[TowerTalk] Fwd: Change in Frequency As Antenna Height Rises

[jimlux]

On 6/1/16 9:02 PM, Paul Braiman wrote:
> The folks from SteppIR mention just such a check in their DB18 assembly manual. It was reassuring to see it resonate somewhere on all bands before I went through the effort to get it onto the tower. What I cannot remember was how it read on my analyzer when the beast was sitting on the sawhorses...I believe it was lower than the frequency I dialed up on the controller, but 2+ years since install has left my memory very fuzzy on that point.
>

Typically, sitting on saw horses (a tiny fraction of a wavelength), 
you're seeing the "loading" effect of having a dielectric near the 
antenna which will reduce the resonant frequency.  It's the same idea as 
the difference between using bare and insulated wire.

There have been a lot of attempts over the years to measure soil 
properties by measuring the Z (or resonant frequency) of a dipole at a 
known height over the soil. Although the technique seems attractive (and 
easy), it's generally not been successful, which is why the Open Wire 
Line (OWL) technique was developed by George Hagn at SRI.

The "measure a dipole" technique isn't great because there's too much 
other stuff that can affect the field, and the soil isn't very uniform, 
and the field is different along the length of the dipole, so the 
contribution to the measured value isn't uniform over the area being tested.

Measuring the Z of a short open wire transmission line immersed in the 
soil turns out to be a much more stable measurement technique. It's in a 
small area, it's actually an easier measurement, and most important, 
you're not dealing with the interface between air and soil. The probe is 
entirely immersed in the medium being measured.

There are coaxial impedance probes made for VNAs.. same sort of logic 
applies.