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

To: towertalk@contesting.com
Subject: Re: [TowerTalk] DIR-DE-DIR on a 40m yagi ?
From: W0MU Mike Fatchett <w0mu@w0mu.com>
Date: Tue, 16 Nov 2021 17:05:32 -0700
List-post: <mailto:towertalk@contesting.com>
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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