneak peek of the all-new AOL.com.
Re: [Repeater-Builder] Coax length between added cavities
Repeating the experiment? Hmmm. We were re-cabling a set of Sinclair duplexers for a 2-meter machine and had access to a high quality vector voltmeter. It took three of us Keystone Cops over 3 hours to make a set of 1/4, 1/2 and even a couple of 1 wavelength cables. Later, we repeated the process for a 220 machine and even though we had experience, it took about the same amount of time. My point? It ain't easy ! We just weren't ready to use the time and materials to add an inch to a set of cables and then make another set of short ones. In both cases, things worked out great. Many hams who are delving in cavity alignment don't have a vector voltmeter, so we were in good shape. Plus, you have a warm fuzzy comfort factor if you know your cables are the right length when you start the job. If alignment ain't happening, you can say I know it ain't the cables!...right ! And I got some ocean front property here in western Arizona de WD7F John in Tucson So my take is that there are critical cable lengths involved for adding a pure pass cavity to a BP BR duplexer, but I would be interested to hear from anyone aboard who has the necessary hardware kicking around to repeat that experiment and either replicate or refute the results I got. As I said, my sole cavity experience has been with Wacoms, but I find it difficult to believe that this parameter is OEM specific. K7IJ Get a sneak peek of the all-new AOL.com. No virus found in this incoming message. Checked by AVG Free Edition. Version: 7.5.476 / Virus Database: 269.10.20/919 - Release Date: 7/26/2007 9:56 AM
RE: [Repeater-Builder] Coax length between added cavities
We just weren't ready to use the time and materials to add an inch to a set of cables and then make another set of short ones. Connector savers can be incrementally added to a cable when experimenting to find the optimum length. Connector savers are male-to-famale adapters of the same connector type. They are frequently installed on test equipment to save wear and tear on the built-in connector, hence their nickname. They are about the shortest length you can add from a mechanical standpoint to cables that use type N connectors. A line stretcher is the best way to do it, but connector savers are quick and dirty, and their incremental electrical length is small enough to make them effective for this purpose at the frequencies we normally deal with below 1 GHz. High-quality elbow adapters can be used in a pinch instead, but there are some caveats with respect to the impedence bump they can create (see the blurb I put together earlier today that touches on this subject). They are also electrically longer than connector savers, typically making them useful only below about 500 MHz in many cases. If you do this kind of work on a regular basis like we do, you end up with a few dozen test cables of varying lengths (nominally 1/2 incremental difference in length) to use when building a system. Once the optimum cable lengths are found, new cables are made for the final installation. BTW, when making up cables based on manufacturers' recommend cable lengths, make sure you know whether the cable length they specify is the cut length or whether it's tip-to-tip. Some manufacturers, like TX-RX, specify their cable lengths based on the cut length before the connectors are installed, so the overall tip-to-tip length ends up being about 1.25 longer when type N crimp connectors are installed. --- Jeff
[Repeater-Builder] Coax length between added cavities
Can we wind our way back to addressing the original query which asked if there is a critical length for the interconnect between a BP BR duplexer and added pure pass cavities? I can't speak for all bottle manufacturers, but I own 8 Wacom BP BR duplexer + pass cavity arrays some of which go back to the early eighties. Lloyd Alcorn was kind enough and patient enough to give me a pretty good nuts and bolts education on cavity characteristics. He said in no uncertain terms that there were optimum cable lengths for both the interconnects between the duplexer cavities and also an optimum length for any pure pass cavities added to the chain. For the added pure pass cavity, the optimum interconnect length would ensure that the pure pass curve would superimpose over the duplexer curve. When I did some experimenting with the pure pass cable length, it validated his point. If I significantly lengthened or shortened this cable, the tracking generator would indicate that the pass curve was no longer superimposed on the duplexer curve. It would either lead or lag the duplexer curve producing two results: 1) the composite curve began to show some distortion and 2) the total attenuation at the desired frequency was higher than when the optimum cable length (supplied by Lloyd) was used. So my take is that there are critical cable lengths involved for adding a pure pass cavity to a BP BR duplexer, but I would be interested to hear from anyone aboard who has the necessary hardware kicking around to repeat that experiment and either replicate or refute the results I got. As I said, my sole cavity experience has been with Wacoms, but I find it difficult to believe that this parameter is OEM specific. K7IJ ** Get a sneak peek of the all-new AOL at http://discover.aol.com/memed/aolcom30tour
