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Re: LF: MF: 630m band dipole tests today

To: <[email protected]>
Subject: Re: LF: MF: 630m band dipole tests today
From: "Markus Vester" <[email protected]>
Date: Thu, 2 Aug 2012 23:11:43 +0200
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Thank you Stefan for this interesting experiment and report. Here are a couple of thoughts I have discussed with Stefan, and which I would like to share with the group.
 
Regarding the decrease of the resonance frequency close to earth: The wire is a line with distributed capacitance and inductance. If you would bring a halfwave line in close proximity to a perfectly conducting plane, the capacitance would be increased. But at the same time the inductance would decrease, as the magnetic flux would be confined by the smaller gap between the wire and the ground plane. So the wave impedance would go down but the resonance frequency would remain nearly unchanged.
 
A couple of meters above real ground, the capacitance would still be increased as long as its impedance is high compared to that of the soil. However the magnetic fields will penetrate the soil to some depth, so the effect on the inductance is much less. Thus the resonant frequency will go down at low height.
 
However when the wire is actually laid on the ground, the capacitance becomes so large that its impedance falls below the ohmic ground resistance. So then you will end up with a nonresonant nearly real impedance - just what Stefan observed.
 
Regarding the radiation pattern: An low dipole, oriented eg. in the E-W direction, would be expected to have three distinct modes of radiation:
 
- a high angle skywave ("Near Vertical Incidence Skywave"), with more or less omnidirectional pattern, and the E-field vector polarized E-W,
 
- a horizontally polarized low angle radiation, broadside to N and S. However this would be heavily attenuated by the ground acting as a short circuit, and would not be able to develop into a propagating ground wave,  
 
- a vertically polarized groundwave to the E and W, generated by an earth loop formed by the wire and the subsurface return current. This is the same mechanism as in the VLF earth-antenna experiments, only with the galvanic end contacts replaced by capacitive coupling. I believe that this mode enabled the daytime QSO between Stefan and myself, 180 km due east of him. This axial radiation would of course not be present for a free space dipole.
 
Best 73,
Markus (DF6NM).

Sent: Thursday, August 02, 2012 5:51 PM
Subject: Re: LF: MF: 630m band dipole tests today

MF,

Here is my report of the MF dipole experiment of last saturday: https://dl.dropbox.com/u/19882028/MF/A%20full%20size%20dipole%20for%20the%20630%20m%20band.pdf

73, Stefan/DK7FC
 
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