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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