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Re: [TowerTalk] antenna choices

Subject: Re: [TowerTalk] antenna choices
From: Jim Brown <>
Date: Mon, 06 Apr 2015 22:47:01 -0700
List-post: <">>
On Mon,4/6/2015 8:14 PM, Matthew King - KK4CPS wrote:
What are you unhappy with about the T11, and what are you looking to

Yes, that's the real question. Part of that is to know the forward gain of the T11. I see a single number for gain for an antenna that covers two octaves of frequency. Somehow I don't trust that number on all bands. Also, by the gain numbers (5.8 dBd, 7.94 dBi, 13.14 dBi @ 1 wavelength height) I see an asterisk, but no note to go with the asterisk. :) There's an LPDA design in the ARRL Handbook -- it's 12 elements on a 26 ft boom, 2 ft longer than the T11. It's gain is 6.6 - 6.9 dBi, which is 1 - 1.3 dB less than the gain claimed for the T11, so there's either some design magic or some specsmanship happening with the T11. :).

Now, look at the Optibeam numbers and description. This is a 2-el Yagi on all bands except 10M and maybe 12M. The gain numbers are 4.3-4.5 dBd which translate to 6.45 - 6.65 dBi (free space). That's slightly better than a near perfect 2-el Yagi, so there's either some design magic or some specsmanship happening here too. :)

Now look at the 3-el SteppIR -- 7.4 dBi on 20M, 8.3 dBi on 17, 8.5 on 15, 8.8 on 12, 9 dBi on 10M, and 10.1 dBi on 6M (with 4th fixed element). Those numbers are quite believable for a 3-el monoband Yagi on an 18 ft boom. The gain is lower on 20M because the boom is a bit short for 20M, and the gain improves as the boom gets longer as a fraction of a wavelength.

So -- repeating the question, what do you hope to achieve by going to another antenna? There seems to be no question that the Optibeam is ruggedly built, but it's unlikely that it will perform measurably better that what you have. Folks are always saying that whatever they have is wonderful, usually based on all the DX they worked. But the basic facts are that if you want more gain, you need more elements PER BAND and a longer boom, also between elements. For example -- with the Optibeam, there are two 20M elements and they appear from the photo to be only about 8 ft apart. That's awfully closed spaced for 20M -- the SteppIR has 3 el 8 ft apart, and that spacing is short for 20M.

The OB 11-5 (3-el/band, 19 ft boom) looks to be the performance equivalent of the 3-el SteppIR, the 16-5 (4-el/band, 33 ft boom) the performance equivalent of the 4-el straight SteppIR. At the current EU-dollar value, the 11-5 is about $1,650, the 16-5 about $2,640. The SteppIRs are about $2,000 and $2,800 respectively.

73, Jim K9YC


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