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Re: [TowerTalk] Cushcraft/MFJ Traps

To: towertalk@contesting.com
Subject: Re: [TowerTalk] Cushcraft/MFJ Traps
From: Michael Tope <W4EF@dellroy.com>
Date: Fri, 13 Sep 2019 10:34:25 -0700
List-post: <mailto:towertalk@contesting.com>
On 9/13/2019 6:24 AM, jimlux wrote:
On 9/13/19 6:17 AM, john@kk9a.com wrote:
I currently have a pair of 2el shorty 40's made by Optibeam, model OB2-40. These use a high Q coil in the center of the elements for loading and have an 18' boom.  For months I have been modeling various full size 40m Yagis and comparing them to my small antennas. Larger antennas have more bandwidth but I have been amazed at the efficiency (at least in my model) of OptiBeam's shortened elements.  If I replace my current small 75 pound antennas with two full sized 350 pound 4 element OWA beams on a 48 ft boom, I will be only 2dB louder.


I think the OB2-40 has 10 meter long elements, which is about 50% of the full size resonant dipole for 40 meters.
Your modeled 2dB change is fully consistent with the calculation below..

As someone else pointed out, you do potentially give up some gain from a physically shorter antenna - or a narrower bandwidth, or something else. (TANSTAAFL)

There's even an equation for it: The Chu-Harrington formula tells you the tradeoff between physical size, Q, directivity

https://en.wikipedia.org/wiki/Chu%E2%80%93Harrington_limit


Hi Jim,

According to the Opti-Beam website, the OB2-40 has a longest element of 49ft.  As long as the inductor Q is decent, that will be a very efficient element. The 4 element OWA has a boom length 2.7 times that of the OB2-40, so that's where the 2dB difference is coming from. Of course, as you and others have pointed out, bandwidth is one of the trade-offs with physically shortened elements, so the OWA will very likely maintain its gain over a wider frequency range than the shortened Opti-Beam antenna, making the off center-frequency gain advantage of the OWA greater than 2dB.

BTW, the element length on the OB2-40M is 10.6 meters long (maybe that is what you were thinking of), but I don't think that is counting the bent tips (it's a Moxon version of the OB2-40).

73, Mike W4EF..........
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