Pieter-Tjerk's sketch of equipotentials around a mast is a good explanation of the effective height of a capacitive probe on a mast. Dioes anyone know a simple analytic formula for that E-field enhancement near the apex, maybe modelling the mast with a tall and slim ellipsoid?

 

Regarding ground on space ships: I have always wondered how the TACAMO guys excite the VLF transmit long wire dangling from their aircraft. Does the aitframe itself provide enough capacitance against free space? Or do they have a second (perhaps shorter) wire being dragged at a higher angle?

 

Best 73,

Markus (DF6NM)

 

On Sat, Jul 06, 2013 at 06:05:01PM +0200, Stefan Schäfer wrote: 

 

> Am 06.07.2013 15:16, schrieb Pieter-Tjerk de Boer: 

> >This view also at least approximately matches VK1OD's NEC4 calculation, 

> >in the sense that he finds an output voltage which is of the order of 

> >the field strength times the antenna height. 

> Not the antenna height but the distance between the probe and the 

> grounded shield of the coax, or the grounded part of the circuit, 

> which is the closest grounded part to the probe. 

 

Actually, I meant the real height above ground, multiplied by the 

free-space field strength. 

 

But of course, you are right that what the device responds to, is the 

field between the probe and the nearby ground. However, the field 

strength there will be much larger, due to the fact that the ground 

has been "brought up". 

This much larger field strength mostly compensates for the much smaller 

distance, so the output voltage is still roughly the same as when 

one just multiplies the real height above ground by the free-space field 

strength. 

 

See the attached sketch: the equi-potential lines in the area near the 

probe are much closer together because the pole is at ground potential. 

 

B.t.w., a member of this mailinglist contacted me off-list and pointed 

out that there is an article in VHF Communications 96/2 in which this is 

discussed in more detail. A German version of the same article has 

appeared in UKW-Berichte in 1994/1995 (but those years unfortunately are 

missing in our radio club's collection). 

 

> The potential 

> difference, the intagral E . ds... Of course the E field is not 

> homogen in the electrode arrangement, not a simple idealized plate 

> capacitor... 

 

Indeed. 

 

73, Pieter-Tjerk, PA3FWM