A Little Dab Will Do You - Sensitivity in Tuning the Bruce Array

At the Washington Island W9EVT QTH George W9EVT has been running a full sized 40 meter Bruce Array Tom KC9JGD and I built from the remainder of a previous Bruce Array that had been lost to the grounds keeping crew’s tractor. You can read about the Bruce Array at “A 40m Bruce Array for WI-001L Operations.”

This last trip we retuned the 10 month old Bruce Array to try and center its natural resonance as close to 7.268 as possible.

The previous array was better optimized for the effects of towers at each end supporting it, but alas those measurements were unavailable and there had not been enough of the original array left after the Bush-Hog “ate it” to get anything more than generalized measurements.

Underestimating some of the effects of the towers, ground situation and being at a 60 degree bottom-forward position, my calculations had lead us to build a usable, but electrically too short design.

So we set off to do a retuning.

My expectations were to roughly add wire in proportion to the amount.

Was not to be – the observed tuning effect was roughly 10 to 14 times more sensitive than expected.

(click on graphic to enlarge)

I have to do some more research, but it would appear that altering the gap was more of an effect than the actual wire added.

Have some further experiments planned to further improve the SWR of the Array, but with only 9 inches added towards closing each gap the center of resonance moved from 7.500 to 7.260

Of course W9EVT teased me about missing our 7.268 target, but if the gaps are truly that sensitive it should be a simple matter to hit any desired center of resonance.

73

Steve
K9ZW

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