[TowerTalk] dipole z vs height

[Jim Lux]

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 at 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.