[Fwd: LF: Re: Technical details of CFH (137kHz)]
Mon, 01 Nov 1999 18:27:37 -0500
> In a message dated 10/24/99 7:06:29 GMT Daylight Time,
> firstname.lastname@example.org writes:
> << I have done a quick calculation on a 200m loaded vertical using G4FGQ's
> program and this shows that such an antenna, assuming 50 ohms earth
> resistance, would have a radiation resistance of 13 ohms and an efficiency of
> 14.3%. A top loading umbrella is likely to improve this somewhat (and the
> earth resistance is likely to be quite a lot lower with a professional earth
> mat). Given the typical efficiency of antennas most of us can use, and the
> limitation to 1W erp I suspect this puts things into perspective!
> This is very interesting Dave.
> In an early slow CW article by Peter Martinez, I remember that he managed to
> receive signals from G4JNT on 73k which were predicted as 20dB below noise
> level in normal bandwidths (LF Source book)
> Some rambling......
> If the ERP from the VE on 137 is 20% of 10kW, that equals about 2kW ERP.
> I can't remember what the best signal to noise is on the signal. Would 10dB
> be about right?
> Therefore, for a 3dB received S:N+N, the ERP could be reduced by 7dB =
> 400 W
> If it's 10dB above noise in a 250Hz bandwidth, then by using an fft program
> to reduce the bandwidth by a factor of 400, (which is not unrealistic) would
> allow us to receive an amateur 1W signal over the same path with a 3dB S:N+N
> So 250Hz /400 = 0.625mHz equivalent bandwidth.
> I think that for normal CW it's going to be almost impossible, but for QRS
> techniques, it looks like a good challenge, which with some appropriate
> planning, may eventually succeed.
> The problem for the very slow data rates used by Andy and Peter would be
> changes in propagation during the transmission.
> How long is the signal audible at reasonable signal levels each night?
> and, I wonder if we'll mange to cross the Atlantic before the 2m Op's?