Skipp, Thanks for the suggestion. I have tentatively concluded that the "desense" problem is not classic desense caused by too much RF from the TX getting into the RX. I have used a spectrum analyzer and a "sniffer" probe to locate the RF. But the only RF I can find is at the TX frequency. I don't see any at the RX frequency. The dynamic range of the spectrum analyzer appears to be at least 40 or 50 dB. With the duplexer adding another 79 or so dB and the receiver having selectivity, I can't see how the RF level from the TX can be a problem. Nevertheless, when the repeater transmits, the receiver doesn't hear as well as otherwise. I'm thinking that the COR board might have a problem that is somehow feeding into the receiver. Have you ever heard of such a case?
The problem seems to be independent of the external cables and feedline and antenna. I have experienced it with dummy load, with antenna, without the duplexer, with various lengths of cables, etc. I'd like to have some RG214 for test purposes, and I'd like to have some additional RG400. Getting cable is a two-hour round trip for me, so I can't do that for a few days. I hope to get to the cable store (The RF Connection) soon. Thanks for all the help, Skipp. With you an others from Repeater Builders holding my hand, this problem might actually get solved. JohnT -----Original Message----- From: Repeater-Builder@yahoogroups.com [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 Sent: Wednesday, September 03, 2008 11:10 AM To: Repeater-Builder@yahoogroups.com Subject: [Repeater-Builder] Re: Wits End -- Desense (actual Cable-Q contributions) Desense (actual Cable-Q contributions) Hi John, There is a case where you can actually be fighting a complex problem with unwanted contributions actually made/introduced by the cable (feed-line) Q. To be more specific some combination of the antenna, duplexer, hardware circuit(s) in addition to or along with the coaxial cable. Where I'm going with all of this is... There have been example cases where unwanted product generation has been "fixed" by replacing portions of the antenna system coaxial cables with a less or lower Q cable. Some transmit antenna combiner low-level generation issues have been addressed with lower-Q coax jumpers. I have replaced higher-Q feed-lines with more resistive cable, which in more than one case has solved an otherwise pesky gremlin - grunge problem. I really like RG-214 and similar (relatively) more lossy double shield coaxial cables in some cases where they provide a slightly higher measure of resistive padding and lower Q versus rigid hard-line and higher Q cable. The double shield is a big deal... but the higher Q cable could very well be a portion of the grunge problem. As well as some problematic cable lengths (and other yet unknown issues). RG-214 is cheap enough to be an easy try... keeping in mind the "no free lunch" rule applies in many examples. As obvious your results will probably vary. One other item... pay attention to the actual RG-214 description aka mfgrs label as there seem to be a larger number of clone cables, which is not actually the mil-spec RG-214 cable "real deal". cheers, skipp > "John Transue" <[EMAIL PROTECTED]> wrote: > Well, I don't understand it, but yesterday afternoon the > repeater seemed to revert to a bad case of desense. Today > I will try to determine why this happened. Such is life! > JohnT > > > > -----Original Message----- > From: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > [mailto:Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com] On Behalf Of John Transue > Sent: Monday, September 01, 2008 9:09 PM > To: Repeater-Builder@ <mailto:Repeater-Builder%40yahoogroups.com> yahoogroups.com > Cc: [EMAIL PROTECTED] > Subject: RE: [Repeater-Builder] Wits End -- Desense > > > > Eric, Bob, and many other good folks, > > Success!!!! This being a holiday, I could not get RG400 to replace > the cable from the TX to the connector on the back of the repeater, so I > built a shield to completely enclose the cable. I also wrapped aluminum > tape around the receive cable in the chassis. Lo! And Behold! No more > desense! > > My sincere thanks go to all of you who have helped me through this > most vexing problem. The repeater will be far more useful now. > > Best 73s to you all, > > JohnT > > AF4PD > > > > > > __________ NOD32 3192 (20080616) Information __________ > > This message was checked by NOD32 antivirus system. > http://www.eset. <http://www.eset.com> com > __________ NOD32 3192 (20080616) Information __________ This message was checked by NOD32 antivirus system. http://www.eset.com
