K9RX (StellarCAT) wrote:
>>> There's no "probably" to it! It is lowest (in frequency) closet to the
>>> ground. [snip] Finally on 40 meters I saw as much as a 300Khz change from
>>> 32" (on saw horses) to 37' on just one element of the yagi... I believe it
>>> was about 150 Khz from 8' to 37'. And think there might be an additional 20
>>> Khz or so shift from here on up to probably 50' or so.
For a 40m dipole model (length 66 ft, diameter 1 inch, aluminum, 29 segments,
over "Average" ground) the chart below shows (red trace) the frequency of
minimum SWR(50) and (blue trace) the frequency of resonance (jX=0 at feedpoint)
at heights from 2.5 ft (30") to 100 ft.
http://s33.postimg.org/vjtgi392n/resonate3.png
While it is true that the frequency (either one, take your pick) is roughly 300
KHz higher at 37 ft vs 30", and is higher still up to about 60 ft, from there
to about 100 ft the frequency *decreases* as the height is increased. It also
*decreases* from about 10 ft to about 25 ft. So it seems that no blanket
statement can be made. Whether the frequency increases as the height increases
depends on where you start looking at things, at least for this simple model,
which may or may not be an accurate representation of Gary's one element from a
Yagi.
Dan, AC6LA
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