From: G0MRF@aol.com
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Date: Sun, 24 Oct 1999 20:25:54 EDT
Subject: LF: Re: Technical details of CFH (137kHz)
To: rsgb_lf_group@blacksheep.org
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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