From owner-qrp-l@Lehigh.EDU Fri Jan 16 09:39:29 1998 Received: from fidoii.CC.lehigh.EDU (fidoii.CC.lehigh.EDU [128.180.1.4]) by oucsace.cs.ohiou.edu (8.8.5/8.8.5) with ESMTP id JAA00188 for ; Fri, 16 Jan 1998 09:39:27 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12507-62042>; Fri, 16 Jan 1998 09:38:57 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12456-41302>; Fri, 16 Jan 1998 09:38:36 -0500 Received: from mailhost.cas.utk.edu (MAILHOST.CAS.UTK.EDU [128.169.76.44]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id JAA120320 for ; Fri, 16 Jan 1998 09:38:34 -0500 Received: from larry by mailhost.cas.utk.edu with SMTP (8.8.7/mailhost.utk) id JAA01893; Fri, 16 Jan 1998 09:38:31 -0500 (EST) Message-Id: Date: Fri, 16 Jan 1998 09:38:31 -0500 (EST) Reply-To: cebik@utkux.utcc.utk.edu Sender: owner-qrp-l@Lehigh.EDU Precedence: bulk From: "L. B. Cebik" To: "Low Power Amateur Radio Discussion" Subject: Re: Bright Idea Award #1; RG174U; Balun loss; c.w. tone In-Reply-To: MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII X-To: Adrian Weiss X-Cc: Low Power Amateur Radio Discussion X-Sender: cebik@larry X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO > Also, while in passing, I ought to comment about a misconception about > inserting a balun between a length of balanced line and a coax run to the > shack. I think this arrangement came about from the frequently seen G5RV, > where, given the specific length of the legs of the antenna, and given > length of balanced line drops from the antenna, is connected to a run of > 50-Ohm coax of any length, which then completes the run to the shack. Now, > the length of the balanced line, given the impedance of the antenna, is > selected to provide an impedance transformation down to the vicinity of > 50-Ohms. Use any other length of line with that antenna, and it won't be a > 50-Ohm termination. In reality, the balanced line dropping from the G5RV > is an impedance transformer doubling as the feedline. It is an integral > part of the antenna system, just as an extra element on a yagi is. The > coax is actually feeding a system terminated in the balun. > A short note on the G5RV scheme. 1. Since only the 102' wire radiates, the pattern is the same, no matter how fed, so long as equal power reaches the wire feedpoint for each scheme. 2. Every length of transmission line is both a feedline and a transformer. The G5RV tries for a wire length and paralel wire (450) that achieves something close to 50 ohms at its end for the harmonic hambands. Usually, it fails, since the wire feedpoint Z and consequently the Z at the end of the parallel line vary as the antenna is raised or lowered, uses different diamter wire, and generally succumbs to all the other variables. Now, if we connect some further parallel line to the system, the line continues the transformation progression started at the wire feedpoint, and what the ATU sees varies from band to band. However, except for some lengths of overall line that defy ATU components, efficiency is best due to low losses in the parallel line--assuming one does not use a toroidal balun of the 4:1 type found in many network ATUs. All bets are off band-to-band if one uses such an ATU. Line lengths can be calculated to bring the Z at the ATU terminals close to 50 ohms (+/-). Here, a 1:1 ferrite choke balun is handy to provide the network ATU with single-ended termination. Alternatively, if one is willing to accept some loss, one can install a 1:1 ferrite choke at the original design parallel line termination (usually given as 34' of 450) and use coax to a network tuner. This works best if the transition leaves less than 20' or so of coax and one uses RG213 or better. At 10:1 SWR at 10 meters, 20' of RG213 loses about 1 dB, and that is just about system max loss. Losses less on lower frequencies and with lower SWRs at the junction of the coax and the ferrite choke balun. The choke is not lossless, but does not do as badly as those 4:1 toroids when reactance is high. Why do this? In some installations, bringing parallel line indoors involves close proximity to unbalancing metal, etc., and losses here (loading up the duct work, telco lines, AC lines, etc.) can be considerably more than losses of the coax/ferrite 1:1 balun system. If you have a clear path, use parallel line all the way. It is a hangover from olden days to treat the antenna and some length of feedline as integral parts of the antenna system, pressing hard on the word "system" while still leaving the impression that the key word is antenna. Treating the antenna as the antenna and the feedline as something one can vary in length to effect various current, voltage, and impedance transformations gives the greatest flexibility of thinking. There are some charts on 102' wires on all the HF bands with some feedline information in one of the articles at my Web site in the Low Down series that may be useful in clarifying performance characteristics of the 102' wire. Incidentally, the 100-102' wire was well known long before G5RV used it to try to make a simplified feed system for the harmonic bands. His call attaches not to the wire length, but to the feed system design. The length--about 3/8 wl at 80 meters--was known in the 30s to be about the shortest effective 80-meter doublet--with the Feed Z going cookoo with shorter lengths. For limited space, it can be an effective radiator (and receptor), especially if you can get it up in the air good. -73- LB, W4RNL L. B. Cebik, W4RNL /\ /\ * / / / (Off)(423) 974-7215 1434 High Mesa Drive / \/ \/\ ----/\--- (Hm) (423) 938-6335 Knoxville, Tennessee /\ \ \ \ / / || / (FAX)(423) 974-3509 37938-4443 USA / \ \ \ \ || cebik@utk.edu URL: http://funnelweb.utcc.utk.edu/~cebik/radio.html