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

[David Gilbert]

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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