Starting new thread..
It will have an effect, but I don't know about a huge one. Here's some
sample data from a dipole at 3.68 MHz, with legs 19.646m long, copper wire.
Two soil conditions 14/.005, 5/.001 the former is moderately wet, the
latter definitely dry.
three heights above ground 10,20,30 meters
ht Z wet Z dry
10 52.4+j7.63 65.5-j1.78
20 84.6+j1.39 81.2-j7.17
30 85.7-j24.1 80.1-j77.9
Ignore spurious digits of accuracy..I just copied lazily
10m is pretty low (1/8th wavelength).. and as Jim K9YC pointed out, the
Z is close to 50 ohms
30m is getting up there >3/8 wavelength and the soil has less effect.
It looks like the antenna is a bit off tuned, but the Z is definitely
closer to the "free space" 72 ohms (and given I modeled with copper
wire, you'd expect a bit higher from the resistance).
And, pretty much as expected, the soil moisture has small effect at 30m
(and even at 20), but a bigger effect at 10 meter heights. The big
effect of moisture is also the change in resonant frequency, more than
the actual Z (that is, it has a bigger effect on the imaginary term)
In any case, my own experience has been that an antenna makes a lousy
way to measure soil properties, even if it's *laying on the ground*
(something that when I researched the literature, I found others seem to
agree with me, hence the popularity of things like OWL probes)
--
Jim, you may well find that the resonant frequency as well as impedance
of your dipole increases as you raise the height above ground. Ground
moisture and resulting conductivity and dielectric constant changes
have quite an influence on both impedance and frequency.
Gerald K5GW
In a message dated 2/20/2013 4:48:17 A.M. Central Standard Time,
jim.thom@telus.net writes:
## I will try and attempt to test that high/low dipole theory out,
next summerwith my 80m rotary dipole on its motorized tower. 40 feet
retracted, and 103’extended.
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