[TowerTalk] DIR-DE-DIR on a 40m yagi ?

[W0MU Mike Fatchett]

That 3 element gets quite large for the a couple of DB.  Interesting the 
magic 20 ft number for 2 elements on 40.  That has been the standard for 
a while.

W0MU

On 11/16/2021 3:03 PM, David Gilbert wrote:
>
> I just got the chance to play with this in EZNEC+.
>
> A 2-element version for 40m at 90 feet up in the air seemed to 
> optimize at 67.6 foot element lengths spaced 20 feet apart.  The main 
> lobe was 10.65 dBi at 20 degree elevation.  Other lengths are not 
> bidirectional, and other spacings have less gain.
>
> A 3-element version for 40m at 90 feet up in the air seemed to 
> optimize with the same 67.6 foot parasitic elements but a slightly 
> shorter driven element (66.8 feet).  The optimum spacing was 31 feet 
> between each element.  The main lobe was 12.13 dBi at 18 degree 
> elevation.
>
> If you stop to think about it, these are basically parasitic W8JK 
> antennas except with higher feedpoint impedances.  A classic 2-element 
> W8JK with both elements fed has a really low feedpoint impedance  ... 
> typically 6 to 8 ohms.   The 2-element parasitic version described 
> here is more like 20 or 25 ohms.
>
> The 3-element version drops to something on the order of 10 ohms 
> feedpoint impedance, and requires a lot longer boom.  It is indeed 
> bidirectional, but I don't think the extra 1.5 dBi is worth it.
>
> I'm sure there are other configurations (lengths and spacings) that 
> are bidirectional.  Some may even perform better than the ones I 
> described above.  I have no idea what the 3-element SteppIR does when 
> it is told to go bidirectional.
>
> 73,
> Dave   AB7E
>
>
> On 11/15/2021 8:43 AM, Mark - N5OT wrote:
>> Dave,
>>
>> Is there an optimal element spacing for this design?  I'd run it 
>> through the computer except that computer's at home and I'm not.
>>
>> 73 - Mark N5OT
>>
>>
>> On 11/14/2021 7:36 PM, David Gilbert wrote:
>>>
>>>
>>> If you want to make a 3 or 2 element array be bidirectional, tune 
>>> the parasitic elements to the driven frequency.  Don't make it 
>>> shorter or longer.  You can run that through EZNEC if you don't 
>>> believe me.
>>>
>>> 73,
>>> Dave   AB7E
>>
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