[TowerTalk] Cushcraft/MFJ Traps

[jimlux]

On 9/13/19 10:34 AM, Michael Tope wrote:
> 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. 

Oh yeah... that's 15 meters, 75% of full size, so yeah, a very tiny hit 
in efficiency


  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).
> 
Probably...

Tnx for catching it.