Saturday, November 17, 2012

Bobtail Curtain for 20 Meters: A True Story

Bobtail Curtain for 20 Meters: A True Story
I love wire antennas, especially those at low heights. It is easy to experiment with them without having to climb trees, towers. or high roofs. True, my wire antennas cannot compete in any sense with a Yagi at 100 feet. On the other hand, they are fun and cheap, and I can fiddle with them to my heart's content. Plus, all of them work very well within North America, and several are excellent for DX. Long live low wire antennas!

Bobtail Curtain for 20 Meters: A True Story

The Bobtail Curtain is one of my all time favorite antennas. Mine is cut for 20 meters and oriented broadside NW/SE. When the band is open even a little, I can almost always work someone in Central/South America or Asia, running 100 watts or less. That is because the antenna has a low angle of radiation and 4-6 db gain over a dipole in its favored, broadside directions.

I substituted an SGC SG-239 automatic tuner for the tuned circuit shown above. The tuner allows me to use the antenna on 30 and 15 meters in addition to 20. Although my antenna is not designed for 30 or 15 meters, it still works pretty well on those bands into South America and Asia.

D1 is cut according to the formula 492/F(Mhz), while D2 and D3 are cut to 240/F(Mhz). Dimensions are not particularly critical, and you will find in the literature various formulas for lengths. The idea is for D1 to be one-half wave in length, while D2 and D3 are one-quarter wave. Some authors make D3 longer than D2 in order to keep the tuned circuit close to physical ground (the tuned circuit tunes D3 to resonance). My antenna is low, with the ends of D2 and D3 only about 3 feet above the ground.

Grounding does not need to be elaborate, since the antenna is connected to ground at a high-impedance point. Often a single ground rod is sufficient. I have soil that is very poor electrically. For that reason, my ground consists of a 6-foot ground rod plus several 17-foot radials buried just under the soil.

The primary disadvantage of the Bobtail Curtain that I have noticed is its narrow beamwidth, which makes it difficult to work stations not directly in the antenna's focus. In my situation, for example, in the northwest direction I can work from Alaska in the north to Hong Kong in the south, but very little further south.. In the southeast direction, the antenna works well into Florida, Texas, Central America, and the eastern part of South America. It is more difficult the work into eastern Canada and Europe. If only I had a very large rotator!

A search of the Internet will yield many articles about the Bobtail Curtain. The antenna is simple and it works well. Recommended.

The bobtail curtain was invented by Woodrow Smith, W6BCX. The original description can be found in his article "Bet My Money on a Bobtail Beam" in the March 1948 issue of CQ Magazine. W6BCX also invented the Half-Square antenna, which is one-half of a bobtail curtain. It reportedly also works well and has a somewhat broader beam with; however, I have not used one.

N4GG Multi-Band Array on One Wire: It Almost Works!

The antenna in the following diagram was briefly suggested in a QST article by N4GG (see citation below). He wrote that he had not modeled or tested the antenna -- that it was just an idea. Even so, his concepts looked very interesting, so I decided to build an antenna in real life based on his design. It did not work out quite as expected. Still it was an instructive experiment that resulted in a usable antenna for 40 and 10 meters.

Array as Suggested by N4GG:

Over the course of two days, my friend, Mike (W7XTZ), and I measured, cut, and erected the antenna shown above. First we cut the 40 meter dipole and tested it. Then we added one band (pair of down leads) at a time, testing the results until all the bands had been installed. The antenna was fed with 52-ohm coax through an MFJ-918 1:1 current balun. The whole process involved a lot of raising and lowering of wires, as new sections were added and adjusted.

The results showed that the 12 and 17 meters would not yield acceptable SWR values (under 2:1) when the antenna is fed with 52-ohm coax. The exact reason is unknown -- at this juncture, just some mysterious interaction between elements. Also, the antenna would not work on 15 meters (the 3rd harmonic of 40 meters). Ten and forty meters looked promising, providing some pruning was done. The final working configuration is shown in the diagram below:

The dimensions are as follows: A = 54 ft 6 inches; B = 17 ft 3 inches; and C = 8 ft 11 inches. The antenna was erected in a straight line at a height of about 25 feet. The center balun is an MFJ-918 current balun, and the feed line is RG8X 52-ohm coax. The SWR for 40 and 10 meters was as follows:

Freq (Mhz)

7.0 1.6:1
7.1 1.3:1
7.2 1.1:1
7.3 1.1:1

28.0 1.5:1
28.1 1.3:1
28.2 1.2:1
28.3 1.1:1
28.4 1.1:1
28.5 1.3:1
28.6 1.4:1

As you can see from the dimensions, the 10-meter sections are about as predicted by the formulas for half-wave and quarter wave lengths (489/fMhz and 257/fMhz, respectively); however, the overall 40-meter length is shorter than the formula by almost the length of C (8' 11").

The antenna was erected in September 2011. It has proven to be a very good performer on 10 meters for DX and stateside (deep South and Alaska). Performance seems to favor long distances, as many SSB contacts have been made into Argentina, Brazil, Chile, parts of Africa, Australia, Asia, and Europe. The antenna's low angle of radiation undoubtedly accounts for the success with DX. While my signal is not particularly strong (only 80 watts), it is consistent. The phased verticals appear to have directivity and gain to the southwest, northwest, northeast, and southeast -- as predicted. For a low, easy-to-erect antenna, this version of the N4GG is a keeper.

Performance on 40 meters is typical of a low dipole -- contacts are no problem along the West coast. East of the Rocky Mountains is a little more difficult but still possible -- surprising for a low dipole!

This antenna was inspired by the last page of N4GG's article in QST: "The N4GG Array." Hal Kennedy. QST. July 2002, pages 35-39.

N4GG Multi-Band Array on Multiple Wires: It Works!

After experimenting more or less successfully with an N4GG array on a single wire (see above), I still wanted to have more than two bands on one antenna fed with 52-ohm coax That wish called for a return to N4GG's original idea -- multiple antenna arrays fed by a single coax. Specifically, I wanted have arrays for 20, 17, and 15 meters.

The formulas for calculating wire lengths are as follows: Horizontal wire length = 489/fMhz and vertical wire length = 257/fMhz.

My version of the antenna is quite low to physical ground, and the dimensions for lowest SWR came out as follows: A (20 meters) = 34 ft 8 inches; B (20 meters) = 18 ft 11.5 inches; C (17 meters) = 27 ft; D (17 meters) = 14 ft 11 inches; E (15 meters) = 23 ft 2 inches; and F (15 meters) = 13 ft 1 inch. The antenna was erected in a straight line at a height of about 25 feet, resulting in the vertical elements being only a few feet above ground. The center balun is an MFJ-918 current balun, and the feed line is RG8X 52-ohm coax.

One again, my friend, Mike (W7XTZ), and I measured, cut, and erected the antenna shown above. We cut horizontal elements to the exact lengths given above and left the verticals a foot or so too long so the antenna could be adjusted for the lowest SWR on each band by decreasing the length of each vertical an inch at a time, then measuring, and so on. The spacing between the horizontal elements was one foot -- the spacers were made from very small plastic irrigation tubing (drip type) placed about every 4-6 feet along the antenna.

SWR is about 2:1 on 20 and 17 meters. It runs 1.5: 1 on 15 meters. Such SWR levels are to be expected from an antenna with elements that are 1.5 wavelengths long. I use a tuner to reduce the SWR seen by the transmitter.

My version of the antenna is oriented broadside to SE and NW. This means that, in theory, it has its strongest lobes into the NE (Europe), SE (southern Pacific Ocean), NNW (north Asia), and SW (Australia). Gain in those directions should be on the order of 3-4 db. Deep nulls are evident to the SE and NW (in the direction of Japan and Korea).

Bottom line results... at last! The antenna was erected in Septermber 2011. Many DX stations have been worked on 20, 17, and 15 meters in Europe, Asia, the South Pacific, the Indian Ocean, and Africa, using SSB (80 watts) and PSK31 (20 watts). For me this is quite an accomplishment, since I am only on the air a couple of hours during the week.

I have now had about nine months experience with the antenna. My station has also been upgraded to about 500 watts output (Ameritron AL-811H amplifier). Now if I can hear a station, I can almost always make contact. Even during the poor conditions of June 2012, I was able to work a number of SSB stations in Europe on 20 meters even though they were of marginal strength in Washington State.

Note: Be sure the balun is water tight. Mine got water inside (probably my fault) and arced at the coax connector where it was wet, resulting in ruined female coax connector (and coax from the water). The balun beads and coax were fine, and I put them in another container with a new coax connector. I detected the bad connector/coax by putting an ohm meter across the coax -- it read 400 ohms!! It should have read infinity.

Source: "The N4GG Array." Hal Kennedy. QST. July 2002, pages 35-39.

Multiband Dipole: Faithful for More Than 20 Years!

This is a plain-jane antenna that I have used for more than 20 years on 40, 20, and 15 meters. The A element is for 40 meters (and 15 as the third harmonic), while the B element is cut for and works fine on 20 meters. The antenna is fed straight from the coax without a balun or choke. The overall height is about 24 feet. The spacing between elements is 4 inches. Element lengths were calculated using the formula 468/F(Mhz) and cut several inches long for any needed adjustment after the antenna was erected.

The antenna is fed with about 75 feet of RG-8 coax that has been up since the antenna was originally installed. It still works fine. The only grounding is at the transmitter in the shack. I have never had trouble with RF in the shack, using this antenna.

Despite its simplicity and low height, this antenna is a consistent performer into the Mid-West from Washington State and, when conditions are fairly good, it can reach reasonably well into the East Coast, Europe, South America, and Asia on 20 and 15 meters.

Broadband Traveling-Wave Antenna ala WA2WVL (Floyd Koontz)

I built this antenna in the summer of 1996. Its main attraction was the advertised flat SWR from 1.8 to 30 Mhz. Who could argue with that? In my version the height (H) was 25 feet and the length (L) was 70 feet. I purchased the 50:450 ohm transformers and used an old 50 ohm dummy load as the termination.

How well did it work? Well...the SWR indeed was flat as advertised, varying from 1.1:1 to 1.5:1 from 1.8 to 30 Mhz. And, importantly, there was no RF in the ham shack as is often found when random wire lengths are used. I made numerous stateside contacts and talked with many DX stations using a 100-watt transmitter. However, in my configuration the antenna was not a stellar performer, particularly given its complexity and the need for a 50-ohm termination. I continued to use the antenna for several years primarily for SWL'ing, as the low SWR yielded consistent performance across the HF spectrum .

Source: "Broadband Transmitting Wire Antennas for 160 through 10 Meters." Floyd F. Koontz. QST, November 1995, pages 22-24.

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