+++++++++++++++++ Reply-To: "Ron D'Eau Claire" From: "Ron D'Eau Claire" To: Subject: Re: [Elecraft] antenna question Date: Thu, 23 May 2002 22:03:33 -0700 >... extended double zepp > with a transmatch. My question is, given that I use a > transmatch anyway, am I better off putting up 137 feet or 142? I've often > felt, "get up as much wire as you can." But being so close to a half wave > makes me wonder. Any thoughts? > dave belsley, w1euy You are using my favorite wire antenna, Dave! The issue over the length is strictly what it does to your feedpoint impedance and that depends upon the electrical length of your feeders of course. I'd go for 142 feet and shorten the antenna as needed only if you have trouble loading on some bands. The difference in the "gain" of the antenna in its most favored directions is going to be very slight even if you shorten it by another 10 or 20 feet. You will still get lobes showing gains over 10 dbi up on the higher bands. If your transmatch uses a balun at the output to provide balanced feed and you are concerned about whether it is still providing a balanced feed on bands where the impedance appears to be very high or very low, I suggest checking the currents in the lines for balance. Two r-f ammeters with sufficient accuracy to see a 10% or 20% unbalance can be had very inexpensively at most stores. They are called "flashlight bulbs", Hi! I have a couple of little sockets that mount some 12 volt screw in bulbs that I use for high current feeds. When the impedance is very high, the current will be very low, and I have a couple of 2.5 volt 25 ma "grain of wheat" bulbs with clips on the leads that show relative currents there. They are very easy to "blow" though - especially with the K2 if you forget to turn down the power THEN transmit so the power control circuits react to the change BEFORE inserting them in the line. I can get useful indications at only 1 or 2 watts on most bands. I ran a few tests using DC to see how much of a difference in currents was readily apparent in the bulb brilliance, and I found that as long as they are only lit dimly it was easy to see a 5% or less difference. I've had great success using them down to where they are only 1/4 wavelength long, provided they are high enough off of the ground. That would suggest that it would still do an efficient job for you on 160 meters if you like to operate there. In my tiny space, I am presently using a shorty 42 -foot center fed doublet with high quality ladder line. It's just about an "extended double Zepp" length on 10 meters, but it works FB on 40 through 10 meters. Ron AC7AC K2 # 1289 +++++++++++++++++ Date: Fri, 24 May 2002 00:29:45 -0500 From: "George, W5YR" Organization: AT&T WorldNet Service To: "David A. Belsley" Cc: elecraft at mailman.qth.net Subject: Re: [Elecraft] antenna question You will probably never notice the difference between 175 ft and 145 ft except perhaps a bit on the lower bands. The tuner will adjust differently, but I really doubt that will see much difference in operation. I would keep the 145 feet rather than cut it to 137 feet (why a resonant antenna when you are tuning the system anyway?) but if you can droop the ends down 15 ft, you will still have your 175 feet with neglgible effect on results. It may tune a little differently, however. Remember that the ends will be a a relatively high r-f voltage, even with 5 watts, so make sure that no one can come in contact with them. I am a firm believer in non-resonant antenna lengths when using ladderline and a tuner. That way you have smaller variations in feedline input impedance that you do with resonant flat-tops. Besides, the antenna will never show a driving-point impedance of 450 ohms resistive on any band, so the antenna length is highly non-critical from that aspect - just so it reaches from one tree to the other! <:} 73/72/oo, George W5YR - the Yellow Rose of Texas Fairview, TX 30 mi NE of Dallas in Collin county EM13qe Amateur Radio W5YR, in the 56th year and it just keeps getting better! QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735 Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437 "David A. Belsley" wrote: > > Well, the ol' K2 has been very happy with my 40 meter extended double zepp > with a transmatch. It has a flat top of about 175 feet, and it's done > beautifully 160-10. But one of the trees it is anchored to is > disappearing. My alternative gives me a stretch of roughly 145 feet max, > probably 142 feet to play it safe. Of course, half wave for the bottom of > 80 meters is roughly 137 feet. My question is, given that I use a > transmatch anyway, am I better off putting up 137 feet or 142? I've often > felt, "get up as much wire as you can." But being so close to a half wave > makes me wonder. Any thoughts? > > thanks, > > dave belsley, w1euy +++++++++++++++++++ From: "Stuart Rohre" To: "George, W5YR" , "David A. Belsley" Cc: Subject: Re: [Elecraft] antenna question Date: Fri, 24 May 2002 14:55:26 -0500 Folks, George hit it on the head. There are more reasons to use non resonant antennas fed with parallel line than to worry about measuring and cutting them to resonant length. You are using the transmatch to make that unneeded. Many handbooks and introductory texts start out talking about resonant dipoles just as a way to teach the basic theory. But that does not mean they are more efficient than a transmatch and parallel line fed doublet of some length. That early intro to resonance gets misinterpreted by new hams as giving resonant dipoles some magical property over non resonant lengths. The resonant dipole is an advantage only on a single frequency, and you need a transmatch everywhere else, so why bother with measuring to resonance? Who works only a single spot in the band? Or a single band? A resonant antenna is higher Q, and might spike the impedance such that the transmatch has to be adjusted if you try to cover another frequency in the same band that is nearby. With a broadly tuned doublet that is not resonant on any band you are using, except perhaps the lowest, you can move around not only on frequencies in a band but from band to band with some easily determined settings of the transmatch, that can be returned to when you QSY. A simple chart sets you up to return to any spot on any band. A few settings work over any one band for our Field Day big loops and Double Zepps. The losses of parallel line are much lower feeding broad coverage doublets than if you tried to use coax for multibands. The measurement of doublets for many bands is simply as George says, have wire to stretch from one support to the other. Fold it in half to find the middle, and cut it there and insert feedpoint insulator, and your feeder. That way you do not need to carry a long tape measure to field days, or emergency set up locations. If your doublet is long enough at a given band, you can realize gain over a dipole. It is the truly multipurpose antenna. 72, Stuart K5KVH +++++++++++++++++++ Reply-To: "Don Wilhelm" From: "Don Wilhelm" To: "Stuart Rohre" , "George, W5YR" , "David A. Belsley" Cc: Subject: Re: [Elecraft] antenna question Date: Fri, 24 May 2002 18:41:16 -0400 Folks, I agree 100%. Now there is just one word of warning to those trying a non resonant antenna for the first time --- some flattop/feedline combinations may present a feedpoint impedance that will make then hard to tune with the tuners commonly available today. If that occurs, there are 2 possible cures - add or subtract some feedline (don't know how much - you will have to experiment), or add some wire to the antenna ends - just let it hang down if necessary. Three to 5 feet added to each end makes a difference at 40 meters - less at 80, and even more difference at the higher frequencies. Remember that if a 'leg length' is close to a half wave will be hard to feed with all but a parallel tank circuit type balanced tuner (we can't hardly buy these now-a-days). The leg length that I am referring to is the length of half the flattop plus the electrical length of the feedline (this is length/0.95 for window line). You did measure your feedline didn't you??? It helps in figuring what the matter may be. 73, Don Wilhelm - Wake Forest, NC W3FPR home page: http://www.qsl.net/w3fpr/ QRP-L # 485 K2 SN 0020 mailto: w3fpr at arrl.net ++++++++++++++++++ From: N2EY at aol.com Date: Fri, 24 May 2002 20:07:41 EDT Subject: Re: [Elecraft] antenna question To: w3fpr at peoplepc.com, rohre at arlut.utexas.edu, w5yr at att.net, belsley at bc.edu Cc: elecraft at mailman.qth.net In a message dated 5/24/02 6:47:11 PM Eastern Daylight Time, w3fpr at peoplepc.com writes: (some good stuff about dipoles snipped to save space) > Remember that if a 'leg length' is close to a half wave will be hard to feed > with all but a parallel tank circuit type balanced tuner (we can't hardly > buy these now-a-days). Yes, but we can build them! > The leg length that I am referring to is the length > of half the flattop plus the electrical length of the feedline (this is > length/0.95 for window line). You did measure your feedline didn't you??? > It helps in figuring what the matter may be. The following website has some simple but useful freeware: http://www.g4fgq.com Program DIPOLE3 lets you describe any dipole/feedline combination, and will tell you the expected efficiency, SWR and feedpoint impedance. (It won't tell you the pattern, though). Program ENDFEED does the same for end-fed wires. Lots of other neat stuff. These are simple DOS-based programs. They are small, run on almost any computer and the instructions are built-in. You may have to convert to metric units but that's no big deal. Many hams use unbalanced tuners and wound-core baluns to feed balanced lines. This method works OK if the feedpoint Z is close to the impedance that the balun is designed to see. It can work very poorly otherwise, causing all sorts of odd effects. The obvious answer is a balanced tuner. This problem, and an easy solution, was described in QST back in February, 1990, by AG6K. A recent QST article also dealt with it. Online, see: http://www.vcnet.com/measures/bbat.html Although he uses ganged roller coils, there's no reason switched coils couldn't be used instead. Note that this design works for balanced or unbalanced loads. Standard disclaimer: I have no connection to either of these hams except agreement with and admiration of what they have written. The KAT1 and KAT2 are unbalanced tuners. Feeding their unbalanced output to a balun and balanced line is an iffy setup. Elecraft could lead the way (hint hint) by producing a true automatic balanced tuner, using switched coils instead of the ganged roller inductors used in the above article. Only downside would be that twice as many toroids would be needed.... 73 de Jim, N2EY +++++++++++++++++++ Date: Fri, 24 May 2002 21:42:03 -0500 From: "George, W5YR" Organization: AT&T WorldNet Service To: Stuart Rohre Cc: "David A. Belsley" , elecraft at mailman.qth.net Subject: Re: [Elecraft] antenna question Thanks, Stuart, for the mention and agreement. If anyone still has any thought that "resonance" is a requirement for efficient radiation, consider that an *infintesimal* dipole - practically speaking that means as small as you want to make it! - will transmit a signal that theoretically is down only 0.45 db from a full-size "resonant" dipole. Of course, the problem is getting the power into that tiny antenna since its driving-point impedance will have an infinitesimal real part (resistance) and a near infinite capacitive reactance. Back in the real world, the point simply is that the property of "resonance" conveys no special performance capability on a dipole except for one thing: its driving-point impedance is purely real at the "resonant" frequency. But, move away from that frequency and the resistive component starts changing slowly and the reactive component becomes either capacitive or inductive fairly rapidly depending upon whether the antenna is shortened or lengthened. The resistive input impedance simplifies the line matching situation and that in turn can result in reduced loss elsewhere in the system. However, the dipole will radiate no more power than is fed into it, regardless of its size, just like any other antenna. Everything that gets delivered to the antenna must be radiated, turned into heat or returned to the source as reflected power to rejoin the forward power at the tuner input and go back to the antenna. If the antenna is "non-resonant* then unless we happen to come up with a magic feedline Z that both matches the antenna to the line and also presents the proper load resistance to the transmitter, then we have to "tune" the system to accomplish the impedance transformations required with the usual ladderline or whatever we use. Is this bad? Hardly ever, if done properly. A properly designed and operated tuner will have very little loss - ever see any tuners with fans in them to get rid of the heat? Open-wire feedline will have negligible loss at HF; conventional ladderline about the same as long as it is dry. So, in the real world, there is little need to worry about getting an antenna precisely the right length unless you are shooting for the combination of 50-ohm coax feeding a 50-ohm antenna to present a 50-ohm resistive load to the transmitter. Nothing wrong with that setup if you want to operate essentially a small range of frequencies within one band. But move frequency away from that sweet spot and the SWR goes up, the transmitter no longer sees its required load, and your resonant antenna is no longer on your side. Remember that we are talking about wire dipoles and verticals here. Yagi arrays and the like depend upon critical resonances among the various elements to obtain the pattern and directive gain for which they are used. 73/72/oo, George W5YR - the Yellow Rose of Texas Fairview, TX 30 mi NE of Dallas in Collin county EM13qe Amateur Radio W5YR, in the 56th year and it just keeps getting better! QRP-L 1373 NETXQRP 6 SOC 262 COG 8 FPQRP 404 TEN-X 11771 I-LINK 11735 Icom IC-756PRO #02121 Kachina 505 DSP #91900556 Icom IC-765 #02437 Stuart Rohre wrote: > > Folks, > George hit it on the head. There are more reasons to use non resonant > antennas fed with parallel line than to worry about measuring and cutting > them to resonant length. You are using the transmatch to make that > unneeded. -- snip -- ++++++++++++++++ From: "Mike McCoy" To: Subject: Re: [Elecraft] antenna question Date: Sat, 25 May 2002 07:03:53 -0500 > Now there is just one word of warning to those trying a non resonant antenna > for the first time --- some flattop/feedline combinations may present a > feedpoint impedance that will make then hard to tune with the tuners > commonly available today. Which is why I'm a big fan of the Johnson Matchboxes. I don't care much for their "two tone 55 Buick" look though. Rather, I like the black 'bad boy' look of the Nye Viking version (and have two of 'em ;) Mike - AD5IU +++++++++++++++++