Re: LF: RE: Earth electrode loop area - I'm struggling

[Roger Lapthorn <rogerlapthorn@gmail.com>]

Many thanks for this calculation Rik - you are a real star! 

I was using L^2 which may have been my problem (I found this in the RSGB LF handbook, which I guess is wrong). This makes the area some 600 times bigger than my calculation and a believable figure now.

Now looking at the actual area of the loop that is "in the air" (as opposed to in the ground) I estimate this as being:

15m^2          (the run along the garden fence)
5m^2             (the part under the sloping roof of the study)
30m^2          (the area under the house between the copper pipes and ground)

i.e. about 50m^2

This suggests the current flow is not deep within the soil as only an additional 20m^2 is in the ground. This would also suggest that the impact of higher conductivity soil should not be that great, as in the limit all the current flows along the surface?

Still, this was a worthwhile exercise and I have learnt something from it, and that is what our wonderful hobby is all about.

Thanks again Rik.

73s
Roger G3XBM

On 18 June 2010 22:10, Rik Strobbe <Rik.Strobbe@fys.kuleuven.be> wrote:

Hello Roger,

 

my pennyworth:

 

assuming

- 50uW ERP

- antenna is traded as a loop (gain = -0.4dBd)

 

Due to the negative gain (-0.4dB versus dipole) a loop antenna must "radiate" 55uW to get 50uW ERP.

Since the antenna feeding current is 0.15A the radiation resistance is 2.44 milliOhm (R= P/I^2)

The radiation resistance of a small loop is : Ra = 320xPi^4*A^2/L^4

where A = loop area (m^2) and L = wavelength (m)

For 500kHz Ra = 5*10^(-7)*A^2 or A = 1416*sqrt(Ra)

If Ra = 0.00244 Ohm then effective the loop area is 70m^2

 

73, Rik  ON7YD - OR7T

 

 

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Van: owner-rsgb_lf_group@blacksheep.org [owner-rsgb_lf_group@blacksheep.org] namens Roger Lapthorn [rogerlapthorn@gmail.com]
Verzonden: vrijdag 18 juni 2010 22:29
Aan: rsgb_lf_group@blacksheep.org
Onderwerp: LF: Earth electrode loop area - I'm struggling

Please can someone help me with the maths? I've been trying to work out my effective loop area and I cannot get sensible answers. These are the measured or estimated numbers I have:

Voltage across "loop"  = 24.48V rms  - measured
Current flowing in "loop" = 0.15A - measured
ERP (from far field levels) = 50-100uW (may be less) based on -18dB S/N with M0BMU

>From these figures I should be able to work out the loop area, but either my calculations are wrong or the figure is correct and the effective loop area is very very small.

Help!

73s
Roger G3XBM

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