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@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.
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