Hi all the Excel spreadsheet for calculating ERP for use at 500kHz (and any
other frequency) is fine if used with the values given. Some wished to
modify the sheet for very short top-loads compared with my 20m minimum. This
can be done but it seems you run into granularity of the lookup table which
will always give the lower value for form-factor if the top to height ratio
is not exactly on a cell value. This means the sheet underestimates the Rrad
slightly for some ratios on very short top loads. Once the top-length is
equal to the height the error is very small. I will rework the sheet
eventually to cover this problem by fitting an equation to the lookup table.
( N.B. this table was intended for use on Battleships !!)
The lastest version ( ERP1.xls) dated 10/3/07 should be correct. I note
though that the table gives a 0.639 form factor for an unloaded vertical
when I thought this should be 0.5. I will need to check this out further. It
came to light as Finbar is experimenting on short verticals with very small
top-hats. We hope to write up his findings in due course.
His present beacon is on a top-inductively loaded 7m vertical with short
capacity wires!! This is an aerial that most should be able to fit into even
a small garden. The trick will be whether we can make it efficient with such
a small top capacity.
Cheers de Alan G3NYK
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