Re: wire antennas
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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: [hidden email] 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 |
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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: [hidden email] 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 |
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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: [hidden email] 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 |
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In reply to this post by Bill Cunningham K4KSR
In a message dated 2/20/07 5:18:23 PM Eastern Standard Time,
[hidden email] 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: [hidden email] 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 |
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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 B&W 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: [hidden email] 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 |
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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: [hidden email] 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 |
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In reply to this post by Bill Cunningham K4KSR
In a message dated 2/21/07 4:59:24 PM Eastern Standard Time,
[hidden email] 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 B&W > 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: [hidden email] 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 |
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