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Without knowing really what you are dealing with, some instant caveats, and comments aimed at the general audience, since this hasn't been posted for a while: Brand-new coax can be bad, bad, bad. Take any new length and run a dummy load on one end. Sweep it to twice the highest frequency you intend to use with an analyzer looking through a common mode choke. Or use a Bird and check it on all bands. Tape some paper to the coax with the result of the test and the date of purchase/date of test. A lot of low pass filters (most?) start to mess with swr on ten meters. This can be true even if the SWR is low into a dummy load. The common impedance analyzers (MFJ 259x, etc) are very susceptible to interference from strong signals nearby, and also can have problems with common mode current on coax when a common mode choke (eg, beads over coax type "balun") is NOT used between analyzer and coax. 50 watts of exciter power and a Bird type wattmeter does not suffer from the strong signal problem. The coax INSIDE an amp to and from it's changeover relay(s) can cause SWR anomalies (when the amp is off). The better engineered HF amps have a matching component or two to make the inside wiring flat on ten meters. Miscellaneous pieces of hookup coax in the shack can have anomalous behavior. This possibility increases as frequency goes up. I had a one foot RG8 jumper with PL259's on either end that was transparent on 80/40 meters. It was a 20 db block at 432 MHz. Never did understand WHY. But it definitely DID screw up 432. The **ONLY** (I repeat, **ONLY**) way to measure SWR without danger of antenna feedline SYSTEM cobbling the SWR and confusing the issue, is to measure AT THE ANTENNA with **NOTHING** (I repeat, **NOTHING**) in between. Any other setup requires careful snooping, with proper attention to possible strangeness in the feedline system. No assumptions allowed. In just the last several years I have seen EVERY SINGLE THING commonly stuck in a feedline, at one time or other, detected as a cause of an SWR difficulty originally blamed on the antenna, INCLUDING a brand-new Bird wattmeter (sent back to the factory and had the part that the slug fits into replaced, admittedly about as uncommon a problem as one could get). Hope some of this helped. Let us know what is *really* going on. 73 & GL, Guy. > > From: Marty Tippin <ki0lo@yahoo.com> > Date: 2001/06/19 Tue AM 10:26:19 EDT > To: Force12Talk@qth.com > Subject: [Force 12 Talk] C3S - Adjusting 10m SWR curve ? > > I have a C3S up and it works fine - only issue I have is that > the SWR is lowest at about 29MHz, rising to well over 2:1 at > 28.300. > > How can I adjust the point of lowest SWR on 10m without screwing > up the other bands? The parasitic nature of the driven elements > makes me think it's not as simple as adding a few inches to the > 10m driven. > > 15 and 20 are not bad - 20 shows acceptable SWR across the band, > and 15 is a little bit low but not worth worrying about. > > -Marty NW0L > martyt@pobox.com > > __________________________________________________ > Do You Yahoo!? > Spot the hottest trends in music, movies, and more. > http://buzz.yahoo.com/ > > > -------------------------------------------- > Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. > Force 12 Web site: http://www.qth.com/force12 > > Submissions: send to Force12Talk@qth.com > To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com > Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk > For problems with the list, contact n4zr@qth.com > > > -------------------------------------------- Force12Talk mailing list provided as a service by Force 12 Antennas, Inc. Force 12 Web site: http://www.qth.com/force12 Submissions: send to Force12Talk@qth.com To unsubscribe: send a blank e-mail to Force12Talk-leave@qth.com Force12Talk Message Archive: http://www.qth.com/force12/list/force12talk For problems with the list, contact n4zr@qth.com |
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