Force 12 C31XR vs. Optimized Monobanders

Since moving to New Mexico in 2003, I have had the pleasure to use NA5S's fine QTH.  Despite having a relatively simple antenna system, I have achieved excellent contesting success from Britt's station (#5 USA M/S IARU, #2 August NAQP CW, #2 SS CW).  Fundamental to the experience I have enjoyed at Britt's QTH is his Force 12 C31XR @ 52'.  I have been extremely impressed with the C31XR, and have wondered whether Force 12 really has achieved monobander performance on 10, 15 and 20 meters on a single 31' boom.

To study this further, I used the dimensions from the C31XR instruction manual and the "Leeson corrections" to create a cylindrical-element model of the C31XR that could be used with MultiNEC running a NEC-2 antenna model.  The element spacings are exactly the same as in the C31XR instruction manual, except that the 15 meter driven element has been moved slightly closer to the 20 meter driven element to achieve an acceptable SWR.  Dean Straw, N6BV, provided useful suggestions for modeling the closely-spaced 15 and 20 meter driven elements.

I then modeled the previously-optimized 10, 15 and 20 meter yagi designs from the 20th edition of the ARRL Antenna Book.  Specifically, I used the 10 meter, 24 foot boom, 15 meter, 24 foot boom, and 20 meter 26 foot boom designs.

Picture of C31XR

C31XR MultiNEC Model

Discussion:

On 10 meters, the C31XR appears to have 7 active elements, with performance comparable to a 5 element monobander.  The "extra" elements on 10 meters compensate for interaction with the 15 and 20 meter elements.

On 15 meters, the C31XR appears to be have about 1 dB less gain than a 5 element monobander. This seems reasonable, since the C31XR has 4 active elements on 15 meters,

On 20 meters, the C31XR is about 0.5 dB down from the 4 element monobander.  The 10 and 15 meter elements have very little impact on the 20 meter performance.

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