[Elecraft] RE: Wire Antennas -- corrected theory comment
Hello, Don, Dave, et al, Don said The only problem with that is the high voltage point on a fullwave loop is electrically opposite the feedpoint, not 1/4 wave away from the feedpoint. Not so. Here's why... Define a quad, full wave loop, as a square -- geometrically equal lengths on all sides. Feed point (zero degree point) is placed on the center of one side. On a closed full wave loop (360 degrees long) there are always 2 current maximum points (loops) and 2 current minimum points (nodes). The same for the Voltage standing wave -- there are always 2 voltage maximum points (loops) and 2 voltage minimum points (nodes). Current and voltage maximas, on a standing wave antenna, are 90 degrees apart. On the defined closed full wave square loop, the current maxima (loops) are at the feed point (zero degree point) and 180 degrees around the loop or half way round the loop on the opposite side. Then, the voltage maximas are 90 degrees away from the feed point (or 90 degrees back toward the feedpoint from the 180 degree point) -- 1/2 way between the feed point and the 180 degree point, physically centered on the adjacent sides to the feedpoint side. Summary and benefit of this characteristic for a multi-band antenna -- Opening a full wave loop at a voltage maximum (current minimum) does not change the closed loop standing wave pattern for the fundamental frequency, but opening it forces a dipole type sinusoid current distribution on the 1/2 sub-harmonic. Works great for when you are trying to control the current distribution on both bands for a specific pattern generation. Hence, my original suggestion. TNX and Regards..Rick -- W5RH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] Re: Wire Antennas
Dave (and Don), Not a silly question, Dave, what so ever. My suggestion was to open it up 90 degrees from the present feed-point, 1/4 way around...this being a voltage maximum (loop) and current minimum (node). This enables the loop to still act like a loop on 80, if you desire to maintain it's present performance. And then on 160 it acts like, as Don states, a folded back 1/2 wave dipole on 160 meters but in this case it will be off center fed. Auto 80/160 band switching, once you get the lengths correct for the two bands, if that is possible in this case, as I do not know what modes you operate. If you open it at, as Don suggests, 180 degrees or half way round, the 80 meter loop turns into a bent 80 meter 2 Half-waves in Phase with a very high Z feed, if you maintain the feed point as is. This also shifts the standing wave current distribution by 90 degrees, which might cause you problems if your original antenna was set up to favor a particular area of the world. Not convinced that is the case though, as it does sound like you have up 270 feet of wire in a triangular loop, cause that's where the trees be. Wish I had those trees -- hi. 60 feet above ground is low for a horizontal 160 meter sky hook, but you do what'cha gotta do. Sounds like a fun modeling and empirical experiment to say the least. Regards -- Rick -- W5RH - Dave, Open it at the point 180 degrees (midway around) from the feedpoint. If your existing loop is a full wavelength on 80 meters, opening it will turn the loop into a folded back 1/2 wave dipole on 160 meters - not as good as straight out, but it should work since you are feeding the line with a tuner anyway. In fact, you may be able to make it automatic by using a trap tuned to 160 meters - that will electrically open the loop on 160 and only add some inductnce on the other bands. 73, Don W3FPR -Original Message- Not sure where you are suggesting I open it. I was considering disconnecting the feedline on one side where it connected to the balun. Is this what you mean. My apologies if it is a silly question. David Wilburn [EMAIL PROTECTED] Rick Hiller wrote: Dave, Another possibility for 160 -- Don't know to what extent/effort you want to go to get on 160, but here is an easy way, although, a bit of a compromise. Simply open the loop at one of the 80 meter voltage loops (also a current node). The 80 meter standing wave current distribution /performance will not change, but it will force the wire to be an off center fed 1/2 wavelength for 160. Although not quite resonant within the 160 meter band, if you feed it with open wire and run it thru the appropriate matching network, you should be right. -- Rick Hiller Toll Free: (866) 658-7527 Manager, CAD Sales Direct:: (713) 278-6310 SDI -- System Development, Inc. Main: (713) 266-5667 1 Richmond Ave., Suite 110 Fax: (713) 974-4911 Houston, Texas 77082[EMAIL PROTECTED] 2D Graphics Tools to View, Edit, Convert, Publish and Print If you deal with CGM, visit www.sdicgm.com ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] RE: Wire Antennas
Rick, The only problem with that is the high voltage point on a fullwave loop is electriclly opposite the feedpoint, not 1/4 wave away from the feedpoint. It is true that opening it at the 80 meter 1/4 wave point would create an off-center fed 160 meter dipole that has been bent back on itself, but it will change the behavior on all other bands too. I still think the 160 meter trap at the 180 degree point is the best solution - or use the loop as-is and tie both sides of the feedline together and operate against ground for 160 meters. Somewhere on his website, L B Cebik simulated a 160 meter inverted L antenna with only 4 20 foot radials and it did not fare too badly - he also simulated with one 20 foot radial and that did not do badly either, so there is hope even if an extensive radial system for 160 meters is out of the question in your case. 73, Don W3FPR -Original Message- Dave (and Don), Not a silly question, Dave, what so ever. My suggestion was to open it up 90 degrees from the present feed-point, 1/4 way around...this being a voltage maximum (loop) and current minimum (node). This enables the loop to still act like a loop on 80, if you desire to maintain it's present performance. And then on 160 it acts like, as Don states, a folded back 1/2 wave dipole on 160 meters but in this case it will be off center fed. Auto 80/160 band switching, once you get the lengths correct for the two bands, if that is possible in this case, as I do not know what modes you operate. If you open it at, as Don suggests, 180 degrees or half way round, the 80 meter loop turns into a bent 80 meter 2 Half-waves in Phase with a very high Z feed, if you maintain the feed point as is. This also shifts the standing wave current distribution by 90 degrees, which might cause you problems if your original antenna was set up to favor a particular area of the world. Not convinced that is the case though, as it does sound like you have up 270 feet of wire in a triangular loop, cause that's where the trees be. Wish I had those trees -- hi. 60 feet above ground is low for a horizontal 160 meter sky hook, but you do what'cha gotta do. Sounds like a fun modeling and empirical experiment to say the least. Regards -- Rick -- W5RH - Dave, Open it at the point 180 degrees (midway around) from the feedpoint. If your existing loop is a full wavelength on 80 meters, opening it will turn the loop into a folded back 1/2 wave dipole on 160 meters - not as good as straight out, but it should work since you are feeding the line with a tuner anyway. In fact, you may be able to make it automatic by using a trap tuned to 160 meters - that will electrically open the loop on 160 and only add some inductnce on the other bands. 73, Don W3FPR -Original Message- Not sure where you are suggesting I open it. I was considering disconnecting the feedline on one side where it connected to the balun. Is this what you mean. My apologies if it is a silly question. David Wilburn [EMAIL PROTECTED] Rick Hiller wrote: Dave, Another possibility for 160 -- Don't know to what extent/effort you want to go to get on 160, but here is an easy way, although, a bit of a compromise. Simply open the loop at one of the 80 meter voltage loops (also a current node). The 80 meter standing wave current distribution /performance will not change, but it will force the wire to be an off center fed 1/2 wavelength for 160. Although not quite resonant within the 160 meter band, if you feed it with open wire and run it thru the appropriate matching network, you should be right. -- No virus found in this outgoing message. Checked by AVG Free Edition. Version: 7.5.441 / Virus Database: 268.18.3/694 - Release Date: 2/20/2007 1:44 PM ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] Re: wire antennas
Jim and the group, At every field day, our club W5KA uses 80 M Inverted Double Extended Zepp element wire doublets. We have had up to 250 feet of two types of window line, and with the large Dentron tuner, we have a low loss match, and it works every signal we hear. That is on multiple bands where we use the above antenna between 80 and 15 meters. (10 having been dead for last several field days down here). No problems matching with the popular nominal 300 and 450 ladder lines of poly insulation. A number of Cebik's models are done without assumption of big ceramic insulators, or home made 600 ohm line. If a particular lenght of feeder presents a matching problem on a given band you add or subtract a few feet of line. Keeping 5 and 10 foot adder sections is not hard and wire nuts make a quick splice of the added section into the feeder to move the optimum impedance position to suit the tuner. I have matched such antennas as 5/8 leg Zepps on 20m with the smaller BW tuner, also without a problem with random feed line length. Such antennas have less weight and wind load than Windom, (OCF) types that require a balun at the feed point. Stuart K5KVH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
RE: [Elecraft] Re: wire antennas
Regarding open wire line, I generally use something in the vicinity of 450 ohms. That is an easy size to buy or fabricate and produces a fairly low SWR when used to feed most doublets. While open wire lines are low-loss, they are not lossless. SWR does matter, just no where near as much as when using coaxial cable. Typically, the voltage loop (highest impedance) point of a wire antenna won't exceed 3,000 or 4,000 ohms impedance, so the SWR on the line won't reach 10:1 even when fed at a voltage loop (such as when the doublet is a full wavelength long). At the frequency at which it is 1/2 wave long, the feedpoint impedance is about 50 ohms which produces an SWR of less than 10:1 again. Everywhere in between will produce impedances somewhere inside that range. That's what keeps the losses on open wire low. The greatest losses in well-made open wire lines are ohmic: the RF resistance of the wire. RF resistance is directly proportional to the diameter of the wire, since RF currents flow only on the outer surface of the conductor. So I like to use a much larger conductor than most commercially-made wires. That is a significant help when I load up the antenna at a frequency below which it is 1/2 wave long. For example, using my 66 foot doublet on 80 meters. It's an efficient antenna when used that way provided feeder losses are kept low. But when loaded on a band where the wire is only 1/4 wave long (e.g. a 66 foot wire used on 80 meters) the impedance is rather low so the SWR on 450 ohm line might be 20:1 or greater. That's one reason I shun ladder line or twin lead with small conductors. Using open wire line to feed other low-impedance antennas, such as small loops presents the same issues of high SWR and increasing losses. As I mentioned before I make my own open-wire for most work using #14 (or larger) copper wire and well-spaced low-loss insulators. Actually, the common dog bone end insulators work well providing a spacing of about 2.5 inches. Hold 'em in place with a small ty-wrap threaded through the same hole and snugged down tight to provide a tight friction-grip with the wire (most ty-wraps do disintegrate in three or four years from UV but I make a point to check my installation at least that often). Another way to hold the wire is to loop a short length around the end of the insulator and wrap it tightly around the feeder directly above and below the insulator. Those will last indefinitely. One insulator every three or four feet is plenty. It is not necessary for them to hold the wires exactly equidistant: only to keep them from touching. I don't try to run my open wire at a low SWR, so the actual impedance or impedance bumps along the line aren't important. Open wire line made that way using white insulated house wire and white dog-bone insulators is actually harder to see from several feet away than the typical brown window line or twin lead. There are lightweight plastic dog bone end insulators as well. Ron AC7AC ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] Re: wire antennas
In a message dated 2/21/07 4:59:24 PM Eastern Standard Time, [EMAIL PROTECTED] writes: our club W5KA uses 80 M Inverted Double Extended Zepp element wire doublets. If my math is right, that works out to about 330-340 feet of wire, center-fed! We have had up to 250 feet of two types of window line, and with the large Dentron tuner, we have a low loss match, and it works every signal we hear. That is on multiple bands where we use the above antenna between 80 and 15 meters. (10 having been dead for last several field days down here). Good job! No problems matching with the popular nominal 300 and 450 ladder lines of poly insulation. A number of Cebik's models are done without assumption of big ceramic insulators, or home made 600 ohm line. If a particular lenght of feeder presents a matching problem on a given band you add or subtract a few feet of line. Keeping 5 and 10 foot adder sections is not hard and wire nuts make a quick splice of the added section into the feeder to move the optimum impedance position to suit the tuner. I have matched such antennas as 5/8 leg Zepps on 20m with the smaller BW tuner, also without a problem with random feed line length. One key point to remember is that the feedpoint Z of those antennas is neither very high nor very low on any band. Nor is it likely to be highly reactive. The end result is that the ladder line is probably operating at an SWR of less than 10:1, and maybe even less than 5:1. Which is a good recipe for low loss. See Ron's (AC7AC) commentary on the situation where the feedpoint Z is very high or low. Such antennas have less weight and wind load than Windom, (OCF) types that require a balun at the feed point. True - but one could also use open line, and a balun near the ground. All depends on the situation. In a domestic contest like Field Day, a lot depends on things like where you are. Hams in the middle of the country face different antenna challenges than those on the coasts and the corners. For example, somebody in NE or KS is within 1500 miles or so of every ham in CONUS. Someone in WWA, SDG, SFL or ME may be twice that distance from a sizable number of CONUS hams. OTOH, a stations in the corners don't have to deal with the fact that the rest of CONUS is all around them. 73 de Jim, N2EY ** AOL now offers free email to everyone. Find out more about what's free from AOL at http://www.aol.com. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
[Elecraft] Re: wire antennas
Why not put up a Windom, which is an off-center fed dipole? It does require a 4:1 balun, but it is coax fed and works very well on both odd and even harmonics. I use two 40 meter Windoms, crossed for complementary coverage and fed separately. They work well on all bands (even 80 in a pinch) except 30 meters where they are a beast to load. Much simpler than multiple dipoles with a common feed. Bill Cunningham K4KSR ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] Re: wire antennas
Even simpler is a 80m dipole fed with balanced line to a tuner for all band use. The window line is less costly than coax. A good quality tuner is less lossy in multiband use than coax/ tuner balun, etc.. Balanced antennas have fewer problems than off center feeds. Balanced line to dipole does not need a balun at the antenna. Stuart K5KVH ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] Re: wire antennas
On February 20, 2007 02:17 pm, rohre wrote: Even simpler is a 80m dipole fed with balanced line to a tuner for all band use. Perhaps simpler, but not necessarily a good solution. The window line is less costly than coax. Usually. A good quality tuner is less lossy in multiband use than coax/ tuner balun, etc.. A link coupled tuner is the most efficient. Proper balanced tuners with baluns at the input should also be efficient. Any tuner with a balun on the output is going to stress the balun on bands where the antenna is an even multiple of a half wavelength. You could easily see thousands of ohms of impedance which is an impossible situation for a balun. Yes, many amateurs use this configuration, and make lots of contacts. The 100 watts output at the rig may only be 10 watts of effective radiated power. Any balun manufacturers care to publish the measured efficiency of their baluns terminated by an impedance of 2500 -j3300 ohms? How about it Elecraft: how efficient are your baluns from 500 kHz to 54 MHz when terminated by an impedance of 2500 -j3300 ohms? How does the efficiency vary with applied power? How effective are they with common mode currents? Balanced antennas have fewer problems than off center feeds. Like what? Very few centre fed dipoles are balanced. You must have symmetrical surrounding terrain and objects within at least a half wavelength, preferably two wavelengths from the antenna, and also symmetrical ground losses. The feedline must run at 90 degrees to the antenna. Very few amateurs have such a perfect site for an antenna. So even with a centre fed antenna, balance is usually not achieved. It becomes a matter of degree of balance. An off-centre fed dipole will present a lower impedance on more bands, and therefore will place less demands on the tuner and balun. Use a good Guanella type balun should be used. What happens when the balance of an antenna is upset due to surrounding objects, uneven terrain, a feedline that does not run at 90 degrees to the antenna, or an offset feed? Typically feedline radiation. That is not necessarily a bad thing. The feedline radiation can improve the radiation pattern as it may add vertically polarized components and fill in where the nulls occur from the antenna. The downside is the possibility of RFI. However this will usually only be evident when an unbalanced current loop exists near the shack. Changing the feedline length will usually help reduce the RFI. Let's face it a multiband antenna is a compromise. As long as we understand the compromise, and can live with the results, there is no problem. Unfortunately most of what we are taught is from modeled antennas that do not take into account asymmetrical surroundings and ground losses, and feedline routing. There are very few, if any, measurements of efficiency of tuners and baluns in high impedance (including high reactance) situations. Therefore many amateurs are simply unaware of the losses that occur in a multiband antenna system. However, any antenna is still better than no antenna. -- Darrell Bellerive Amateur Radio Stations VA7TO and VE7CLA Grand Forks, British Columbia, Canada ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
Re: [Elecraft] Re: wire antennas
In a message dated 2/20/07 5:18:23 PM Eastern Standard Time, [EMAIL PROTECTED] writes: Even simpler is a 80m dipole fed with balanced line to a tuner for all band use. The window line is less costly than coax. A good quality tuner is less lossy in multiband use than coax/ tuner balun, etc.. Balanced antennas have fewer problems than off center feeds. Balanced line to dipole does not need a balun at the antenna. In general, the above is true. But it's not always as simple as it is made out to be. First off, the classic lowloss open line numbers we see in the books are usually for lines with ceramic insulators and heavy wire - say, #14 spaced 4 inches with a ceramic spreader every couple of feet. Those numbers don't necessarily match those of common window line (Twin Lead with holes) and other parallel lines. Second, there are all sorts of tuners out there, and they're not all created equal. The Ancient Ones used big split-coil balanced tuners, which required a lot of cut-and-try. And if they were lossy, it was obvious because the coils would heat up at medium to high power levels. The modern single-ended-tuner-with-balun-at-the-output is a different animal. It can work well with some loads, and be a loss leader in others. At QRP levels, losses may not result in much heating, either. I read all sorts of stuff about recommended dipole lengths, but not much about feedline lengths. But what really matters is the overall system, and what impedance it presents at the shack end of the line at the frequencies of interest. A good antenna fed with the wrong line can present a shack-end impedance that is very low, very high, and/or very reactive. The tuner may match it, but it may not be very efficient. One excellent tool we have today that the Ancient Ones did not is antenna modeling software. Reg Edwards' simple DIPOLE3 gives a good idea of the actual efficiencies and losses of various dipole/transmission line setups. Free and easy to use. But it won't tell you how lossy your tuner or balun is. OTOH, don't let the search for the ideal antenna prevent you from putting up *something* and trying it out. 73 de Jim, N2EY ** Check out free AOL at http://free.aol.com/thenewaol/index.adp. Most comprehensive set of free safety and security tools, millions of free high-quality videos from across the web, free AOL Mail and much more. ___ Elecraft mailing list Post to: Elecraft@mailman.qth.net You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com
