---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Fri 02/24/12: 5 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 07:41 AM - Re: Servo problem (Eric M. Jones) 2. 09:38 AM - Dipole antenna fabrication (Robert L. Nuckolls, III) 3. 10:46 AM - Re: Dipole antenna fabrication (Eric M. Jones) 4. 10:57 AM - Re: Re: Dipole antenna fabrication (Dj Merrill) 5. 07:14 PM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 07:41:04 AM PST US Subject: AeroElectric-List: Re: Servo problem From: "Eric M. Jones" Etienne drew a T-antenna on his discussion of the servo problem. This incluced stripping back the insulation on a coax, separating the shield and core, and stretching out each to form a Tee. Clever. Actually too-clever-by-half. Would that it could be so easy! The problem lies in the termination, since the core and shield do not share the signal. In fact neither the core nor the shield even CARRY the signal. See: periheliondesign.com/downloads/Dabbling%20with%20electricity.pdf Then grab your SWR meter and Google any of the ham radio sites and see how to design a T-antenna. Might it work the way Etienne shows? A little probably, but let's do it right. -------- Eric M. Jones www.PerihelionDesign.com 113 Brentwood Drive Southbridge, MA 01550 (508) 764-2072 emjones@charter.net Read this topic online here: http://forums.matronics.com/viewtopic.php?p=367126#367126 ________________________________ Message 2 _____________________________________ Time: 09:38:01 AM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: Dipole antenna fabrication At 09:39 AM 2/24/2012, you wrote: Etienne drew a T-antenna on his discussion of the servo problem. This incluced stripping back the insulation on a coax, separating the shield and core, and stretching out each to form a Tee. Clever. Actually too-clever-by-half. Would that it could be so easy! The problem lies in the termination, since the core and shield do not share the signal. In fact neither the core nor the shield even CARRY the signal. See: periheliondesign.com/downloads/Dabbling%20with%20electricity.pdf Then grab your SWR meter and Google any of the ham radio sites and see how to design a T-antenna. Might it work the way Etienne shows? A little probably, but let's do it right. Can you cite a website specific to the fabrication of VHF dipoles fed with coax? Bob . . . ________________________________ Message 3 _____________________________________ Time: 10:46:02 AM PST US Subject: AeroElectric-List: Re: Dipole antenna fabrication From: "Eric M. Jones" Bob, I see that there are thousands of sites when you Google them. But here a general purpose one that will get one started and discusses the issues (sorry for all the equations...): http://en.wikipedia.org/wiki/Dipole_antenna But Bob, in your vast experience, have you seen many dipoles per Etienne's drawing? -------- Eric M. Jones www.PerihelionDesign.com 113 Brentwood Drive Southbridge, MA 01550 (508) 764-2072 emjones@charter.net Read this topic online here: http://forums.matronics.com/viewtopic.php?p=367147#367147 ________________________________ Message 4 _____________________________________ Time: 10:57:21 AM PST US Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication From: Dj Merrill On 2/24/2012 1:44 PM, Eric M. Jones wrote: > http://en.wikipedia.org/wiki/Dipole_antenna > > But Bob, in your vast experience, have you seen many dipoles per Etienne's drawing? > > I'm not Bob, but his diagram looks like the one at the top right of the page on that link you sent, and looks normal to me. I've made many of these types of antennas for ham equipment when a quick-n-dirty setup was needed. In fact, I just made one a couple of weeks ago for an APRS setup. They tend to work okay. Not great, just okay. Much better than the flexible antennas that come on a handheld radio. What specifically are you seeing wrong with it? -Dj -- Dj Merrill - N1JOV Sportsman 2+2 Builder #7118 N421DJ - http://deej.net/sportsman/ Glastar Flyer N866RH - http://deej.net/glastar/ ________________________________ Message 5 _____________________________________ Time: 07:14:50 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication At 12:44 PM 2/24/2012, you wrote: Bob, I see that there are thousands of sites when you Google them. But here a general purpose one that will get one started and discusses the issues (sorry for all the equations...): http://en.wikipedia.org/wiki/Dipole_antenna But Bob, in your vast experience, have you seen many dipoles per Etienne's drawing? Oh yeah . . . tens of thousands. For many years during the vacuum tube days of aviation radio, tens of thousands of single engine Cessna's went out the door with VOR whiskers in the vertical fin cap that were nothing more sophisticated than Etienne's coax fed dipole. Granted, the whiskers were stuck out in the breeze and assembled to the end of the coax but electrically, no different. I think we had a brief romance with adding BALUNS to these antennas. But without spending money to do really scientific tests on the antenna range, we generally relied on the anecdotal observations of production and flight test pilots. Cessna was putting 10,000 airplanes a year out the door on 5 production lines. Some years later, Jim Weir suggested adding ferrite toroidal cores the coax right at the junction with the antenna. The idea was to reduce coax radiation depicted in the Wikipedia article Emacs! . . . radiation caused by less than ideal termination of the feedline. I tested this work-around in the EMC lab at HBC about 15 years ago and found that an array of 10 cores had no appreciable effect on these stray shield currents. Years later I suffered this "hot-d@#n" moment. I saw an antenna product wherein the string of ferrites was replace with a single ferrite core. The coax was wound through it several times. Emacs! But of course! Inductance in a single magnetic circuit varies with the SQUARE of turns. The picture seen here has 6 passes of the coax through the core. It is the electrical equivalent of 36 single cores. Now, having 'discovered' that, it would still be interesting to go to the lab and see how effective it is. Had we possessed the knowledge and materials to duplicate this technique on a 1967 Cessna 172, I'll bet the pilots would have continued to report "no observable difference" in hearing the VORs. Of course, transmit is a different story. Energy levels are perhaps 130db higher (10 trillion stronger). The effects of poorly terminated coax don't have much to do with transmit/receive performance but MIGHT have an impact on how much energy is coupled into unwanted systems radiated from coax feed lines bundled in with other wires. But then, the potential for interfering signals radiated DIRECTLY from the antenna is already very high on a small airplane. If it's a plastic airplane, it's worse yet. And those 'el cheeso vertical dipoles tend to be really popular for installation in vertical fins of plastic airplanes. Admittedly, these antennas are not the best we know how to do . . . but they are easy to implement. They pose some risks for EMC issues. But the relative success with these installations suggests that the ship's 'other' appliances are reasonably designed immunity to strong external fields. The genesis thread discovered that making the servo more robust to the strong RFI environment was a good solution. But given that it's a plastic airplane, it's not clear that refining the antenna's feed point design would have made any difference at all. Bob . . . ------------------------------------------------------------------------------------- Other Matronics Email List Services ------------------------------------------------------------------------------------- Post A New Message aeroelectric-list@matronics.com UN/SUBSCRIBE http://www.matronics.com/subscription List FAQ http://www.matronics.com/FAQ/AeroElectric-List.htm Web Forum Interface To Lists http://forums.matronics.com Matronics List Wiki http://wiki.matronics.com Full Archive Search Engine http://www.matronics.com/search 7-Day List Browse http://www.matronics.com/browse/aeroelectric-list Browse Digests http://www.matronics.com/digest/aeroelectric-list Browse Other Lists http://www.matronics.com/browse Live Online Chat! http://www.matronics.com/chat Archive Downloading http://www.matronics.com/archives Photo Share http://www.matronics.com/photoshare Other Email Lists http://www.matronics.com/emaillists Contributions http://www.matronics.com/contribution ------------------------------------------------------------------------------------- These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.