Hi John,
I believe that the 'basic rule' with an elevated loading coil is that the
current remains constant from the bottom end (feeding point) up to the coil
and will drop linear from the coil toward the end of the antenna.
So if you have the coil at the top of the vertical section you will get a
constant current over this section, the same that you would get with a
infinite topload (that is Andy's theory in other words ..).
But I am 'triggered' by something else :
I base this on measurements under our US no-license regulations at 1750
meters. One of my last attempts at operation from this qth, surrounded by
trees, resulted in 200ma at the base and roughly 150 to 170ma entering the
top hat with base tuning only. (I say "roughly" because I could only get
myself to about half the height of the vertical run and had to sight through
binoculars. Also, my notes have long since gone missing, so I'm having to
depend a lot on my write-only memory. This was all with a vertical run of a
little over 8m and a top radius a little less than 7m.)
At a rather poor location (trees) you manage to get 0.2A with 1W input,
what means a loss resistance of less than 25 Ohm.
When I read the results similar of other lowfers (antenna currents up to
0.4A, what would mean a loss of only 6 Ohm) I am always suprised by the low
losses they achieve.
From what I know from European stations on 136kHz the losses go from +/- 30
Ohm (those with a close-to-perfect location) to 150 Ohm at a poor location.
So either :
1. ground losses are very frequency dependent
2. American soil is so much better (for LF) than European soil
3. the conversion rate from American Watt to European Watt is not 1/1
Assuming that :
1. most lowfers stick to the 1 Watt input rule
2. soil can be different, but not over an entire continent
I believe that ground loss must be very frequency dependent
73, Rik ON7YD
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