---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Mon 02/27/12: 8 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 03:09 AM - Re: Re: Dipole antenna fabrication (Jan de Jong) 2. 03:13 AM - Re: Re: Dipole antenna fabrication (Jan de Jong) 3. 07:55 AM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III) 4. 09:38 AM - Re: Re: Dipole antenna fabrication (Jan de Jong) 5. 02:26 PM - Re: Re: Dipole antenna fabrication (Noel Loveys) 6. 02:28 PM - Re: Re: Dipole antenna fabrication (Noel Loveys) 7. 03:17 PM - Re: Re: Dipole antenna fabrication (Noel Loveys) 8. 08:39 PM - Re: Re: Dipole antenna fabrication (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 03:09:53 AM PST US From: Jan de Jong Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication Bob, When you're at comparing I would be interested in the effectiveness of a Pawsey stub balun as it is so easy to make from scrap coax. The following antenna gets good results using such, apparently: http://chrusion.com/BJ7/InvVeeAntenna4ULs.pdf I also wonder about the effectiveness of a coax air coil in the feedline (people use 10 turns of 2 or 3 " diameter or so). Also easy to make. As I understand it the Pawsey stub works by compensating, the air coil (and ferrites) by choking. Jan de Jong Robert L. Nuckolls, III wrote: > > > 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 2 _____________________________________ Time: 03:13:28 AM PST US From: Jan de Jong Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication Bob, Sorry about this. The balun you describe making IS a Pawsey stub - should have taken a look before responding. In any case - I'm interested in the comparison. Jan ________________________________ Message 3 _____________________________________ Time: 07:55:22 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication At 05:12 AM 2/27/2012, you wrote: > >Bob, > >Sorry about this. The balun you describe making IS a Pawsey stub - >should have taken a look before responding. >In any case - I'm interested in the comparison. You are correct. If one compares the Pawsey stub with a 1/4 wave, de-coupling sleeve or "sleeve balun" as shown in Wikipedia: Emacs! . . . we see that the top end of the sleeve is the open end of a shorted 1/4-wave transmission line. I.e., the impedance of the top rim of the sleeve looking at the shorted end is very high. Hence, connecting it to the center conductor adds no 'load' to purposeful energy but it would have the effect of collecting differential currents between coax shield and the center conductor and force them toward equivalency. The Pawsey stub functions in the same manner except that its behavior is more like a short piece of balanced transmission line as opposed to a piece of coax. This brings to light an error in my balun article cited earlier. Since the stub IS NOT operating as a free-space 1/4-wave but instead as a transmission line, dielectric effects on velocity come into play. The velocity factor of this piece of transmission line is on the order of .95 to .97 which means that my 26" dimension for the stub would be closer to optimum were it shortened by an inch or so. The 'coiled coax ploy" has good foundations in physics. Consider this image from the Wikipedia article: Emacs! The current balun is a common mode choke which may be implemented by wires on a high-efficiency core (as in many TV couplers), a coil of coax immediately adjacent to the attach point on the dipole or consider this image also from Wikipedia article on baluns: Emacs! As you can see here, the common mode choke effects are greatly enhanced by winding the coax onto the core. Inductance in these windings is proportional to the permeability of the magnetic path material. Air is 1.0, the ferrite may be 5-20 . . . thus multiplying the beneficial effects by that factor. The neat thing about this balun is that it is not resonant or tuned; thus effective over a wider range of frequencies. The stubs in coax or balanced transmission lines are tuned and optimum at one frequency only. The 'hard' thing about the coax on core ploy is finding coax that will wind tightly through a relatively small core WITHOUT dropping below the recommended bend radius for that coax. Coaxes with solid center conductors are at risk for center conductor migration through the insulation over time and temperature cycles. RG-141 wound through a core like that has potential for becoming a shorted feed line years from now. The commercial antenna I saw with this technique used a small diameter, very flexible coax and offered a BNC connector on the stub for extending the feed line to the appliance with more conventional coax. Bob . . . ________________________________ Message 4 _____________________________________ Time: 09:38:44 AM PST US From: Jan de Jong Subject: Re: AeroElectric-List: Re: Dipole antenna fabrication Bob, thank you for the explanation. There was some discussion on the web whether the velocity factor to be used for finding the length of the Pawsey stub was that for the coax insides (NO) or the coax outsides (YES). The latter would be the .97 to .99 then. Jan ________________________________ Message 5 _____________________________________ Time: 02:26:21 PM PST US From: "Noel Loveys" Subject: RE: AeroElectric-List: Re: Dipole antenna fabrication Dipole antenna erected vertically is similar to the 1/4 wave whip that we now use. The tricks are to make sure you have a large enough groundplane (section of the aircraft skin). When I first saw the talk of dipole antennae I assumed incorrectly you wanted to be able to remove a radio and take it with you and still have good range in case of a crash. Aircraft radios are restricted to power. Basically the powers to be don't want you to be transmitting thousands of miles otherwise they would up the power of transceivers and drop the frequency closer to six meters than to two meters. Main problem with that is you could end up with a lot of crosstalk like they used to have on the CB band (11 meters). While flying in mountainous areas for safety I would not be without the new 400 mHz ELT. There are lots of things about those ELTs I don't like but we are stuck with it. Noel -----Original Message----- From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Eric M. Jones Sent: February 26, 2012 2:12 PM Subject: AeroElectric-List: Re: Dipole antenna fabrication 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 6 _____________________________________ Time: 02:28:24 PM PST US From: "Noel Loveys" Subject: RE: AeroElectric-List: Re: Dipole antenna fabrication "Good enough" is everyone's enemy when it comes to ceasing to operate at 3000' ! Noel -----Original Message----- From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Richard Dudley Sent: February 26, 2012 3:06 PM 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 7 _____________________________________ Time: 03:17:33 PM PST US From: "Noel Loveys" Subject: RE: AeroElectric-List: Re: Dipole antenna fabrication Bob: Have you ever used a broadband windom antenna? Noel -----Original Message----- From: owner-aeroelectric-list-server@matronics.com [mailto:owner-aeroelectric-list-server@matronics.com] On Behalf Of Robert L. Nuckolls, III Sent: February 27, 2012 1:06 AM 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 8 _____________________________________ Time: 08:39:29 PM PST US From: "Robert L. Nuckolls, III" Subject: RE: AeroElectric-List: Re: Dipole antenna fabrication At 05:12 PM 2/27/2012, you wrote: > >Bob: > > Have you ever used a broadband windom antenna? No, I've known hams that did. Most of my activity for the past 40 years has been on 2M repeaters. In the picture at . . . http://aeroelectric.com/Pictures/Misc/KTVH.jpg you can see the toes of my boots while peering down the center of KTVH tower east of Hutchinson KS. I was standing on the 1200' platform were the 22/82 repeater was quartered back then. Antennas to die for on that system were things like DB-228 arrays of 8 dipoles on a mast about 40' long and weighing in at 75 pounds. Got to hang two of those off the leg of this tower about 100' apart. 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