The DS50-5 50 MHz Yagi

DS50-5.PDF

The DS50-5 was designed utilizing computer optimization and a healthy dose of real world experience to provide a good all around antenna for six meter DXing. There really is not one type of antenna that is the best for all occasions. Each size and type of antenna has its own advantages and disadvantages. The DS50-5 was chosen to provide as many advantages as possible, coupled with as few disadvantages to go along with it. This translates in to a design that will give good performance over a wide range of conditions, both weather and propagation-wise!

We opted for a lightweight antenna that was easily installable by one person in many cases. We chose a boom length that was long enough to provide enough gain to produce a serious signal for ionospheric scatter propagation, but with a manageable beamwidth that was not so sharp as to require constant aiming corrections or monstrous rotators and towers. Considerable effort was spent in fine tuning the pattern and minimizing sidelobes, while maintaining very high forward gain. The front to back ratio is 20 dB at the low end of the band, and improves slightly as you go higher in frequency. While not a great number, 20 dB is more than adequate, considering that reflections from local obstructions and hills approaches 20 dB in many cases. In a number of locations, very high F/B ratios are not realized in real life due to local geography. The positive tradeoff is in forward gain. 9.1 dBd is a very good number for a five element beam.

When we say "lightweight", we do not mean flimsy. The mechanical strength had to be adequate for ice storms and high winds, and not include bracing or wires that manage to collect more dead weight during heavy icing. The approximate 17 foot boom length was a good choice for the 1 1/2" tapered boom diameter. It is very rugged and has performed admirably under high wind and icing conditions. Electrically, the antenna had to perform with uniform results in wet and dry weather, snow, and sleet. Mast sizes up to 2" are accomodated.

The driven element uses a tried and true modified T match with all open construction and the highest quality materials. Open construction means that there is no place for water to collect and corrode aluminum or other metals and provide conducting paths from the feed line to ground. The input connector is a type N UG-58/U receptacle. The only enclosed area is the type N plug on your feedline. Below is a close up picture of the backside of the T match feed. A half wave balun is constructed from RG-400/U MIL teflon, double shielded, silver plated braid, coaxial cable. Such a balun is impervious to water entry, and will provide plenty of high power capability as well. Note that the balun, shown below, should be taped to the main boom for strength.

The original DS50-5 prototype yagi is currently perched on a short 30 ft tower at the factory. It utilized an RG-303 (RG-176 sized) teflon coax balun, and has been working for years with 1100 rf watts applied. It has been tested to 2000 watts output for short periods. It is hard to believe that such small teflon coax will withstand that power level, but it does. The much larger RG-400 balun will handle the legal limit with ease. The T match rods are constructed from cold drawn 1/4" solid aluminum rod. The 3/8" dia. machined delrin standoff insulators are very rugged, and the whole T match assembly has proven trouble free for years of service. One look at the T match shorting bars will confirm our committment to quality and ruggedness. It is made from a solid block of 3/8" X 1" aluminum bar. No flimsy formed straps here!! The compression method is 100% effective and, unlike setscrews, defies working loose under all forms of abuse and stress.

You can look at the specifications and bandpass plots by clicking on this link to the PDF manual. The gain of the DS50-5 is slightly over 9 dBd and the gain is constant from 50.0 up to about 51.0 MHz. The antenna has been designed for SSB service and not FM operation. gain drops rapidly above 51.0 MHz, and it not useable above about 51.75 MHz. On the low frequency side, the rolloff is not as fast, and the antenna will work well as a DX video carrier detector down to about 48 MHz.

The computer generated patterns are located here. E Plane Polar Diagram and the H-Plane Polar Diagram We spent much time optimizing the pattern in both planes. The reasons for a clean E-plane (The electric field or horizontal) pattern are obvious. Large minor lobes will rob forward gain and narrow up the main lobe. This results in poor discrimination against strong local signals in many cases, and is a problem in high activity areas. A clean pattern provides high gain with the widest possible main lobe. The H plane (magnetic or "H" field) pattern is just as important. Clean vertical patterns are required in any good design. Proper stacking of yagis in the vertical plane is important when higher gains are needed. The new digital modes such as JT65 for EME offer a prime example. A messy H plane pattern will produce a narrow vertical lobe with high gain robbing sidelobes, stealing forward gain, and more importantly, requiring closer vertical spacing to achieve proper sidelobe levels for efficient stacking. Simply put, a clean H plane allows wider spacing and a greater gain improvement when two (or more) yagis are stacked vertically. The common rule of thumb is to stack yagis at .66 or two thirds of the boom length. The DS50-5 can be stacked at up to 1.12 times the boom length! Any spacing from about 13 ft up to 19' 5" will produce acceptable to sterling results. The wider the stacking distance, (up to 19 ft 5") the more gain you will get. The DS50-5 makes a great building block for larger EME arrays. There is at least one eight yagi DS50-5 array now in operation on 50 MHz. A four yagi stack is actually quite manageable with a simple supporting framework.

 

What is the best height to mount your DS50-5 (or any other yagi for that matter)? Again, that is a hard one to answer. Anyone who has operated six meters during Es or F2 band conditions, with a multiple stack array will tell you that there is no magic height. Probably the best compromise is to put an antenna at 30-45 ft above ground. Better yet is to install that 130 ft tower and coaxial switch, and put yagi #1 at 130 ft, and then hang yagi #2 at 40 ft. There are some times when a very high yagi is the only thing that will work. Other times favor the lower choice. Naturally we will tell you to install as many DS50-5s as your supporting structure will hold. (Just kidding!) The main idea is to tailor your installation to agree with what your goals are. After all, it IS a hobby, right?