Sigma© 280 Yagis
These Yagis follow the same efficient design as the Sigma verticals that have set several World Records. The Sigma designs have also been proven over the past 2 years in commercial applications running up to 10KW continuous service, not intermittent peaks as in amateur work.
There are 3 production models: 2 were planned and another was originally a proto-type for a new rotatable 160, but was turned into an 80/75 meter Yagi. Advantages of the horizontal Sigma compared to other designs include: T-bars effectively extending the electrical length; center loading using huge air-core coils (not on PVC cores) and 3/8" tubing for true high-Q; center coil location avoids high voltage arc potential of mid-point coil loading; overall heavy-duty, strong, storm-tested construction.
The Sigma-280S is the smallest and can fit in just about all locations. It has 54' elements and a 35' boom. The T-bars are 18', so the effective electrical length of the elements is over 70' and operating bandwidth is on the order of 70 kHz. The model shows the efficiency is 90%, which means the total loss in all antenna components is 0.46dB. This is the first Sigma Yagi design and was on the air for several months. It proved to be an eye-opener. The pattern was excellent, with F/B of more than 20dB and impressive signal reports were common. The on-air performance followed the history of the Sigma verticals expreienced in setting World Records from 6Y.
Next came the Sigma-280 Magnum that was designed to compete with larger 3 element Yagis with mid-point loading, both in gain and in pattern. The elements were extended to 76', with 26' T-bars and a 43' boom, which expanded the operating bandwidth to about 100 kHz. The efficiency increased to 97%, for a total loss in the antenna of only 0.14dB. Depending on the model used for the larger 3 element Yagi (90' elements, 60+' boom), the smaller Sigma-280 Magnum is within about 0.3-0.4dB in gain and the patterns are similar, with excellent front-to-side rejection, as well. The Magnum is a step up in physical size from the Sigma-280S, with a 5" and 4" boom and about 200 pounds, so be ready to have a substantial rotator and tower!
A "rainy day" project was to re-design our rotatable 160 dipole using Sigma techniques. We chose to make a lightweight unit using welded sections, similar to aluminum tower. The first element had 3 tapering sections of 12' per side, plus a 15' tip and 30' T-bars. The overall physical length was 102'. To check out the design, the model showed it should resonate around 3.800 at 50 ohms and it did, at 3.850 kHz. It was used on 75 phone for a while, easily working several long path stations (even with my poor long path shot). It was then decided to make a second element and use it as a 2 element Yagi. The boom length for a good pattern was selected at 35' and up it went. The 2:1 points on the dipole were over 300 kHz and the 2 element is about 150 kHz, with a wide frequency range for pattern and gain. One would expect this, of course, from an antenna with essentially full size elements. The center loading to move the reflector down was minimal, on the order of 1.5 uH. The antenna has also been used on evenings into EU, Africa, the Caribbean and South America. If the local power company could keep the line noise below its usual S7, many more stations could be worked; however, it is always nice to work DX on 75, regardless of the difficulty on receive. Relay boxes were added to operate CW, with excellent results. There was an immediate demand for this incredible Yagi, so we tooled up our welding facility and it is now being built. Since this is the ultimate in efficiency at 99.5%, it is simply called the Sigma-280 Ultra.
Sigma-280S 2 element 80/75 Mtr Yagi (at W7CB) w/relay boxes for phone and CW |
Sigma-280 Magnum 2 element 80/75 Mtr Yagi w/relay boxes for phone and CW |
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I had just painted the
coil covers at WA5VGI. A 2el 40 (EF-240S) is above and
WARC-7 is below the Sigma-280 Magnum. The tower is a US
Tower 572 (72') The cradle mount for the 5" and 4" boom is professionally welded and one of the coils is shown below with the cover removed. |
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Sigma-280 Ultra 2 element 75 Mtr Yagi at N6BT |
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35'). As another experiment, a single relay box has been added to the driver and it now runs on both phone and CW. The parasitic element functions as a reflector on phone and director on CW. Antenna direction is reversed on the rotator when on CW. First QSO was with 5B4AGN a few minutes after sunrise on New Year's day on a band that was not very good - one call and 579 with 1200 watts. |
Welded element construction, T-bars. | Looking at LP through a 12 degree hill that is only 300 mtrs away - nice to work anyone! |
Yes, the Sigma Yagis can be made in a single dipole. Two have been delivered and are at W0YK in the Santa Cruz mountains in California and at N6XI in the Sierras.
Please contact Force 12 for additional information.