[Fwd: LF: Re: Technical details of CFH (137kHz)]

[Andre' Kesteloot]

G0MRF@aol.com wrote:

> In a message dated 10/24/99 7:06:29 GMT Daylight Time,
> sergeantd@compuserve.com 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?
>
> 73
>
> David