Topband: RX 4 square

[Michael Tope]

----- 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................................................