Amy:
You may want to re-think the longwire idea after a look at what I published in the October 2000 edition of the Oklahoma DX Association Newsletter: (
http://www.qsl.net/okdxa)
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Here at Casa QRM, the weather switched from ‘Broil’ to ‘Simmer’ last month and I did a little antenna work that was rewarded by an abundance of ‘first call’ contacts during the Worked All Europe contest. I was hot into Eastern Europe and probably relied on good band conditions more than an over-achieving wire at the far end of the coax, but I’m not complaining.
Last spring I got a wild hair and took down the 20 Meter Extended Double Zepp to try out a non-resonant terminated long-wire. Bad Move. Although I had a dead flat SWR on 40 Meters and above, the performance on 20 and up was dismal with spurs from two local AM stations popping up throughout the HF range. I know multi-band antennas tend to be a compromise, but this was ridiculous. Maybe the EDZ had me spoiled, as it was one of the best mono-band wires I’ve used.
I was tempted to try a non-resonant Folded Terminated Dipole but, after reading the modeling comments by L. B. Cebik, W4RNL about mediocre TX performance, the hassle of building a 16:1 balun and weatherproofing the 800W non-inductive resistor didn’t appeal to my inner sloth. As a general rule, any antenna that requires a complicated matching device is probably more trouble than you need. I opted for an old favorite, the Off-Center Fed Dipole or, as it’s often called, a “Coaxial Windom”.
I put one up a few years ago that worked well, but the rig at that time had no Auto-Tuner. On some bands the SWR power fold-back circuit was working overtime and I wondered if I could do better. Never could tweak it into complete sweetness. But, have you noticed how the second try usually turns out better than the first? Same here. In its simplest form, the 136’ OCF Dipole doesn’t work on 30 or 15 Meters, and mine needs a little help from a tuner on 75 Meter phone from 3.75 to 3.99 MHz, but the SWR is almost flat on 40, 20, 17, 12 (all full band) and 10 Meters (28.2 through 29.2 MHz). Plus, it loads nicely on 50.125 MHz and that’s a good assortment. I didn’t expect much fire in the wire on Six, as the balun is working above its design limit, but a little testing on a local Net tells me it radiates ‘Not Bad’.
As for performance, Saturday night on 20 Meters during the WAE contest saw Lithuania, Croatia, Russia, Slovenia and England in the log while Sunday morning added Bulgaria, Latvia, Germany, Poland Lithuania, Slovenia and Belgium on 10 Meters. All barefoot at 100 Watts, all on the first call. I could have worked many more, but I had things to do and wasn’t in a contest mood. All I wanted was a little antenna check, and the QSO on 12 Meters with OM5DP in the Slovak Republic indicated the OCFD does haul the freight.
If you want an OCF Dipole in your yard, you’ll need a pair of supports in the 30’ or better range at least 100’ apart. Running the wire on a true North-South line with the short wire at the South end works well for me. I used a pair of 30’ push-up TV masts (easy to find at your local Handy-Guy mega-store) topped with 5’ of gray 1.5” PVC conduit to give 3’ of separation between the wire and mast. Glue a “T” fitting on the top as a wire guide, then cut four slits in the bottom and hose clamp it to the mast. You’ll need 140’ of 12 or 10 gauge THNN wire, solid preferred, two toroid cores of Mix 61 or 43 material, plus the usual rope and hardware. In my case, I bought some mid-sized Mix 43 (850mu) toroids on eBay last year. Although 125mu is the textbook choice for HF, the 850mu cores have worked well with fewer turns.
You can make a career out of comparing all the variations of the OCF Dipole and, as the old joke says, six web pages will give you the eight best ways to build one. Here’s mine: First, you build a two-core 4:1 Guanella ‘current’ balun. Don’t use a Ruthroff ‘voltage’ balun, stacked cores on beaded coaxial lines, or a single-core Guanella with this antenna. The dual-core (or dual rod) 4:1 Guanella is as close to ‘transparent’ as any balun I’ve used and I tried plenty of different baluns when I was tinkering with the terminated long wire. Trust me on this. You’ll find a ferrite rod design (same wiring as a toroid balun) at www2.dynamite.com.au/vk1brh/CM4Balun.htm. Mine are wound with 18ga stranded silver-teflon on beefy little 1” OD toroids about 3/4" long that hang an inch or two below the insulator. Drop the RG-8X coax feed straight down to the ground and I used RG-213 from there to the shack. This makes a break point for maintenance, mowing and the ground rod I’ll install whenever I get around to it. Waterproof the barrel splice with Scotch 33 or 88 PVC tape. (don’t waste your money on cheap electrical tape as you may regret it after the first good rain) For the ‘Magic Wire Lengths’, I found 45’ 6” on the short side and 138’ overall tip-to-tip is a good start. Prune the long side (only) in 3” steps until the bands drop in where you want them. If you’re a CW guy, you probably won’t cut anything. And, in case you’re wondering how you squeeze 138’ of wire between supports 100’ apart, the ends droop. The masts here are 120’ apart with the wire tips coming down at a 25 degree angle.
You can learn more about the OCF Dipole at
http://www.cebik.com/groundup.html as well as other antennas worth considering. It’s an excellent series of articles and well worth your time, although some of the charts for the 80 Meter OCF Dipole are a bit contradictory and some of the text needs a little editing. Another good source for links to antenna projects can be found at
http://www.dxzone.com. Be sure to visit www2.arrl.org/tis/info/pdf/9008028.pdf to see how a pair of shorter wires can be added for 30 and 15 Meters. I’ll be doing that to mine, but it’s been working so well I’m not inclined to ‘fix’ it just yet. You’ll also find a variation on the feed method at
http://users.erols.com/k3mt/windom/windom.htm, but I’m a bit leery about the balun design used by K3MT as I’ve had better luck with Jerry Sevick’s three-core model. Look it over, and when you need a break from antennas,
http://www.hypertools.com can be amusing for the formerly OD OM.
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Hope this gives you something to consider, as two local AM broadcast stations made the longwire a poor choice at my QTH. My OCF Dipole continues to work well and the only thing that's going to take it down is bad weather.
73 de AC5UP, OKDXA Newsletter Editor.
