Hi Ray,
Although the 61.5 dimension seems odd, it appears to be correct. Note that
the 1.25" diameter sections of each element become progressively shorter
with each successive element. Shortening D1 by 10" will significantly
reduce F/B performance and you will have a hard time adjusting your beta
match to get any significant bandwidth.
Something to be aware of... I found the dimensions listed in in ARRL YW
file place D1 in conflict with the boom connector plate - at least on my
pair older 204BA's. Here's some dimensions that I used that move D1 forward
just far enough to clear the boom connector and then re-optimize
performance, also corrected the element bracket length & overlap to actual
dimensions:
Inches - 1 - 2 -3 - 4 - 5 - 6
Diameter - 2.14 - 1.25 - 0.875 - 0.625 - 0.438
Reflector - 0.0 - 3.625 - 41.875 - 46.0 - 50.5 - 24.0 - 54.0
Driver - 95.0 - 2.925 - 32.075 - 46.0 - 50.5 - 24.0 - 52.0
Dir 1 - 167.0 - 3.625 - 18.375 - 46.0 - 50.5 - 24.0 - 60.5
Dir 2 - 306.0 - 3.625 - 14.125 - 46.0 - 50.5 - 24.0 - 49.125
Predicted performance:
Freq - 14.000 - 14.175 - 14.350
Gain - 8.07 - 8.15 - 8.26 dbi free space
F/R - 21.49 - 29.89 - 21.79 db
Z - (25.1-34.6j) - (25.8-25.5j) - (24.0-15.8j)
SWR - 1.46:1 - 1.02:1 - 1.46:1
Hairpin length is predicted at 16.7 inches.
Also be aware that the dimensions in YW are modeled from the centerline of
the boom, which is 2" diameter. Don't forget to add that extra inch of
boom radius if you are measuring your elements lengths from the outside of
the boom.
One last thing to be aware of is that I have found that the recommended
torque compensators are unnecessary - which can be shown by preparing a free
body diagram and applying the wind loads on the elements at any pitch angle.
The drag force vectors act parallel to the boom and do not create a torque
moment . I did have to add some weight to the front end of the boom to
balance the antenna.
Hope this info is helpful & good luck on your project.
73
Matt
KM5VI
>> I am rebuilding a old HyGain 204BA using the ARRL Handbook CD dimensions
that was run on YW. The CD has the D1 tip at 61.50", but this is longer that
any of the other elements tips. The rest of the element sections are the
same dimensions, so the only variable is the tip. I think this is a typo and
the tip length for D1 should be 51.50".
Can anyone confirm or correct my suspicion?
Tnx,
Ray,
N6VR
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