Topband: RX 4 square

[Joe Subich, W4TV]

Mike, 

> If the sky noise power (read propagated noise +  local QRN) is
> 10dB above the receive system thermal noise at 2.1 KHz 
> bandwidth, it will still be 10dB above the receive system thermal 
> noise when you ratchet down the bandwidth to 200 Hz bandwidth.

This is only true if the bandwidth is determined in the detector 
as happens with a DSP demodulation.  Since most amateur transceivers 
set the demodulated bandwidth with a crystal or mechanical filter 
early in the IF, the noise bandwidth is effected by the noise of 
the IF system (particularly the last IF stage) and is independent 
of the RF "window."  The effect is more significant with weak 
signals as the IF stages will tend to run "wide open" and wideband 
noise will be maximum. 

73, 

   ... Joe, W4TV 
     



> -----Original Message-----
> From: topband-bounces at contesting.com 
> [mailto:topband-bounces at contesting.com] On Behalf Of Michael Tope
> Sent: Thursday, August 23, 2007 1:37 AM
> To: Tom Rauch; Topband at contesting.com; W0UN -- John Brosnahan
> Subject: Re: Topband: RX 4 square
> 
> 
> 
> ----- Original Message ----- 
> From: "Tom Rauch" <w8ji at contesting.com>
> To: <Topband at contesting.com>; "W0UN -- John Brosnahan" 
> <shr at swtexas.net>
> Sent: Wednesday, August 22, 2007 6:10 PM
> Subject: Re: Topband: RX 4 square
> 
> 
> >> I built an RX array in 1989 using three sets of crossed 
> >> dipoles.  Each dipole
> >> was about 10 feet above the ground and a total of 24 feet 
> >> long (2 by
> >> 12 ft pieces
> >> of 1" aluminum tubing).
> > 
> > One big factor people miss is the bandwidth of the receiver.
> > 
> > Noise power is proportional to receiver bandwidth and a wide 
> > receiver or detector system limits on propagated noise much 
> > more easily than a narrow receiver. It doesn't take nearly 
> > the receive system noise figure to limit on propagated noise 
> > when BW is wide. For example when I go from 100-250Hz wide 
> > CW filters to a 2.1kHz wide SSB filter I can drop ten dB out 
> > of the system gain and noise figure without any problem.
> > 
> 
> If the sky noise power (read propagated noise +  local QRN) is
> 10dB above the receive system thermal noise at 2.1 KHz 
> bandwidth, it will still be 10dB above the receive system thermal 
> noise when you ratchet down the bandwidth to 200 Hz bandwidth. 
> 
> Conversely, a system that is thermal noise limited at 2.1 KHz 
> bandwidth will be thermal noise limited at 200 Hz bandwidth. 
> Sensitivity will improve when you ratchet down bandwidth, but 
> the ratio between the receiver thermal noise and sky noise will be 
> unchanged (and in this example dominated by the receiver thermal
> noise). 
> 
> Mike, W4EF................................................
> 
>  
> 
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