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Re: Topband: RX 4 square

To: <>,"W0UN -- John Brosnahan" <>
Subject: Re: Topband: RX 4 square
From: "Tom Rauch" <>
Reply-to: Tom Rauch <>
Date: Wed, 22 Aug 2007 21:10:29 -0400
List-post: <>
> 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.

Another thing often ignored is the fact signals from the 
elements in an electrically small phased array are not 
additive in any direction. The overall output from the 
combination of elements is less than the output of a single 
element for both signals and external noise. When close 
spaced elements are combined there not only is a reduction 
in overall output level of signals and noise, there is also 
an even greater reduction of available noise power because 
the array looks at a smaller window of noise.

Of course local noise floor is also a factor, but most 
people already know the very obvious.

This is why, as I clearly stated earlier, there are 
applications where people can get away with a simple 
transformer. I can here with a simple dipole or vertical and 
with SSB bandwidth, except at the most quiet times.

The point I made, and it is a completely accurate point, is 
a transformer is NOT even remotely close to being the 
equivalent of a voltage follower. The loss of 20-30dB gain 
following a few dB noise figure is significant in many 
applications, probably more than when it isn't significant.

There's a big danger in assuming since one particular 
application manages to get away with 20-30 dB less gain and 
5-10dB less noise figure it generally applies to all other 
systems. My experience is it rarely applies to systems like 
a compact array used for weak signal DX'ing.

I'm sure there are cases where a transformer is fine, but I 
already said that in my very first post.

73 Tom

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