---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Sun 02/26/12: 6 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 09:43 AM - Re: Dipole antenna fabrication (Eric M. Jones) 2. 10:38 AM - Re: Re: Dipole antenna fabrication (Richard Dudley) 3. 11:34 AM - Re: Dipole antenna fabrication (Eric M. Jones) 4. 12:13 PM - Re: Re: Dipole antenna fabrication (Dj Merrill) 5. 08:38 PM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III) 6. 11:10 PM - Re: Re: Dipole antenna fabrication (Mike Wynn) ________________________________ Message 1 _____________________________________ Time: 09:43:39 AM PST US Subject: AeroElectric-List: Re: Dipole antenna fabrication From: "Eric M. Jones" The Wiki site shows a simple dipole and the caption says: "A schematic of a half-wave dipole antenna connected to an unbalanced coaxial cable. Better practice is to connect the balanced dipole to the unbalanced line with a balun." I've been thinking of putting aluminum foil on the TV rabbit ears.... Proper termination is the key to antenna efficiency. If Cessna didn't do it, that doesn't mean the homebuilder shouldn't do it right. Bob et al. having flown extensively in the Western states and Mexico, I have plently of experience with being a zillion miles from a station. At those times I start to think that having a good radio and good antennas is a wise and cost effective thing to do. But do what you want. -------- 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=367293#367293 ________________________________ Message 2 _____________________________________ Time: 10:38:19 AM PST US From: Richard Dudley Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication A wise (and practical) person once said: "Perfection is the enemy of good enough." Anymouse RHDudley On 2/26/2012 12:41 PM, Eric M. Jones wrote: > --> AeroElectric-List message posted by: "Eric M. Jones" > > The Wiki site shows a simple dipole and the caption says: > > "A schematic of a half-wave dipole antenna connected to an unbalanced coaxial cable. Better practice is to connect the balanced dipole to the unbalanced line with a balun." > > I've been thinking of putting aluminum foil on the TV rabbit ears.... > > Proper termination is the key to antenna efficiency. If Cessna didn't do it, that doesn't mean the homebuilder shouldn't do it right. > > Bob et al. having flown extensively in the Western states and Mexico, I have plently of experience with being a zillion miles from a station. At those times I start to think that having a good radio and good antennas is a wise and cost effective thing to do. But do what you want. > > -------- > 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=367293#367293 > > ________________________________ Message 3 _____________________________________ Time: 11:34:41 AM PST US Subject: AeroElectric-List: Re: Dipole antenna fabrication From: "Eric M. Jones" A good job is sufficient. We disagree on what constitutes a good job. -------- 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=367298#367298 ________________________________ Message 4 _____________________________________ Time: 12:13:16 PM PST US Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication From: Dj Merrill On 2/26/2012 2:28 PM, Eric M. Jones wrote: > --> AeroElectric-List message posted by: "Eric M. Jones" > > A good job is sufficient. We disagree on what constitutes a good job. > Having made and used at least a few dozen of these types of antennas, I can say they generally do perform a "good job". They perform far better than a handheld antenna, and perhaps not as good as some other types of antennas, but overall they are okay, and of course, cheap to make! :-) Comparing $$ spent to value received they rank very high. -Dj -- Dj Merrill - N1JOV Sportsman 2+2 Builder #7118 N421DJ - http://deej.net/sportsman/ Glastar Flyer N866RH - http://deej.net/glastar/ ________________________________ Message 5 _____________________________________ Time: 08:38:23 PM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication At 11:41 AM 2/26/2012, you wrote: The Wiki site shows a simple dipole and the caption says: "A schematic of a half-wave dipole antenna connected to an unbalanced coaxial cable. Better practice is to connect the balanced dipole to the unbalanced line with a balun." . . . and I published instructions for adding this feature to a coax fed dipole at http://aeroelectric.com/articles/BALUN/Balun_Fabrication.html . . . the same technique could be applied to a comm dipole. I've been thinking of putting aluminum foil on the TV rabbit ears.... Which will lower the resonant frequency of the antenna and perhaps lower the Q . . . wider bandwidth. But no benefit for relative efficiency compared to an "ideal" dipole at the same frequency. Proper termination is the key to antenna efficiency. If Cessna didn't do it, that doesn't mean the homebuilder shouldn't do it right. I can assure you that Dr. Wood read the same books we've all read about antennas with professional attendance to "better practice." The point I endeavored to make was that after he came to understand all the deleterious effects over which we had no control on a C-172, adding baluns on the VOR antennas yielded very small gains compared the sum of other losses. Next time I can get into the test lab, I'll do some measurements on the difference between a balun-fed dipole and a bare foot, coax-fed dipole. Bob et al. having flown extensively in the Western states and Mexico, I have plently of experience with being a zillion miles from a station. At those times I start to think that having a good radio and good antennas is a wise and cost effective thing to do. But do what you want. But you paint a picture suggesting that a balun would make the difference between communicating with some distant station . . . and not. Given the light of sight character of VHF+ frequencies, you're more likely not to have a useable path because you can't see the other station than because your effective radiated signal is down by a few db due to violations of feed point protocols. Further, it's insufficient to be able to see the other station peeking over the horizon (or mountaintop). The effects of intervening "roughness" in the "Fresnel zone" can have some profound effects on what might otherwise be considered a free-space, line of sight situation. http://web.arundale.co.uk/docs/ais/PathlossCalculationsforAmateurs.pdf Mountain peaks and valleys intruding into the Fresnel zone can kick your free space performance predictions in the teeth. Currents flowing on the coax shield are but part of the 'violations' . . . the center impedance of a perfect dipole is on the order of 73 ohms. This means that the BEST SWR to be achieved without application of some impedance matching is on the order of 73/50 = 1.46:1 So we've tossed off some efficiencies right out of the box. When we plot radiation patterns on VHF antennas on airplanes, one generally expects 10 db or greater 'lumpiness' in the pattern due to geometry of the conductive elements around the antenna. So again, if one is at 'extreme range' for useful communications, it's more likely that you'll improve the path by turning the airplane a few degrees as opposed to optimizing feed point design. Yes, a BALUN can be "better" but experience on small aircraft has shown that the difference is so small compared to all other effects combined that the balun was not 'cost effective'. Further, it won't fix the fundamental mis-match so 1.46:1 is still the best we can expect. The textbooks are full of examples of data taken from the idealized antenna and feed line examined in the anechoic chamber with test equipment that can split a DB into small pieces. As soon as you put that same antenna is a real world environment, lots of things with potential for altering performance happen. This is why the guy flying 'around with a radio capable of talking to another one just like it 1000 miles away finds that the practical limits are much shorter for reasons mostly beyond control of the antenna designer. But just for grins, I've got some DIY antenna's I've used in my weekend seminars and articles. I'll fit one with a balun and leave the other barefoot. Then see if my buddy Don P can tell the difference without looking to see which one is hooked up. Hmmmm . . . I think I know how to do the experiment with equipment I have. Need warmer weather though . . . Bob . . . ________________________________ Message 6 _____________________________________ Time: 11:10:52 PM PST US From: "Mike Wynn" Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication Extremely educational dissection [of antenna logic] from bystander viewpoint. Many thanks Bob. Mike W. Moab, UT ****************8's ----- Original Message ----- From: "Robert L. Nuckolls, III" Sent: Sunday, February 26, 2012 9:35 PM Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication > > > At 11:41 AM 2/26/2012, you wrote: > > > The Wiki site shows a simple dipole and the caption says: > > "A schematic of a half-wave dipole antenna connected to an unbalanced > coaxial cable. Better practice is to connect the balanced dipole to the > unbalanced line with a balun." > > . . . and I published instructions for adding this > feature to a coax fed dipole at > > http://aeroelectric.com/articles/BALUN/Balun_Fabrication.html > > . . . the same technique could be applied to a comm > dipole. > > I've been thinking of putting aluminum foil on the TV rabbit ears.... > > Which will lower the resonant frequency of the antenna > and perhaps lower the Q . . . wider bandwidth. But > no benefit for relative efficiency compared to an "ideal" > dipole at the same frequency. > > > Proper termination is the key to antenna efficiency. If Cessna didn't do > it, that doesn't mean the homebuilder shouldn't do it right. > > I can assure you that Dr. Wood read the same books > we've all read about antennas with professional > attendance to "better practice." The point I endeavored to > make was that after he came to understand all the > deleterious effects over which we had no control on > a C-172, adding baluns on the VOR antennas yielded very > small gains compared the sum of other losses. > > Next time I can get into the test lab, I'll do some > measurements on the difference between a balun-fed > dipole and a bare foot, coax-fed dipole. > > Bob et al. having flown extensively in the Western states and Mexico, I > have plently of experience with being a zillion miles from a station. At > those times I start to think that having a good radio and good antennas is > a wise and cost effective thing to do. But do what you want. > > But you paint a picture suggesting that a balun would > make the difference between communicating with some distant > station . . . and not. Given the light of sight character > of VHF+ frequencies, you're more likely not to have a useable > path because you can't see the other station than because > your effective radiated signal is down by a few db due > to violations of feed point protocols. Further, it's > insufficient to be able to see the other station > peeking over the horizon (or mountaintop). The effects > of intervening "roughness" in the "Fresnel zone" can > have some profound effects on what might otherwise be > considered a free-space, line of sight situation. > > http://web.arundale.co.uk/docs/ais/PathlossCalculationsforAmateurs.pdf > > Mountain peaks and valleys intruding into the Fresnel > zone can kick your free space performance predictions > in the teeth. > > Currents flowing on the coax shield are but part of > the 'violations' . . . the center impedance of a perfect > dipole is on the order of 73 ohms. This means that the > BEST SWR to be achieved without application of some > impedance matching is on the order of 73/50 = 1.46:1 > So we've tossed off some efficiencies right out of the > box. When we plot radiation patterns on VHF antennas > on airplanes, one generally expects 10 db or greater > 'lumpiness' in the pattern due to geometry of the > conductive elements around the antenna. So again, > if one is at 'extreme range' for useful communications, > it's more likely that you'll improve the path by turning > the airplane a few degrees as opposed to optimizing > feed point design. > > Yes, a BALUN can be "better" but experience on small > aircraft has shown that the difference is so small > compared to all other effects combined that the balun > was not 'cost effective'. Further, it won't fix the > fundamental mis-match so 1.46:1 is still the best > we can expect. The textbooks are full of examples of > data taken from the idealized antenna and feed line > examined in the anechoic chamber with test equipment > that can split a DB into small pieces. > > As soon as you put that same antenna is a real world > environment, lots of things with potential for > altering performance happen. This is why > the guy flying 'around with a radio capable of > talking to another one just like it 1000 miles > away finds that the practical limits are much > shorter for reasons mostly beyond control of > the antenna designer. > > But just for grins, I've got some DIY antenna's I've > used in my weekend seminars and articles. I'll > fit one with a balun and leave the other barefoot. > Then see if my buddy Don P can tell the > difference without looking to see which one is > hooked up. Hmmmm . . . I think I know how to > do the experiment with equipment I have. Need > warmer weather though . . . > > 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.