From owner-qrp-l@Lehigh.EDU Fri Jan 16 11:17:08 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 LAA08474 for ; Fri, 16 Jan 1998 11:17:05 -0500 (EST) Received: from Lehigh.EDU ([127.0.0.1]) by fidoii.cc.Lehigh.EDU with SMTP id <12755-41302>; Fri, 16 Jan 1998 11:15:15 -0500 Received: from nss4.cc.Lehigh.EDU ([128.180.1.13]) by fidoii.cc.Lehigh.EDU with ESMTP id <12726-62042>; Fri, 16 Jan 1998 11:13:13 -0500 Received: from server1.maxwell.com (server1b.maxwell.com [199.120.55.3]) by nss4.cc.Lehigh.EDU (8.8.8/8.8.5) with ESMTP id LAA120426 for ; Fri, 16 Jan 1998 11:11:43 -0500 Received: from inferno.scubed.com (inferno.scubed.com [192.31.66.42]) by server1.maxwell.com (8.8.7/8.7.2) with SMTP id IAA27145 for ; Fri, 16 Jan 1998 08:11:31 -0800 (PST) Received: from [192.31.66.158] by inferno.scubed.com (S3.4/s3-sgi-5) id QAA11434; Fri, 16 Jan 1998 16:04:42 GMT Message-Id: Date: Fri, 16 Jan 1998 09:13:53 -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: Odd Thoughts on Balanced Feedline and Tuners (long) Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" X-Sender: ji3m@192.31.66.42 X-Listprocessor-Version: 8.1 beta -- ListProcessor(tm) by CREN Status: RO The recent threads on antenna tuners and balanced feedline construction fired a few neurons. I waited until my thoughts filled the output buffer and have decided to give a dump to the list. 1. Feedlines. I alternate between commercial "window line", homemade balanced feedline, and coax. Each has its advantages and disadvantages. The disadvantages are usually such to make me switch feedline types when a new antenna goes up. a) The commercial window line is easy to work with and maintains separation without a lot of additional work. The commercial line I have used deteriorates rather rapidly in the environments I have used it in. Small cracks develop in the dielectric which absorb water and change line impedance and SWR with humidity. Things really get fun when it rains. To be fair, I have used this line in the LA area where pollution contributes to its rapid deterioration, and at my present QTH at about 7000 ft asl where the UV certainly contributes to its deterioration. The stuff I have used had the old brown polyethylene (is this what the Brits call polythene?) dielectric; I have seen modern stuff with what looks like an improved black dielectric. It may be better. I am leaning towards using premium 300 Ohm balanced feedline for TV use from Radio Shack for my next try in this area. b) Home made ladder line is cheap and pretty straightforward, if somewhat tedious, to make. My present line is made from wire I scrounged when they replaced the PBX system at work with spacers made from the spines of report covers. I don't recommend this material, it is hard to fit the wire in the spine, and all colors but black have deteriorated (depolymerized and become brittle) after 3 yrs exposure to the sun at my current QTH. The line must be kept under tension or the spacing is not preserved. It is a hassle to get into the house, I will use a bridge of 300 Ohm TV line through the house next time to resolve this issue. When I make my next line I will use stranded wire and make the spreaders from something that is UV stabilized. It appears that the cheapest source of such material is the grey PVC electrical conduit. c) Coax is easiest to handle. Its cost can be high, but good deals can often be had on quality surplus coax removed from computer networks. At low frequencies it has low losses. It is best for matching resonant half wave dipole antennas. Usually no external matching is required, or one can get away with simple matching. It is heavy and puts lots of strain on centerfed antennas supported at the ends. It is ideal for inverted Vee installations. It cannot be used as a feed for a single antenna on multiple bands, but parallel fed dipoles for harmonically related bands are a way around this limitation, as are trapped antennas and other multiband antennas. The jackets of most coax have UV stabilizers so I have not suffered the same deterioration with Coax as I have with the balanced feeders. In addition to the commonly used 50 and 70 ohm coax, 92 ohm coax is available and is useful for antennas such as quads and 3/2 wavelength dipoles. A dipole fed with coax should always include a 1:1 choke balun, either ferrite or air core, at the feedpoint. This reduces currents on the outside of the feedline. 2. L. B.'s comments on tuners are right on the money. For coax use a network, for balanced feeders use a link coupled tuner. I would like to expound further on this though; a) For networks to use on coax I recommend "l" (ell) networks. In an l network there is one unique setting of the inductance and capacitance that results in a match. With other commonly used networks, such as the Tee, and pi, there can be several settings that will result in a match and these are not always the same in terms of loss. Unless one tunes with a field strength meter, one does not know which setting is optimum. To be fair, the correct setting of the L match may require very large or very small sizes of L and C, but these can usually be accommodated. A good l network can be easily constructed with a 200 pf or greater variable capacitor, and a good air core inductor. Variable capacitors can be found at swap meets as can old Air Dux coils. You can also make your own air core inductor from instructions recently published in QST. b) For balanced feeders a good low loss tuner can be made by switching in and out varying lengths of feed line to obtain a good match to coax. Cecil, W6RCA has details of this on his web page. c) When it comes to used tuners, everybody knows about the Johnson Matchbox. An equally good T match tuner, which is lesser known, is the Millen Transmatch. A look inside either of these tuners will put modern tuners to shame. The capacitors are wide spaced, inductors are silver plated, and heavy duty ceramic switches are used. 3. If you want good performance from a resonant dipole at 160 M or 80 M consider carefully how it is fed. One can often have lower losses by feeding it with coax than with balanced feeders and a tuner. At these frequencies the loss of even RG-58 is low for runs less than 100 feet. Low loss inductors are much harder to make at these frequencies and the combined losses from tuner and balanced feeder (even without a 4:1 balun) may well be greater than with a coax feeder fed straight from the transmitter. Of course one looses the ability to use the antenna on higher bands, but one can always erect another antenna for these bands or feed it with Cecil's methods. Just my $0.02 worth. I hope somebody benefits from it. - Duffey KK6MC/5 James R Duffey KK6MC/5 DM65 30 Casa Loma Road Cedar Crest, NM 87008