NJ0F Bobtail Curtain Antenna

Posted with permission "Things you always wanted to know about the Bobtail Curtain but were afraid to ask" by George Tomlinson III NJ0F (Now NE0S) Revised 11/28/00 Before tackling construction detail with respect to the bobtail curtain, let me mention a brief history, as well as background of the antenna. According to W6BCX, who appears […]

Posted with permission

"Things you always wanted to know about the Bobtail Curtain but were afraid to ask" by George Tomlinson III NJ0F (Now NE0S)


Revised 11/28/00

Before tackling construction detail with respect to the bobtail curtain, let me mention a brief history, as well as background of the antenna. According to W6BCX, who appears to be the bobtail's "Daddy" by the way, the antenna with three elements is a "broadside array of co-phased radiating elements configured as inverted ground planes". His bobtail, as described, was fed at the bottom (voltage fed), across a matching network, and used a "ground screen" at the feed point. The W6BCX bobtail was used mostly for work at frequencies lower than 7 MHz for DX work. From what I have read in his old articles the bobtail must have been, and still could be a darned effective antenna when fed at the bottom, and used on the lower frequencies. In fact, I have no data to support any contention that it would not be more effective than the current fed version that I am about to describe. I would simply say that current feeding the bobtail seems to work well. This photo is me hard at work along with my assistant engineer.


This simplified diagram shows an overview of this very good performer. . . . . . .


These are the dimensions of my antenna…optimized for the local installation conditions. You may find that slightly different dimensions are necessary at your location.


About a year ago I was searching for an antenna for use on 17m, which I hoped would out perform typical dipoles and verticals. I decided that the antenna, above every other consideration, had to be simple to build, easy to "hang" and I wanted it resonant. The biggest criteria were, of course, that it had to work. With the help of W1XU, Jim (SK) a version of current fed bobtail evolved for use on 17m. His reasoning, and now mine, was that if a current fed half-square could be effective, then two end to end half squares would be better. As was typical of Jim, over the many wonderful hours we visited about the antenna, he did not mention that he had published an article in 73 Magazine, February 1982, about the vary subject. By the way, I see no reason not to scale the antenna to other bands above 20m. While I won't burden you with extra notes, it is worth mentioning that the configuration evolved over a span of about 6 months. A lot of changes didn't work. Others worked well.

We did not know, but did suspect, that the fed in the corner bobtail had a different lobe pattern than the classic "figure 8 of the more conventional variety. I was logging every contact with a pin in the map, both stateside and DX. I suspected a cloverleaf pattern but wasn't totally sure. Both W1XU and I had been visiting from time to time with Sam KT4QW. Sam took an interest and volunteered to computer model the antenna. (Those models are on the web page, by the way). Sam's model confirmed the cloverleaf. That meant that the bobtail was now "end-fire" instead of "broadside". Sam's model confirmed the low takeoff angle, because DX was calling me. Jim was of the opinion that the bobtail was actually getting into DX areas as they opened. Also of interest was the fact that the bobtail signal reports were almost always a bit better than I gave in return with respect to DX. Eventually, we determined that a lot of the DX I was working on the bobtail were lower power, low antenna height stations. We noted as well that the bobtail has an affinity for stateside mobiles. We learned not to dismiss the lesser lobes behind the feed-point, as contacts onto South America, the South Pole, and to a lesser extent, VK and ZL were more than possible.

Construction of the 17m current fed bobtail is simple, but like any wire antenna, accuracy is required. First, placement of the antenna is important because it is directional on paths over 1500 miles. It behaves like a wire beam on DX…BUT, for gosh sakes, if you lack a "proper direction" to hang it, don't let that stop you.


Continued refinement of computer modeling data, as well as real world on air testing, requires that we modify some of what has been written here with respect to the bobtail curtain. Construction notes should come first, I believe, with the formula, construction detail, and the usual dos and don’ts. Some consideration of antenna placement is necessary to take advantage of the lobe pattern of the bobtail. Those models can be viewed on the links. Let me add that what you see in the modeling is about what you will get. Next, it would be fun to tackle some of the history WRT the bobtail curtain and it’s performance in the real world.

Construction is straightforward and simple. In concept, the antenna is composed of three one-quarter wave verticals; each separated by half wave phasing lines. The formula required are a bit unconventional in that the verticals lengths are calculated by using 254/f, and the horizontal phasing lines 501/f. Doing the math for 18.130mHz suggests that the vertical elements should be 14 ft exactly, and the phasing lines 27ft 7 inches. Do not be concerned about the choice of 18.130 MHz as this will cover the entire band at 1:1 SWR. Let me inject here that use of the MFJ or other so-called antenna analyzer will be of little or no value in adjustment of the antenna. In fact, it has been noted that use of the analyzer at the shack end of the coax line leading to the antenna will cause one to believe things that simply aren’t so.

We have applied the formula to the wire portion of the antenna. I suggest the use of good wire, such as 7 strand hard drawn copper, or flex-weave, in 14 Ga. Wire stretch is our enemy.

I cut each vertical, and each phasing line, as individual prices of wire. Then, I use a "dogbone" glass insulator at the feed point. At the center, and the far end, of the phasing lines, I use the glass egg insulators. By using one in the middle of the phasing wires I believe that I can relieve some of the strain, and prevent possible stretch on the antenna when I pull it up to "middle C" on the ole violin. Attach the first vertical element at the feed point, using the dogbone insulator. Attach the first horizontal phasing line to the other end of the same dogbone insulator. (Note that his point is the only open point on the antenna). Next, attach the other phasing line to the far end of the first, using an egg strain relief insulator. Attach the second vertical at this point, and bond this vertical to the phasing line. Repeat the process with the last vertical, again using an egg insulator at the end of the phasing line. (A lot of words, but a simple process, as the picture shows). Remember that only the first vertical is electrically open from the phasing line, and that the balance of the antenna, verticals and all are bonded to present one piece of wire.

Now, we’re in the short rows. At the feed point, connect the shield of the coax to the first vertical, and the center conductor to the rest of the antenna. Before connection, wrap a 7 turn current choke, just before the feed point, to prevent "common currents" traveling back up the coax. I find that use of wire ties makes the task of attaching the coax to the support rope much easier. Continue to attach the coax to the support rope for as far as you can away from the feed point. A half wavelength would be good, if you can do it.

At this point there are two things left to do. Waterproof the feed point connection as best you can. I have toyed with somehow encapsulating the thing but haven’t yet decided how.

Last thing to do is connect approximately 8 inch pieces of 1/2-inch diameter PVC pipe to the bottoms of the wire verticals. Do this by drilling a small hole in the top and bottom of the length of PVC pipe. Attach the bottom of the vertical wire to the pipe. At the bottom of the pipe, attach a small weight using good string. I find that cheap tubes of caulking are ideal. They provide just enough weight to hold the wires straight down, and believe it or not, dampen their swing in the wind. Again, note the detail of vertical bottom treatment in the photo.

Tuning and adjust of the bobtail curtain is accomplished in a simple way. Plot an old fashioned SWR graph on a piece of paper. If you have measured accurately, the formula should put you very close to resonance. The SWR curve must be a flat line. At this point, SWR value is not as important as the flat line. Remember, this is a very broad banded antenna, and that any curve might indicated either a vertical out of balance, or simply long or short. Hopefully, if your measurements and assembly are correct, no adjust will be necessary. To adjust for length, either wrap wire up on the PVC pipe or tape in place, or pull the wire through the drill hole and twist off. Either works for me.

From an operational point of view, I note that the current fed bobtail curtain is really an end fire antenna, and not broad side as the classic. The antenna is nearly Omni directional within 1500 miles, but that there is some re-enforcement of signals within the lobes, both front and back, along the directional plane of the wire. Most likely, you will note a deep null directly "off the nose" of the antenna. My first bobtail, or BT1, is up about 35 ft above ground, and is pointed out about 030 degrees true. From Kansas, this provides good coverage into Europe, the Middle East, and parts of Africa from the NE lobe. The NW lobe does well into points to the left of the North Pole. Behind the feed point, the SE lobe does well into S America, and the SW lobe into ZL and VK. Not as strongly as forward of the feed point but still well.

Here is a short list of dos and don’t for the BT. Don’t use an antenna analyzer to tune the antenna, it will just cause grief! Don’t use stretchy wire. Don’t forget the current choke at the feedpoint. Don’t read too much about the older configurations of the bobtail, and let that keep you from putting one up. Don’t think it will set the world on fire, because it won’t. Don’t be surprised if you hear frequent comments like "you are the loudest thing on the band". Don’t forget to smile when you hear that.

The so-called "do list" is simple. Measure close. Remember it is pointy thing, but put it up anyway. Keep some notes of both US and DX contacts, because that is the best test of what the antenna is doing. Always remember that here are a lot of good folks in this wonderful hobby of amateur radio. With few exceptions, most have opinions. Form your own.

To hang the antenna, I use WOVEN (not twisted) rope of about 3/8-inch diameter, or bigger. The antenna doesn't weigh much but when you pull it up to operating height you will have a tendency to haul it to "middle C" on a violin. I do! That is also the logic behind the hard-drawn copper wire. I find that previously mentioned Radio Shack wire to be tough stuff. Wire that stretches a couple of feet every time the wind blows, or rope that moves when it gets wet and dries out, aren't good!

As always, I must express my thanks to Sam, KT4QW, not only the use of his web-site, but his unfailing offers of help. It is Sam who did the modeling, not only on this version, but several others as well. It was Sam who volunteered that he wasn't too sure about some of the model information. Let me tell you, either that computer model works well, or Sam has a well polished crystal ball" Sam's predicted DX lobes are pins in the map! As for Jim, W1XU..Well, he was the guy I only met on the radio. As frequently happens, DX stations will break-into my QSO's to comment on my signal strength. It's then I smile, and think of Jim.

Thanks for your interest. I hope this article inspires you to build instead of buy!


George, NJ0F
November, 2000

(George is now NE0S Sept 2013)

 

If you have