Topband: re: Ultralinear amps

[Tom Rauch]

> is 5-500 MHz, with 3.5 dB NF. Better would be the AM-109, 0.5-60MHz,
> with about +50 dBM 3rd order intercept. The NF is about 4.5 dB at
> 160M, however, which might limit its usefulness for pennant or loop
> application.

I think there may be some exaggeration of how much "noiseless 
feedback" helps. It is not required for high intercept or compression 
points, it mainly allows the designer to greatly reduce gain and 
control impedances through feedback without adding components 
that reduce noise figure.

The deterioration in NF is so minor with normal resistor feedback 
systems, I've never found it necessary to use "noiseless feedback". 
(Although I built some "noiseless feedback" amps in the 70's after 
the RF Design article appeared.)

With a 2.0 dB noise figure and 250 Hz BW, we could easily copy 
CW signals that are at a level of -144.6 dBm at the preamplifier  
input. There would be plenty of "headroom". 

With a deterioration to 6.5 dB NF, the sensitivity for the same 
(S+N)/N ratio only drops -140dBm! Hardly a big change.

The noise figure required depends on more than antenna size. With 
wider bandwidths or less directivity, or with higher external noise 
levels, noise figure is even less important.

I can't imagine a case where someone would need a 2 or 3 dB 
noise figure, especially with a low-directivity antenna, unless it is 
badly mismatched. I've settled on standard push-pull 2N5109's, and 
as Gary Breed indicates I've found MMIC amplifiers to have very 
poor performance. I found MAV-11's had too much harmonic 
distortion for use in the MFJ Antenna analyzers, even at 100 mW 
output with just a single signal! They required incredible amounts of 
quiescent current compared to the power levels being amplified for 
reasonable distortion.

My very directional arrays in a very quiet location, some of which 
have 20 foot resistor loaded elements, 
  
73, Tom W8JI
w8ji@contesting.com


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