[TowerTalk] Cushcraft/MFJ Traps

[Michael Tope]

On 9/13/2019 6:24 AM, jimlux wrote:
> On 9/13/19 6:17 AM, john at 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..........