Hello Toni, hello all,
> From HB9ASB, JN36pt
> 2. I have the means to measure the fieldstrenght (V/m) close to
my
> antenna (up to 100m away). Is there a reliable method to
calculate the
> ERP from the fieldstrenght?
Yes and no.
Yes - you could measure the field strength at a known distance from
your antenna and calculate the ERP from that. The equation is:
EIRP = [D * E]^2/30
EIRP = effective isotropic radiated power [W]
D = Distance [m]
E = electrical fieldstrength [V/m]
No - you cannot deduce ERP from taking Fieldstrength at a distance
of
100m, as you are still in the reactive near field of your antenna at
137kHz
and equation above is valid ONLY in the far field.
The nearfield/farfield transition is said to occur at 1/(2*PI) the
wavelength.
This would be 350m for 137kHz alias 2200m wavelength.
As I saw in articles regarding nearfield/farfield transition
distance, actual
measurements should be made at distances which are a function of
the radiators sizes related to the wavelength for the results to be
reliable.
Quater wave verticals require at least 1 wavelength distance.
Half wave verticals require at least 1.5 wavelength distance.
For shorter than 1/4 wavelength verticals the recommended measuring
distance is 1 wavelength.
If you cannot take measurements more remotely, then you may think
of using numerical methods (NEC) to calculate near field values of
electrical field strength. Assume a ERP value, go through the NEC
procedure, if resultant field strength is lower than measured one,
then increase assumed ERP etc.
Best 73
Gamal Soegiono (SWL)
50N01 / 08E27 / JO 40 fa
[email protected]
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