From owner-qrp-l@Lehigh.EDU Tue Dec 17 12:41:45 1996 Received: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA00107 for ; Tue, 17 Dec 1996 12:41:44 -0500 (EST) X-Received-x: from fidoii.CC.Lehigh.EDU (fidoii.CC.Lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.7.6/8.7.1) with ESMTP id MAA00107 for ; Tue, 17 Dec 1996 12:41:44 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.lehigh.edu with SMTP id <35449-24991> convert rfc822-to-8bit; Tue, 17 Dec 1996 12:41:25 -0500 Received: from nss2.CC.Lehigh.EDU ([128.180.1.26]) by fidoii.cc.lehigh.edu with ESMTP id <35414-29089>; Tue, 17 Dec 1996 12:40:24 -0500 Received: from scubed.com (server1.scubed.com [192.31.70.21]) by nss2.CC.Lehigh.EDU (8.8.4/8.8.4) with ESMTP id MAA36351 for ; Tue, 17 Dec 1996 12:40:02 -0500 Received: from inferno.scubed.com by scubed.com (S3.4/solaris-main) id JAA10077; Tue, 17 Dec 1996 09:39:45 -0800 Received: from [192.31.66.229] by inferno.scubed.com (S3.4/s3-sgi-5) id RAA21015; Tue, 17 Dec 1996 17:36:17 GMT Message-Id: Date: Tue, 17 Dec 1996 10:45:19 -0700 Reply-To: ji3m@maxwell.com Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: ji3m@maxwell.com (James R. Duffey) To: "Low Power Amateur Radio Discussion" Subject: Loop Performance vs. G5RV Performance Mime-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 8BIT X-To: "Kelly Ellison" X-Cc: X-Sender: ji3m@192.31.66.42 X-Listprocessor-Version: 8.0 -- ListProcessor(tm) by CREN Status: RO Kelly - Jim Here - In your post of Dec 15; you described your loop; "Š 282 ft loop. The top wire is up at least 50 feet and the bottom is about 5 ft off the ground. Š the G5RV beats the loop by 1 to 2 S-Units on receive. Is there way to make this a better antenna?" Without more specifics it is hard to make definitive (read correct!) comments, but that has never stopped me before :-). You don't state the shape of the loop, but since you say you are feeding it at a corner, I assume that on 80M you are using a square or rectangle in the vertical polarization mode. If you are making your comparisons on 80M a horizontally polarized antenna such as the G5RV should probably be stronger on close in signals, say out to 700 miles, where its high angle of radiation is superior, and the vertical loop should be better for distance signals, say 900 miles or greater, where it's low angle of radiation should be better. On 40 M the primary radiation from the loop will be off the ends and it should be significantly down from a G5RV in the same plane. Above 40 M the loop pattern will have many lobes, some at high angles of radiation and a smaller antenna would be better. I would say off hand that your problem with the loop compared to the G5RV are ground losses, which will probably be significant with the bottom end only 5 feet from the ground. You did not state the shape of the loop, but if it is a square or rectangle with a substantial length parallel to and only 5 feet from the ground, ground losses will certainly be significant on 80 M. You may or may not have feed line radiation as well which can be in an undesired direction, straight up, and cause additional losses when a comparison test is run. If I were you, and I wanted to retain the loop, I would reconfigure the antenna as a delta loop, apex down. You can feed the loop at the apex for horizontal polarization, or part way up the side for vertical polarization (actually it will probably be mixed polarization). Values from 1/6 or 1/4 wavelength up the side are often recommended for vertical polarization. I think that L. B. has posted modeled information for these configurations in the recent past and you may wish to review the archives for this information. I would also put a balun or choke at the feed point to eliminate the possibility of feed point radiation. You can make a "self resonant choke" by simply coiling the feed line at the feed point; see the Handbook for some excellent designs by W7EL. I think they are his designs, they don't give him credit in the 1996 edition. If you want to get fancy, you can make a simple transmission line or current balun by coiling 10 to 12 turns through a ferrite core with permeability of 125 or less. For particularly stubborn cases of feed line radiation you may wish to add additional choke in the shack next to the transmitter. You also say " Š it seems this G5RV is tough to beat for an all band wire antenna. Your practical comments welcome." If you are using the classical G5RV I would suggest replacing the coax with ladder line all the way to a tuner in the shack. A balanced tuner here is best, if you can't do that an ungrounded tuner with a 1:1 balun at it's input (transmitter side) is the next best choice, the last choice is a 4:1 balun at the output (antenna side). If you use the 4:1 balun at the output I suggest you read Jerry Sevick's comments on this configuration in his book published by CQ. I don't have a copy handy, but it's title is something like "Baluns and UnBaluns for the Radio Amateur". He demonstrates that with some consideration for the load being matched, and a hefty balun this configuration can work with acceptable losses. Proper choices of antenna and feed line length can avoid the extremes of matching where the losses are the greatest. You may wish to replace your large loop with a smaller one, say 40 M to get the bottom up off the ground to reduce ground losses. Feed it with ladder line and use a tuner for multiple band operation. On 80M you can tie the ends of the feeder together and feed it as a vertical against radials or a counterpoise. I think that the simplest multiple band antenna consists of paralleled dipoles. An 80 M dipole, paralleled with a 40 M dipole, paralleled with a 20 M dipole, paralleled with a 10 M dipole will provide good coverage of 80, 40, 20, and 10 with relatively low SWR. This works, at least in theory because the dipoles on the bands not in use present a high impedance relative to the low impedance, 70 ohms or so, of the dipole on the band in use. In practice these can be somewhat tricky to tune to minimum SWR due to residual interactions between the dipoles, but tuning is greatly simplified if the ends of the higher frequency dipoles are brought down perpendicular to the dipole for a foot (10M) or 2 ft (40M). This reduces the capacitive coupling between the dipoles. Also tuning should start at the highest frequency and proceed to the lowest. Resist the urge to improve on good enough. 15 M can be operated on the 40 M dipole as a 3/2 dipole with a tuner or by the addition of capacitive loading (a foot figure eight of wire will do) a quarter wavelength (on 15 meters) out from the center(see the handbook). If you do this you may wish to run the 40 M dipole 45 degrees from the axis of the other dipoles. This will align 2 of the main lobes on 15 with the main lobes on the other bands, and will have little effect on the 40 M pattern for antenna heights less than a half wavelength on 40 M (66 feet or so). This antenna should be fed with a balun or choke at the feed point to prevent feed line radiation. At 50 feet it should have excellent performance and will require less wire than your present loop. You can use the coax feed line as a vertical on 80M by shorting center to the shield together and feeding against a radials or a counterpoise. I have used this antenna, a classic G5RV, several G5RV variations, including balanced feeders to a tuner, and a center fed Zepp 100 feet long. I think that the parallel dipoles are the easiest to deal with and are, for the most part, equivalent in performance. The G5RV moderate gain at higher frequencies is offset by the fact that it does not occur in the same direction on all bands, and the G5RV ends up having gain in an uninteresting direction on some bands. My current antenna is a 30 M loop fed with ladder line to a tuner. I use this on 160 M, 80 M, and occasionally 40 M as a vertical and on 40 M to 17 M as a loop. On 15 to 10 M I use a 12 M loop fed with ladder line to a tuner. You will have to make other provisions for optimum performance on the WARC bands, as dipoles on these bands will not have high impedances relative to their adjacent conventional band, but an 80 M dipole cut for the low CW end of the band will be a reasonable load on 30 M as a 3/2 antenna with an SWR of 5:1 or so, which with a tuner will have losses that probably will be no worse than a classic G5RV (one with coax feed) on that band. It is not optimum though and a separate dipole or loop for that band will be better. The same 80 M dipole cut for the low end of the band will also have similar resonances near the 12 M, 5/2 wavelength and 17 M, 7/2 wavelength, bands and can be operated with similar moderately high SWRs. This is not a good solution for the best signal on these bands, but it is probably no worse than the classic G5RV on these bands. Multiple band antennas are of necessity a compromise. You trade off performance in terms of radiation efficiency, antenna patterns, and use of entire antenna's length for convenience in terms of no feed line switching or tuning. There is no free lunch and there are no miracle antennas. You can choose where to take your losses though if you know a little about the antennas. At least then you know where your limitations are and what you can do to improve them if you want to. Good luck and keep us posted on your antenna experiments. - Duffey KK6MC James R. Duffey Principal Scientist Maxwell Technologies Incorporated/Albuquerque Division Suite 300 2501 Yale Blvd SE Albuquqerque, NM 87106 (505) 764-3143 (505) 843-7995 (FAX)