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## [Towertalk] Stub tuning

 To: [Towertalk] Stub tuning k2av@contesting.com (Guy Olinger, K2AV) Fri, 13 Sep 2002 13:43:28 -0400
 ```Without going into the math, however correct or incorrect it may be, the problem with using the math at all is that the actual velocity factor and characteristic impedance of a given piece of coax vary from "nominal" and can easily put your desired notch or peak well off your intended frequency. The last bit of coax I set up had a characteristic impedance of 54 ohms and a velocity factor of .693 (instead of 50 ohms and .67). That's enough to put a math derived dip at 14.225 actually out at 14.713, and likely useless at 20m cw frequencies, if even effective at 20m phone. The impedance thing is why a grid-dipper (however currently out of favor and manufacturing) is really the best method for nailing a section right to the quarter/half wave, etc. You can dip a section to within a few kHz, which is a practical requirement to get one to work for both the phone and cw segments on a band. Any real distance from dead on middle band, and phone or cw end starts losing effectiveness quickly. If one wants to get a good dipper, the old Millen jobs, a still functional workhorse, are available (for less than the analyzers) on EBay. Or pass one around the local radio club. 73 ----- Original Message ----- From: "Dino Darling" To: "TowerTalk" Sent: Friday, September 13, 2002 1:17 PM Subject: Re: [Towertalk] Stub tuning If you were trying to make a half wave stub, couldn't you just do this... 11802.8/ (frequency in Megahertz) * (velocity factor of coax) /2. This would give you an electrical half wave length of coax in inches. Example: 11802.8 / 14.250 = 828.27 * .85 = 704.03" ^ ^ 20 Meters LMR-400 (85%) 704.03 / 2 = 352.02 (Electrical half wave in inches). Or am I totally off here? Dino...K6RIX k6rix@arrl.net ----- Original Message ----- From: "Steve Katz" To: Sent: Friday, September 13, 2002 9:05 AM Subject: RE: [Towertalk] Stub tuning > It will, if you are trying to make a 1/2-wave, open-ended stub. --- StripMime Report -- processed MIME parts --- multipart/alternative text/plain (text body -- kept) text/html --- _______________________________________________ Self Supporting Towers, Wireless Weather Stations, see web site: http://www.mscomputer.com Call 888-333-9041 to place your order, mention you saw this ad and take an additional 5 percent off any weather station price. _______________________________________________ Towertalk mailing list Towertalk@contesting.com http://lists.contesting.com/mailman/listinfo/towertalk ```
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