OptiBeam OB9-5HD-P Technical Article

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OptiBeam Tech Tip

OptiBeam OB9-5HD

9 Element 5-Band Yagi

Yagi, 20/17/15/12/10 meters, 9 Elements

5.1 m (16.73 ft.) Boom Length, 0.72 sq. m (7.8 sq. ft.) Wind Area

Includes Balun

This Tech Tip is from Tom, President of OptiBeam Antenna Technologies, call sign DF2BO, known on the air as DF2BlueOcean.

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A customer sent in a message about selecting a new antenna to replace a SteppR :

I currently use (actually did use) a 3 element SteppIR at 100 ft. and it has again developed issues that will require me to remove it from the tower. I would like to continue to use my Rohn 25g tower at 100' triple guyed and thinking of replacing the current antenna with another multi band antenna that would not need maintenance once installed and be able to cover at least 5 bands (20,17,15,12,10). Of course I would like 40M but I am afraid that adds too much wind load . I also want to have no traps and be able to run legal limit plus. I also have VHF/UHF yagis above the HF antenna

This is where Optibeam comes in. I like the robust looking quality and have looked at the specifications for the OB95HD. Is it correct to say it operates as a 2 element yagi per band or is there more at work here? It is difficult for me to compare manufacturers specs when it comes to gain and F/B figures. Some use Dbi and others Dbd and even when comparing numbers some seem to be either quite low or usually quite high based on the number of active elements and boom length.

Does the OB9-5HD seem like a good choice for my application or do you have another model to recommend? I really would like to not give up on gain vs the SteppIR but also don't want to work on it every other year.

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Thank you for your note and for your interest in OptiBeam antennas. Welcome to the club! You would be surprised on the number of e-mails we receive from SteppIR and Ultrabeam antenna users who tell us that they are tired of repairing their dynamic antennas and often give up on a DXpedition or fully participating in a contest.

Regarding your triple guyed 100 foot high Rohn tower. - I cannot tell how much wind loading this tower with this kind of guying is able to handle. But I am rather sure way more than just a 3 element SteppIR or an OptiBeam OB9-5.

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You have expressed interest regarding the OB9-5, in the following I will supply you with some detailed information about this antenna model.

We do a lot of computer analyzing followed by a lot of practice measurements. Just for comparison, a trapped tri-bander with 3 active elements per band has a gain of about 3.5 dbd (gain over dipole in free space) as an average on all three bands, which counts as long as the traps are new. The older they become the more they loose efficiency.

And, it also depends on the quality of the traps, most of the companies, sorry to say, really fabricate poorly in this respect. The good old Hy-Gain traps were of better quality, what measurements have proven, Mosley traps are of lower quality, also proven by measurements which were done in the US some years ago.

Trapped Yagis have a rather steep vertical take off angle due to the destroyed current distribution on the elements and the smaller active antenna surface (reduced capture area).

As another comparison, the usual Log Periodic will have a small active region per band and the elements, therefore gain is limited and the take off is rather steep as well.

The OptiBeam OB9-5 has about 4.3 dbd on 20 meters and about 4.5 to 4.6 dbd on 17-15-12-10 meters, clean patterns and very good SWR.

Gain and pattern wise it is situated between a 2 and a 3 element Yagi on all 5 bands. Theoretically it has 2 active elements per band, but due to the multi element structure and the positive

interaction of elements of close-by frequencies from the one to the other band in practice it has more than just 2 active elements on each frequency range. I dare to say, it is the one and only solution for the ham who wants to have all 5 bands in a perfect compact form.

Several practice side by side tests with a 3 element SteppIR realized by a couple of corresponding users have shown that the OB9-5 is as good, on two bands even better ! Why is that so since the 3 element SteppIR is a 3 element Yagi?

To the disadvantage of the dynamic antenna, because of the fixed spacing on all five bands (optimum only on 15 and 17 meters), due to the thin Beryllium bands which do not allow the skin effect to develop properly and since the surrounding fiberglass tubing absorbs some percentage of the radiated energy. Besides that, I am not telling you anything new, a dynamic antenna is a ticking time bomb. Failures seem to be pre-programmed is what the reports from users have proven. Some hams are lucky without any issues for many years, and others have failures of this and that (hung up electronics, broken motor shafts, etc.) even after just a few months of use.

Compared to a trap Yagi, the OB9-5 easily will beat it by minimum one S-unit and more ! Likewise it will beat a 2 element five-band quad on several bands. And a standard log periodic is far behind as well (not to be compared with our new Log Yagis).

 The OB9-5 would be a good choice also in terms of mechanical reliability.  The boom is square and consists of three sections.  It is a 40 x 40 x 2.0 mm boom which also is trussed by a 4 mm stainless steel wire support.

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 We have not lost a single OB9-5 boom since we have this antenna in line.  We have many OB9-5s on the Orkney Islands, and there the wind is just incredible.  Three hams were using 3 element steppers before and lost them due to the severe weather. So

they have switched over to OB9-5s and did not have any more problems (also they have been reporting that performance wise, they have not lost anything)

 The boom to mast mounting is a 220 x 160 x 8 mm plate, built of extra hard aluminium.

Also the elements are heavy duty designs, tapered mulitple times:

 20 meters: from 30 x 2.0 to 12 x 1.5  17 meters: from 25 x 2.0 to 12 x 1.5  15 meters: from 20 x 1.5 to 12 x 1.5  12/10 meters: from 16 x 1.5 to 12 x 1.5

So if there is an antenna which is made to withstand harsh weather environments, it really is the OB9-5. I hope this supplies the answers to your questions with some additional information regarding the OB9-5. Best 73s, OptiBeam Antenna Technologies President (Thomas Schmenger), Tom, DF2 Blue Ocean

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