Hello Roger,
I am curious what you will hear from them.
For a coil (or loop antenna) the magnetic field at a certain distance along the coil axis can be calculated
as:
B = (u0*N*a^2*I)/(2*(a^2+z^2)^1.5)
where B = magnetic field (T) , uo = 4*Pi*10^-7 (vacuum permeability), N = number of turn of the coil,
a = coil radius (m), I = coil current (A), z = distance from coil (m).
See http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/curloo.html.
The relationship between magnetic field (B in T) and magnetic field strength (H in A/m) is B = u0*ur*H (for
air the relative permeability ur = 1)
So based the allowed magnetic field (strength) at a certain distance and coil
(loop antenna) dimensions the maximum current through the coil can be calculated.
And from the I and Rrad you can calcualte the allowed ERP.
Limitation: for a large coil the maximum field stength at 10m distance (from any
point of the antenna) will not be along the coil axis.
73, Rik ON7YD - OR7T
Still awaiting OFCOM responses after about 8 working days since first email sent. Will post reply here, assuming they ever manage one. I am not hopeful.
73s
Roger G3XBM
---------- Forwarded message ----------
From:
Roger Lapthorn <[email protected]>
Date: 10 February 2014 13:46
Subject: Questions about 73kHz and sub 8.3kHz
To:
[email protected]
Early last week I emailed OFCOM but I have still not received responses:
My questions were:
Am I correct in assuming that radio amateurs may legally experiment in the old 73kHz band as long as output power is less than 72dBuA/m at 10m? This is the limit for licence-exempt inductive devices.
Also, please can you confirm that no licence is needed to operate below 8.3kHz (assuming no harmful interference to services above 8.3kHz). This part of the spectrum is unallocated.
Thanks
Roger Lapthorn
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